Aquacultural Engineering 22 (2000) 33–56
www.elsevier.nl/locate/aqua-online
Review
Solids management and removal for intensive
land-based aquaculture production systems
Simon J. Cripps *, Asbjørn Bergheim
RF-Rogaland Research, P.O. Box 2503 Ullandhaug, N-4091 Stavanger, Norway
Abstract
This review aims to identify and examine realistic aquaculture waste solids management
strategies. The main reason for treating solids to be discharged from flow-through systems is to
reduce potential negative impacts on the surrounding aquatic environment. In reuse and recycle
systems, solids management will be required to maintain culture water quality. In such cases,
solids management will often be designed to be combined with other unit processes. Solids
concentrations in the untreated effluent from flow-though farms are low at around 5-50 mg l-1, and
do not appear to have altered greatly within the last 20 years. These solids can commonly carry 732% of the total nitrogen and 30-84% of the total phosphorus in the wastewater. Feed quality and
feeding management can be manipulated to reduce the quantity of waste solids produced.
Decreases in the specific water consumption within a farm, increase the concentration of solids in
the effluent, which results in an increase in particle separation efficiency. Particles should be
separated from the culture stock and the primary effluent flow quickly and efficiently (i.e. at high
solids concentrations). This can be achieved by within-tank separation systems, often comprising a
separate low-flow particle outlet. Rotating microscreens are commonly used at land-based
intensive fish-farms in Europe. Screen mesh pore sizes of 60-200 um are common. There is little
advantage in using pore sizes smaller than 60 um. Low concentration aquaculture solids usually
settle discretely in sedimentation tanks, i.e. with no agglomeration of particles that would increase
their settling velocity. Such basins are prone to many technical problems, and in most situations are
unlikely to be suitable for the treatment of solids in the primary wastewater from aquaculture
facilities. Sedimentation basins may be appropriate for secondary de-watering or thickening.
Overflow rates of 1.0-2.7 m3 m-2 h -1 have been reported. Bead filters and
* Corresponding author. Tel.: +47-51875000; fax: +47-51875200. Email address: scripps@wwfnet.org (S.J. Cripps)
0144-8609/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII
: S 0 1 4 4 - 8 6 0 9 ( 0 0 ) 0 0 0 3 1-5
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S.J. Cripps, A. Bergheim/Aquacultural Engineering 22 (2000) 33–56
flotation columns have been shown to perform well for solids removal in limited flow-rate recycle
systems. The sludge produced by separation technology can be thickened and stabilised by the
addition of lime, to kill pathogenic diseases and restrict putrefaction. The resulting sludge has been
spread on agricultural land. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Solids; Particles; Wastewater; Effluent; Aquaculture; Treatment
1. Introduction
Wastes from aquaculture include all materials used in the process which are not
removed from the system during harvesting. The main wastes from aquaculture
operations are uneaten feed, excreta, chemicals and therapeutics, as described by
Ackefors and Enell (1990, 1994), Beveridge et al. (1991), Braaten (1992). Dead and
moribund fish, escaped fish and pathogens (Bergheim and Asga˚rd, 1996) can also be
considered forms of waste. The main source of potentially polluting waste either
discharged in the farm effluent, or to be made available for reuse within the farm, is feedderived (Pillay, 1992), such as uneaten feed, undigested feed residues and excretion
products.
The feed-derived wastes include components that are either dissolved, such as
phosphorus (P) and nitrogen (N) based nutrients, or are in the solid phase as suspended
solids, as described by Losordo and Westers (1994). This review will present information
relating to the management of particulate wastes in intensive land-based aquaculture
facilities.
The management of solids, through feed design, feeding management and flow
regulation, in addition to the eventual removal of solids using separation and sludge
treatment technology, becomes increasingly important as aquaculture systems intensify.
For recycle systems, in which reused water must be of an adequate quality to maintain
the culture organisms in a healthy and fast-growing condition, it is especially important
to remove waste products.
A build-up of solids in an aquaculture system can lead to a decline in culture water
quality that will increase the stress on the culture organisms (Rosenthal et al., 1982;
Klontz et al., 1985; Braaten et al., 1986). This will be as a result of: a direct impact of the
solids on the animals, e.g. through the partial smothering of gills; or indirectly, e.g. by
offering a suitable habitat for the proliferation of pathogenic organisms (Braaten et al.,
1986; Liltved and Cripps, 1999), and the consumption of dissolved oxygen as the solids
decay (Welch and Lindell, 1992).
This review aims to identify and examine realistic aquaculture waste solids
management strategies. This will be achieved by the critical analysis of the components
that could comprise a multi-stage treatment system.
The aims of waste treatment in general and solids management in particular differ,
depending on whether the intensive culture system is single-pass flow-through, water
reuse with little reconditioning, or recycle, as summarised by Losordo and Westers
(1994).
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35
The main reason for treating solids to be discharged from flow-through systems will
be to reduce potential negative impacts on the surrounding aquatic environment. Feedderived wastes from intensive aquaculture facilities can degrade the environment and
conflict with other aquatic resource users. Many authors have described these impacts,
including impacts from solids, such as: Enell and Lof (1983), Ackefors and Enell (1990),
Gowen (1991), Gowen et al. (1991), Pillay (1992), Costa-Pierce (1996).
In reuse and recycle systems, solids management will normally be aimed at providing
at least an adequate culture water quality. In such cases, solids management will often be
designed to be combined with other unit processes for the control of dissolved nutrients,
biochemical oxygen demand (BOD), dissolved gases (O2 and CO2), pH and pathogens.
Irrespective of the type of culture system, modern solids management commonly
comprises a series of stages, or unit processes. More advanced systems integrate these so
that solids are prepared by one unit process or management regime, to be better handled
at the next stage (Cripps and Bergheim, 1995). Hence, solids control stages such as feed
management, pre-treatment, primary separation, secondary solids handling and disposal
may comprise an integrated solids management system (Alana¨ra¨ et al., 1994;
Summerfelt, 1998).
