Application of low
water exchange
microbial floc
technology for
production of Penaeus
monodon under
Australian conditions
A Start - Up
Guide
Project No.
2012/729
Version 1
December 2012
This guide was developed by Anni Conn (Conn & Associates) and Matt West (Australian Prawn
Farms) on behalf of the Australian Seafood CRC and in collaboration with the Australian Prawn
Farmers Association.
ISBN: [insert number]
Copyright, 2012: The Seafood CRC Company Ltd, the Fisheries Research and Development
Corporation and Australian Prawn Farmers Association
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publication may be reproduced by any process, electronic or otherwise, without the specific written
permission of the copyright owners. Neither may information be stored electronically in any form
whatsoever without such permission.
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change without notice.
CONTENTS
FOREWORD ...............................................................................................................................7
ACKNOWLEDGEMENTS ...........................................................................................................9
CONTRIBUTORS .......................................................................................................................9
1.
INTRODUCTION............................................................................................................. 10
WHAT IS BIOFLOC TECHNOLOGY? ..................................................................................................................... 10
THE ADAPTATION OF BIOFLOC TECHNOLOGY BY AUSTRALIAN PRAWN FARMS .......................................... 12
LOW WATER EXCHANGE, MICROBIAL FLOC SYSTEMS – DISPELLING THE “BIOFLOC” MYTHS. ...................... 12
2.
IS A LOW WATER EXCHANGE, MICROBIAL FLOC PRODUCTION METHOD
APPROPRIATE FOR YOUR BUSINESS? ................................................................................. 15
WHY ADOPT THIS TYPE OF POND MANAGEMENT SYSTEM? ............................................................................ 15
DOES IT SUIT YOUR BUSINESS MODEL? ............................................................................................................. 15
DO YOU HAVE THE RIGHT PEOPLE, INFRASTRUCTURE AND EQUIPMENT?...................................................... 17
People............................................................................................................................................................. 17
Aeration ......................................................................................................................................................... 17
Aeration System Back Up ........................................................................................................................ 18
Pond Design ................................................................................................................................................. 18
Operational Equipment ............................................................................................................................ 19
3.
AN OVERVIEW OF THE MICROBIAL COMMUNITY AND WATER CHEMISTRY
DYNAMICS OF A LOW WATER EXCHANGE, MICROBIAL FLOC SYSTEM ......................... 20
PRAWN NUTRIENT METABOLISM AND TOTAL AMMONIUM NITROGEN ........................................................ 20
INFLUENCE OF PH ON AMMONIA TOXICITY ...................................................................................................... 21
BACTERIAL COMMUNITIES IN AQUATIC SYSTEMS ............................................................................................. 22
NITROGEN CONVERSION PATHWAYS ................................................................................................................. 22
Photoautotrophic ammonia removal .................................................................................................. 23
Litho-autotrophic bacterial ammonia removal (nitrification)..................................................... 24
Nitrification and Alkalinity...................................................................................................................... 26
Heterotrophic bacterial nitrogen removal ......................................................................................... 28
Denitrification.............................................................................................................................................. 30
FLOCS..................................................................................................................................................................... 31
NUTRITIONAL BENEFITS OF FLOCS ..................................................................................................................... 31
4.
GETTING STARTED - PRELIMINARY STEPS TO IMPLEMENTATION ......................... 33
SET WATER QUALITY PARAMETERS AND TOLERANCE LEVELS .......................................................................... 33
IDENTIFY YOUR PRODUCTION SYSTEM MANAGEMENT “TOOLS” .................................................................... 33
UNDERSTAND THE CONSEQUENCE OF THE USE OF EACH “TOOL” .................................................................. 34
SOURCE AND APPLY ADDITIVES IN THE RIGHT FORM ....................................................................................... 35
Sources of Silica .......................................................................................................................................... 35
Sources of Inorganic Carbon .................................................................................................................. 35
Sources of Organic Carbon ..................................................................................................................... 35
4
MANAGING THE C:N RATIO ............................................................................................................................... 36
METHODS OF ADDITIVE APPLICATION .............................................................................................................. 38
5.
POND MANAGEMENT .................................................................................................. 39
THE PROCESS OF POND DEVELOPMENT ............................................................................................................. 39
GRAPHICAL DEPICTION OF POND MICROBIAL DYNAMICS AND AMMONIA LEVELS DURING POND
DEVELOPMENT ...................................................................................................................................................... 42
STOCKING DENSITY.............................................................................................................................................. 43
ALGAL MANAGEMENT ......................................................................................................................................... 43
AERATION PLACEMENT........................................................................................................................................ 44
MONITORING OF PONDS .................................................................................................................................... 45
POND MANAGEMENT DECISION MAKING PROCESS....................................................................................... 46
WATER EXCHANGE ............................................................................................................................................... 47
FLOC DENSITY....................................................................................................................................................... 47
SLUDGE MANAGEMENT....................................................................................................................................... 48
ACTIVE SUSPENSION ............................................................................................................................................ 49
NUTRIENT DISCHARGE ........................................................................................................................................ 49
CONTINGENCY PLANS ......................................................................................................................................... 50
6.
KEY POINTS: THE AUSTRALIAN PRAWN FARMS’ APPROACH TO MANAGING A
LOW WATER EXCHANGE, MICROBIAL FLOC PRODUCTION SYSTEM FOR P. MONODON.
51
7.
REFERENCES AND RECOMMENDED READING........................................................... 53
8.
APPENDIX 1 ................................................................................................................... 56
RESEARCH PAPER SUMMARY - SMITH AND WEST 2009 ................................................................. 56
BACKGROUND ...................................................................................................................................................... 56
METHODS.............................................................................................................................................................. 56
Fertiliser Comparison Trial ...................................................................................................................... 57
RESULTS ................................................................................................................................................................. 57
Biofloc Formation ....................................................................................................................................... 57
Total Organic Carbon ............................................................................................................................... 57
Total Ammonium Nitrogen .................................................................................................................... 58
pH and Ammonia....................................................................................................................................... 58
Total Inorganic Carbon ............................................................................................................................ 59
Silica and Silicate ....................................................................................................................................... 59
Filterable Reactive Phosphorus.............................................................................................................. 59
Nitrate and Nitrite ..................................................................................................................................... 59
Molasses, Bicarbonate, and Biofloc ...................................................................................................... 59
FERTILISER COMPARISON TRIAL RESULTS ......................................................................................................... 60
Water Quality .............................................................................................................................................. 60
Microalgae .................................................................................................................................................... 60
DISCUSSION OF FERTILISER COMPARISON TRIAL RESULTS ............................................................................. 60
CONCLUSIONS ...................................................................................................................................................... 60
5
The Effect of Nutrient Conditions on Establishing Biofloc ............................................................ 61
Effectiveness of Different Carbon Sources.......................................................................................... 61
NUTRITIONAL BENEFITS OF BIOFLOCS AND REDUCTION IN FEED COSTS ..................................................... 62
ENVIRONMENTAL BENEFITS AND EFFECTS ON FARM PROFITABILITY ............................................................ 62
6
FOREWORD
T
his start-up guide contains important recommendations and advice for Australian prawn farmers
who are considering the adoption of a low water exchange, microbial floc production system for
use within their farming operations.
The low water exchange, microbial floc production system described in this guide is based upon the
principles of “Biofloc Technology” but follows a much-modified approach to conventional Biofloc
Technology practices. By definition, the term “Biofloc Technology” refers to a method of prawn or fish
pond / tank management that combines very high stocking densities, a dense heterotrophic bacterial
community, zero or little water exchange, high levels of aeration, removal of sludge and suspended
solids during the crop and active harvesting of flocs by the animals under culture. The pond
communities in these Biofloc systems also comprise algae and litho-autotrophic bacteria, but the
primary focus of these systems is to exploit the nutrient immobilising capacity of heterotrophic
bacteria to control water quality.
Those Australian prawn farmers that are presently exploiting the ability of bacterial communities to
help control water quality are employing a modified approach to that described above. The first
chapter in this guide therefore aims to dispel some of the myths of “Biofloc Technology” in the
Australian prawn-farming context and to accurately explain the low water exchange, microbial floc
methodology that is currently being utilised within the Australian prawn farming industry.
The successful implementation of a low water exchange, microbial floc production system depends
largely on the level of operator knowledge of pond water chemistry and microbial community
dynamics. Operators must also be committed to investing in the necessary farm infrastructure,
equipment and inputs required to meet the specific needs of this method of pond management.
This guide is not written as a “how to” for farmers wanting to learn the practical methods of
implementing a low water exchange, microbial floc production system. Low water exchange, microbial
floc systems are a relatively new area of pond management and more research is required into the
application of this technology for intensive commercial black tiger / banana prawn farming under the
range of variable Australian conditions before a “how to” manual can begin to be considered. Rather,
this guide is aimed at providing farmers with an overview on what they need to know and what steps
they will need to take before being able to adopt a low water exchange, microbial floc approach for
their production.
The development of this guide is in direct response to prawn farmer needs identified during an
Australian Seafood CRC extension program associated with the research project “Increasing the
profitability of Penaeus monodon farms via the use of low water exchange, microbial floc systems at
Australian Prawn Farms” (Australian Seafood CRC Project No. 2009/748. David Smith and Matt West).
This research demonstrated that the effective co-management of algal and bacterial communities for
control of water quality can deliver increased productivity and other benefits for P. monodon
production under Australian conditions. See Appendix1 for a summary of these results.
7
This guide also recommends research papers, books and other publications that have been written on
the subject of biofloc technology, water chemistry and the behaviour and ecology of microbial
communities within aquatic systems, that farmers may find useful when researching implementation
methods and management strategies.
For the purposes of this guide, the focus will be on the application of low water exchange, microbial
floc systems for intensive commercial P. monodon farming. However the information provided will also
be of significance for commercial farmers of banana prawns (Penaeus mergiuensis) and may be of
interest to the Barramundi (Lates calcarifer) farming community also.
The term “prawn” will be used when describing this species or its culture in the Australian context, as
this is the description used within the Australian prawn farming industry. In most of the current global
research literature, Penaeidae species are referred to as “shrimp” and therefore where quoting and
referencing from this literature, the term “shrimp” will be utilised.
8
ACKNOWLEDGEMENTS
Lorem Impsum
CONTRIBUTORS
Mr Brad Colcott
Pacific Reef Fisheries, Ayr, Queensland, Australia
Ms Annemette Conn
Conn and Associates, Darwin, Australia
Mr Wayne DiBartolo
Pacific Reef Fisheries, Ayr, Queensland, Australia
Mr Alistair Dick
Pacific Reef Fisheries, Ayr, Queensland, Australia
Mr John Moloney
Pacific Reef Fisheries, Ayr, Queensland, Australia
Mr Matt West
Australian Prawn Farms, Ilbilbie, Queensland, Australia
9
1. INTRODUCTION
WHAT IS BIOFLOC TECHNOLOGY?
B
iofloc technology (BFT) is based upon a comprehensive philosophy of viewing the culture of fish
or prawns as part of a complex interactive aquatic eco-system. By understanding this eco-system
and controlling the pond microbial community it is possible to improve water quality and maximise
feed recycling.
The underlying principle of a BFT culture system is the promotion of a stable, heterotrophic bacteria
dominated pond environment instead of a phytoplankton (algae) dominated one that is light sensitive
and can be highly unstable.
In a BFT culture system, where there is little or no water exchange, water quality is managed through
the development of a co-culture of heterotrophic bacteria and algae that are grown in flocs under
controlled conditions within the culture pond. The heterotrophic microbial community colonises the
particles of organic waste suspended in the water column, forming aggregates or flocs comprising
bacteria, algae, protozoa and zooplankton.
In very simple terms a biofloc technology culture system works as follows
Limit water exchange
Organic residues accumulate
Mix and aerate
Heterotrophic bacteria thrive and 'floc'
Flocs assimilate waste nutrients
Water quality is stable
Animals eat flocs
Nutrients are recycled
IN A BIOFLOC POND THE POND BECOMES THE BIOFILTER
10
This active microbial community absorbs nitrogen, phosphorous and other nutrients from the water.
The process gives a stable pond environment and recycles nutrients that are not retained as prawn or
fish biomass into a reusable form of feed. Grazing on the flocs by the culture animal importantly
enables reuse of protein that would otherwise by lost as a waste product (Avnimelech 2006)
BFT systems have several advantages over conventional flow-through culture systems:
BFT System Advantages
Stable culture environment
Higher survival rates
5% - 10% greater production capacity
Reduced FCR’s due to nutrient recycling
Optimised biosecurity
Lower production costs e.g. reduced pumping costs
However there are also the disadvantages:
BFT System Disadvantages
High set-up costs
Increased aeration requirements
Back up power critical
Recommended Reading…
Pond lining recommended
Facility to remove sludge during crop
For an interesting article on
Higher level of managerial skill required
Technology - ‘Google’
BFT has been successfully applied in the Americas, China, Indonesia
and Thailand for the production of Pacific white shrimp
(Litopenaeus vannamei). Belize Aquaculture Ltd. in Central America
the history of Biofloc
Shrimp News International
“Meet The Flockers”
(Rosenbury 2006)
was one of the pioneers of this technology and remains the most well
known commercial producer of shrimp under biofloc conditions, so
much so that many in the industry still refer to biofloc shrimp production methodology as the “Belize”
method.
BFT is a new field and is still maturing as an applied science. There are numerous unknown variables
and a need for more research and greater experience, however farmers from all over the world are
expressing great interest in this technology due to its potential for facilitating intensive, profitable, yet
environmentally sustainable aquaculture production.
11
THE ADAPTATION OF BIOFLOC TECHNOLOGY BY AUSTRALIAN PRAWN FARMS
R
esearch into the use of biofloc technology as a production method for Penaeus monodon was
funded by the Australian Seafood CRC and FRDC and conducted by Australian Prawn Farms Pty.
Ltd. and CSIRO between 2007 and 2009.
