 The treatment of effluents by biological
means is of particular importance to any
consideration of environmental biotechnology,
since it represents the central point of the
previously mentioned intervention triangle,
having simultaneous relevance to
manufacturing, waste management and
pollution control.
 A large number of industrial or commercial
activities produce wastewaters or effluents
which contain biodegradable contaminants
and typically these are discharged to sewers.
 The character of these effluents varies greatly,
dependent on the nature of the specific
industry involved, both in terms of the likely
BOD loading of any organic components and
the type of additional contaminants which
may also be present.
 Accordingly, the chemical industry may
offer wastewaters with high COD and rich
in various toxic compounds, while
tannery water provides high BOD with a
chromium component and the textile
sector is another high BOD effluent
producer, with the addition of
surfactants, pesticides and dyes.
• The direct human biological contribution to
wastewater loading is relatively light.
• Of course, the actual effluent arriving at a
sewage works for treatment contains the
nitrogen, phosphorus and other components.
Sewage Treatment
 The aims of treatment can be summarised as
the reduction of the total biodegradable
material present, the removal of any coexisting toxic substances and the removal
and/or destruction of pathogens.
 The typical sewage treatment sequence
normally begins with preliminary
screening, with mechanical grids to
exclude large material which has been
carried along with the flow.
 Primary treatment involves the removal of fine solids
by means of settlement and sedimentation, the aim
being to remove as much of the suspended organic
solid content as possible from the water itself and up
to a 50% reduction in solid loading is commonly
achieved.
 At various times, and in many parts of the world,
discharge of primary effluent direct to the sea has
been permissible, but increasing environmental
legislation means that this has now become an
increasingly rare option.
 Throughout the whole procedure of sewage
treatment, the effective reduction of nitrogen
and phosphorus levels is a major concern,
since these nutrients may, in high
concentration, lead to eutrophication of the
waterways.
 Primary stages have a removal efficiency of
between 5–15% in respect of these nutrients.
 Secondary treatment phase, This contains the
main biological aspect of the regime and
involves the two essentially linked steps of
initial bioprocessing and the subsequent
removal of solids resulting from this enhanced
biotic activity.
 Aerobic bacteria are encouraged, thriving in
the optimised conditions provided, leading to
the BOD, nitrogen and ammonia levels within
the effluent being significantly reduced.
 Achieves nutrient reductions of between 3050%.
 In some cases, tertiary treatment is required as
an advanced final polishing stage to remove
trace organics or to disinfect effluent.
 Tertiary treatment can add significantly to the
cost of sewage management, not least
because it may involve the use of further
sedimentation lagoons or additional processes
like filtration, microfiltration, reverse osmosis
and the chemical precipitation of specific
substances.
Process Issues
 At the end of the process, the water itself may be
suitable for release but, commonly, there can be
difficulty in finding suitable outlets for the
concentrated sewage sludge produced.
 Spreading this to land has been one solution which
has been successfully applied in some areas, as a
useful fertiliser substitute on agricultural or amenity
land.
 Anaerobic digestion, which is described more fully in
the context of waste management in Chapter 8, has
also been used as a means of sludge treatment.
 Sludge is readily biodegradable under this regime
and generates sizeable quantities of methane gas,
which can be burnt to provide onsite electricity.
 Water resources are coming under increasing
pressure.
 This clearly makes the efficient recycling of water
from municipal works of considerable importance to
both business and domestic users.
 The biological players and processes involved are
little modified from what would be found in nature
in any aquatic system which had become effectively
overloaded with biodegradable material.
 In this way, a microcosmic ecological succession is
established.
 Hence, heterotrophic bacteria metabolise the
organic inclusions within the wastewater; carbon
dioxide, ammonia and water being the main
byproducts of this activity.
 Inevitably, increased demand leads to an
operational decrease in dissolved oxygen
availability, which would lead to the establishment
of functionally anaerobic conditions in the absence
of external artificial aeration, hence the design of
typical secondary treatments.