2. Solids characteristics
2.1. Solids loads
In order to maintain the solids at acceptable levels for discharging or recycling, it is
important to understand the nature of the wastes. Appropriate management practices
and/or treatment technology can then be applied.
Many studies and reviews, including Cripps and Kelly (1996), have shown that
aquaculture waste characteristics are not conducive to easy treatment, because of their
low concentrations in the effluent. In the 1980s, Alabaster (1982), Muir (1982) Solbe´
(1982), and Warrer-Hansen (1982) showed that effluent total solids (TS) concentrations
from intensive cold-water, flow-through fish-farms were about 9, 11.1, 5–50, and 5–50
mg l-1, respectively. Fish-farm operations have changed in recent years, due to an
intensification of farming, through culture density increases and specific water
consumption (flow of water required per biomass of stock per unit time — l kg-1 min-1)
decreases, improvements in feed formulations, and improved feeding systems that reduce
losses. Nevertheless, solids concentrations in farm effluents prior to treatment, do not
appear to have altered greatly. This was shown by Hennessy et al. (1991), Bergheim et
al. (1993a,b) and Cripps (1995), who reported a wide range of total solids concentrations
of 1.6–14.1, 0–20.1, and 6.9 mg l-1, respectively, though strict comparisons with earlier
data are difficult.
Data presented by Kelly et al. (1997) and Bergheim et al. (1998) showed that
treatment efficiency, in terms of the separation of particles from the effluent, increased
with increased solids concentration. This indicates both that aquaculture
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wastes solids are difficult to treat, and that by increasing the concentration prior to
treatment, an increase in treatment efficiency, or clarification rate, would be expected.
2.2. Nutrients in the solids
Suspended solids (SS), total nitrogen (TN) and total phosphorus (TP) concentrations
are also commonly low in aquaculture effluents, at about 14, 1.4 and 0.13 mg l_1,
respectively (Cripps and Kelly, 1996). Further, of these total waste concentrations, not all
are bound to the particulate fraction (Ackefors and Enell, 1994). About 7-32% of the TN
and 30-84% of the TP is in the particulate fraction (Foy and Rosell 1991a; Bergheim et
al., 1993a,b). The remainder is transported out of the farm in the dissolved fraction. This
is largely not possible to remove by particle separation techniques, which are commonly
employed for aquaculture wastewater treatment.
Much of the bio-degradable organic matter, which produces the BOD and reduces
dissolved oxygen (DO) levels, is also present in the particulate fraction (Amirtharajah
and O’Melia, 1990). In one of the few aquaculture studies to quantify this, Kelly et al.
(1997) showed that 21% of the BOD load remained after screening a particle-rich
effluent through a 60-^m pore size microscreen.
2.3. Physical properties of suspended particles
Whilst there have been many studies of the nutrient loading of aquaculture
wastewaters, little work has been conducted to quantify the physical characteristics of the
solids particles.
A study by Clark et al. (1985) is one of the few, or only, to consider the shear
resistance of suspended particles, which they considered to be low compared with
sewage flocs.
Cripps (1995) and Kelly et al. (1997) examined the distribution of SS and nutrients
within different particle size fractions of salmon (Salmo salar) smolt farms in Sweden
and Scotland, respectively. Chen et al. (1993a,b) and Cripps (1995) using particle
characterisation techniques (Cripps, 1993) showed that though the majority of the
number of particles were smaller than 30 ^m diameter, the total volume of these particles
was much lower than relatively infrequently occurring particles larger than 30 ^m. Cripps
(1995) further proposed that the particle-bound P and N concentrations were significantly
greater in the larger particles, but because these larger particles occurred infrequently,
overall, no individual size fraction of particles contained an especially high nutrient load
(amount rather than concentration). Kelly et al. (1997) achieved a much greater reduction
in SS and TP concentration with the use of smaller pore-size filters than Cripps (1995) —
about a reduction of 77 and 33% at 100 \m; 71 and 64% at about 25-30 ^m, respectively.
Both studies indicated the value of screening out solids down to a filter pore-size of about
60 ^m, smaller than which produced negligible benefits in terms of treated effluent
quality.
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37
3. Management of solids produced
3.1. Feed quality
In addition to treatment technology, waste management techniques can be employed
to reduce the production of particles in the first place, or to facilitate their removal
should their production be unavoidable. Improved feed quality in recent years has
resulted in enhanced bio-availability of phosphorus and proteins, at lower concentrations.
This results in a reduction in the quantity of faecal solids produced. Improved pellet
integrity, with subsequent slower breakdown rates, further reduces feed losses.
Optimised feeding systems and protocols have also reduced wastage.
The close connection between feed quality and feed-derived waste production has
been demonstrated in several reports, such as Cho et al. (1991) and Asgard and Hillestad
(1998). The development of ‘high-energy diets’ with increased fat content, reduced
carbohydrate levels, reduced protein levels, and improved digestibility has significantly
decreased waste production in salmonid farming. In a standard diet for salmonids, the
following fractions of the main components were shown to be indigestible and excreted
as faecal waste: 13% of the protein, 8% of fat, 40% of carbohydrate (fibre completely
indigestible), 17% of organic matter, 50% of ashes and 23% of dry matter (Asgard and
Hillestad, 1998). About 40% of ingested protein N is excreted as dissolved N (TAN =
NH3 + NH4+) by salmon. Recent studies, indicate that a minimum of 11 g kg -1 dietary P
is required by juvenile Atlantic salmon (Asgard and Shearer, 1997).
