10-05-2012, 03:21 PM
The Biological Basis of Wastewater Treatment
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Introduction
This booklet was written to fulfil the need for a simple explanation of the biological processes that underpin wastewater
treatment. It attempts to show how the bacteria involved deal with the organic carbon in the sewage. Remarkably, there
are just 3 major processes involved, and these mirror exactly the 3 major processes at work in the plant viz: biodegradation,
oxygen removal from the water, and the production of sludge.
The article is divided into two parts. The first section deals with the biology of the bacteria. In the second section, the
ways in which these processes underpin the management of a wastewater treatment plant are explained. Inevitably in a
brief overview, such as this, much has had to be left out. However, it is to be hoped that this will be justified by the
clarification and simplification of the underlying principles.
For an in depth treatment, the ‘Biology of Wastewater Treatment’ by N.F Gray (Imperial College Press, 2004) gives an
excellent and readable review of the subject.
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Strathkelvin Instruments Ltd
Biological Processes
Biological treatment by activated sludge
Wastewater comes from two major sources: as human sewage and as process waste from manufacturing industries. In
the UK, the total volume of wastewater from industry is about 7 times that of domestic sewage. If untreated, and
discharged directly to the environment, the receiving waters would become polluted and water-borne diseases would be
widely distributed. In the early years of the twentieth century the method of biological treatment was devised, and now
forms the basis of wastewater treatment worldwide. It simply involves confining naturally occurring bacteria at very much
higher concentrations in tanks. These bacteria, together with some protozoa and other microbes, are collectively referred
to as activated sludge. The concept of treatment is very simple. The bacteria remove small organic carbon molecules
by ‘eating’ them. As a result, the bacteria grow, and the wastewater is cleansed. The treated wastewater or effluent can
then be discharged to receiving waters – normally a river or the sea.
Whilst the concept is very simple, the control of the treatment process is very complex, because of the large number of
variables that can affect it. These include changes in the composition of the bacterial flora of the treatment tanks, and
changes in the sewage passing into the plant. The influent can show variations in flow rate, in chemical composition and
pH, and temperature. Many municipal plants also have to contend with surge flows of rainwater following storms. Those
plants receiving industrial wastewater have to cope with recalcitrant chemicals that the bacteria can degrade only very
slowly, and with toxic chemicals that inhibit the functioning of the activated sludge bacteria. High concentrations of toxic
chemicals can produce a toxic shock that kills the bacteria. When this happens the plant may pass untreated effluent
direct to the environment, until the dead bacteria have been removed from the tanks and new bacterial ‘seed’ introduced.
Globally, the composition of effluents discharged to receiving waters is regulated by the national environment agencies. In
Europe the regulatory legislation is the Urban Waste Water Treatment Directive (1991) and the more recent Water
Framework Directive (2000). In the USA, the Environmental Protection Agency (EPA) ensures compliance with the Clean
Water Act (1977). The legislation is concerned with the prevention of pollution, and therefore sets concentration limits
on dissolved organic carbon (as BOD or COD), nitrogen and phosphates – which cause eutrophication in receiving waters.
It also attempts to limit the discharge of known toxic chemicals by setting allowable concentration limits in the effluent.
Recently, in recognition that effluents contain unknown toxic chemicals, a more pragmatic approach to regulation is being
introduced in Europe, using Direct Toxicity Assessment (DTA) tests. In the US these have been in use for many years and
are known as Whole Effluent Toxicity (WET) tests. These tests are used to measure the toxic effects of effluents on
representative organisms from the receiving waters. Any toxicity detected in the effluents will obviously have been present
in the sewage entering the plant. Surprisingly, direct toxicity assessment of influents to wastewater treatment plants
that could impact on the functioning of the bioprocesses is not yet included in legislation.
The nature and composition of wastewater
Domestic sewage is made up largely of organic carbon, either in solution or as particulate matter. About 60% is in
particulate form, and of this, slightly under a half is large enough to settle out of suspension. Particles of 1nm to 100μm
remain in colloidal suspension and during treatment become adsorbed on to the flocs of the activated sludge.
The bulk of the organic matter is easily biodegradable, consisting of proteins, amino acids, peptides, carbohydrate, fats
and fatty acids. The average carbon to nitrogen to phosphorus ratio (or C : N : P ratio) is variously stated as approx 100
: 17 : 5 or 100 : 19 : 6. This is close to the ideal for the growth of the activated sludge bacteria. However, industrial
wastewaters are very much more variable in composition. Those produced by the brewing, and pulp and paper industries,
for example, are deficient in nitrogen and phosphate. These nutrients need to be added therefore to achieve the correct
ratio for microbial growth, and to allow treatment to proceed optimally.
Degradable and non-degradable carbon
For control of the biological processes in a treatment plant, it is necessary to have some knowledge of the organic
strength, or organic load, of the influent wastewater. Three different measures of this are available, and they each have
their merits and weaknesses. The Total Organic Carbon (TOC) is analytically straightforward to measure. It involves
oxidation by combustion at very high temperatures and measurement of the resultant CO2. However, TOC values include
those stable organic carbon compounds that cannot be broken down biologically.
Organic carbon can also be measured by chemical oxidation. The sample is heated in strong sulphuric acid containing
potassium dichromate, and the carbon oxidised is determined by the amount of dichromate used up in the reaction. The
result is expressed in units of oxygen, rather than carbon, and the procedure is referred to as the Chemical Oxygen
Demand (COD). Again it is an analytically simple method. However, its weakness is that a number of recalcitrant organic
carbon compounds that are not biologically oxidisable, are included in the value obtained. Conversely, some aromatic
compounds, including benzene, toluene, and some pyridines, which can be broken down by bacteria, are only partly
oxidised in the COD procedure. Overall however, COD will overestimate the carbon that can be removed by the activated
sludge.