Aerobes and Effluents

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Aerobes and Effluents
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• 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.

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• 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.

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• 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.

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• 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.

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Sewage Treatment

• The aims of treatment can be summarised as the
  reduction of the total biodegradable material
  present, the removal of any co-existing toxic
  substances and the removal and/or destruction of
  pathogens.

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• 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.

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• 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.

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• 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 515 in respect of these nutrients.

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• 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 30-50.

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• 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.

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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.

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• 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.

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• 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.

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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.

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• 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.

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• 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.

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• 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.

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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.

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• By contrast, a septic tank is a less passive
  system, settling and partially digesting the
  input sewage, although even with a properly sized
  and well-managed 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.

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Figure 6.1 Diagrammatic septic tank
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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.

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• 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.

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• 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.

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• 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.

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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.

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• 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.

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• Approximately 2030 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.

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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.

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• The systems used fall into one of two broad
  categories, on the basis of their operating
  criteria
• Diffused air systems.
• Mechanical aeration.

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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.

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• 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.

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Mechanical aeration systems
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Figure 6.2 Turbine sparger aeration system
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• 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.

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Trickling Filters
Figure 6.3 Trickling filter
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• 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.

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• 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.

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• 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.

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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.

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Figure 6.4 Schematic activated sludge system
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• 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.

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• 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.

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• 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.

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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.

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• 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.

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• 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.

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Organic loadings

• Calculating the organic loadings for a given
  activated sludge system is an important aspect of
  process control.

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• 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.

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• 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

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Deep Shaft Process

• An activated sludge derivative.
• Is based around a shaft 50100 metres
  deep.

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Figure 6.5 ICI deep shaft process
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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.

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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.

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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.

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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.

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Figure 6.6 The UNOX pure oxygen system
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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.

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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.

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Figure 6.7 Rotating biological contactor
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Membrane Bioreactors

• Membrane bioreactor (MBRs) - 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.

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Figure 6.8 Schematic membrane bioreactor
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Advantages

• 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.

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• 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.

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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.
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• 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.

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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.

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• 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.