The daily nutrition discharges per fish (DND) for nitrogen and phosphorus are
predicted by the following equation (Einen et al., 1995):
DND (N, P) = nutrient fed - nutrient gain
(1)
nutrient fed = ration fed (g) × nutrient in feed (g g-1 diet)
(2)
nutrient gain = growth (g) × nutrient in fish (g g-1 fish)
(3)
where
and
At a feed conversion ratio (FCR) of 1.0 kg feed kg -1 gain, the estimated discharges
from juvenile salmonids, in terms of g (N, P) kg -1 fish gain, are about 33 g N (26 g
dissolved and 7 g solid-bound) and 7.5 g P (80-90% solid-bound).
Based on digestibility estimates of typical diets (Asgard and Hillestad, 1998), the
calculated discharge of suspended solids from salmon and trout farms should be 150-200
g SS kg-1 fish gain at a FCR of 0.9-1.0.
3.2. Feeding management
Clearly the best means of reducing the quantity of waste discharged by an aquaculture
facility is to reduce its production in the first instance. The feeding
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regime and technology used to both deliver rations to the stocks and monitor its intake
can be used to minimise waste losses, as described by many authors specialising in
feeding management, such as Alana¨ra¨ (1992), Durant et al. (1995), Summerfelt et al.
(1995) and Derrow et al. (1998). Connections have been demonstrated between feeding
management practices and either production economics and efficiency (Storebakken and
Austreng, 1987; Hankins et al., 1990), or waste production (Seymour and Bergheim,
1991; Asga˚rd et al., 1991; Westers, 1992).
A full evaluation of different feeding regimes is outside the scope of this review. The
concept is however, that feed utilisation should be maximised by supplying feed to the
stock, such that the uneaten quantity is minimised. The required capacity of treatment
systems can then be minimised, thus reducing capital and operating costs. Additionally,
in water reuse systems that generally have a fixed carrying capacity based on the ability
of the system to handle the daily feed application, removing the waste feed load can
effectively increase fish rearing capacity (Summerfelt et al., 1995).
Technology for monitoring uneaten pellets has been shown to be a useful means of
reducing wastage (Summerfelt et al., 1995). Durant et al. (1995), Summerfelt et al.
(1995) and Derrow et al. (1998) described devices that use ultrasound to detect feed
particles. When pellets are detected in the tank effluent, the devices discontinue feeding.
A pre-set timer is then activated to control the interval between feedings. Whilst these
studies indicated an increase in growth of fish using these devices of up to 60%,
compared with ration feeding (fixed portions), the effect on the solids loading in the
effluents was not quantified.
As well as optimising the timing of feeding, the location of feeding can affect both the
quantity of solids wasted and their distribution within the culture facility. Many fish for
example will not take feed pellets off the bottom, so tank hydrodynamics, pellet structure
(which affects the sinking rate) and location of the feeder, need to be adjusted in order to
maintain the solids in suspension as long as possible. Auto-feeders that broadcast pellets
to the centre of a tank, or to the effluent end of a pond may be used to reduce the length
of time the pellets remain within the culture facility, but also reduce the time available
for the stock to eat those pellets.
3.3. Flow management
Culture stock density increases and water resource shortages have in some cases
necessitated the use of oxygen addition and CO2 removal systems. Wastewater flow rates
from these systems tend to be markedly reduced, with a subsequent rise in waste
concentrations. Higher solids concentrations are usually easier to treat in order to obtain
significant benefits in wastewater quality.
The specific water consumption in intensive farming has been reduced during the last
decade by the use of supplemental oxygen. This affects the within-tank water quality and
the self-cleaning efficiency of the tanks (e.g. Colt et al., 1991; Fivelstad and Binde,
1994). The water flow can be reduced from 1–2 l kg-1 min-1 down to about 0.2 l kg-1 min1
, or even lower, by using inlet water supersaturated to 160–200% oxygen saturation.
Consequently, the tank outlet solids concentration is
S.J. Cripps, A. Bergheim /Aquacultural Engineering 22 (2000) 33–56
39
then a factor of 2-5 times greater than without oxygenation. Fivelstad and Binde (1994)
found that a water flow below 0.16-0.20 l kg-1 min-1 reduced growth and caused tissue
damage in freshwater fish. The concentration of carbon dioxide in freshwater should not
exceed 10-20 mg l-1 in freshwater (Colt and Watten, 1988). A reduced water
consumption, often by combining recirculation and addition of oxygen, is a means to
improve the utilisation of the water supply and to reduce the discharged effluent load
because of improved treatment efficiency (Cripps and Bergheim, 1995).
3.4. Variation in solids loading
Several parameters have been found to influence the waste load, and hence cause
variability in the quality of the wastewater. Kelly et al. (1994) found that the waste
quantity discharged from a fish farm increased with temperature. Foy and Rosell (1991b)
however showed that the proportion of nutrients in the particulate fraction increased with
temperature. These nutrients were not then becoming more soluble with increasing
temperature as may have been expected. At sites where water temperature changes
throughout the year, care must be taken at the planning stage of development, to ensure
that the capacity of treatment facilities will be adequate to consistently produce the
required discharge or recirculation water quality. Changes in ionic composition,
dissolved nutrient concentration and physical parameters, such as flow rate and scouring,
also influence the partitioning of nutrients between the dissolved and particulate phases,
as reviewed by Hamann et al. (1990). Again changes in these parameters will form
design criteria.
Intermittent solids loading increases can occur as a result of intermittent tank cleaning
operations (Kelly et al., 1997), or from unit processes that function irregularly, such as
back-pressure activated rotating micro-screens (see below). Studies also indicate the
advantage of continuous pre-treatment to concentrate wastes (Twarowska et al., 1997).
Bergheim et al. (1998) further provided evidence that if the waste solids concentration is
increased, then the efficiency of the clarification processes will increase (see below).