The research, entitled “Increasing the profitability of Penaeus monodon farms via the use of lowwater exchange, microbial floc production systems at Australian Prawn Farms. David Smith and
Matt West. Project No. 2009/748” gave promising results during the course of the three year study.
These included:
Increase in productivity by 50%;
Reduction of nitrogen discharge into the environment by 77%;
Reduction in water exchange by 70%;
Reduction in feed costs per unit weight of production by 30%.
Table 1. Smith and West (2009) research results
Prior to and during the course of the research, Australian Prawn Farms (APF) concluded that
conventional biofloc technology practices used in intensive L. vannamei production are not suitable
for the culture of P. monodon under Australian farming conditions, due primarily to differences
between the behavioural characteristics of the two species, subsequent stocking density limitation for
P. monodon and the limitations of farm infrastructure at APF. Instead, Australian Prawn Farms trialled a
modified approach, based upon biofloc technology principles but tailored to a lower stocking density
and APF’s infrastructure capacity such as aeration capability.
Australian Prawn Farms has thereby been able to develop a holistic pond management system that
utilises some of the principles of biofloc technology whilst retaining particular characteristics of an
algal-based system.
LOW WATER EXCHANGE , MICROBIAL FLOC SYSTEMS – DISPELLING THE “BIOFLOC”
MYTHS.
T
he principles behind APF’s low water exchange, microbial floc pond management system differ
from those employed in conventional biofloc systems for several reasons. The most notable is
that APF do not achieve the very high density of heterotrophic bacterial biomass that is commonly
observed in a conventional biofloc system and therefore do not usually observe the formation of
large floc particles within their ponds. The flocs that do develop are, on the whole, much finer.
12
The APF system is based firstly upon the development and maintenance of a desirable algal bloom
and secondly upon the development of a healthy community of bacteria that work in parallel with the
algal community to stabilise water quality. With limited or no water exchange APF “develop” their
ponds using a range of additives and careful monitoring to achieve a balanced microbial community
comprising algal, litho-autotrophic bacteria and heterotrophic bacteria.
The APF system favours the development of a stable algal bloom and a robust litho-autotrophic
bacterial community over a dense heterotrophic bacterial community, which tends to have a higher
oxygen demand. As such, the primary carbon requirement of the system becomes inorganic rather
than organic.
The philosophy of this approach is that the algae and litho-autotrophic (nitrifying) bacteria do most of
the work in immobilising the undesirable inorganic nutrient loading in the water leaving the
heterotrophic bacteria to work behind the scenes to immobilise or “mop up” the rest.
APF then aim to maintain this balanced microbial community utilising the various “tools” at their
disposal which can range from additives i.e. molasses, hydrated lime, sodium bicarbonate etc, to water
exchange and physical influences such as aeration placement and pond bottom chaining.
APF do not, as is commonly practiced in conventional biofloc technology system management,
measure and manipulate the Carbon : Nitrogen ratio of their organic matter inputs in order to
manage their heterotrophic bacterial communities. Instead they closely monitor their water quality
parameters, primarily focusing on algal species dominance and levels of pH and DO to provide the
necessary clues as to what is happening within the microbial communities in the pond.
Water exchange is simply utilised as one of a range of pond management “tools” in the APF approach.
Most of the microbial community dynamics can be managed and manipulated using pond additives,
applied in response to water quality parameters. These additives can influence the water chemistry or
provide the nutrients necessary to promote growth or stability of the target community or can have
the opposite effect in the instance where a target community may need to be suppressed.
APF occasionally use water exchange to “reset” a pond where a
particular undesirable algal species has taken hold and is not
responding to additives alone. Importantly, water exchange can
flush out both undesirable and desirable microbial communities
and therefore is looked upon as a last resort when the suite of
other available pond management “tools” has not been effective.
Recommended Reading…
The Smith and West 2009
research results are freely
available from the Seafood
CRC. A summary of these
results can be found in
Appendix 1 at the end of
this guide.
13
SUMMARY
The Australian Prawn Farms’ modified use of Biofloc technology
View pond as a “holistic community” comprising water, algae, bacteria and prawns.
Development of a “mixotrophic” microbial community focusing on a good algal bloom
first and litho autotrophic and heterotrophic bacteria second.
Algae and litho-autotrophs do most of the work to stabilise water quality,
heterotrophs “mop up” what is left.
Use a range of “tools” to manage microbial communities in pond, of which water
exchange is just one.
Tend to get very fine flocs – not large dense flocs
Don’t measure and manage C:N ratio to manipulate heterotrophic bacteria.
Instead primarily use pH, DO and algal species as measure of what is happening in
the pond.
Use water exchange occasionally to “reset” a pond.
14
2. IS A LOW WATER EXCHANGE, MICROBIAL FLOC
PRODUCTION METHOD APPROPRIATE FOR YOUR
BUSINESS?
What follows is a series of vital questions or key issues that operators should consider before deciding
whether to adopt a low water exchange, microbial floc production system.
WHY ADOPT THIS TYPE OF POND MANAGEMENT SYSTEM?
I
t is important to know why you want to employ a low water exchange, microbial floc system,
because this will determine your entire approach and ultimately the level of success you may be
able to achieve with the system.
Do you want to reduce the need for water exchange? Do you want to increase production capacity?
Do you want to increase your yields? Do you want to lower your FCR’s? Do you want to reduce
pumping costs? Do you want to have greater control over your pond system? Do you want to increase
your biosecurity? Do you want to achieve all of the above?
The research conducted at Australian Prawn Farms suggests that there are many potential benefits to
utilising low water exchange, microbial floc methodology as a pond management tool, however it is
important to note that what works at Australian Prawn Farms may not necessarily work at another
farm due to the range of influencing variables such as climate, soil type, water chemistry, operator
knowledge, and infrastructure.
By identifying your motivation for adoption of a low water exchange, microbial floc system you can
then begin to evaluate whether this type of pond management system suits your particular business
model.
DOES IT SUIT YOUR BUSINESS MODEL?
I
t is vital to evaluate whether this type of pond management system is right for your business.
Implementing any new pond management system carries a degree of risk; particularly so for a
technology such as low water exchange, microbial floc, that is still in its early days of application and
that requires more research before the processes involved are fully understood.
It is therefore critical to carry out a feasibility study and develop a business plan (or incorporate this
within your existing business plan) in order to properly evaluate the viability to the business of
adopting low water exchange, microbial floc technology as a pond management system. Without a
good business plan, mistakes will be made that could have been solved on paper. A written business
plan is a well-recognised method for documenting corporate objectives and can help to expose
assumptions and provide reasoning. What is our budget? How many ponds will we trial this method
in? What resources do we need in the short, medium and long term? What performance targets
should we set? Can we afford to take the risk? What if it doesn’t work? By writing down and analysing
15
the answers to these and other questions it is possible to develop strategies that will assist in
minimising the risk to the business. This guide aims to assist operators in recognising the critical
components involved in low water exchange microbial floc technology and will help to provide a
focus for business planning towards a change to this type of pond management system.
A feasibility study and subsequent business plan will include examining factors such as business
structure, financial and market position, environmental impact, resource capacity, production schedule
and may also encompass a SWOT analysis (Strengths, Weaknesses, Opportunities, Threats) and a
comprehensive risk assessment. The potential financial impact of a change in pond management
system needs to be carefully examined. There will almost certainly be upfront capital costs and
ongoing fixed and variable operational costs that may be different to that of the existing pond
management system.
A feasibility study should include the analysis of the system requirements across cost centres such as
but not limited to:
Infrastructure
Plant & Equipment
Electricity (Aeration & Pumping)
Inputs / Additives
Labour
Staff Training
Maintenance
Table 2. Areas to address when carrying out a feasibility study and developing a business plan
A feasibility study should also include analysis of the impact of controlled and uncontrolled variations
in production components such as but not limited to:
Survival %
Stocking Density
Growth Rate
Yield
FCR
Table 3. Controlled and uncontrolled variations in production components.
.
Be flexible in your estimates. It is useful to develop three sets of projections based on high, medium
and low production performance. If you have carried out your feasibility study and written your
business plan and you are convinced that a change to a low water exchange, microbial floc pond
management system can meet your business objectives then the next step would be to develop an
16
Recommended Reading…
For more information on
Business Planning in
Aquaculture see “Business
Planning for Aquaculture –
Is It Feasible?” StrombonRutgers and Tweed (1992)
action plan for implementation. An action plan would include a
detailed description of each action required, a timeline of steps
that need to be taken (and who carries responsibility for each
action) and a measure by which you can determine that each
goal has been achieved.
Refer to the Queensland State Government business planning
website for further assistance in the development of a business
plan:
www.business.qld.gov.au/business/starting/businessplanning/preparing-business-plan
DO YOU HAVE THE RIGHT PEOPLE , INFRASTRUCTURE AND EQUIPMENT?
A
low water exchange, microbial floc system requires close monitoring, the ability to react
quickly and appropriately to changing parameters and a minimum level of infrastructure and
equipment for it to be successful. Successful implementation of a low water exchange,
microbial floc production system will largely depend on whether the business has the appropriate
resources in place.
People
The person or persons directly responsible for making daily decisions on pond management will
require a high level understanding of water chemistry and microbial community dynamics to fully
understand the complex processes occurring within the pond system. They will also need to know the
cause and effect of the range of pond management “tools” at their disposal. This may require initial
up-skilling or training of staff and will require an ongoing commitment by owners and managers to
continued learning and people development.
Labour resource requirements should be considered and the person-hours needed to monitor and
maintain this system should be closely evaluated. The intensity of water quality and algal monitoring
is generally greater for a low water exchange, microbial floc system than for a conventional flow
through system and there can be additional labour requirements due to increased operational
demands such as applying additives and pond bottom chaining. It is therefore important to factor in
any additional labour costs into your business plan.
Aeration
A low water exchange, microbial floc system requires additional aeration compared with a
conventional flow through system due to the aerobic activity of a significantly denser microbial
community. The role of aeration is primarily to supply oxygen to the system but also to keep flocs in
suspension and to control sludge deposition on the pond bottom. A high microbial biomass without
adequate aeration will not only starve the prawns of available oxygen but will also decrease the rate of
microbial growth and activity, leading to rapid increases in potentially toxic levels of waste nutrients.
17
The intensity of dissolved oxygen (DO) consumption by the microbial community is largely a function
of feed inputs required for the particular stocking density (Boyd 2009). A rough approximation of total
oxygen consumption in a pond is therefore one kg oxygen for one kg feed (Avnimelech 2012).
Common aeration rates applied in intensive biofloc technology shrimp production ponds are between
20hp and 60hp/ha. At APF the average is 20hp/ha which gives a comfortable holding capacity of
8t/ha; however it should be noted that APF monitor their pond parameters very closely and have the
capacity to react quickly to deteriorating pond conditions if that situation arises and therefore may be
able to have a higher holding capacity at this level of aeration.
The practical way to determine aeration requirements of a low water exchange, microbial floc system
is through trial and error, working out the minimum aeration required to maintain a sufficient oxygen
level and measuring oxygen levels at different sites and depths in the pond to determine balanced
oxygen distribution. Most operations work on maintaining a DO concentration of no less than 4
mgO2/l.
The recommendation is to ensure your aeration capacity can cope with an organic carbon system i.e.
if the pond shifts to a more heterotrophic dominated bacterial community rather than an algal / lithoautotrophic bacterial community.
Aeration System Back Up
Due to the critical need for a fail-safe oxygen supply, it is essential to have a reliable power supply to
the aeration system as well as a reliable back up of all electrical components such as generators,
circuit components and parts. In addition, spare aerators and aerator parts must be available.
Appropriate staffing to cover nights and weekends and a supporting oxygen / power alarm system
and / or automatic switching to back up systems are also essential requirements.
Pond Design
It may be possible to maximise the viability of a low water exchange, microbial floc system if ponds
are designed in accordance with the specifications recommended for intensive biofloc technology
systems. These specifications include pond shape to maximise active suspension and oxygen
distribution, maximum and minimum depths, drainage capability, and lining. For detailed information
on pond design refer to Avnimelech 2012.
However, the influence of pond design on system viability has not been fully researched and at
present the Australian operators implementing a low water exchange, microbial floc system are doing
so in unlined, rectangular or square, earthen, 1.5 – 2m depth ponds that don’t have a centralised
sludge draining point and have been designed for conventional flow through management.
18
Operational Equipment
The following operational equipment is recommended for the successful management of a low water
exchange, microbial floc system:
Recommended Operational Equipment
Reliable water quality meter to read pH, DO and temp (and back-up meter) E.g.
HACH HQ40D.
Test equipment for Total Ammonium Nitrogen
Test equipment for Alkalinity
Microscopic equipment for identification of algal species
Molasses tank. (30,000 Litres).
Additive spreader that allows for application of additives in solution and wide
dispersal of this solution across pond. E.g. Lime tank used on farms.
Chaining boat for pond sediment re-suspension. E.g. 12’ tinny with 15hp
outboard.
19
3. AN OVERVIEW OF THE MICROBIAL COMMUNITY AND
WATER CHEMISTRY DYNAMICS OF A LOW WATER
EXCHANGE, MICROBIAL FLOC SYSTEM
A
rguably the most vital pre-requisite to being able to successfully implement a low water
exchange, microbial floc system is to have an in depth understanding of the microbial community
and water chemistry dynamics that take place within a pond.
The following chapter contains an overview of the pertinent information regarding the pond microbial
community and associated water chemistry dynamics. There is a great deal of comprehensive
literature available on these subjects and it is strongly recommended that operators and technicians
to do their own extensive reading and research in order to fully understand the microbial and
chemical influences on their pond water quality. (See Chapter 7 for References and Recommended
Reading).
The relationships between prawns, the algal and the microbial communities, the pond inputs and
water quality must be properly understood. It has been said of a low water exchange, microbial floc
system, that if the algal and bacterial communities are managed so that they are stable and healthy,
then the prawns will be stable and healthy also. In effect, with this system you are managing your
microbial communities, rather than your prawns.
