How Sewage Gets Cleaned at Sewage Treatment Plants?

Sewage treatment is the process of removing contaminants from wastewater and household sewage. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants to produce environmentally safe treated wastewater (or treated effluent) that can be returned to the water cycle with minimal environmental issues. Sewage treatment plants play a vital role in protecting public health and the environment by treating the sewage from homes, businesses, and industry to make it safe to release back into the environment or reuse. We will examine the major stages involved in sewage treatment at a typical municipal wastewater treatment plant.

1- Preliminary Treatment

The first stage of the sewage treatment process is known as preliminary treatment. This involves basic physical methods to remove large objects and other items that could damage or clog the pumps and equipment in subsequent treatment stages. Preliminary treatment typically includes:

Screening - Large objects like rags, sticks, plastics are removed by passing wastewater through screens and grates of varying sizes. This protects the downstream equipment.

Grit removal - Grit like sand, eggshells, gravel that can settle is removed by passing wastewater through grit chambers where velocity is adjusted to allow grit to settle to the bottom while the remaining wastewater flows out.

Flow equalization - Sewage flowrate varies widely during the day. Flow equalization buffers these flows using baffles in tanks to attenuate peak flows and regulate minimal flows. This allows the downstream processes to be handled more efficiently at a constant rate.

2- Primary Treatment

The main stage for removing settleable organic and inorganic solids from the wastewater is called primary treatment. This uses physical processes including sedimentation. Steps involved include:

Sedimentation - Wastewater flows through large sedimentation tanks that allow suspended solids to settle at the bottom as primary sludge.

Skimming - Oils, greases and lighter solids float to the top and are skimmed off as scum.

Chlorine addition - Chlorine is added to disinfect the wastewater of pathogens.

Around 50% of suspended solids and 30% of BOD can be removed in primary treatment through sedimentation and skimming. The sludge collected is pumped to sludge thickeners and digesters for further processing while the partially treated effluent moves to secondary treatment.

3- Secondary Treatment

Secondary treatment uses biological processes to further purify wastewater after primary treatment.

Aeration - Effluent is mixed with air and aerobic microorganisms. This encourages growth of microbes that consume organic matter thereby removing BOD.

Activated sludge process - Air is pumped into large aeration tanks that contain effluent and microbial floc or "activated sludge". Bacteria and protozoa consume organic compounds. This sludge is re-circulated to increase efficiency.

Sedimentation - The activated sludge mixture then flows into sedimentation tanks where the sludge settles at the bottom and is recycled back to the aeration tanks. The clarified effluent moves ahead.

Up to 90% of BOD and TSS can be removed in the secondary stage through the breakdown of organic matter by microbes.

4- Tertiary Treatment

Tertiary treatment uses advanced chemical and biological treatment methods to further improve effluent quality before final disposal. Steps include:

Filtration - Effluent passes through filters like sand, dual media or activated carbon to remove fine particulates.

Nutrient removal- Excess nutrients like phosphorus and nitrogen are removed by chemical precipitation.

Disinfection - Final disinfection of effluent is done before discharge using methods like chlorination, UV radiation, or ozonation. This removes harmful microorganisms.  

Additional methods can also be used including membrane filtration processes like reverse osmosis. The final treated effluent can be safely discharged into the environment. In some cases, it may also be further treated to make it potable for non-potable re-use purposes like irrigation or industrial use. This multi-stage sewage treatment process thus makes the wastewater safe for return to nature or reuse.

Conclusion

Sewage treatment is a complex process that involves progressive levels of treatment using various physical, chemical and biological methods. Starting from preliminary treatment to remove solids, it goes through primary sedimentation, secondary biological processes, and finally advanced tertiary treatment. Each stage helps remove increasing amounts of pollutants to turn the raw sewage into a treated effluent that meets environmental regulations for discharge or reuse. Proper treatment protects the environment and public health by reducing the pollution, pathogens and contaminants in wastewater from human activities. Sewage treatment plants thus play an indispensable role in modern societies.

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