4. Pre treatment
4.1. Advantages
The main parameter determining flow-through tanks is the need for adequate gaseous
exchange, as described above. Solids handling is normally a lower priority that has to be
accommodated into an already designed and managed tank or pond. The three main
advantages to preparing (i.e. concentrating) solids prior to clarification processes are an
improved culture environment, an increased treatment efficiency and a reduction in
required treatment capacity.
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4.2. Tank hydrodynamics and culture environment
Numerous studies have indicated the advantages of maintaining a ‘clean’ culture
environment, with the minimum of waste matter present that could lead to an increase in
the incidence of diseases (Klontz et al., 1985; Braaten et al., 1986), and/or an increase in
stress as a result of sub-optimal water quality (Pickering, 1981; Rosenthal et al., 1982;
Klontz et al., 1985; Braaten et al., 1986). In particular, environmental gill disease, which
is a chronic, non-infectious condition that occurs under intensive culture, is thought to be
caused directly by the accumulation of suspended solids and/or un-ionised ammonia
(Burrows, 1964). Braaten et al. (1986) propose that it is these physical/chemical
parameters, including prolonged exposure to organic particles, that cause the gill lesions,
rather than micro-organisms that are secondary invaders. In order to optimise culture
production, it is therefore important to remove solids quickly and efficiently from culture
facilities.
Various studies have been conducted into the effects of tank hydrodynamics on
suspended particle movement, such as feed distribution (Backhurst and Harker, 1989),
self-cleaning and efficient use of available water (Watten and Beck, 1987; Westers,
1991; Cripps and Poxton, 1992; Yoo et al., 1995), and periodic cleaning (Stabell, 1992).
This information, combined with a knowledge of the water quality requirements of
different species (e.g. Alabaster and Lloyd, 1980; Wickins, 1981; Poxton and Allouse,
1982), has lead to the development of tank designs in which solids transport and
separation are important design criteria (e.g. Boersen and Westers, 1986; Watten, and
Johnson, 1990; Wagner, 1993).
Various devices have been proposed or used for moving particles within tanks or
ponds. Such devices include: the juxtapositioning of inflow and outflows; vanes; or
screens. Inflow direction and distribution, with outflow location and size, can be used: to
create high energy and/or quiescent zones, as described by Cholette and Cloutier (1959),
Rosenthal et al. (1982), and Burley and Klapsis (1985); for the effective transport of
particles to the effluent (e.g. Cobb and Titcomb, 1930; Burrows and Chenoweth, 1955,
1970; Watten and Johnson, 1990); or for transport to a quiescent collection point
(Westers, 1991).
4.3. Treatment efficiency and capacity
Kelly et al. (1997) showed a large variation in effluent solids loading as a result of
farm management regimes such as intermittent tank cleaning. The greater loads
increased the efficiency (i.e. proportion of the total waste load removed by any screen
pore size) with which the effluent was clarified using micro-screens (see below). Cripps
(1995) proposed that the reason for such an effect may be the build-up of a filter cake
that would restrict the passage of particles smaller than the nominal pore diameter, as
described by Hamann et al. (1990).
A similar effect was described by Bergheim et al. (1998), who found that the settling
efficiency of an aquaculture sludge sedimentation chamber increased from about 58% at
about 1 mg SS min-1 to nearly 90% at 18 mg SS min-1 at the same flow rate. Such an
effect is likely to be due to an increase in the probability of
S.J. Cripps, A. Bergheim /Aquacultural Engineering 22 (2000) 33–56
41
particles contacting and coalescing in more concentrated suspensions, and so increasing
their mass, thereby settling faster, as described by Tchobanoglous and Burton (1991) (pp.
220–240).
Solids concentrations from culture vessels that have been increased in order to
increase treatment efficiency, can be discharged either as a continuous stream of a lesser
flow than the primary effluent, or as a discontinuous stream of short duration–high flow
pulses. In either case, the total mass flow of the wastes should be approximately similar
to the flow with no pre-treatment, however the total mass flow of the wastes and water in
the pre-treated effluent will be substantially less than with no pre-treatment. Hence,
facilities for treating pre-treated wastes can be reduced in size, because of the reduced
capacity required to treat a smaller volume of waste. This should result in savings in both
capital and operational costs at the treatment stage.
4.4. Pre-treatment technology
Technology used to pre-concentrate solid wastes prior to treatment can, as proposed
above, be classified into equipment or procedures that produce an intermittent plug, or
continuous flow, of high solids content waste.
The need to expel waste solids from a tank in an intermittent plug can arise from the
accumulation of material, either deliberately or accidentally. Solids can be deliberately
deposited within a section of a tank that is either close to the effluent and/or partitioned
from the culture stock, e.g. particle traps described by Westers (1991). Accidental
deposition can arise as a result of sub-optimal tank flow management. Intermittent tank
cleaning is the traditional means of removing solids from a tank. Wastes that have
accumulated are scrubbed and then flushed out of the tank by increasing the inflow rate
and lowering the water level. This method is not as good as continual removal because it
can lead to a stressed stock and poor water quality in the tank. Rather than allowing this
plug to exit the farm in the main effluent, it may be stored in separate holding facilities to
allow treatment devices sufficient time to function at a lower hydraulic load than if
treatment of the primary flow was required.
A preferential alternative to flushing is the use of a combined concentrator and
separate waste solids outlet, as reviewed by (Cripps and Bergheim, 1995). Particle
concentrators, though sometimes expensive, are becoming more popular. They are
devices at the tank outlet which assist the settlement and consolidation of solids. This
concentrated waste is removed from the tank periodically, or preferably continuously,
through an outlet which is separate from the primary flow (Fig. 1). This sludge outlet is
then led to the treatment device. The primary flow commonly does not require treatment
for solids removal.
Eikebrokk and Ulgenes (1993) described a system which combined a within-tank
particle concentrator with a separate outlet and sludge dewatering unit (whirl separator).