With a floc system, the focus is on managing the
microbial communities first and the prawns second…
PRAWN
NUTRIENT METABOLISM AND
P
TOTAL AMMONIUM NITROGEN
rawn feed, whether natural or formulated, contains protein. Formulated feed usually contains 3040% protein. The feed utilisation efficiency of prawns is only about 20%, meaning that only about
20% of the carbon, nitrogen and phosphorous in feeds is assimilated within the animal for energy and
growth. The remainder is lost into the culture system as uneaten feed and excretory waste.
The processes of prawn protein metabolism and the decomposition of organic matter in uneaten feed
produce two major end products; ammonium (NH4+) and ammonia (NH3). Together these compounds
are released into the pond water and are collectively termed Total Ammonium Nitrogen or TAN.
The two species that make up TAN; NH4+ and NH3; exist in a state of equilibrium that is determined by
pH and (to a lesser extent)
temperature and salinity.
NH4+
NH3 +
20
H+
It is the un-ionized species NH3, or ammonia, that is toxic to aquatic animals and that causes
significant problems for aquaculture farmers. The percentage of toxic NH3 increases with pH and
temperature and decreases with salinity. Reference charts such as that shown in the next section
“Importance of pH” are commonly used to determine the amount of un-ionised ammonia in a water
sample (Bower and Bidwell 1978).
In small concentrations NH3 can cause stress, gill damage, bacterial infections, reduced growth and, in
higher concentrations can quickly cause death. The maximal safe level of NH 3 concentration for
prawns is considered to be 0.2mg/l. The lethal level of NH3 is typically in the range of 1-2mg/l.
(Avnimelech 2012
INFLUENCE OF PH ON AMMONIA TOXICITY
I
t is important to understand the influence of pH on water chemistry. Changes in pH can cause
changes to the solubility or reactive form of various compounds or elements within a pond, which
subsequently can be problematic or beneficial for pond management and prawn health. Algal bloom
activity has a direct effect on pH levels. CO2 is utilised during algal photosynthesis during the day,
which increases pH, whereas at night during algal respiration, CO2 is released and reacts to form
carbonic acid, which decreases pH.
The most important influence of pH is on ammonia. When pH is low,
most of the TAN is stored in the water in the ionised, less volatile
NH4+ state and ammonia toxicity is lower. When pH rises, the
fraction of NH3 rises and toxicity levels increase. See Table 4.
Recommended Reading…
For further information on the
influence of pH on ammonia
toxicity see Bower & Bidwell
1978. “Ionization of ammonia
in seawater: effects of
temperature, pH and salinity”
Temp
(0C)
pH
pH
240C
320C
7
0.5
0.9
8
4.9
8.7
9
34.4
49
10
84
90.5
Table 4. Percentage of NH3 species within total TAN (Avnimelech 2012)
For example, if TAN is measured to be 1.2mg/L and the pH is 9 and temperature is 32 oC then reading
from Table 4 the percentage of NH3 is 49%. This then equates to 0.58mg/L (1.2mg/L x .49) of
unionised ammonia (a potentially lethal level).
21
Two other notable areas of pH influence are on silicate solubility where a high pH gives greater
solubility, and on hydrogen sulphide toxicity where a low pH will result in an increase in toxicity. These
two areas are discussed further in Chapter 4 “Getting Started” and Chapter 5 “Pond Management”.
BACTERIAL COMMUNITIES IN AQUATIC SYSTEMS
T
he feed supply of natural aquatic systems is based upon the primary production of algae that
produce organic materials through photosynthesis. Algae are consumed by different microscopic
and macroscopic herbivores, which are then in turn consumed by larger carnivorous organisms and so
on. A microbial community is always found in parallel to this food web described. Algae excrete
organic matter, and microorganisms such as bacteria feed on this organic matter. Microorganisms
degrade the organic matter and use it firstly for energy and secondly for growth and development of
new cells.
In an aquatic system, bacterial production is positively related to net phytoplankton primary
production, even when there is no feed added. (Cole et al, 1988). When external feed is added,
bacterial production is much higher. Microbial population growth dynamics suggest that the microbial
potential to modify and control water quality in ponds is practically unlimited as long as there is
adequate oxygen and the water is mixed well. (Monod, 1949).
Recommended Reading…
For further information on
bacterial communities in
aquatic systems refer to Cole
et al, 1988. “Bacterial
production in fresh and
saltwater ecosystems: a crosssystem overview”
NITROGEN CONVERSION PATHWAYS
T
he three nitrogen conversion pathways traditionally used for the removal of ammonia-nitrogen in
marine prawn aquaculture systems are:
22
1
Through the conversion of ammonia-nitrogen via
photoautotrophic algae directly to algal biomass
2
By litho-autotrophic (nitrifying) bacteria to
nitrate nitrogen
3
By heterotrophic bacteria directly to microbial
biomass
Photoautotrophic ammonia removal
One of the most commonly used pathways for ammonia removal in aquaculture production systems is
through the use of phytoplankton based culture systems which use the high nutrient content of
ponds to grow algae at high density. Almost all phytoplankton such as algae and cyanobacteria (bluegreen algae) are photo-autotrophic organisms, which means that they produce their own food by
conversion of solar energy into chemical energy (carbohydrate) through the process of
photosynthesis. This chemical energy is then used for various metabolic processes in the organism
through the process of respiration. (Ebeling 2006)
In conventional flow-through prawn production systems under a normal feeding regime, TAN levels
will increase significantly to dangerous levels within a few days unless it is removed or changed to
other forms of nitrogen. In these algal dominated culture systems, TAN is mainly removed through
the exchange of nitrogen rich pond water for external clean water.
During photosynthesis, inorganic nitrogen in the form of ammonia-nitrogen and to a much lesser
extent the form of nitrate-nitrogen is converted to organic nitrogen through new algal cell
production. However the algal capacity to immobilise nitrogen in this manner is limited by its rate of
carbon assimilation, which is typically in the range 2 – 5 g C/m2 in growing algae. This gives a daily
nitrogen immobilisation capacity of 0.4 – 1 g N/m2 which is high enough to control TAN build up in
ponds holding biomass of less than 1.2 kg/m2 but not sufficient on its own in more intensive culture
systems. (Avnimelech 2012).
The major disadvantages of algal-based systems are the wide diurnal variations in DO, pH and
ammonia and the long term changes in algal density and frequent ‘die-offs’ (Burford, et al. 2003).
Algal assimilation of carbon (and therefore immobilisation of TAN) is dependent upon many variable
23
factors such as insufficient light (e.g. cloud cover) for photosynthesis, inadequate pond nutrients (a
bloom too dense to be supported by available nutrients and oxygen) and/or bloom senescence (the
algal cell line becomes too old to continue reproduction). Rapid decomposition of dead algae
reduces DO and pH levels whilst increasing ammonia and carbon dioxide concentrations.
SUMMARY
Photoautotrophic (algal based systems) - the water quality impacts
of photosynthesis
o Ammonia is preferentially taken up over nitrate
o Both ammonia and nitrogen source reactions metabolise only a small quantity
of phosphorous 0.14g P / gN
o For both reactions the C:N ratio of algal biomass is the same, 5.69 gC / gN
o Alkalinity is consumed when ammonia is the nitrogen source.
o Alkalinity is produced when nitrate is the nitrogen source
o Both ammonia and nitrogen source reactions utilise CO 2 as their carbon
source
o Volatile (organic) Suspended Solids consist of 36% C and 6.3% N
o pH increases during the day due to the consumption of CO2
o Both ammonia and nitrogen source reactions generate oxygen
o pH decreases, oxygen is consumed and carbon dioxide is released at night or
during low light levels through respiration
(Ebeling and Timmons 2006)
Litho-autotrophic bacterial ammonia removal (nitrification)
TAN undergoes a process of biological oxidation termed nitrification, which is a two-step process by
which ammonium is biologically oxidised by litho-autotrophic (nitrifying) bacteria into nitrite (NO2-)
and then into nitrate (NO3-).
24
In the primary stage of nitrification, the oxidation of ammonium is performed by bacteria that include
the genera Nitrosomas, Nitrosococcus, Nitrospira, Nitrosolubus and Nitrosovibrio, which covert
ammonia to nitrites (NO2-). A second group of bacteria that include the genera Nitrobacter,
Nitrococcus, Nitrospira and Nitrospina, then oxidise nitrites into nitrates (NO3-).
As previously mentioned, ammonia (NH3) is toxic to aquatic animals. Importantly, so too is nitrite
(NO2-) the product of the first stage of nitrification, however nitrate (NO3-) the end product of
nitrification is not toxic unless present at very high levels more than 100’s mg/l. (Colt and Armstrong,
1979)
Ammonia
form
(NH3) is
toxic to
aquatic
organisms
Nitrite
(NO2-) is
toxic to
aquatic
organisms
Nitrification
Ammonium
Ion
NH4- + 1.5O2
NO2Nitrite
NO2- + 0.502
Nitrosomas
2H+ + H2O +
Nitrobacter
Nitrate
NO3-
Nitrate (NO3-)
is not toxic
unless at very
high levels
Nitrifying bacteria are slow growers, with a generation period in the order of 12 hours versus about 30
minutes for heterotrophic bacteria. Nitrification therefore is a slow process and it can take a few weeks
to fully develop the nitrifying community within a pond. Furthermore, nitrification responds very
slowly to fast changes in TAN or nitrite (NO2-) concentration. In addition, if water is exchanged then
the nitrifying bacteria can be flushed out and if exchange rates are high enough they do not have
enough time to recuperate. (Diab et al., 1992)
Nitrifying bacteria need oxygen for respiration. Ammonium oxidation requires less oxygen than nitrite
oxidation. Therefore when oxygen concentration is limited, ammonium may still be oxidised but toxic
nitrite may accumulate in the water as the rate of the second stage of nitrification slows.
The nitrification processes result in a large amount of alkalinity being consumed (7.05g CaCO 3 / gTAN)
and high levels of carbon dioxide produced (5.85g / gTAN) (Ebeling and Timmons 2006). If alkalinity is
not compensated for by supplementation, pH will drop. A decrease in pH will result in an inorganic
carbon species shift in the water carbon balance from bicarbonate to dissolved carbon dioxide (see
next chapter on nitrification and alkalinity).
25
Nitrification is affected by a variety of parameters such as substrate, DO concentrations, organic
matter, temperature, pH, alkalinity, salinity and turbulence level. Nitrifying bacteria are extremely
susceptible to a wide variety of inhibitors such as high concentrations of ammonia and nitrous acid,
low dissolved oxygen levels (< 1mg/l), pH outside the optimal range (7.5 – 8.6) and the presence of
reduced compounds such as sulphides. These conditions, in a similar way to low oxygen, also lead to
the build-up of nitrite concentrations.
Importantly, the presence of a high ratio of biodegradable organic carbon to nitrogen can result in
faster growing heterotrophic bacteria outperforming the nitrifying bacteria in the system. Under these
conditions, nitrifying bacteria can be significantly inhibited and their already slow growth rates can be
further impeded. (Ebeling & Timmons 2006). Subsequent attempts to control the TSS levels in the
production system (elevated due to heterotrophic growth) can compound the problem as nitrifying
bacteria are very easily washed out of the system during water exchange.
SUMMARY
Litho-autotrophic bacterial ammonia removal - the water quality
impacts of nitrification
o Nitrification is a two step process: ammonia to nitrite, then nitrite to nitrate
o The second step does not start until the first step is well underway
o The C : N ratio of nitrifying bacterial biomass is 4.29g C / gN
o The net production of biomass is very small: 0.20g VSS / gN
o Nitrification utilises alkalinity as the primary carbon source: 7.05g Alk / gN
o pH of the system drops as the alkalinity is consumed and carbon dioxide is produced
o Actual carbon requirement is 1.69g C / gN
o Oxygen requirement is 4.18g O / gN
o Carbon dioxide production is 5.85g CO2 / gN
o Generation times for biomass are very slow: 2 – 5 days.
o Nitrifying bacterial growth can be significantly inhibited by heterotrophic bacterial
growth
o Nitrifying bacteria can easily be washed from the system during water exchange.
(Ebeling and Timmons 2006)
Nitrification and Alkalinity
The term “alkalinity” refers to a measure of the acid neutralising or “buffering” capacity of water.
Maintaining a high alkalinity is important in an aquaculture production system as it serves to reduce
26
the daily fluctuation in pH, ensures optimum nitrification and is essential for animal growth and
development (Van Wyk and Scarpa 1999).
Although there are other chemical buffer systems, the carbonate-bicarbonate buffer system is
dominant in saltwater and is normally the one measured by prawn farmers as a indicator of alkalinity.
Bicarbonate (HCO3-) and carbonate (CO3-2) are the main bases responsible for the total alkalinity in
this system and alkalinity is usually expressed in equivalents of calcium carbonate (mg CaCO 3/L). The
total alkalinity is the capacity to maintain the following acid-base equilibrium:
C02 + H2O
H2C03
H+ + HC03-
2H+ + C03-2
When CO2 is produced in water during respiration it forms carbonic acid (H2CO3). Carbonic acid can
then dissociate to form hydrogen ions and bicarbonate ions. In poorly buffered water this can cause a
drop in pH, but if there is an adequate pool of carbonate ion present, the free hydrogen ions will
combine with carbonate to form bicarbonate. The result is that the pH will not change until the
carbonate pool is exhausted. Conversely, the removal of carbon dioxide during photosynthesis can
result in rapidly rising pH but in well-buffered water, hydrogen ions are released and pH levels are
maintained.
Alkalinity is lost during nitrification. Litho-autotrophic (nitrifying) bacteria use bicarbonate as a carbon
source, i.e. they use an inorganic form of carbon. Hydrogen ions (H+) are produced when ammonium
ions are oxidized to nitrite. Nitrous acid (HNO2) is also produced during the oxidation of ammonium
ions. These processes result in a reduction in the alkalinity of the system with a ratio of 7.14 parts
alkalinity being consumed for each part ammonium immobilized.