Land Spread
 Treatment by land spread may be defined as the
controlled application of sewage to the ground to
bring about the required level of processing through
the physico-chemical and biological mechanisms within
the soil matrix. In most applications of this kind, green
plants also play a significant role in the overall
treatment process.
 Inherent abilities of certain kinds of soil microbes to
remediate a wide range of contaminants, either in an
unmodified form, or with optimisation, enhancement
or bioaugmentation.
 The primary mechanisms for pollution abatement are
physical filtration, chemical precipitation and
microbiological metabolism.
 The activity is typically concentrated in the upper few
centimetres of soil, where the individual numbers of
indigenous bacteria and other micro-organisms are
huge and the microbial biodiversity is also enormous.
 With so high a resident microbial biomass,
unsurprisingly the availability of oxygen within the
soil is a critical factor in the efficiency of treatment,
affecting both the rate of degradation and the nature
of the end-products thus derived.
 Oxygen availability is a function of soil porosity and
oxygen diffusion can consequently be a rate-limiting
step under certain circumstances.
 In general, soils which permit the fast influx of
wastewater are also ideal for oxygen transfer.
 In land that has vegetation cover, even if its presence
is incidental to the treatment process, most of the
activity takes place within the root zone. Some plants
have the ability to pass oxygen derived during
photosynthesis directly into this region of the
substrate. This capacity to behave as a biological
aeration pump.
 In this respect, the plants themselves are not directly
bioremediating the input effluent, but acting to
bioenhance conditions for the microbes which do
bring about the desired treatment.
Septic Tank
 For homeowners in rural areas (or places with no
connection to main sewage pipes), septic tanks are the
principal means of waste water disposal. makes use of
an intermediate form of land treatment.
 Underground tank, collects and stores all the sewage
arising from the household. At regular intervals, often
around once a month dependent on the capacity, it
requires emptying and tankering away, typically for
spreading onto, or injection into, agricultural land.
 By contrast, a septic tank is a less passive system,
settling and partially digesting the input sewage,
although even with a properly sized and wellmanaged regime the effluent produced still contains
about 70% of the original nutrient input.
 Since a system that is poorly designed, badly
installed, poorly managed or improperly sited can
cause a wide range of environmental problems, most
especially the pollution of both surface and
groundwaters, their use requires great care.
Figure 6.1 Diagrammatic septic tank
Limits to land application
 Efficacy of the approach for human sewage and animal
manures, its application to other effluents is less well
indicated and the only truly ‘industrial’ wastewaters
routinely applied to the land in any significant proportion.
 A significant proportion of the water is used for washing
purposes and thus the industry as a whole produces
relatively large volumes of effluent, which though not
generally dangerous to human health or the
environment, is heavily loaded with organic matter.
 The alternative options to land spreading involve
either dedicated on-site treatment or export to an
existing local sewage treatment works for
coprocessing with domestic wastewater.
 The choice between them is, of course, largely
dictated by commercial concerns though the decision
to install an on-site facility, tanker away to another
plant or land spread, is not based on economic
factors only. Regional agricultural practice also plays
an important part.
 Food and beverage industry, heavy potassium load. (for
microbial metabolism and plant uptake) which obviously
lends itself to rapid utilisation and in addition, the
majority of effluents from this sector are comparatively
low in heavy metals.
 High levels of organic matter and nitrogen and,
consequently, a low C/N ratio, which ensures that they
are broken down very rapidly by soil bacteria under even
moderately optimised conditions
 Heavy sodium and chloride loadings originating from the
types of cleaning agents commonly used.
 The land application of such liquors requires care
since too heavy a dose may lead to damage to the
soil structure and an alteration of the osmotic
balance.
 Long-term accumulation of these salts within the soil
produces a gradual reduction of fertility and
ultimately may prove toxic to plants.
 low carbon to nitrogen ratio tends to make these
effluents extremely malodorous.