The particle-enriched outlet flow was 5–6% of the total flow, thus pre-concentrating
particles by a factor of 20 (assuming approximately 80% of the particles were trapped).
An overall system removal of 71% SS, 38% TP and 14%
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S.J. Cripps, A. Bergheim/Aquacultural Engineering 22 (2000) 33–56
TN was estimated. The dry matter content of the dewatered sludge from the whirl
separator was about 14%.
Ma¨kinen et al. (1988) studied the effluent P budget in tanks with two separate outlets:
a ‘bottom effluent’ and a main ‘surface effluent’ with flow rates of 10 and 90% of the
total flow, respectively. Without tank flushing, the average P concentration in the
effluent from the tank bottom was more than double the surface concentration. Using a
stationary microsieve removing particles from the bottom effluent, the overall P
treatment efficiency was 46%, incorporating the flushing load.
A commercially available particle concentrator system has recently been further
developed which is a combination of a Norwegian Ecofish tank separator system and a
US recycle system (Twarowska et al., 1997). This system combined both a specially
designed particle trap that separated excess feed pellets from faecal wastes so that
feeding could be more closely monitored, and collectors for sludge and dead fish
removal.
Fig. 1. Within-tank pre-treatment and separate particle treatment system (courtesy of Aqua Optima-Ecotank/Eco-trap®).
S.J. Cripps, A. Bergheim /Aquacultural Engineering 22 (2000) 33–56
43
5. Solids separation technology
5.1. Choice of unit process
Several types of particle separators, or clarifiers, are commercially available for
integration into an intensive aquaculture treatment system and are capable of accepting
the pre-concentrated wastes from tanks or ponds. Not all are suitable for treating
aquaculture wastewater that has not been pre-treated. Several authors have reviewed
particle separators, including Wheaton (1977), Huguenin and Colt (1989), Landau
(1992), Lawson (1994), Chen et al. (1994a) and Cripps and Kelly (1996) (Fig. 2).
It is generally more feasible to remove the solids (and thus the nutrients and BOD
associated with them) in high flow–low concentration commercial aquacul-ture
wastewaters, than to treat the dissolved fraction using some form of filter bed. The
exception to this is where a large proportion of the wastewater is to be reused, such as in
intensive recirculation systems, or where discharge legislation is restrictive. Solids
separation technology can be conveniently divided into mechanical and gravitational
methods. The most popular method of mechanical particle separation is by the use of
screens. Traditionally, the aquaculture industry in Europe and North America have used
sedimentation for first stage particle separation. The low residence times, and hence high
overflow rates associated with intensive systems makes sedimentation, as a first stage
technique, inefficient (Cripps and Kelly, 1996; Summerfelt, 1998). It may however prove
a suitable process for the clarification of lower flow rates, such as the sludge flow
produced by a screen separator.
Fig. 2. Particle sizes removed by different solids separation processes (adapted from Chen et al., 1994a).
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5.2. Microscreens
Makinen et al. (1988) reported the use of a triangle filter which is a static screen
inclined so that the primary flow through the screen also serves to carry the separated
particles to the waste trough. Its current lack of widespread use may be due to capacity
limitations, though no data have been found to verify this.
Rotating microscreens are an alternative to primary sedimentation (Tchobanoglous
and Burton 1991) and so have been more commonly installed at farms in recent years.
These usually comprise a fine mesh screen (often 60 to 200 ^.m pore size) in the form of
a rotating drum or disc through which the wastewater is passed. Particles held back on
the mesh are backwashed or scraped, to a waste collection trough. Rotating microscreens
are especially suited to applications where blockage is likely (Wheaton, 1977), and so are
used in fish farms because of the large flow of wastewater which must pass through the
screen and the small screen pore size which is required to separate out the solids.
Cripps and Kelly (1996) reviewed commercially available rotating microscreens and
concluded that the majority currently used in aquaculture were developed for the
treatment of drinking water. The near perpendicular flow of water through the screens
was designed to stop the particles rather than remove them. Setting the screen at a
gradient of about 30° to the water surface towards the water flow, such as is achieved
using band or belt ‘filters’, had the potential to gently remove particles, with minimal
damage, out of the primary waste stream.
Rotating screens can have a substantial backwash sludge water flow that usually
requires further thickening/dewatering (see below). The backwashing however can be
initiated automatically with little intervention required. Pore size, rotation speed and
backwash flow can be adjusted to the application.
In order to minimise the quantity of backwash water required and maximise the solids
concentration, advanced units can be operated intermittently. As particles build up on the
static screen, a filter mat becomes established. The back pressure increases and a level
indicator is used to initiate the rotating and backwashing operation when the water height
difference across the screen exceeds a pre-deter-mined value. For special operations,
such as high fat content wastes, warm/hot water can be used for backwashing. When
discharging to a marine recipient or recycle system, sea-water can be used for
backwashing. Little of the salt is retained within the sludge that may be later applied to
farm land (Bergheim et al., 1998).
Usually, wastewater flows into a drum microscreen through one of the ends of the
drum. It then passes out through the walls of the drum, which are composed of the filter
mesh. The backwash jets are located out of the water on the outside of the drum, so that
collected particles are washed into a trough inside the drum (Huguenin and Colt, 1989).
These are directed out of the drum through one of the open ends.
Wheaton (1977) described an alternative design of drum screen which acted more like
a water mill and was more suited to applications with large debris-type particles. In this
device, the effluent is passed through the screen from the outside to the inside of the
drum, whilst the particles are carried over the drum to a
S.J. Cripps, A. Bergheim /Aquacultural Engineering 22 (2000) 33–56
45
collection channel. It is important to ensure that the hydraulic capacity and pore size is
matched to the characteristics of the effluent to be treated (Cripps and Kelly, 1996).
