In most parts of the world, increases in population, the use of huge quantities of fertilizers and pesticides in modem agriculture, the expansion of food processing industry and the growth of other industrial processes contribute to the volume of sewage and waste water.
Therefore wastewater treatment attempts to remove compounds with a high BOD, pathogenic organisms and harmful chemicals.
The wastewater treatment is done as follows:
Primary Treatment:
Primary processing is done to remove solids and large objects by passing the wastewater through a series of screens. Rapidly sedimentable solid particles (‘grit’) then settle out during flow through a grit chamber. The effluent is then allowed to settle in a sedimentation tank (Fig. 9.1). Primary treatment removes about 60% of settleable solids and about 35% of oxygen-demanding waste. In other words, it results in 35% reduction in BOD. 
Secondary Treatment:
Secondary processing is required to degrade the dissolved organic compounds. This is effected by natural aerobic microorganisms. The resulting sludge is either disposed of or sent to a digester. In the activated sludge process, some is returned to the aeration tank (Fig. 9.1). At the end of the secondary treatment the strength of the effluent is reduced to 30 : 20 (i.e., suspended solids 30 mg per litre and BOD 20 mg per litre), which can be discharged into water sources. However, major part of the nitrogen and phosphorus compounds still remains in the effluent.
Tertiary Treatment:
Tertiary treatment involves chemical precipitation and separation of nitrogen and phosphorus. However, in many cases tertiary treatment is considered optional. The resulting effluent at the end of tertiary treatment is of 10: 10 strength (i.e., suspended solids 10 mg per litre and BOD 10 mg per litre). However the effluent cannot be used as drinking water without giving the normal chemical treatment.
The drinking water may be disinfected by chlorine. However, organic material present in water reacts with chlorine to produce disinfection by-products (DPBs) such as chloroform. Some alternate disinfectants that produce smaller amounts of DPBs are ozone, UV light, chlorine dioxide and chloramines.
Digestion Processing:
It is used to treat the sludge from the primary and secondary treatments, by an anaerobic process. This treatment reduces the solid volume, the odour and the number of pathogens and also generates the valuable organic fuel, methane. 
Biotechnological Improvements:
Treatment of industrial wastewater uses processes similar to those described above. Therefore, any biotechnological improvements to these processes are likely to have immediate industrial application.
Biotechnological improvements may include:
(i) An increase in the capacity of treatment plants;
(ii) increased recovery of useful by-products;
(iii) Replacement of synthetic chemical additives that are currently used; and
(iv) Removal of metals, recalcitrant compounds and odour.
Aerobic Treatment Systems:
The main purpose of secondary treatment is to reduce the BOD of liquid waste. BOD is a measure of the amount of DO consumed by aerobic microorganisms as they metabolize the degradable organic material in the waste (milligrams of DO consumed per litre on incubation for 5 days at 20°C). Aerobic effluent treatment is the largest controlled use of microorganisms in the biotechnological industries.
It includes – substrate adsorption to the biological surface; adsorbed solid breakdown by extracellular enzymes; dissolved material absorption into cells; growth and endogenous respiration; release of excretory products; and ingestion of primary population by secondary grazers. These steps should ordinarily result into complete mineralization of the waste. For aerobic processing of waste either trickling (percolating) filter system or activated sludge tanks are used.
A trickling filter tank is 3 to 10 feet deep, usually packed with crushed rock on which the microbial population forms a thin film. The liquid waste is applied to the top of the filter and percolates downward, depositing organic matter on the support and the microbial film. An upward flow of air through the spaces between the particles of the packed rock material maintains aerobic conditions.
Heterotrophic bacteria and fungi in the upper part of the filter obtain nutrients and energy by oxidizing the organic matter. In this way their numbers multiply and new film is formed. However, a major limitation of the trickling filters is excessive growth of the microbes in the filter, which restricts ventilation and flow, eventually causing blockage and failure. In a recent modification, called the alternating double filtration, the order of the filters first receiving the effluent is periodically reversed.
