Contaminated water is involved in the outbreak of most of the major water borne epidemics and hence treatment of waste water before release into environment is becoming more important. With the increasing use of pesticides and other chemicals in our daily lives, this has been given the highest priority in survival of human life. The cases of neglect have been disastrous as seen in the outbreak of cholera and plagues in the past.

Sewage is the main contaminant of waste water. The contaminated water undergoes three stages of treatments. It is initially passed through iron screen which filter out the larger debris. These grit chambers are automatic and the waste water is flown at a constant velocity against the screens which does the filtering.

Primary treatment

In this, sedimentation of solid wastes is done by passing the waste water through the tanks. The sludge is then fed to a sludge digester in which further processing is carried out. Alternatively biological treatment is used. The efficiency is higher in terms of unit removal of pollution for the sedimentation process. The primary sludge formed contains almost fifty percent of the suspended solids.

Secondary treatment

It involves removal of dissolved and colloidal compounds by the process of oxidation. It is usually done through microorganisms for removal of organic compounds. There are three methods employed depending on the nature of effluent obtained after primary treatment.

1. Biofiltration
It employs the use of intermittent sand filters, contact filters or trickling filters for fine filtration. Filters are costly and employed for smaller volumes of sewage treatment. They also occupy more area. Among these, the trickling filters are the most common and efficient. They are packed bed made of plastic, broken rock, gravel, clinker or slag. The ideal material should be uniformly graded to provide for sufficient voidage.
The effluent can be made to percolate through the bed once again for finer removal of suspended solids. The surface area of the medium, hydraulic loading and temperature of the primary effluent determines the rate of removal of BOD. These filters provide a suitable environment for oxidation of contaminants due to presence of oxidizing microbiota which settles on the filter.
Heterotrophic bacteria and fungi are predominant among the microbes. Autotrophic bacteria occupy the lower layers of the percolation tank. Apart from microbes, macro invertebrates also are present which increases the efficiency due to their grazing nature. It also helps to increase oxygen diffusion.

2. Aeration/ Activated sludge process

These systems treat the waste water by mixing it with a flocculent suspension of microorganisms and aeration of the mixture for long hours sometimes even up to 30 hours depending on the nature of primary effluent. The suspended solids and colloidal matter gets adsorbed on the microbial aggregates. The microbes metabolize these flocs and dissolved nutrients into smaller compounds in a process known as stabilization.

There are three types of activated sludge processes such as conventional, stepped aeration sand contact sterilization systems. The activated sludge is essentially an aquatic system in which the higher links of food webs are absent. The microbial mass has to be maintained by periodic withdrawal of excess sludge from the system. Filter beds are more efficient in oxidizing nitrogen than activated sludge plants.
The microbial community in the sludge is established at two stages one with the untreated waste and another with the purified effluent. Filter beds harbor a succession of communities at different depths. Activated sludge has higher species diversity. They contain more gram negative bacteria and about 200 species of protozoans. The basic process has undergone more revisions and technological improvisations and now it is the most widely used biological waste water treatment process to treat organic and industrial effluents.

3. Oxidation ponds
These are used in warmer climates and makes use of natural water bodies such as lagoons. The waste water is allowed to pass through the lagoon and retained for about 2 to 3 weeks. The organic contaminants undergo bacterial decomposition and carbon dioxide, ammonia and nitrate are released for use by the algal community. Organic sludge settles at the bottom of the pond and methane is finally released. These ponds are prone to harbor pathogens and insects.

Tertiary treatment
This is applied to the secondary effluent for maintaining the water quality. The process essentially removes phosphates and nitrates from the system. Rapid sand filters, micro straining and fluidized bed systems are commonly used in tertiary treatment. Activated carbon and sand are typically used. Beds of aquatic macrophytes and reed bed systems are also used in tertiary treatment. The biomass should be harvested frequently to maintain the productivity of the system for efficient functioning.

Whatever be the choice of the treatment method, the aim is to remove pathogens as far as possible.

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