The optimal pH for nitrification is near 8.0. Values outside of 6.0 - 8.5 can be expected to reduce
nitrification efficiency. Alkalinity provides pH buffering. A reduction in alkalinity results in more rapid
shifts in pH, reduced inorganic carbon availability for growth of nitrifying bacteria and suboptimal pH,
all of which stress nitrifiers, decreasing growth efficiency and rates of nitrification. From a nitrogen
removal perspective, it is therefore important to maintain adequate levels of alkalinity to both
promote pH stability and provide a food source for nitrifiers. This is usually done through the addition
of sodium bicarbonate. (Van Wyk and Scarpa 1999).
Most literature recommends maintaining an alkalinity level of between 50 to 300mg CaCO3 /l for
optimum nitrification and reduction of pH swings. In addition, sub-optimum alkalinity can adversely
affect biological processes of aquatic animal. Specifically, Smith and West 2009 state that P. monodon
require a total alkalinity level of between 80 and 120mg/l for ideal growth and development.
27
SUMMARY
Nitrification and Alkalinity
o P. monodon require an alkalinity between 80 and 120mg/l for ideal development
o High alkalinity serves to reduce the daily fluctuation in pH
o Alkalinity is lost during nitrification at a rate of 7.14 parts alkalinity per part
ammonium immobilized.
o Optimal pH for nitrification is 8.0
o Low alkalinity causes rapid pH shifts reduced inorganic Carbon availability –
causing decrease in nitrification rate
o Alkalinity is increased through addition of sodium bicarbonate
Heterotrophic bacterial nitrogen removal
Bacteria use carbohydrates (sugars, starch and cellulose) as a food to generate energy and to grow:
Organic Carbon
CO2 + Energy + Carbon assimilated in microbial cells
Nitrogen is required as an important building block of the microbial cell since the major component
of the new cell material is protein. The Carbon : Nitrogen ratio of most microbial cells is about 4:5.
Removal of ammonia-nitrogen by heterotrophic bacteria is a single step process where the ammonianitrogen is assimilated directly into bacterial biomass. (Ebeling & Timmons 2006)
Therefore, when bacteria are fed with organic substrates that contain mostly carbon and little or no
nitrogen, they have to take up nitrogen from the water in order to produce the protein needed for cell
growth and multiplication. By doing so they reduce the concentration of inorganic nitrogen in the
water (particularly Total Ammonium Nitrogen – TAN).
Many operators practice the use of carbohydrate addition (such as molasses) to correct potentially
dangerous levels of TAN following increases due to events such as cloudy days, algal crashes etc.
However care must be taken not to add large quantities of carbohydrates at once as this can lead to
high oxygen consumption and oxygen deficiency. It is important to be careful adding molasses to a
pond when the morning DO’s are under 3.5mg/L. This allows for a decrease in the following morning
DO’s due to the increase in oxygen demand brought about by the increased bacterial growth. The
addition of 100L of molasses to a pond can reduce the ammonia by at least 0.5mg/L, depending on
the constant load on the pond, but it can also decrease the following morning DO by 1mg/L. (Smith
and West 2009).
28
A different, pro-active approach, and the approach used within the application of a low water
exchange, microbial floc system is to encourage heterotrophic bacterial community growth before
the situation of an un-wanted TAN increase arises to optimise the process of inorganic nitrogen
immobilisation.
There are some important aspects to the heterotrophic bacterial nitrogen removal reaction.
Paramount is the extremely large amount of bacterial biomass produced compared to the lithoautotrophic reaction: 8.07g Volatile Suspended Solids (VSS) / gN versus 0.20g VSS / gN. This translates
into extremely high growth rates and substantial quantities of total solids produced. In a dominant
heterotrophic system, some form of solids management is required to remove excess Total
Suspended Solids (TSS) production. This intensive growth of bacteria can limit algal activity due to
reduced light penetration such that only a small part of the water column recives enough light for
photosynthesis. It is possible therefore to manage photoautotrophic activity by controlled removal of
TSS to increase water clarity.
Another important issue is the modest amount of alkalinity consumed as the carbon source (3.57g Alk
/ gTAN) and the resulting high levels of carbon dioxide (9.65g CO2 / gTAN). Again, addition of sodium
bicarbonate is the usual method used to supplement alkalinity levels.
Generally, some form of carbon supplementation is required to develop a heterotrophic dominant
system, as there is usually insufficient carbon available from the feed alone. For example a feed with
35% protein has only sufficient available carbon for the heterotrophic bacteria to sequester about 36%
of the ammonia-nitrogen. The remaining nitrogen is therefore available to the autotrophic bacterial
population. (Ebeling, et al., 2005b).
29
SUMMARY
Heterotrophic bacterial nitrogen removal - the water quality impacts of
heterotrophic bacteria
o Heterotrophic conversion is a one step process
o The C:N ratio of bacterial biomass is the same as for nitrifying bacteria 4.29gC / gN
o Net production of biomass is 8.07g VSS / gN
o Carbon from carbohydrate is the primary carbon source at a rate of 6.07g C / gN or
15.17 g Carbohydrate / gN
o pH of the system drops as alkalinity is reduced: 3.57g Alk / gN
o Oxygen requirement is 4.71g O / gN
o Carbon dioxide produced is 9.65g CO2 / gN
o Generation time for biomass is very fast (30 minutes)
(Ebeling and Timmons 2006)
Recommended Reading…
For a good synopsis on the
ammonia removal pathways
in microbial floc based
systems refer to Chapter 12,
Aquaculture Production
Systems, Tidwell, J.H., 2012.
Denitrification
The complete removal of nitrogen from a pond system occurs via denitrification. When oxygen is
absent, denitrification can occur when certain heterotrophic bacteria, that are essentially aerobic but
that can adapt to low or even zero oxygen conditions, use nitrate (NO3-) as an oxygen source to
decompose organic matter. They obtain their oxygen by removing it from the nitrate (NO 3-) ion, which
reduces it to nitrogen gas (N2) or organic nitrogen compounds. Nitrogen gas is then either volatilized
to the atmosphere or converted by biological fixation to ammonia.
Denitrification efficiency in prawn ponds is very low (<2%) (Burford,and Longmore 2001). To promote
denitrification, several conditions must be met: the availability of sufficient sites with appropriate
redox potential (300-350mv), a source of organic carbon, adequate levels of nitrate, and lack of
denitrification-inhibiting compounds. In a study on the nitrogen budget of P. monodon farms by
Preston et al, (2000), it was found that only 3% of total input nitrogen (in feed, stock and intake water)
30
was unaccounted for after calculating nitrogen destination. This 3% complete nitrogen loss from the
system was attributed to denitrification.
Microbial floc systems need to be kept well aerated and mixed to avoid the development of anaerobic
accumulations of organic material that might release toxic sulphides and other compounds. However
it has been speculated that denitrification might occur at significant levels in the oxygen depleted
interior of the floc particles suspended within the water column. The annamox pathway; a process in
which a portion of the denitrification reaction is bypassed and ammonia is converted directly to N2
gas, may also occur in the anaerobic interiors of biofloc particles. (Tidwell 2012).
FLOCS
W
hen the microbial biomass of an aquaculture system becomes dense enough, microbes tend to
congregate and create flocs that are conglomerates of microbes and have a diameter in the
range of 0.1mm to several mm, which at this larger size are clearly visible to the naked eye.
Floc particles are agglutinated by a polysaccharide slime produced by bacteria and suspended
materials become absorbed onto the floc where they are hydrolysed by extracellular bacterial
enzymes. One interesting facet of floc formation is that most organisms are negatively charged and
induce a mutual electrostatic repulsion. If this repulsion is lowered then strong attractive forces can
take precedence. This happens when salt concentrations are high or poly-valent ions prevail. It means
that addition of calcium or magnesium ions can help to induce stable flocculation (Avnimelech 2012).
Flocs are made up of a mixture of live organisms including bacteria, filamentous algae, protozoa,
zooplankton and also dead cells (detritus). Flocs have an open structure and are therefore very
porous. This enables water and chemicals to flow through the floc resulting in an effective supply of
nutrients and removal of waste into and out of the biomass within the floc.
This high porosity makes the density of flocs very low; just a little above that of water. This effectively
keeps the flocs suspended in water, slowing down sedimentation, facilitating aerobic processes and
providing extended contact with the prawns in the system to enable nutrient recycling.
Another interesting feature of flocs is that the due to various factors such as consumption by prawns
and other organisms, microbial degradation and production of new microbes, that the biota in flocs
change about twice a day and is mostly made out of young and active cells.
The density of floc can be measured using a device called an Imhoff cone. Typical floc density in true
biofloc ponds are 2 – 40 m/L (shrimp) and up to 100ml/L (fish) . Smith and West (2009) only took into
account floc density over 0.1ml/L during their research, as 0.1ml is the smallest amount of settle-able
solids that can be measured using a 1 L Imhoff cone. In addition, this amount of settle-able solids can
also occur through the presence of material other than biofloc.
NUTRITIONAL BENEFITS OF FLOCS
31
S
uspended flocs in intensive limited water exchange ponds represent a potentially valuable
resource of recycled protein and other nutrients that would otherwise be lost to the pond bottom
or into the environment through water exchange.
Studies on biofloc contribution to protein nutrition of L. vannamei have shown 20% – 30% of the
protein assimilation by shrimp originates from biofloc harvesting. (Burford et al. 2003., Nunes et al.,
1997; Velasco et al, 1998). It has been stated that the shrimp eat the protein twice: once with the feed
and then as microbial protein. The average protein utilisation in commercial biofloc systems is 46%,
which is twice as high as that in conventional systems. (Avnimelech et al., 1994; Chamberlain et al.,
2001).
More research is required into the efficiency by which P. monodon harvest and utilise microbial
protein from flocs and the nutritive value of flocs in P. monodon development. P. monodon have a
different behaviour to L. vannamei and tend to inhabit the bottom of the pond rather than feeding
throughout the water column. This may limit the capacity for P. monodon to graze on and utilise flocs
in active suspension. (Peter Van Wyk 2004)
Smith and West 2009 found it difficult to determine the direct nutritional benefit of flocs for P.
monodon cultured in ponds without knowing what effect the stable water quality in the ponds was
having on feed assimilation and growth rates. However, using production data for 2007/08 it was
calculated that an FCR of 1.2 to 1.3 was obtained from ponds in which a good floc had been
established in contrast to an FCR of 1.5 to 1.6 obtained from ponds in which only a poor floc had been
established. Based on this data, in a system with the same management regime, the presence of a
good floc resulted in a saving of 3 tonnes of feed or about $6000 per pond (20% of the total feed
cost). (Smith and West 2009).
32
4. GETTING STARTED - PRELIMINARY STEPS TO
IMPLEMENTATION
SET WATER QUALITY PARAMETERS AND TOLERANCE LEVELS
T
he first step in moving towards adopting a low water exchange microbial floc system is to
determine the ranges or limits within which you want to keep your water quality criteria in order
to optimise prawn growth. The key is to know your species limitations and set your water quality
criteria limits accordingly.
Water quality criteria limits might be for example:
Dissolved Oxygen >4mg/L
Ammonia
<1mg/L (TAN) and <0.1mg/L (toxic)
pH
7.8 – 8.3
Salinity
>15ppt
It is recommended to initially set these limits or tolerance levels at a “low risk” level and work towards
pushing these limits when you become more comfortable with managing the system. Once you have
determined your water quality ranges then you can create a series of rules for each criterion that
include warning signs and breach limits.
IDENTIFY YOUR PRODUCTION SYSTEM MANAGEMENT “TOOLS”
T
he next step would be to determine the range of pond management “tools” that you have at your
disposal with which to manipulate the system.
These will include, but may not be limited to:

Water

Chaining

Hydrated Lime

Inorganic Carbon

Silicates

Aeration

Fertiliser
33

Organic Carbon

Tea Seed

Dye

Feed
UNDERSTAND THE CONSEQUENCE OF THE USE OF EACH “TOOL”
T
he application of each pond management tool will have one or more consequences within the
system. It is therefore important next to identify what the consequence of the use of each of your
tools may be and to fully understand the cause and effect of the application of more than one tool at
a given time.
The influences of each tool include, but are not limited to:
Tool
Influence
Water
Introduction of algae, nutrients and disease, lowers NH4,
increases DO, influences pH, can flush out algae, heterotrophic
and litho-autotrophic bacteria
Chaining
Decreases algae, NH4 and DO, increases nutrients, algae, floc,
cleans pond bottom
Hydrated Lime
Increases pH, toxic ammonia, alkalinity, decreases CO2 by
shifting carbonate ratio towards bicarbonate.
Inorganic Carbon
Increases alkalinity, increases litho-autotrophic bacteria
E.g. Sodium Bicarbonate
Inorganic Carbon
Increases pH, increases litho-autotrophic bacteria
E.g. Sodium Carbonate
Silicates
Increases algae (diatoms), decreases ammonia
Aeration
Increases DO, increases current and therefore increases FCR
Fertiliser
Increases algae, can increase or decrease NH4
Organic Carbon
Increases heterotrophic bacteria, increases algae during startup, decreases DO, decreases ammonia, decreases pH
Tea Seed
Decreases dissolved oxygen, stimulates moulting
Dye
Reduce light penetration and reduce benthic algae
Feed
Increases growth, increases NH4, decreases DO, pH
Table 5. The influence on water quality of a range of pond management tools
34
SOURCE AND APPLY ADDITIVES IN THE RIGHT FORM
U
tilising the right source of nutrient or additive is critical when aiming to manipulate the microbial
communities and associated pond water dynamics of a low water exchange, microbial floc
system. Recommended sources of silicates, inorganic and organic carbons are outlined below.
In addition, it is vital to get the method of application right according to the additive being applied to
ensure proper distribution throughout the pond. Additives are costly and represent an expensive
wasted resource if not applied to the pond correctly as their influence will be lessened.