Nitrogenous Wastes
 For those effluents, however, which are consigned to
land treatment regimes, the fate of nitrogen is of
considerable importance. In aerobic conditions, the
biological nitrification processes within the soil
produce nitrate from ammonia and organic nitrogen,
principally by the chemotrophic bacteria,
Nitrosomonas and Nitrobacter, which respectively
derive first nitrites and then finally nitrates.
 However, in anoxic conditions nitrate compounds can
be reduced to nitrogen gas as a result of the activities
of various species of facultative and anaerobic soil
bacteria, in which the nitrate ion acts as an
alternative electron acceptor to oxygen in respiration,
as mentioned in Chapter 2.
 Nitrogen losses via denitrification and plant uptake as
control mechanisms for the nitrogenous component
in wastewaters in land applications.
 Approximately 20–30% of the applied nitrogen
is lost in this way, a figure which may rise to as
much as 50% under some circumstances, as
factors such as high organic content, fine soil
particles and water-logging all provide
favourable conditions for denitrification within
a soil.
Aeration
 Stimulating resident biomass with an adequate
supply of oxygen, while keeping suspended
solids in suspension and helping to mix the
effluent to optimise treatment conditions,
which also assists in removing the carbon
dioxide produced by microbial activity.
 The systems used fall into one of two
broad categories, on the basis of their
operating criteria:
• Diffused air systems.
• Mechanical aeration.
Diffused air systems
 The liquid is contained within a vessel of suitable
volume, with air being introduced at the bottom,
oxygen diffusing out from the bubbles as they rise,
thus aerating the effluent.
 Ultra-fine bubble (UFB) systems maximise the oxygen
transfer effect, producing a dense curtain of very
small bubbles, which consequently have a large
surface area to volume ratio to maximise the
diffusion.
 The UFB system is the most expensive, both to install
in the first place and subsequently to run, as it
requires comparatively high maintenance and needs
a filtered air supply to avoid air-borne particulates
blocking the narrow diffuser pores.
Mechanical aeration systems
Figure 6.2 Turbine sparger aeration system
 The value of aeration in the treatment process is not
restricted to promoting the biological degradation of
organic matter, since the addition of oxygen also
plays an important role in removing a number of
substances by promoting direct chemical oxidation.
 This latter route can often help eliminate organic
compounds which are resistant to straightforward
biological treatments.
Trickling Filters
Figure 6.3 Trickling filter
 The trickling or biological filter system involves a
bed, which is formed by a layer of filter medium
held within a containing tank or vessel, often cast
from concrete, and equipped with a rotating
dosing device.
 The wastewater percolates down through the filter,
picking up oxygen as it travels over the surface of the
filter medium.
 The aeration can take place naturally by diffusion, or
may sometimes be enhanced by the use of active
ventilation fans.
 Though the resident organisms are in a state of
constant growth, ageing and occasional oxygen
starvation of those nearest the substrate leads to
death of some of the attached growth, which
loosens and eventually sloughs, passing out of the
filter bed as a biological sludge in the water flow
and thence on to the next phase of treatment.
 The filter medium itself should be durable and
long lasting, resistant to compaction or crushing in
use and resistant to frost damage such as clinker,
blast-furnace slag, gravel, crushed rock and
artificial plastic lattice material.
 But a clinker and slag mix is generally said to give
some of the best results.
 The ideal filter bed must provide adequate depth
to guarantee effluent retention time, since this is
critical in allowing it to become sufficiently
aerated and to ensure adequate contact between
the microbes and the wastewater for the desired
level of pollutant removal.
 To maximise the treatment efficiency, it is clearly
essential that the trickling filter is properly sized
and matched to the required processing demands.
Activated Sludge Systems
 Treatment is again achieved by the action of aerobic
microbes, but in this method, they form a functional
community held in suspension within the effluent itself
and are provided with an enhanced supply of oxygen by
an integral aeration system.
 Has a higher efficiency than the previously described
filter system and is better able to adapt to deal with
variability in the wastewater input, both in terms of
quantity and concentration.