The treatment efficiency of a Hydrotech drum filter has been tested by several workers
including Ulgenes (1992a), Ulgenes and Eikebrokk (1992) and Twarowska et al. (1997).
During extensive tests conducted by the Norwegian Hydrotechnical Laboratory, the
treatment efficiency of a 60-^m pore size drum screen varied considerably within the
ranges SS (67-97%), TP (21-86%) and TN (4-89%). Again, efficiency was found to vary
proportionally with the waste effluent concentration. Twarowska et al. (1997) however
achieved lower solids removal rates of 36.5% using the same type of 60-^m pore size
screen. Clearly then the efficiency of such screens is dependent on the characteristics of
the effluent and hence, and pre-treatment techniques applied.
Unpublished information has been located that describes a design of drum screen with
axially positioned troughs located on the inside of the drum. These troughs were
designed to catch large particles, such as uneaten pellets or faeces, and deposit them in
the backwash trough. Such a design would appear to be a useful method to quickly and
gently transport large particles to waste, but no operational data to verify this has been
located.
Rotating disc screens are composed of a flat circular disc of microscreen material held
approximately perpendicular to the primary wastewater flow. Large screens are required
in high flow-rate situations, such as is common in aquaculture, but these are difficult to
rotate, so a sequence of screens with pore sizes decreasing downstream (e.g. 200 and 60
|am) may be required.
The main effluent flow travels through an axial flow screen perpendicular to the plane
of the screen, which is disc-shaped. Commonly, backwash water jets extend across the
full radius of the downstream side of each screen. The waste, which accumulates on the
screen during a rotation cycle, is then collected in a trough on the upstream side. The
sludge streams from multiple screen units are usually combined prior to disposal or
further treatment (Cripps and Kelly, 1996).
Several workers (Liltved, 1988; Liltved and Hansen, 1990; Bergheim et al., 1991;
Ulgenes, 1992b; Bergheim et al., 1993a,b) have tested the treatment efficiency of a
commercially available Unik disc microscreen. Similar to the drum screen results,
treatment efficiency estimates using this unit vary considerably, both due to variations in
effluent quality and characteristics, and with the pore size of the screens chosen (Table
1).
Ulgenes (1992b) testing 250- and 120-^m pore screens together achieved a wide range
of SS removal efficiencies of 16–94%, whilst Bergheim et al. (1991) achieved an
average 40% suspended dry matter (SDM) removal using 350- and 60-^m pore size
screens. Liltved (1988) obtained even lower TS removal efficiencies of 20%, but this
was using large pore size screens of 1600 and 600 ^m. During tank washing operations,
this value was however increased to greater than 80%. The low removal efficiency could
be attributed to the low particle concentration in the effluent and the large pore sizes of
screens employed.
46
S.J. Cripps, A. Bergheim/Aquacultural Engineering 22 (2000) 33–56
Table 1
Published removal efficiencies using Unik disc screens
Screen pore size
Coarse
250
1600
350
Removal efficiency (%)
Fine
120
600
60
SS
16-94
80 > 40 > 40
TP
Reference
TN
18-65 201–49
Ulgenes (1992b) Liltved
(1988) Bergheim et al.
(1991)
7–30
These results indicate that screen pore size should be chosen to suit the application and
that the choice should be based on the characteristics of the wastewater to be treated.
The capacity of a drum screen is proportional to its length and its diameter, while the
capacity of a disc screen is limited by the diameter (Wheaton, 1977). Drum microscreens
are therefore not as capacity limited as disc screens. In practice however, at high flow
rates, such as those in aquaculture applications, several disc or drum units are operated in
parallel. This also allows for a unit to be out of operation, for repair or maintenance.
5.3. Sedimentation
The use of sedimentation in aquaculture has been reviewed by Wheaton (1977) (pp.
505–513), Lawson (1994), Cripps and Kelly (1996) and Summerfelt (in press).
Sedimentation is the process by which settleable suspended solids, that have a greater
density or specific gravity than water, can settle out of suspension and so be separated
from the main flow.
It is gravity, in the absence of other confounding influences, that causes particu-late
waste matter to sink. The settling velocity is controlled by the viscosity of the fluid
(water) and the diameter of the particle (if the particle is assumed spherical), as described
by Stokes’ Law. The physical properties of sedimentation have been described by Weber
(1972), Wheaton (1977), Gregory and Zabel (1990) and Lawson (1994).
There are four types of sedimentation: type 1, discrete; type 2, flocculent; type 3,
hindered or zone; and type 4, compression settling (Gregory and Zabel, 1990). The type
of sedimentation that occurs in a vessel is dependent on the concentration of the particles
and their interaction with each other. The low particle concentrations of aquaculture
wastes, that have not been pre-treated, usually settle discretely without interacting with
each other (Chesness et al., 1975).
Only in pre-concentrated aquaculture backwash water or sludge is there likely to be
flocculent or hindered settling, as described by Bergheim et al. (1998). The processes of
flocculation, whereby waste particles combine, either through natural collision or
attraction, or are induced artificially, can assist settlement by increasing particle size and
settling velocity.
S.J. Cripps, A. Bergheim /Aquacultural Engineering 22 (2000) 33–56
47
In view of the large number of parameters affecting sedimentation and the complicated
physical characteristics of the wastewater, it is common to conduct site-specific analyses
of settling prior to planning a settling basin for a specific application.
5.4. Settling basins
Settling tanks for aquaculture applications are usually designed to have a plug flow
(Wheaton, 1977) in which turbulence and resuspension are minimised (Sum-merfelt, in
press), though both circular and rectangular settling basins are common (Lawson, 1994).
