Sewage and its Treatment Processes

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Water required per person per day is approximately 200 liters as per Indian Standard, which includes drinking, bathing, flushing the toilet, washing sinks, gardening, etc (Bureau of Indian Standards, IS:1172-1993). Do you all know what happens to the water and where does it reach? Yes, it has to be drained down into a pipe and should be treated in treatment plants. The wastewater, thus generated is called sewage, the pipes carrying this wastewater are called sewers, and the place where the wastewater gets treated is called sewage treatment plants (STPs).

The sewage entering the STPs passes through a sequence of treatment processes and is regulated at several stages.  The characteristic of sewage are measured in terms of organic waste (oxygen equivalent of organic matter as chemical oxygen demand and biological oxygen demand), inorganic waste such as nitrates, phosphates, pathogens, etc.

The first stage is preliminary treatment where the wastewater passes through huge screeners to remove the large solid objects, for example, broken bottles, nappies, cotton buds, plastics, diapers, rags, sanitary items, face wipes, etc. Second stage of treatment is primary treatment, where the liquid separated from the screener will be allowed to stay for few hours, at which the heavier solids settles to the bottom of the tank and the lighter materials float at the top. The floatable waste will be removed on the top of the tanks and solid particles settled at the bottom will be removed as sludge.

The third stage of treatment is the secondary treatment where bacteria degrades the organic content present in the wastewater in the presence of oxygen, and this processes is called activated sludge processes. The aerated water flows to the final settling tanks and allowed to settle for few hours which are similar to the primary settling tanks.  In these tanks, the heavy particles and other solids settle to the bottom as secondary sludge. Further, the water passes to tertiary treatment where the water is disinfected with chlorine to kill the pathogens.

Finally, the treated water is released in the local waterways. It can be used for gardening, non-portable purposes, etc. The sludge settled in the primary and secondary settling tanks is partially recycled to the activated sludge processes as per the requirement, and the remaining sludge will be processed to thickening of sludge and dried in the sludge drying bed. This is used as organic fertilizer. Actually, natural water bodies have self-cleansing capacity to treat domestic sewage. However, due to increase in population, lakes and rivers are overloaded. Hence, it has become a necessity to treat the wastewater in STPs before discharging them into local water bodies. Now advanced technologies are been developed to convert the waste to valuable bio-based products and bioenergy.

Author: Dr G.Velvizh, CSIR-Pool Scientist, Bioengineering and Environmental Sciences Laboratory, EEFF Division, CSIR-IICT, Hyderabad.

Posted By : ScienceIndia Administrator
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