Sources of Silica
Sodium silicate is the common name for a compound sodium metasilicate, Na 2SiO3, also known as
water glass or liquid glass. Silica is an essential nutrient for diatoms though not required by other
microalgal species (Gilpin et al. 2004). APF determined that their pond water silica concentration
needed to be greater between 0.2mg/L and 0.5mg/L in order to promote a desirable diatom bloom
(Smith and West 2009). Sodium metasilicate can be bought in either aqueous solution or in solid form.
With regard to application, it is important to know that silica is poorly soluble in water with a pH of
<9.0 but becomes increasingly soluble as pH increases to above 9. Therefore if making up a silica
solution in water that has a lower pH, care must be taken to compensate for the percentage of silica
that won’t be available to the system.
Sources of Inorganic Carbon
Total alkalinity within a low water exchange, microbial floc system reduces progressively due to the
chemical processes occurring during litho-autotrophic and heterotrophic bacterial growth.
Bicarbonate (HCO3-) and carbonate (CO3-2) are the main bases responsible for the total alkalinity
within a culture system.
Sodium bicarbonate (baking soda) is the preferred inorganic carbon source to replenish the
bicarbonate and thereby increase the total alkalinity if pH levels do not require changing. If both pH
and total alkalinity are low then the application of calcium hydroxide (hydrated lime) will raise both
these parameters. However it is important to note that calcium hydroxide does not provide an
inorganic carbon source required for litho-autotrophic bacterial growth and subsequent maintenance
of nitrifications rates. (Smith and West 2009).
The application of sodium carbonate will also serve to increase pH and function as a food source for
litho-autotrophic bacteria but is not as effective as sodium bicarbonate for increasing alkalinity.
(Furtado, P.S., et al. 2011).
Sources of Organic Carbon
35
The criteria for an effective organic carbon source are that it: (a) is degraded and utilised rapidly by
bacteria; (b) is in the form of a fine powder or liquid that is readily dispersed within the pond; (c) is
preferably that it is water soluble or miscible so that bacteria can absorb it rapidly; a (d) is
predominantly composed of simple carbohydrates or low molecular weigh lipids and with a minimum
amount of nitrogen. (Smith and West 2009)
Carbohydrates such as sugar, flour, cassava meal and molasses are commonly used as supplementary
organic carbon sources in aquaculture systems. In Australia, molasses appears to be the most costeffective, practical and effectual carbon source available. However it must be mixed thoroughly with
water before being added to the pond to ensure effective dispersal rather than settling on the bottom
of the pond. (Smith and West 2009).
MANAGING THE C:N RATIO
A
s previously described, the microbial utilisation of carbohydrate (or any other low nitrogen feed)
is accompanied by the immobilisation of inorganic nitrogen. This is a basic microbial process and
means that by adding carbonaceous substrates to ponds in prescribed amounts, it is possible to
control the rate of bacterial immobilisation of inorganic nitrogen. Achieving the right ratio of carbon
to nitrogen (C:N ratio) within a prawn culture system is therefore important for the successful
implementation of microbial floc technology.
NB. APF do not actively measure their C:N ratio but understand the impact that sub-optimal organic
and inorganic carbon levels can have on their pond conditions and water quality parameters and
therefore manage their carbon addition in response to these.
Measurement of C:N ratios in ponds is not a simple task because the carbon and nitrogen end up in a
lot of different places: the faeces, the organic floc, the bacteria, the water and the
prawns. Researchers use labeled isotopes of carbon and nitrogen in the feed to study C:N budgets in
ponds, however this isn’t practical in a production pond. Having access to in-house laboratory
equipment or the budget to send samples to an external laboratory, measurement of total organic
carbon and total Kjeldahl nitrogen is another way of obtaining an accurate C: N ratio but again this is
not usually viable for most operators.
Managing the C:N ratio in a pond is more easily achieved by managing the C:N ratio of the feed.
Carbon accounts for roughly 50% of the dry weight of most feeds. This is a crude estimate, but
carbon content is remarkably constant even for feeds with widely varying compositions. The nitrogen
content of the feed is calculated from the protein content. Protein is approximately 16%
nitrogen. Although this method for calculating C:N ratios is admittedly crude, it provides a reasonably
close estimate of actual C:N ratios.
The feeds used for intensive prawn production typically contain at least 35% protein which gives a
carbon/nitrogen ratio of around 9:1.
C = 50% : N = 5.6% (Protein 35% x 0.16)
36
With such a low C:N ratio, carbon becomes the limiting nutrient, and the bacterial populations don’t
expand beyond a certain point; however, when the C:N ratio is increased, bacteria proliferate and the
rate of nitrogen immobilization increases accordingly.
If the C:N ratio is increased, either by feeding lower protein feeds with a higher percentage of
carbohydrate, or by adding a carbohydrate source such as molasses in addition to the regular feed,
the increased availability of carbon allows the bacterial population to consume a higher percentage of
the protein in the organic material. This results in more complete digestion of the organic material in
the pond by the bacteria. As the C:N ratio increases, the bacteria resort increasingly to ammonia
metabolism to meet their nitrogen requirements. As C:N ratios are increased even further, a point is
reached where nitrogen, rather than carbon, becomes the limiting nutrient. This occurs when the C:N
ratio reaches about 15:1. At this point ammonia concentrations should be close to zero in the
pond. Manipulation of C:N ratios is an effective tool for managing ammonia levels in prawn ponds.
Increasing the C:N ratio can be accomplished by either holding the feed protein level constant and
supplementing the feed with carbohydrate, or by feeding a feed with a lower percentage of protein
and a higher percentage of carbohydrate. Both approaches will result in much higher bacterial counts
in the pond. It is important to note that the oxygen required to support additional bacterial biomass
will increase proportionally with the increase in bacterial population. Likewise, CO2 production will
increase, driving pH down. Therefore these two factors must be taken into account when
manipulating the level of heterotrophic activity in the pond
When supplementing carbohydrate to increase C:N ratios, it is important to ensure that the pond is
well aerated and circulated to keep the organic detritus suspended in the water column where there is
sufficient oxygen for the bacteria. Once a dense population of bacteria has developed, it is also
important not to suddenly discontinue the carbohydrate supplementation, as this will starve the
bacteria of carbon, causing a die-off to occur and ammonia to spike. (Peter Van Wyk, 2004)
It is generally agreed that carbohydrate supplementation requires careful management to prevent
excessive biofloc build up as the result of this is an increased oxygen demand and a need to filter or
remove some of the floc either from suspension within the water column or as sludge (see Chapter 5
for Sludge Management).
Using prawn feed as a carbohydrate source can be expensive due to the production costs involved in
manufacturing. It can be cheaper to use a high quality feed that contains the right percentage of
protein and then supplement with a cheaper source of carbohydrate. Operators therefore need to
know the protein percentage and the C:N ratio of the feed and take this into account when calculating
the overall C:N ration of the organic substrates in the pond if supplementing with another source of
carbohydrate also. The C:N ratios of feeds containing different protein percentages are shown in Table
6.
Current literature suggests that a C:N ratio of between 15:1 and 20:1 is the most effective for the
development of dominant heterotrophic community in a pond and its subsequent capacity to
immobilise inorganic nitrogen. (Avenimelech 2012. Chamberlain et al 2001.)
37
Protein Content
(%)
C/N Ratio
15
21.5
20
16.1
25
12.9
30
10.8
35
9.2
For more detail on managing
40
8.1
Avnimelech 1999
Recommended Reading…
C:N rations refer to
Table 6. C/N ratios of prawn feeds (Avnimelech 2012)
“Carbon/nitrogen ratio as a
control element in
aquaculture systems”
METHODS OF ADDITIVE APPLICATION
The preferred way to achieve effective distribution of an additive throughout a pond is to first get it
into solution using a suitable distribution device such as a lime spreader tank, and then to apply this
solution directly into the circulating water current in the pond. It is usually essential to know the
volume of additive being applied, and therefore having a mixing tank with a weighing hopper or
similar application system that allows operators to apply known amounts of additive to each pond is
extremely important.
It is also important to understand any changes that may occur to the “available” or “active”
component of the additive in question when being made up in solution. For example, Silica is poorly
soluble in water with a pH of <9.0 but it becomes increasingly soluble as pH increases to above 9.0.
Therefore if making up a silica solution in water that has a lower pH, care must be taken to
compensate for the percentage of Silica that won’t be available to the system.
38
5. POND MANAGEMENT
T
he following chapter is intended to provide general guidance to operators as to the factors they
may need to address, and to the issues they could expect during the development and
management of a floc based production pond. It is important to note that the information presented
here does not aim to provide a failsafe step-by-step guide to the successful implementation of a low
water exchange microbial floc system. Each region, farm and even each pond is subject to numerous
variables that can affect the implementation, and in turn, the level of success of a floc based
production system. For this reason the authors urge caution and recommend that operators to do
their own research and undertake their own on-farm trials to determine the methods of floc based
pond development and system management that work best for them.
THE PROCESS OF POND DEVELOPMENT
D
eveloping a low water exchange, microbial floc production pond starts off in much the same way
as a conventional flow through pond. An algal dominated microbial community will develop
normally a few days following the start of feeding. The rate of development of a heterotrophic and
nitrifying bacterial community thereafter will depend on the rate of feeding, stocking density and
whether or not inoculums are used to introduce these bacteria.
Inoculums are not essential, as even lined, clean ponds will contain a variety of micro-organisms.
However using an inoculum can speed up bacterial community development and may limit the
transient build-up of TAN and nitrite described below. A simple way to add inoculum is to add pond
soil (ca 100 kg soil per ha) or mature pond water from ponds that have had good productivity and if
possible, good flocs. Note that the risk of pathogen transfer must be taken into account when using
an inoculum.
At APF, throughout the first 30 – 45 days after filling the pond, the main nutrient input into the ponds
is urea and mono-ammonium phosphate (MAP). These are applied at a rate of 2-3kg urea and 1–
1.5kg per hectare every day or every second day. The amount and frequency of application is
determined by the progress of the microalgal population and prevailing weather conditions. This
continues for about 4 weeks until a dense microalgal bloom is established or the TAN exceeds 1mg/L.
Although nitrogen immobilisation into microbial cells may start to happen within the first few days of
pond stocking, it usually takes a few weeks until there is enough organic matter in the pond for
heterotrophic nitrogen immobilisation rates to reach levels at which TAN levels are stabilised. During
this period there is often observed to be an increase in TAN concentration. Until there is enough
organic matter in the system, photoautotrophic assimilation and nitrification will be the major process
controlling TAN concentrations in the water.
As described earlier in this guide, ammonium oxidising to nitrite starts slowly when TAN concentration
is built up. Nitrite oxidation to nitrate starts later when nitrite build-up has occurred. Therefore there is
a typical pattern in the development of a microbial floc pond: ammonium concentration rises up to a
maximal value 2 – 3 weeks after stocking. Subsequently it declines with a parallel rise of nitrite. The
39
subsequent oxidisation of nitrite depends on the development of nitrite oxidisers, which takes place
only when nitrite starts to build-up.
Newly stocked ponds start by having meagre populations of nitrifying bacteria - both ammonium and
nitrite oxidisers. Nitrite oxidisers can be encouraged to develop ahead of this natural succession
pattern by fertilising the pond upon stocking with a nitrite salt such as Sodium Nitrite (NaN02).
(Avnimelech 2012).
These first few weeks may be critical for the newly stocked prawns until immobilisation of inorganic
nitrogen by heterotrophic and litho-autotrophic is in full swing.
Noticeable floc buildup takes some time and depends on the rate of feeding and the rate of organic
substrate buildup. The higher the stocking and feeding, the faster the buildup of biofloc. With shrimp
production, noticeable biofloc appear only in intensive systems. (Avnimelech 2012).
The following phases of pond development have been
observed in zero exchange biofloc systems
Clear water
Algal bloom dominates within days - weeks
Green water colour, large amount of foam on the surface of the ponds due to
accumulating dissolved organic material and inadequate bacterial community
Change to brown water, disappearance of foam and generation of flocculated
masses of bacterial cells, organic detritus and adsorbed colloids
The ponds start out with a phytoplankton bloom. But within 8 –
10 weeks of culture these blooms are replaced by microbial
flocs. Ponds can be easily differentiated as to their stage of
development. Young ponds will be green and create large
amounts of foam on the surface. Older ponds will turn
brownish / blackish in colour and be free of foam on the water
surface. Water from the older ponds is dominate by large
organic / microbial flocs that rapidly settle out if water
40
Recommended Reading…
For a thorough study of
microbial floc production
pond development refer to
Avnimelech 2012 “Biofloc
Technology”
circulation is stopped. Once the ponds reach this older stage they are highly stable and can assimilate
large amounts of organic inputs. Ammonia levels are generally under 2ppm, pH ranges from 7 – 7.5
and dissolved oxygen levels range from 4 – 6mg/l.
41
GRAPHICAL DEPICTION OF POND MICROBIAL DYNAMICS AND AMMONIA LEVELS
DURING POND DEVELOPMENT
T
he following graphs show the typical relationships observed at Australian Prawn Farms between
algae, ammonia and bacteria levels under conventional pond start up conditions. The final graph
shows the influence of increased bacterial biomass in the pond through a floc based management
approach. Please note that algal and
bacterial densities are not shown to scale.
1. Algal density and ammonia levels
increase as nutrient input via feed
increases and prawn biomass goes
up. Ammonia is then removed
through algal uptake.
2. With time, and under the right
conditions nitrifying and
heterotrophic bacterial populations
will slowly increase. Ammonia rises
again due to continued feeding.
3. Prevailing bacterial populations
begin to have an effect on ammonia
levels. The algal population is stable.
4. Low light conditions or a senescence
event causes an algal population crash.
Ammonia increases dramatically. Prevailing
bacteria cannot take up excess ammonia
quickly enough. If left unchecked, the
ammonia stays at this new constant level
even if the algal population returns to its
original density. The higher ammonia level
can also cause rapid and unstable algal
growth under sunny conditions.