A basin will have three design functions. Any failure in one of these functions will
impair the performance of the tank and, if serious, destroy the effectiveness of the
process almost completely (Weber, 1972). These are: to effectively remove SS, leaving a
clear effluent; to collect and discharge the settled sludge; to provide a thickened sludge
with minimal volume. In practice however, especially at the high flow rates encountered
in aquaculture applications, short-circuiting (movement of inlet water directly to the
outlet without mixing in the basin), turbulence and resuspension (scouring, by the water
flow, of settled material off the bottom) can occur, so that it becomes difficult to collect
waste concentrations of less than 10 mg l-1 (Henderson and Bromage, 1988).
In ideal sedimentation tanks, four zones can be identified: inlet, settling, sludge and
outlet (Wheaton, 1977; Lawson, 1994). Baffles and an outlet weir are often incorporated
to promote quiescent conditions. Whilst settling efficiencies are independent of basin
depth (Hazen’s Law), hence overflow rates (the maximum flow through a basin that will
still allow a given particle to settle) are calculated in terms of volume of effluent per unit
area per h (m3 m-2 h-1), the tank must have sufficient depth to collect the sludge and to
minimise the cross-sectional area through which the wastewater flows. An exception to
this is the use of inclined plate or tube settlers, in which settling depth is minimised to a
few centimetres.
Various design criteria have been proposed specifically for aquaculture applications.
To avoid the scouring of settled solids and the risk of turbulence reducing settling
velocity, Henderson and Bromage (1988) recommended that flow velocities should not
exceed 4 m min-1, though preferably 1 m min-1. Warrer-Hansen (1982) adopted more
conservative guidelines of 1.2–2.4 m min-1.
Within wastewater treatment, it is more common to quote design recommendations in
terms of retention time or overflow rate. Mudrak (1981) recommended a retention time
of longer than 30 min, which was in agreement with the findings of Henderson and
Bromage (1988) who calculated the retention times at 16 farms. Overflow rates of about
1.5–3.0 m3 m-2 h-1 would seem to produce adequate settlement of aquaculture solids, with
Mudrak (1981) reporting 1.7, Warrer-Hansen (1982) 2.4 and Bergheim et al. (1998) 1.0–
2.7 m3 m-2 h-1.
The use of coagulants to assist the agglomeration and hence faster settlement of small
particles was not considered economically viable by either Chesness et al. (1975) or
Cripps (1994).
48
S.J. Cripps, A. Bergheim/Aquacultural Engineering 22 (2000) 33–56
Sedimentation basins of various designs are common throughout the industry. They
range in design from simple ponds dug downstream of the farm, to compact second stage
cones, or advanced basins incorporating automatic sludge removal and flow
manipulation (Tchobanoglous and Burton, 1991). Their main advantage is that spare
ponds or tanks can be adapted for this use. Despite their widespread use, the practical
problems inherent in their operation limit their application within aquaculture
wastewaters so that they are, in any form, rarely suitable for the treatment of the primary
effluent from land-based, flow-through facilities, because of inadequate flow dynamics
and sludge removal problems. Though particle settlement velocities of aquaculture
wastes are sufficiently fast to allow the use of sedimentation as a means of separation,
flow rates from farms are high. This can lead to various flow dynamics problems
including: insufficient residence time to allow the particles time to settle out; scouring of
settled particles off the bottom; and short circuiting of influent water direct to the
outflow. The use of sedimentation is not inherently wrong. It is the application to which
the operation is applied that is often inappropriate. Flow rates of the primary untreated
effluent are high, but sludge flows from treatment devices, such as screening, are far
lower, commonly less than 1% of the primary flow. This sludge almost always requires
further thickening. Sedimentation is one of the most suitable methods to accomplish this.
Sedimentation therefore is appropriate for the localised (i.e. within tank) pre-concentration of wastes, and for second stage de-watering of separated sludge within a multistage treatment system (Cripps and Kelly, 1996). It is not suitable for clarifying the
untreated main wastewater flow from a farm.
5.5. Bead filters
Bead filters or expandable granular biofilters (EGBs) can function as both mechanical
and biological filters (Chen et al., 1993a,b) and because of this they have been used for
recycle systems. Chen et al. (1993a,b) claimed that the filter offered both high hydraulic
loading rates and removal of particles smaller than 100
Chen et al. (1993a,b, 1994a), Wheaton et al. (1994a) and Malone et al. (1998)
described the functioning of bead filters. Buoyant, inert, 3- to 5-mm diameter
polyethylene beads retained within the filter housing are fluidised as the wastewater is
up-flowed through the bead bed. Suspended particles are either strained out or deposited
on the bead surface. Flow to the filter is then stopped and the beads are aggressively
backwashed. The bed is expanded to remove the retained particles from the system.
Solids are then allowed to settle to the bottom of the filter chamber, where they are
discharged to waste. The volume of backwash water produced using this method is said
to be 1 -5% of that produced by comparable sized sand filters, whilst filtration flux rates
can be as high as 0.5-1.5 m3 m-2 bead surface min-1 (Chen et al., 1994a). The dual
function of the filter, whilst advantageous within recycle system, may lead to problems
of optimising operation for both biological and physical processes simultaneously.
Wheaton et al. (1994b) confirmed this by noting that bead filters achieve satisfactory
solids removal at loading rates
S.J. Cripps, A. Bergheim /Aquacultural Engineering 22 (2000) 33–56
of 80 kg feed m
feed m~3 beads.
3
49
beads, whilst the nitrification capacity is reached at about 24-32 kg
5.6. Flotation
The removal of fine solids smaller than about 50 ^m from wastewater is difficult.
Sedimentation rates for these particles are slow and the flow capacities of micros-creens
with such small pore sizes are low. In flow-through systems, these are less of a problem
because they do not represent a large fraction of the discharged solids, either by weight
or volume (Cripps, 1995). These sized particles do however tend to build up in
recirculating systems (Timmons, 1994) and hence need to be removed (polished). Foam
fractionation can be used to remove these small particles. Whilst Timmons (1994),
referring to bubble and surfactant interface theory, expected foam fractionation to
remove only particles smaller than about 30 |im, Chen et al. (1992) showed that the mean
particle size distribution in the wastewater and the foam were similar at 10.6 |im,
indicating that a wide range of sizes were being separated.