 Initial installation costs are higher and it requires greater
maintenance and more energy than a trickling filter.
Figure 6.4 Schematic activated sludge system
 In use, the sludge tanks form the central part of a
three-part system, comprising a settlement tank, the
actively aerated sludge vessels themselves and a final
clarifier for secondary sedimentation.
 The first element of the set-up allows heavy particles
to settle at the bottom for removal. After this physical
pretreatment phase, the wastewater flows into, and
then slowly through, the activated sludge tanks,
where air is introduced, providing the enhanced
dissolved oxygen levels necessary to support the
elevated microbial biomass present.
 At the end of the central activated phase, the
wastewater, which contains a sizeable sludge
component by this stage, leaves these tanks and
enters the clarifiers.
 Typically, collector arms rotate around the bottom of
the tank to collect and remove the settled biomass
solids.
 Accordingly, some of this collected biomass, termed
the return activated sludge (RAS), is returned to the
beginning of the aeration phase to inoculate the new
input effluent.
 In effect, then, the ‘activated sludge’ is a mixture of
various micro-organisms, including bacteria, protozoa,
rotifers, and higher invertebrate forms, and it is by the
combined actions of these organisms that the
biodegradable material in the incoming effluent is
treated.
 Thus, it should be obvious that to achieve process
control, it is important to control the growth of these
microbes, which therefore makes some understanding
of the microbiology of activated sludge essential.
 Bacteria account for around 95% of the microbial mass
in activated sludge.
Process disruption
 Toxicity is a particular worry in the operational plant
and can often be assessed by microbiological
examination of the sludge.
 A number of key indicators may be observed which
would indicate the presence of toxic components
within the system, though inevitably this can often only
become apparent after the event.
 Typically, flagellates will increase in a characteristic
‘bloom’ while higher life forms, particularly ciliates and
the rotifers, die off.
 The particular sensitivity of these microbe species to
toxic inputs has been suggested as a potential
method of biomonitoring for toxic stress.
 Foaming can be a significant and unsightly
nuisance in operational facilities and, as has
been discussed, may occur as a result of either
nutrient deficiency or the growth of specific
foam-generating filamentous organisms.
 Microscopic examination of the fresh foam is
often the best way to determine which, and
thus what remedial action is necessary.
 Large numbers of amoeba often suggests that a shock
loading has taken place, making large quantities of
food available within the system, or that the dissolved
oxygen levels in the tanks have fallen, since they are
better able to tolerate conditions of low aeration.
 The population of rotifers seldom approaches large
numbers in activated sludge processes, though they
nevertheless perform an important function.
 Their principal role is the removal of dispersed
bacteria, thus contributing to both the proper
development of floc and the reduction of wastewater
turbidity.
Organic loadings
 Calculating the organic loadings for a given activated
sludge system is an important aspect of process
control.
 The F/M ratio is a useful indication of
anticipated micro-organism growth and
condition, a high F/M value yielding rapid
biomass increase, while a low one suggests
little available nutrients and consequently slow
growth results.
 Clearly, the total active biomass content in an
activated sludge system, which is termed the mixed
liquor suspended solids (MLSS), is an important factor
in process efficacy. Accordingly, it is routinely
measured at sewage works being important in the
calculation of the F/M ratio, which can be more
properly defined as:
Deep Shaft Process
 An activated sludge derivative.
 Is based around a shaft 50–100
metres deep.
Figure 6.5 ICI deep shaft process
Advantages:
1) The high pressures at the base force far more oxygen
into solution than normal, which aids aeration
enormously and allows the process to achieve an
oxygen utilisation of around 90%, which is some 4.5
times better than conventional activated sludge
systems.
2) The bubble contact time produced, averaging 90
seconds or more, is over 6 times longer than in
standard diffused air systems.
3) It has a low footprint, making it ideal for use in
restricted areas.
Pure Oxygen Systems
 With process efficacy so closely dependent on
aeration and the ability to support a high
microbial biomass, the use of pure oxygen to
enhance the effective levels of the gas
dissolved in the effluent has an obvious
appeal.