42
5. This graph shows the same scenario as
above but in this instance the heterotrophic
bacteria population has been increased as a
result of floc base system management.
Here the extra bacteria in the system prevent
the ammonia spike from occurring and
ammonia levels are controlled despite the
algal population crash.
STOCKING DENSITY
O
bservations at APF indicate that a better and more stable biofloc occurs in ponds with stocking
densities of >35 prawns per m2. This observation is consistent with comments by Avnimelech
about the benefit of the additional feed going into the pond and the effect that this has on increasing
the C:N ratio.
ALGAL MANAGEMENT
A
key part of managing a low water exchange, microbial floc system is the development and
maintenance of a desirable algal species in the pond. Monitoring and identifying the dominant
algal species is vital because if the algae are undesirable and the conditions are not changed then this
algal species will continue to dominate. Changes in your algal community can be detected by changes
in pH and DO some time before secchi disk readings are affected but routine algal screening of each
pond 1 – 2 times per week is recommended.
The relationship between the establishment of microalgal blooms of different species and their effect
upon subsequent establishment of a robust microbial floc is still not clear and more research is
required in the this area. However, Smith and West 2009 found that the presence of a moderate to
high-density bloom of diatoms is significantly correlated to the subsequent establishment of a
microbial floc of satisfactory density (>0.1ml/L). They also found that a strong blue green algae bloom
produced a weak floc.
Smith and West 2009 also found that the large peak in TAN (>0.9mg/L) produced within 50 days of
filling the ponds is well correlated to the establishment of the bloom of large diatoms, generally about
30 days after the TAN peak. They also suggest, based upon observations at APF and information in
the literature that it is beneficial for the establishment of a strong bloom of large diatoms if the
concentration of reactive silica is in the order of 0.5 mg/L at about the same time as the TAN peak and
that Filtered Reactive Phosphorous concentration is at least 0.1mg/L at about the same time.
It is not necessary to conduct accurate algal density counts but rather aim to determine which algal
species is dominating the system. A useful method is to determine your level of tolerance of the
difference species and then rank your algal species from desirable e.g. diatom species (high rank –
score 10) to undesirable e.g. oscillatoria species (low rank – score 1) and then after each algal
screening, assign this rank to the pond which then indicates the dominant algal species in the pond at
the time of screening.
It is then essential to have management plans in place to deal with each different algal species e.g. if
the pond algae species is ranking high (for example 6 or above) then the management plan for that
pond should be one of maintenance as the rank indicates the presence of a desirable algal species.
However if the pond is ranking lower than that, this indicates the presence of an undesirable algal
species and the management plan for that pond would switch from maintenance mode to attack
mode to try to reduce the dominance of this particular species.
If the right algal species develops and dominates a pond e.g. a diatom species, then the pond can be
allowed to maintain that particular course as long as the algae is stable. If a good stable bloom is
43
achieved then the litho-autotrophic and heterotrophic bacterial communities can be manipulated in
effect behind the scenes. If the bloom is undesirable and unstable then this needs to be rectified first
before manipulation of bacteria will be of benefit.
Reducing the dominance of an undesirable species can be challenging, may take some time and may
require the use of different pond management tools to combat. It can be useful to utilise the
characteristics of the algal species against themselves. i.e. some algae float so the aeration can be
turned off and the algae will float to the surface which will allow for the algae to be skimmed from the
surface.
One of the key points about the APF pond management approach is that they use molasses primarily
to control fluctuations in microalgal density rather than trying to reduce TAN concentrations. The
objective is to control the algae crowing on this ammonia by stimulating bacterial growth with the
molasses. The addition of molasses to a pond is generally required when there is a weak or poor
biofloc present.
AERATION PLACEMENT
I
t is critical to keep the flocs in motion and under aerobic conditions as the microbial activity and
nitrogen uptake of the floc is more effective in mixed, oxygen rich water. In addition to
oxygenation, aerators serve to provide sufficient mixing to prevent anaerobic zones of sedimented
sludge. If water motion ceases and/or if air or oxygen injection cease, the flocs will consume all
available oxygen, settle to the bottom of the rearing unit, and become anaerobic. Anaerobic
sediments generate toxic waste products such as nitrite, hydrogen sulfide and methane.
Mixing also prevents water stratification, i.e. development of an oxygen rich warm layer at the surface
of the pond and an oxygen poor layer at the bottom. McIntosh (2001a) reported that water velocities
of 10 to 30 cm/sec are required to keep organic material in suspension in intensive shrimp ponds.
Placement of aerators in ponds is based to a large extent upon the farmer’s experience, meaning that
farmers need to adapt their aeration placement to accommodate different pond shapes and depths
and environmental conditions. An important local parameter is the typical wind pattern a factor that
affects water movement, de-stratification and re-suspension of sediments. Important pioneering work
on the modelling of water and sludge movements in aerated shrimp ponds has been done by
Peterson et al (2001) who investigated aerator placement in rectangular shaped ponds used for
intensive shrimp culture.
The number, power, positioning and type of aerator will affect the way the water moves and oxygen is
distributed throughout a pond. Generally, radial positioning of aerators perpendicular to the pond
walls is the simplest and most energy efficient aerator deployment model. One drawback is that this
can create a relatively large stagnant region in the centre of the pond, however by tilting the aerators
at 30o towards the centre or by moving the aerators away from the pond walls and closer to the
centre of the pond, it is possible to achieve a different water flow that can significantly reduce the
stagnant fraction of the pond.
As a general guide it is an advantage to combine different types of aerators or water circulators with a
paddlewheel aeration system in order to mix the whole pond.
44
An important point is the need to put aerators in position before the accumulation of significant
sludge piles. Mixing the pond bottom after the build-up of large sludge piles may disperse anaerobic
particles into the water column which can result in a sharp drop in available oxygen and can lead to
animal stress or mortalities. Treating anaerobic sludge piles in cases where they have been allowed to
build up should be done gradually in tandem with sufficient aeration and in conjunction with careful
monitoring. (Avnimelech 2012).
An example of effective aeration placement in P. monodon ponds at Pacific Reef Fisheries, Australia is
shown below:
5M
100 m
18 M
18 M
100 m
= Air Jet Aerator
= Paddlewheel
MONITORING OF PONDS
S
uccessful management of a low water exchange, microbial floc system relies on effective routine
monitoring and testing of pond conditions and the ability to rapidly respond to changes in pond
conditions when they occur.
Each farm should develop its own frequency schedule for monitoring, testing and other pond
management activities such as chaining.
45
A minimum level of monitoring, testing and other activities is suggested below:
Water Quality Parameter /
Minimum Frequency
Activity
DO, pH and Temp:
Twice per day. pH to be calibrated daily.
Ammonia:
Every 3 days or more if high levels
detected.
Alkalinity:
Once per week
Algal Screen:
Once per week
Salinity:
Once per week or more if high rainfall
Floc Volume:
???
Silicate:
???
Turbidity:
Daily
Prawn Health:
Daily observations on feed trays and
routine weekly cast net examinations.
Pond Bottom Check:
Once per week
Chaining:
Each pond fully once per week but only
½ pond at a time. More often if
necessary e.g. after an algal over bloom
or crash or if undesirable algal species
present i.e. blue green algae.)
As discussed earlier, it is up to the individual farm to determine its own species limitations and the
subsequent water quality and pond condition parameters that they will need to work within. The key
is for operators to identify the species tolerance levels plus the level of risk that the business is willing
to accept, and then set the water quality criteria limits accordingly. These parameters can be refined
later once operators are more skilled at floc based production system management.
POND MANAGEMENT DECISION MAKING PROCESS
M
onitoring is only effective if proper and timely responses are taken following the analysis of
pond water quality, pond conditions and prawn health status. It is recommended that operators
start by developing a series of responses to breaches in water quality or to warning signs of pond
condition deterioration.
In the early stages of adopting a low water exchange, microbial floc based system the action/s taken
in response to a breach in water quality parameter limits or to other signs that intervention is
necessary, should be recorded and the resulting changes in pond condition should be closely
monitored.
46
Through analysis of all potential influences on pond condition such as, (but not limited to), weather,
biomass, stock health, feed volume and water quality conditions; in conjunction with those
management tools applied / pond additives used, the duration of application, quantity or volume
applied and then closely recording the corresponding changes that occur in the pond; it is possible to
develop a catalogue of the most effective action needed in order to optimise, maintain or restore
pond conditions in response to a particular scenario.
APF have been recording this type of data for several years now and as a result have developed a
unique decision support program that will, based on rules and formulae refined over the years by APF,
analyse daily monitoring data and then generate a series of pond management recommendations for
each individual pond.
This type of decision support program is not transferrable between farms due to the many variables
between each region, farm and even each pond. APF have developed and use their program primarily
to reaffirm pond management decisions rather than solely relying upon it to generate management
recommendations, as they believe it poses a risk in that operators may start to rely too heavily on the
recommendations of the program and won’t actively think about the processes that are taking place
in the ponds. The human brain is still a far superior analytical tool compared with a computer when it
comes to pond management.
WATER EXCHANGE
A
t APF, water exchange is used as one of the range of pond management “tools” with which to
control the pond microbial and chemical environment and is carried out in response to particular
pond conditions on a pond-by-pond basis rather than being applied routinely. Occasionally, APF may
use water exchange to “reset” a pond that has deteriorated and that can’t be managed using
alternative “tools” but on the whole water exchanged is only used in conjunction with the suite of
other pond management tools described earlier, with the focus squarely on optimising microbial
population health and stability.
FLOC DENSITY
T
he presence of flocs can be observed by taking a water sample in a transparent container and
looking at the presence of suspended particles. Floc volume can be measured using calibrated
Imhoff cones. This is done by filling the Imhoff cone with 1 lire of water taken from in front of an
aerator stream (to ensure the sample is a representative mixed water sample), and then letting it stand
for 15 – 20 minutes. The volume of the settled plug can then be read.
Typical floc volumes in intensive shrimp biofloc systems are 2 – 40ml/l. Smith and West (2009)
reported the floc density at APF to be at the lower end of this scale at approximately 1 – 2 ml/l. APF
do not routinely measure floc density but it is good practice when managing a zero exchange biofloc
system to ensure that the pond is receiving adequate organic matter or to gauge when excessive flocs
should be drained. In a zero exchange shrimp biofloc system, floc volume below 2ml/l indicates that
47
organic matter supplementation may be required. Conversely if floc volume increases to above 20ml/L
it may be too high and require draining. (Avnimelech 2012).
SLUDGE MANAGEMENT
A
naerobic sludge may accumulate in specific locations even in highly aerated and mixed intensive
ponds, including BFT ponds. The local accumulation of the sludge may be due to imperfect
mixing of the water, e.g. in corners, in centres of radially mixed ponds, in low-lying conditions and
often under and beyond the paddle wheel aerators. Even though the pond is highly aerated, these
anaerobic pockets may inflict damage to the prawns. One of the most essential pond management
strategies is to minimise the extent of sludge accumulation sites through proper pond design, proper
aeration placement and, where appropriate, mechanical removal of accumulated sludge.
The presence of anaerobic sludge often affects the nitrification process. The second stage of
nitrification, NO2 – NO3 is more sensitive to anaerobic conditions that the first (NH4 – NO2) and to the
presence of anaerobic metabolites. Therefore the presence of active anaerobic pockets leads to the
rise in nitrite concentrations. An increase in the rise of nitrites in a pond may be an early warning of
the prevalence of anaerobic pockets that calls for fast corrective action.
A key feature of the pond management protocol at APF is the chaining of the bottom of the ponds.
This is carried out to resuspend material that has settled on the bottom to keep the pond bottom
clean and free of anaerobic sludge except for at the centre of the pond. Chaining of all ponds is
carried out once each week except for very early in the season when it is not carried out at all.
Chaining is only carried out over half the pond at a time, allowing an undisturbed area for the prawns
to occupy. The chaining has a great benefit in that the organic matter is lifted off the bottom and
mixed with the well-aerated water in the pond. This encourages aerobic bacteria to oxidise the
organic matter. The benefit is that aerobic bacteria will oxidise the organic matter at a far greater rate
than can be achieved by anaerobic bacteria – and with less oxygen demand. (Avnimelech, 2009)
Sludge accumulation increases with an increase in stocking and feeding rates. The accumulation of
sludge and the development of anaerobic conditions in the sediment are known to be a factor
limiting microbial processes and the recycling of nutrient in the sediment layer in a BFT system.
Effective solids management has been shown to increase shrimp growth and productivity in biofloc
systems (Ray et al. 2010a)
To ensure adequate nutrient cycling and to be able to further increase production, the accumulation
of anaerobic bottom sludge has to be prevented through proper aeration placement as previously
described, and also by either removing the sludge or resuspending it. In previous studies, sludge
removal has resulted in the remediation of 67% of the nitrogen added to the system in feed, resulting
in improved water quality (Hopkins et al. 1994).
Removal of sludge during a crop presents significant logistical and disposal problems unless ponds
are constructed to allow for this. For example in Belize Aquaculture bottom sludge is drained by
opening a valve connecting a sump in the center of the pond to drainage canals leading to retention
ponds (McIntosh 2001). An alternative method is to continually resuspend the sludge particles and
48
prevent the build up of anaerobic sediment by aggressively cropping the biofloc solids from the
system while they are in active suspension by exchanging water into a settling pond.
ACTIVE SUSPENSION
W
hen water exchange is limited, organic matter in the water builds up. The level of organic matter
in ponds tends to reach a stable steady state due to a balance between the addition of organic
matter and its microbial degradation.
If suspended organic matter (more commonly termed Total Suspended Solids or TSS) is too high and
the pond mixing system cannot keep it in suspension, organic matter accumulates at the pond
bottom, anaerobic conditions develop and the pond environment deteriorates. In such cases the insitu biological balance cannot control the system without intervention. The most common
intervention is drainage of the excessive suspended water, either through water exchange or by
draining the sludge that accumulates at the bottom of the pond.