In addition to ADF, recirculation and intermittent dosing are used to dissipate the load deeper into the filter. Other modifications to plant design and operation are the slowing down of the distributor to even the spread of biomass, and the use of direct double filtration in which a larger size of medium is used for the first filter, allowing higher loading.
A major modification to plant design has been in the form of the Rotating Biological Contractor (RBC). This is a rotating honeycomb of plastic sheets alternately in contact with the waste and air, thus providing a large surface area for the biomass and good aeration.
In the activated sludge process, the wastewater and sewage that have received primary treatment, is mixed with “activated sludge”(an inoculums of microorganisms) and continuously aerated with oxygen for about 15 hours. The heterotrophic organisms that degrade organic matter in an activated sludge tank are the same as those in the percolating filter tank, but the major group of bacteria involved in the treatment process is Zoogloea ramigera , a slime- forming bacteria.
As these bacteria grow, they form clunks called floes to which soluble organic matter, as well as protozoa and other organisms, become attached. When the effluent from an activated sludge tank passes into a sedimentation tank, these floes with the material they are carrying, sediment out and are transferred to an anaerobic sludge digester.
As in the case of filtration processes, recently a number of biotechnological modifications particularly associated with aeration have been introduced into the activated sludge system, some of them are:
(a) Tapered aeration, which relates aeration capacity to the oxygen demand, which is less at the outlet than the inlet;
(b) Step aeration, which introduces the waste it intervals throughout the length of the tank; (c) Contact stabilization, in which the returned sludge is aerated for organisms to utilize any stored nutrient;
(d) The use of pure oxygen in closed tanks, which enables them to operate at higher biomass concentrations; and
(e) The use of deep-shaft air-lift fermenter, which is more economic than the conventional process through reduced residence times and low running costs of the system.
The efficiency of activated sludge process can be improved through a better understanding of the metabolic control in the micro flora of the system. However, it is not easy to control biodegradation, but an appreciation of biochemistry of these pathways may allow manipulation of the control process.
For example, low concentration addition of Krebs cycle intermediates, glucose, amino acids and vitamins, such as alanine and nicotinic acid, to the sludge can accelerate the rate of oxidation of specific components. Addition of these intermediates to biomass produces an energy requirement that results in ATP production by increased oxidation of inorganic compounds such as ammonia or sulphur.
Fluidized bed:
This is a combination of the trickling filter and activated sludge systems.
The two basic designs are:
(a) Simon Hartley captor: In this, the biomass is grown in spaces inside polyester foam pads which are retained in the reactor by mesh. The pads are periodically removed from the reactor, the thick biomass (about 15 kg. in each m3of fluidized support element) machine squeezed out and the empty pads returned to the reactor,
(b) Dorr- Oliver oxitron: In these sand particles are used as the support medium. The sand is allowed to overflow the reactor, is cleaned and then recycled.
Anaerobic Treatment Process:
The digestion of sewage sludge is the most common anaerobic treatment. All the reactions in the anaerobic secondary treatment process can be divided into two groups – acid- forming and methane- forming (Fig. 9.2). Numerous different bacterial species that participate in acid- forming stage include some that are known to be obligate aerobes, but that may grow by utilizing alternative electron acceptors, such as nitrate, sulphate and carbonate.
In the methane- forming stage, anaerobic forms of the genera Methanobacillus, Methanobacterium, Methanococcus and Methanosarcina convert the acetate hydrogen and carbon dioxide produced by the fermenters to methane (CH4). All methanogens, microorganisms responsible for methane production, are archaebacteria that derive their energy by reduction of compounds such as CO2, acetate, or methanol.
Methanogens occur in diverse natural anaerobic habitats.
Some reactions are shown below: 
There are obstacles to implementing anaerobic digesters for gas fuel production. The commercial application of anaerobic digester systems is increasing for the treatment of farm, industrial and food manufacturing wastes, in addition to the processing of energy crops.