Advantages:
1) Pure oxygen obviously gives a better oxygen transfer
rate per unit volume of the bioreactor than can be
achieved using conventional aeration methods.
2) This allows a heavier organic loading per unit volume
to be treated compared with ordinary air-fed
systems.
3) Which enables this system to be used to deal with
stronger effluents.
4) and permits a high throughput where space is
restricted.
Drawbacks:
1) The capital costs involved in installing them in the
first
place are considerable, as are their running costs and
maintenance requirement.
2) The pure oxygen itself represents an explosion risk,
thus necessitating intrinsically safe operational
procedures.
3) and, in addition, leads to accelerated corrosion of
the equipment used.
 However, for some applications and for certain kinds
of effluents, they can prove particularly appropriate.
Figure 6.6 The UNOX pure oxygen system
The Oxidation Ditch
 Characterised by a constructed ellipsoidal ditch, in
which the effluent is forced to circulate around the
channel by brush aerators.
 The ditch itself is trapezoidal in cross-section to
maintain uniform effluent velocity throughout the
channel.
 Effluent is fed into the system without any prior
primary sedimentation and typically gives rise to
only 50% of the surplus sludge produced by a
typical activated sludge process.
The Rotating Biological Contactor
 Is a derivative of the biological filter.
 It effectively combines the advantages of this
previously described approach, like the absence of
a complicated settlement system for sludge return
and a low maintenance requirement with the
smaller footprint and long microbial exposure
characteristic of the active sludge process.
 They have submerged internal disc baffles which
act as sites for the attached growth of biomass,
which are slowly turned by electric motor causing
the microbes to be alternately aerated and
immersed in the effluent.
Figure 6.7 Rotating biological contactor
Membrane Bioreactors
 Membrane bioreactor (MBR’s) - A wastewater
treatment system that combines the use of filters
(membranes) and bacterial processes (bioreactor) to
treat wastewater.
 Bioreactor - The section of the wastewater treatment
system which contains microorganisms or cells to
remove biodegradable pollutants.
 Membrane - Filter to remove solid waste.
Figure 6.8 Schematic membrane bioreactor
Advantages:
1) The membrane bioreactor can offer a greater
degradation capacity for persistent chemicals,
making possible the biological removal of benzene,
nitrobenzene, dichloroaniline and polyaromatic
hydrocarbons (PAHs), for example, which represent
a significant risk, both to the environment and
human health, due to their high toxicity.
 Removal efficiency for these substances can
approach 99%.
2) Not all of the contaminants present in the effluent
are typically completely converted into carbon
dioxide and water, a certain percentage being turned
into metabolic byproducts instead, though this can
amount to less than 5% in a well-managed bioreactor
system (produce a much smaller quantity of excess
sludge).
 These systems are, of course, more expensive than
the conventional activated sludge or trickling filters.
Cellulose Ion-Exchange Media
Ion exchangers are used for separation
of bio-molecules on the basis of their
interaction with media due to their
charge.
 For effluents requiring a highly selective removal of
high molecular weight proteins.
 The ion exchange medium is replenished with brine
as required, and the proteins collected are removed
in the resulting saline solution, for subsequent
coagulation and drying.
 This enables a valuable material to be recovered,
typically for use as an animal foodstuff, while
reducing the wastewater BOD by 90% or more.
Sludge Disposal
 Many of the treatment processes described in this
chapter give rise to primary or secondary sludges.
Typically, these byproducts require disposal and, like
many forms of solid waste, a proportion have been
consigned to either landfill or incineration.
 For some treated sludges, especially those derived
from domestic sewage or food residuals, agricultural
use has been an option, often requiring additional
treatments to ensure its freedom from human
pathogens.
 That most treated sludges have a degree
of heavy metal contamination, which
itself makes possible the accumulation of
these contaminants in soils.
 In addition, there are increasingly
stringent controls on the release of
nitrogen to the environment.