The determination of the point at which intervention is needed is quite complicated and site specific
according to the capacity, type and placement of the aeration system and according to the ponds size,
shape and depth. A very rough estimate is that intervention is needed when the TSS steady level is
above 200-500mg/l, though each farm must develop its specific set of rules (Avnimelech 2012).
It must be noted that stocking densities in intensive L.
Vannamei production, are generally much greater than in
intensive P. Monodon production. Physiology and behaviour
aside, this point may explain why floc volume or TSS can be
easier to manage in a Vannamei culture system as the dense
biomass of shrimp rapidly harvest the flocs and assist in
stabilising production of organic matter.
Recommended Reading…
For greater detail on sludge
management and active
suspension of solids refer to
Avnimelech 2012 “Biofloc
Technology”
NUTRIENT DISCHARGE
M
oving to a low water exchange system will reduce the daily pond discharge volume and
therefore also result in a decrease in daily nutrient discharge. Smith and West (2009) calculated
a 78.5% reduction in water discharged in 2006/07 and a reduction of 68%, 69% and 73% in the
following three years.
The total nitrogen discharge from APF in the last year of conventional flow through pond
management (2003/04) was 1.36 kg/ha/d or 9000kg/crop. Despite the increased production of on
average 4.2 tonnes/ha gained from the adoption of a low water exchange, microbial floc system, the
total nitrogen discharge in 2008/09 was only 0.5kg/ha/d or 3,400 kg/crop. This equates to a reduction
in total nitrogen discharge by 77%.
49
It is important to remember however that although a low water exchange, microbial floc system will
reduce overall nutrient loading, that nutrient maximums remain near EPA limits, and at present the
EPA regulates on both.
CONTINGENCY PLANS
A
vital component in the successful management a low water exchange, microbial floc system is to
have a set of contingency plans developed and in place for when things go wrong. Typical worst-
case scenarios should be evaluated and plans drawn up for what action to take in the event that these
situations occur.
Such situations might include; unusual weather events such as flooding / drought / cyclones; staffing
capacity changes due to resignation or illness or long service leave or changes in managerial structure;
power failure; disease outbreak; using the wrong additives or amount of additive at the wrong time or
in the wrong pond; feed or additive supply issues etc.
50
6. KEY POINTS: THE AUSTRALIAN PRAWN FARMS’
APPROACH TO MANAGING A LOW WATER EXCHANGE,
MICROBIAL FLOC PRODUCTION SYSTEM FOR P.
MONODON.
Key points: The Australian Prawn Farms’ approach
Good operator knowledge of water chemistry dynamics and microbial
ecology
Water quality ranges precisely defined i.e. what makes prawns happy
Water quality tolerance levels defined i.e. what level of risk is acceptable
Range of pond management tools defined i.e. what additives are available
Precise consequence of each additive understood
Condition for use of each additive understood i.e. what form additives take
when applied under different conditions and subsequent activity / influence
Right infrastructure & equipment
Right monitoring structure / staffing
Primary focus is on establishing and maintaining a desirable algal community
Carbon: Nitrogen ratio is not measured for pond management purposes
Algal screening and standard procedures for dealing with undesirable spp.
Utilise water exchange as one of the range of pond management “tools”
Occasionally use water exchange to “reset” a pond that has deteriorated and
can’t be managed using any other tool
Aim to get established diatom bloom and utilise algal and nitrifying bacteria
for inorganic nitrogen assimilation
When a pond is ticking along can manage algae and nitrifying bacteria by
addition of bicarbonate and hydrated lime.
Rarely get big flocs such as in conventional Biofloc systems.
51
Heterotrophic bacteria in the background mopping up.
52
7. REFERENCES AND RECOMMENDED READING
Avnimelech, Y., Weber, B., Hepher, B., Milstein, A., Zorn, M. 1986. Studies in circulated fish ponds.
Organic matter recycling and nitrogen transformation. Aquaculture and Fisheries Management 17:
231-242
Avnimelech, Y., Kochva, M., Diab, S. 1994. Development of controlled intensive aquaculture systems
with a limited water exchange and adjusted carbon to nitrogen ratio. Isreal Journal of Aquaculture,
Bamidgeh, 46:119-131.
Avnimelech, Y. 1999. Carbon/nitrogen ratio as a control element in aquaculture systems. Aquaculture.
176: 227-235.
Avnimelech, Y., 2006. Bio-filters: the need for a new comprehensive approach. Aquaculture
Engineering. 34 (3), 172–178.
Avnimelech, Y. 2012. Biofloc Technology – A Practical Guide Book, 2nd Edition. The World Aquaculture
Society, Baton Rouge, Louisiana, United States.
Bower, C. E., Bidwell, J. P. (1978). Ionization of ammonia in seawater: effects of temperature, pH and
salinity. Journal of Fish Research. Bd Can. 35: 1012-1016
Boyd, C. E. 2009. Estimating mechanical aeration requirements in shrimp ponds from the oxygen
demand of feed. In: The Rising Tide, Proceedings of the Special Session on Sustainable Shrimp
Farming (Ed. By C. L. Bowdy & D. E. Jory), pp. 230-34. World Aquaculture Society, Baton Rouge.
Boyd, C. E. (Eds.). 1995. Bottom soils, sediments and pond aquaculture. Chapman & Hall. N.Y.
Burford, A. A., Longmore, A. R. 2001. High ammonium production from sediments in hypereutrophic
shrimp ponds. Marine Ecology Progress Series. Vol. 224: 187 – 195.
Burford, M.A., Thompson, P.J., McIntosh, R.P., Bauman, R.H., Pearson, D.C., 2003. Nutrient and
microbial dynamics in high-intensity, zero-exchange shrimp ponds in Belize. Aquaculture, 219, 393411.
Chamberlain, G., Avnimelech, Y., McIntosh R. P., Velasco, M. 2001. Advantages of aerated microbial
reuse systems with balanced C/N. In: Nutrient Transformation and Water Quality Benefits. Global
Aquaculture Alliance Advocate 4: 53-56.
Cole, J.J., Findlay, S., Pace, M.L. 1988. Bacterial production in fresh and saltwater ecosystems: a crosssystem overview. Marin Ecology – progress series 43:1-10
Colt, J., Armstrong, G. 1979. Nitrogen toxicity to fish, crustaceans and molluscs. Department of Civil
Engineering, University of California Davis.
Diab, S., Kochba, M., Mires, D., Avnimelech, Y. 1992. Combined intensive – extensive pond system, a:
inorganic nitrogen transformations. Aquaculture 101: 33-39.
53
Ebeling, J., Timmons, M. 2006. Understanding photoautotrophic, autotrophic and heterotrophic
bacterial based systems using basic water quality parameters. 6th international Conference on
Recirculating Aquaculture, Roanoke, Virginia, USA July 21-22.
Furtado, P. S., Poersch, L. H., Wasielesky Jr., W. Effect of calcium hydroxide, carbonate and sodium
bicarbonate on water quality and zootechnical performance of shrimp Litopenaeus vannamei reared in
bio-flocs technology (BFT) systems. Aquaculture. 321: 130-135.
Hopkins, J. S., Sandifer, P. A., Browdy, C. L. & Stokes, A. D. 1994. Sludge management in intensive pond
culture of shrimp: effect of management regime on water quality, sludge characteristics, nitrogen
extinction and shrimp production. Aquacultural Engineering 13: 11-30.
McIntosh, R.P. 2011. A retrospective look at the project that changed the shrimp culture
industry. World Aquaculture Meeting. New Orleans, Louisiana, USA. March 3, 2011.
McIntosh, R. P. 2001. High rate bacterial systems for culturing shrimp. In: Proceedings from the
Aquacutural Engineering Society’s 201 Issues Forum. (Ed. By S.L. Summerfelt, B. J. Watten & M. B.
Timmons), pp. 117-29. Aquaculture Engineering Society, Shepardstown.
McIntosh, P. R. 2000. Changing paradigms in shrimp culture, III: Pond design and operation
considerations. Global Aquaculture Alliance Advocate 42-45
Monod, J. 1949. The Growth of Bacterial Cultures. Annual Review of Microbiology, v. 3, p. 371.
Nunes, A., Gesteira, T., and Goddard, S., 1997. Food ingestion and assimilation by the Southern brown
shrimp Penaeus subtilis under semi-intensive culture in NE Brazil. Aquaculture,
149:121-136.
Panjaitan, P. 2010. Shrimp culture of Penaeus monodon with zero water exchange model (ZWEM)
using molasses. Journal of Coastal Development. 14: (1) 35-44
Peterson, E. L., Wadhwa, L, C., Harris, J. A. 2001. Arrangement of aerators in an intensive shrimp
growout pond having a rectangular shape. Aquaculture Engineering 25: 51-65.
Preston N.P., Jackson, C.J., Thompson, P., Austin, M., Burford, NM. A., 2000. Prawn farm effluent:
composition, origin and treatment. Fisheries Research and Development Corporation final report.
Project No. 95/162. FRDC, Canberra.
Queensland Department of Primary Industries and Fisheries. 2006. Australian Prawn Farming Manual;
Health Management for Profit. ISSN 0727-6273.
Ray, Q. J., Shuler, A. J., Leffler, J. W. & Browdy, C. L. 2009. Microbial ecology and management of
biofloc systems. In: The Rising Tide, Proceedings of the Special Seesion on Sustainable Shrimp
Farming (Ed. By C. L. Browdy & D. E. Jory), pp. 255-66. World Aquaculture Society, Baton Rouge.
Rittman, B. E., McCarty, P. L. 2001. Environmental biotechnology – principles and applications.
McGraw-Hill International Edition, Singapore 754 pp
Rosenbury, B. 2006. Meet The Flockers. Shrimp News International.
www.shrimpnews.com/FreeReportsFolder/PondEcologyFolder/MeetFlockers.html
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Smith, D. M., West, M. 2009. Increasing the profitability of Penaeus monodon farms via the use of lowwater exchange, microbial floc production systems at Australian Prawn Farms. Australian Seafood CRC
Project 2009/748.
Strombon-Rutgers, D. B., Tweed, S. M. 1992. Business planning for aquaculture – is it feasible?.
Northeastern Regional Aquaculture Centre. Fact Sheet No. 150. University of Massachusetts.
Tidwell, J. H., 2012. Aquaculture Production Systems. World Aquaculture Society. Pub. Wiley-Blackwell.
ISBN 978-0-8138-0126-1
Van Wyk, P., 2004. The Shrimp List: Discussions October 2004. Shrimp News International.
Van Wyk, P., Scarpa, J., 1999. Water quality and management. In: Van Wyk, P., et al. (Ed.), Farming
Marine Shrimp in Recirculating Freshwater Systems. Florida department of agriculture and consumer
services, Tallahassee, pp. 128 – 138.
Velasco, M, Lawrence, A and Neill. W., 1998. Effects of dietary phosphorus level and inorganic source
on survival and growth of Penaeus vannamei post larvae in zero water exchange
culture tanks. Aquatic Living Resources, 11:29-33.
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8. APPENDIX 1
RESEARCH PAPER SUMMARY - SMITH AND WEST 2009
Increasing the profitability of Penaeus monodon farms via the use of low-water
exchange, microbial floc production systems at Australian Prawn Farms. David M.
Smith and Matt West. Seafood CRC Project No. 2009/748
BACKGROUND
During their 2005/06 season, Australian Prawn Farms (APF) successfully cultured black tiger prawns,
Penaeus monodon, via a low water exchange, microbial floc production culture system (Smith and
West, 2011).
However, the reason for the success of the trial 2005/06 season was not well
understood, and thus, required further investigation in order to repeat the successful harvest of this
season. In 2008, a three-year study funded through the Seafood CRC program was commenced by
APF and Dr. David Smith (CSIRO) in an attempt to better understand the successful harvest of P.
monodon during the 2005/06 season. The key objective of the study was to determine how varying
nutrient concentrations, carbon source, and water exchange affect the formation, composition, and
persistence of a microbial floc (Smith and West, 2011). Also of interest was the effectiveness of
different carbon sources in the formation of a microbial floc, the nutritional benefits provided to P.
monodon by microbial flocs and consequently the reduction in feed expenses, and ultimately the
environmental and economical benefits that can be reaped via utilization of a low water exchange,
microbial floc production system.
METHODS
The research approach adopted was to intensively monitor the nutrients, microalgal population and
biofloc density over a three year period in five production ponds at APF that were being managed
according to the farms established low-water exchange protocol.
Five production ponds at APF were filled at the beginning of each season, and fertiliser was added as
per Australian Prawn Farms protocol until either a dense microalgal bloom was established, or the
Total Ammonia Nitrogen (TAN) surpassed 1 mg/L (usually after approximately 4 weeks). Two different
types of fertilisers were used for the sake of comparison in this study. During the first two seasons,
the fertiliser used was urea, while during the third season Easy N was used. Easy N is a liquid
fertiliser consisting of a mixture of urea and ammonium nitrate with a 1:1 nitrogen ratio.
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During the 2008/09 and 2009/10 seasons, sodium silicate (waterglass, 35% sodium silicate solution)
was added to the ponds in several doses during the first two weeks after filling to provide
approximately 0.5 mg/L of reactive silica to the pond.
Stocking density ranged from 35, 45 and 60 PL’s per m 2 in 2007/08 (two, two and one pond
respectively) to 50 PL’s per m 2 in 2008/09 and 2009/10 (all five ponds).
Sampling of the ponds was carried out for 120 days, until harvesting preparation began. The main
parameters analysed were; Temperature, pH, Dissolved Oxygen, Total Ammonium Nitrate (TAN),
Biofloc Volume, Salinity, Total Alkalinity. Total Nitrogen, Total Kjeldahl Nitrogen, Reactive Silicate,
Nitrate and Nitrite, Total Phosphorus, Filterable Reactive Phosphorus, Total Inorganic Carbon, Total
Organic Carbon, Microalgal Species and Microalgal Density
Fertiliser Comparison Trial
After experimental difficulties encountered during the study in situ at APF, a more rigorous fertiliser
comparison trial was conducted during the 2009/10 season at CSIRO, Cleveland using 2.5 tonne
fiberglass tanks in a controlled environment facility.
