Title: Aerobes and Effluents
1Aerobes and Effluents
2- 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.
3- 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. -
4- 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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7- 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.
8Sewage 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.
9- 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.
10- 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.
11- 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.
12- 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.
13- 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.
14Process 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.
15- 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.
16- 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.
17Land 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.
18- 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.
19- 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.
20- 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.
21Septic 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.
22- 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.
23Figure 6.1 Diagrammatic septic tank
24Limits 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.
25- 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.
26- 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.
27- 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.
28Nitrogenous 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.
29- 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.
30- 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.
31Aeration
- 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.
32- The systems used fall into one of two broad
categories, on the basis of their operating
criteria - Diffused air systems.
- Mechanical aeration.
33Diffused 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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35- 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.
36Mechanical aeration systems
37Figure 6.2 Turbine sparger aeration system
38- 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.
39Trickling Filters
Figure 6.3 Trickling filter
40- 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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45- 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.
46- 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.
47Activated 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.
48Figure 6.4 Schematic activated sludge system
49- 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.
50- 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.
51- 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.
52Process 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.
53- 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.
54- 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.
55Organic loadings
- Calculating the organic loadings for a given
activated sludge system is an important aspect of
process control.
56- 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.
57- 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
58Deep Shaft Process
- An activated sludge derivative.
- Is based around a shaft 50100 metres
deep.
59Figure 6.5 ICI deep shaft process
60Advantages
- 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.
61Pure 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.
62Advantages
- 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.
63Drawbacks
- 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.
64Figure 6.6 The UNOX pure oxygen system
65The 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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68The 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.
69Figure 6.7 Rotating biological contactor
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72Membrane 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.
73Figure 6.8 Schematic membrane bioreactor
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75Advantages
- 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.
76- 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.
77Cellulose Ion-Exchange Media
Ion exchangers are used for separation of
bio-molecules on the basis of their interaction
with media due to their charge.
78- 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.
79Sludge 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.
80- 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.