Descriptions of the process of foam fractionation, also known as flotation, protein
skimming or air stripping, are given by Gregory and Zabel (1990), Lawson (1994),
Timmons (1994) and Summerfelt (in press). The wastewater is passed downwards
through a contact chamber. Bubbles, produced near the bottom of the chamber float
upwards against the wastewater flow. Surface-active particles become attached to these
bubbles so that the density of the bubble-solids aggregates is less than water. They rise to
the surface, the bubbles break and the associated surface-active material is released into
the foam. Should a stable foam build up, this can be collected over a weir and discharged
to waste. Ozone gas can be used in foam fractionation to remove fine particulate organics
(Otte and Rosenthal, 1979; Williams et al., 1982).
Flotation is dependent on bubble diameter, solids concentration, air-to-water ratio,
surface chemistry of the solids, and the surfactant concentration in the water (Huguenin
and Colt, 1989; Summerfelt, in press). Wheaton (1977) considered that foam systems
function better in seawater than freshwater and cited Dwivedy (1973) who showed that it
was even possible to remove bacteria from water using foam fractionation.
In order to remove particles with the minimum of mechanical disruption that would
cause them to be sloughed off the bubbles, and to increase the bubble surface contact
area available, it is normally the aim to produce small bubbles (about 10 |im), even
though they have less buoyancy than larger bubbles (> 100 |im). Even these small
bubbles, singly or in groups, should be capable of removing the small particles and
compounds that foam fractionation is aimed at. Hence, the method used to produce the
bubbles is important. The two main types of bubble generation that can be applied to
aquaculture systems are dispersed and dissolved air. Dissolved air injection is achieved
by the manipulation of pressure. Weber (1972) and Gregory and Zabel (1990) make a
case that the dispersed air systems produce large, inefficient bubbles. Air is injected into
the system using some form of diffuser such as a venturi constriction. Nevertheless these
have been shown to be suitable for aquaculture applications (Weeks et al., 1992; Chen et
al., 1994b,c).
50
S.J. Cripps, A. Bergheim /Aquacultural Engineering 22 (2000) 33–56
6. Secondary sludge thickening, stabilisation and disposal
Although the backwash water flow from the microscreens may be less than 0.1% of the
primary flow, this still requires dewatering. Sedimentation is well suited for this purpose
because the flow rates are markedly reduced (Cripps and Kelly, 1996; Summerfelt, in
press). There are several potential ways for beneficial disposal of organic waste from
aquaculture: application on agriculture land, composting, vermiculture and reed drying
beds (Tchobanoglous and Burton, 1991; Summerfelt, in press). Newly produced sludge
from aquaculture is considered a good ‘slow-release’ fertiliser in agriculture with a high
concentration of organic matter, nitrogen and phosphorus, but with a low potassium
content (Bergheim et al., 1993a,b; Westerman et al., 1993). Test application rates of such
sludge for plant production have been reported (Willett and Jakobsen, 1986; Myhr,
1989). Composting has been reported as suitable for treating fish farm sludge (Bergheim
et al., 1998).
A system combining effluent treatment and sludge processing by sedimentation and
lime stabilisation is presented in Fig. 3. In tests, the sedimentation unit of this
commercially developed system removed 85-90% of the solids at an overflow rate of
about 1 mh-1, producing a sludge with a dry matter content of 5-10% after 24 h settling
(Bergheim et al., 1998). Throughout the whole process, from primary flow to stabilised
sludge, the solids content increased by a factor of about 20 000. This system produced on
average 0.7 l of settled sludge (10% dry matter) per kg feed supplied at a FCR of about 1
kg feed kg-1 gain. In order to achieve a stabilised sludge (pH > 12), a lime dosage of
about 15 g CaO per l of sludge was required.
After storage for up to 3 months the sludge should then be suitable for land application
as an agricultural fertiliser. Trace metal concentrations in the sludge may need to be
monitored to ensure that they are below regulatory limits.
Fig. 3. Diagram of an integrated effluent treatment and sludge processing system developed in conjunction with
Sterner Aquatech a.s., Sweden.
S.J. Cripps, A. Bergheim /Aquacultural Engineering 22 (2000) 33–56
51
The capital and running costs for effluent treatment that are considered acceptable are
dependent on the size of the farm and the potential profit margin (Muir, 1982). In a
review of some effluent treatment systems used in European salmonid production,
Bergheim et al. (1998) found that the treatment costs amounted to 2–10% of the total
production costs.
7. Conclusions
1. Whilst the proportion of waste nutrients (P and N) in the particulate phase is variable
and commonly low, it is important to remove solids quickly and gently from the
wastewater. This will significantly lessen the nutrient and organic loading discharged
into the surrounding environment in flow-through systems, or maintain the quality of
water returned to the culture stock in recycle systems.
2. The different stages of solids management in aquaculture systems should be
integrated so that the overall rate of solids removal and reuse is optimised. Such
stages will comprise feed quality manipulation, feeding management, pre-treat-ment,
primary separation, secondary thickening, sludge stabilisation and sludge reuse or
disposal.
3. Sedimentation basins do not normally appear to be a viable technology for the
treatment of solids in the primary wastewater from aquaculture facilities, but are
appropriate for secondary de-watering or thickening.
4. Clear differences in research and development strategies have developed between
Europe and North America. In Europe the emphasis has been on the development of
high flow-rate screening units for flow-through farms. In North America however,
the lower flow, higher water quality requirements of recycle systems have
necessitated a greater focus on more intensive solutions, such as fractionation and
media filters. It would probably be beneficial to combine the two approaches more in
the future.
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