26 tanks were filled with seawater, well aerated and maintained at an ambient temperature of 30 oC.
13 tanks were fertilized with urea and 13 tanks were fertilized with Easy N. The dosage rate was
calculated so all tanks received the same amount of total Nitrogen. Sodium silicate was also added to
the tanks. Water quality and microalgal density was analysed daily and dominant algal species were
identified three times per week. The experiment ran for four weeks.
RESULTS
The purpose of sampling was to monitor nutrient concentrations in the ponds, microalgal populations,
and biofloc density. Of the nutrients monitored throughout the course of the study, the measurement
of those nutrient forms categorized as reactive nutrients (TAN, reactive silica, and filterable reactive
phosphorus (FRP)) were found to be most useful as these are the nutrient forms available to bacteria
and microalgae. As a consequence the focus in the discussion is on the reactive nutrients.
Biofloc Formation
The results showed a significant positive relationship between diatom bloom density and the
establishment of a biofloc of > 0.1 mg/L. A strong bloom of blue green algae was also found to favour
the formation of a biofloc, though this floc tended to be weaker and thus less stable than the biofloc
following a diatom bloom.
Total Organic Carbon
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Total Organic Carbon (TOC) measurements were used as an indicator of microalgal biomass in the
ponds and there appeared to be a positive correlation between the TOC concentration and diatom
dominance. TOC levels are a useful indicator in pond management intervention to pre-empt and
prevent a microalgal crash or to minimize its magnitude.
Total Ammonium Nitrogen
TAN, the sum of the ammonia (NH3) and ammonium ion (NH4+) concentrations, was measured.
Peaks in TAN were found to be significantly related to diatom blooms. The peak in TAN that occurred
30 to 45 days after filling the pond appeared necessary for the development of a strong bloom of large
diatoms within the following 30 to 40 days. The lack of a TAN peak renders it unlikely that a medium
or strong bloom of large diatoms will occur and consequently, makes the formation of a biofloc of
adequate density (> 0.1ml/L) unlikely as well.
In the 2009/10 season a TAN peak of > 0.9mg/L did not occur in the first 45 days and consequently a
medium or strong bloom of large diatoms didn’t become established until late in the season and
hence a good biofloc failed to develop. The lack of a TAN peak in 2009/10 was considered due to the
use of Easy N fertiliser. It appears that Easy N assisted the establishment of a pioneering species of
algae – typically chlorella – that has exceptional stability and maintained dominance. The peaks in
TAN later in the 2009/10 season were due to very overcast conditions.
pH and Ammonia
Prawn ponds are usually maintained at a pH between 7.6 and 8.6, as at higher pH values, a greater
proportion of TAN exists in the form of ammonia, which is toxic to prawns at elevated concentrations.
pH was observed to increase over the first 20 days from about pH 8.3 to a maximum of > 9 as the
microalgal bloom became denser and reduced the dissolved CO 2 concentrations in the water.
Thereafter the pH tended to decline as the prawn biomass increased and the amount of respired CO 2
increased. As the season progressed, sodium carbonate or hydrated lime were occasionally used to
increase pond pH following events such as microalgal crashes or a period of cloudy weather during
which the rate of photosynthesis was reduced.
At APF the pond managers accept TAN concentrations of up to 1.5 mg/L for a few days when the pH
is between 9.0 and 9.5, in the early weeks after stocking. Whereas many microalgal species do not
tolerate high pH (pH > 9) very well, certain species of diatom dominate in environments of high pH. It
is suggested that the early peaks in TAN developed as a result of the increased pH causing less
tolerant microalgal species to die off, which would result in less uptake of TAN and more TAN being
released due to decomposition of the dead microalgae. The appearance of large diatoms following
this event reaffirmed the fact that certain species of diatom are more tolerant of high pH.
It was therefore concluded that because most conventionally managed prawn farms would not be
willing to allow the pH of the ponds to reach 9.0, nor would they willingly let the TAN in ponds
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increase to the levels seen at APF, they would not see the development of blooms of large diatoms
and the development of biofloc.
Total Inorganic Carbon
Total inorganic carbon (TIC) is the sum of dissolved CO2, bicarbonate (HCO3-) and carbonate (CO3-)
and therefore is closely related to Total Alkalinity. TIC declined in all ponds in the first 20 – 30 days
after filling as the dissolved CO2 was used by the microalgae and some of the bicarbonate was taken
up by the litho-chemoautotrophic bacteria. Thereafter TIC concentration was variable but TIC and
alkalinity were maintained within defined limits (60 – 120 mg/L) by the addition of sodium bicarbonate.
Silica and Silicate
While silica is not required for other microalgal species it is an essential nutrient for diatoms. Silica is
poorly soluble in water at pH < 9.0 but becomes increasingly soluble as the pH becomes more
alkaline, markedly increasing in solubility when the pH increases above 9.0. Because strong diatom
blooms were found to lead to the formation of a large biofloc, sodium silicate was added to the ponds
in order to support a diatom bloom.
All the five study ponds were dosed with sodium silicate but as a result, without any control ponds to
compare against it was not possible to rigorously test the effect of silica. However, higher
concentrations of silica were measured in the study ponds in the 2009/10 season in the period
between 40 to 80 days when diatoms were not the dominant species in comparison to seasons
2008/09 and 2007/08 when diatoms were the dominant microalgal species. This suggests that the
silica was being taken up and utilised by diatom microalgae when the diatom species were dominant
and silica was available in the pond.
Filterable Reactive Phosphorus
Statistical analysis demonstrated a significant inverse relationship between diatom density and FRP
concentrations.
Nitrate and Nitrite
The combined concentration of nitrate-nitrogen and nitrite-nitrogen (NOX) found in all the ponds was
very low which indicated that toxic nitrite was not a problem and suggests that de-nitrification of nitrate
was occurring.
Molasses, Bicarbonate, and Biofloc
When a strong biofloc was present (> 0.1ml/L), the total alkalinity of the ponds was found to
progressively decrease, suggesting the utilization of bicarbonate by the bacterial community in the
pond. Thus, sodium bicarbonate was added to the ponds in order to keep the alkalinity between 80
59
and 120 mg/L. If a strong biofloc was not present, molasses was added to the ponds in order to
manage the microalgal bloom and the TAN by encouraging the productivity of heterotrophic bacteria.
Under these conditions Total Alkalinity does not decrease markedly. Statistical analyses found a weak
relationship between the amounts of molasses or bicarbonate added to the ponds and the presence
of a strong biofloc but with only 15 sets of data there was not enough information available to detect a
significant correlation.
FERTILISER COMPARISON TRIAL RESULTS
Water Quality
There was no significant difference in water salinity levels, dissolved oxygen, water temperature, pH,
total alkalinity, TAN and nitrate levels between the two treatments with water quality parameter levels
remaining constant across all tanks.
Microalgae
Measurements of the density of microalgae were highly variable with time and were not found to differ
significantly with the use of urea versus Easy N fertilizer. Fertilizer type was also not found to have a
significant effect on the microalgal species composition. These results differ from observations made
at APF where ponds fertilized with Easy N had exceptionally stable microalgal blooms and ponds
fertilized with urea appeared to have a slower response in the establishment of a bloom.
DISCUSSION OF FERTILISER COMPARISON TRIAL RESULTS
The key observations that can be gathered from the fertiliser comparison study are as follows:
1. The use of Easy N fertilizer as opposed to urea was found to have no significant effect on the
establishment and growth of microalgal populations (in contrast to the stable Chlorella blooms
observed after using Easy N at APF).
2. All measurements of TAN throughout this study were less than 0.1 mg/L. The fate of the
nitrogen added to the tanks throughout the course of the study remains uncertain. One
possible explanation is that the nitrogen was taken up by heterotrophic bacteria and by
benthic or epiphytic algae growing on the bottom of the tanks.
3. pH was found to increase progressively at the start of the experiment, with periodic decreases
correlated to culture longevity and conditions of the microalgal bloom.
4. Different types of microalgal species were dominant at different times throughout each
season of the study. For the first week moderate densities of diatoms were frequent, while
from day 3 – 14, dinoflagellates reached a peak, and finally either diatoms or blue green
algae became more prevalent.
CONCLUSIONS
60
The Effect of Nutrient Conditions on Establishing Biofloc
Throughout the course of this study, it was determined that a bloom of large diatoms of moderate to
high density is significantly correlated to the establishment of a satisfactory biofloc (> 0.1 mg/L). The
large peak in TAN (> 0.9 mg/L) that is produced within 50 days of filling the ponds is well correlated to
the establishment of the bloom of large diatoms, generally about 30 days after the TAN peak.
Although data from the test ponds did not show that the establishment of a bloom of large diatoms
was dependent upon having reactive silica and FRP concentrations considered necessary for diatom
growth (0.5 mg/L and 0.1mg/L respectively), the data were heavily skewed in favour of adequate
levels of these nutrients. Observations from other ponds at APF and information in the literature
suggest that it would be beneficial for the establishment of a strong bloom of large diatoms if the
concentration of reactive silica was in the order of 0.5 mg/L and that the FRP concentration be at least
0.1mg/L at about the same time as the TAN peak .
Observations at APF indicate that a better and more stable biofloc occurs in ponds with stocking
densities > 35 prawns per m2. The current APF stocking protocol is for ponds to be stocked with 50
PL/m2.
Weekly chaining of pond bottoms was a vital method for pond management. By dragging the long
loop of chain across the bottom of the pond with the exception of the middle, material that had settled
to the bottom was resuspended, keeping the bottom free of anaerobic sludge. Once the organic
material is resuspended, it is available for oxidation by aerobic bacteria.
Peaks in TAN also occur after microalgal crashes and during overcast conditions when photosynthetic
activity is reduced. During these periods action may need to be taken to minimize the impact of the
high TAN levels. One strategy for the management of TAN is the addition of molasses as an energy
source for heterotrophic bacteria production. During the process of heterotrophic bacterial growth
some of the TAN from the water is used for synthesis of amino acids, protein and nucleic acids
thereby reducing the ammonia concentration in the water.
Another benefit of reducing TAN concentrations is that it results in less ammonium being available for
microalgal productivity, which provides a management strategy to control an unsustainable increase
in density of the microalgal blooms. This is considered by APF to be the most important application for
the addition of molasses to prawn ponds.
Effectiveness of Different Carbon Sources
Carbon sources are added to the ponds in order to supply a complementary nutrient or energy source
for the bacteria. The benefit of adding a carbon source to the pond is twofold: by encouraging
bacterial growth, excess ammonia is removed from the pond via bacterial metabolic processes.
Secondly, while it was assumed prior to this study that only organic carbon sources such as molasses
or bagasse would be of use in the low water exchange production system, it was observed that
61
inorganic carbon, in the form of bicarbonate, is also important as it is required by both microalgae and
litho chemoautotrophic bacteria as their carbon source.
Large diatoms such as Bellerochia and Helicothica spp associated with the establishment of a stable
biofloc appear also to be associated with lithoautotrophic bacteria. These lithoautotrophic bacteria
appear dominant in the biofloc and, along with the microalgae, use the bicarbonate in the water.
Sodium bicarbonate is the preferred inorganic carbon source to replenish the bicarbonate and hence
increase the Total Alkalinity if the pH does not need to be changed. If both Total Alkalinity and pH are
too low, the application of sodium carbonate will raise both of these parameters. It should be noted
that whilst hydrated lime will increase both pH and Total Alkalinity, it won’t add any inorganic carbon
(or bicarbonate).
Organic carbon sources compared in this study were molasses and bagasse. The degradation rate of
bagasse was determined to be far too slow for effective uptake by heterotrophic bacteria, leaving
molasses as the organic carbon source of choice. Molasses is also very cost effective and a practical
carbon source as it is easily applied to the ponds.
The approach at APF is to use molasses primarily to control fluctuations in microalgal density, rather
than trying to reduce TAN concentration. The objective is to control the algae growing on this
ammonia by stimulating bacterial growth with the molasses. The addition of molasses to a pond is
generally required when there is a weak or poor biofloc present. Under these situations, the large
diatoms such as Bellerochia spp and Heliothica spp are either absent or present at low density.
NUTRITIONAL BENEFITS OF BIOFLOCS AND REDUCTION IN FEED COSTS
While the main purpose of establishing a biofloc is to manage water quality in the ponds, the flocs
may also provide a secondary source of nutrition for the prawns. It is also believed that the stable
environment created by the biofloc contributes to improved growth and better feed-conversion ratios.
During the 2007/08 season, ponds in which a good biofloc had been established produced a 10 tonne
harvest 10 days earlier than those with a poor biofloc. These good biofloc ponds also returned an
FCR of 1.2 – 1.3 compared with an FCR of 1.5 – 1.6 from the poor biofloc ponds. Thus, the presence
of a good biofloc was calculated to have saved 3 tonnes of feed or about $6,000 per pond, equivalent
to 20% of the total feed cost.
ENVIRONMENTAL BENEFITS AND EFFECTS ON FARM PROFITABILITY
As compared to the conventional flow-through system of prawn farming, the low water exchange
production system used in this study reduced water usage by 68%, 69%, and 73% in the first, second
and third year of this study. Furthermore, total nitrogen discharge was decreased by 77%.
Perhaps the most enticing benefit to the low water exchange production system is the increase in
production from the ponds. In 2003/04 a harvest of 6.7 t/pond was obtained using the conventional
flow-through system. In comparison, in the 2008/09 season, a harvest of 12 t/pond was obtained from
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ponds in which a biofloc was established. Overall, reductions in power usage, feed costs, and labor
combined with the increase in prawn production using the biofloc system equate to an estimated
$65,000 increase in production per pond.
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