Sewage water purification is achieved by directing it through sewage treatment plants, utilizing the action of microorganisms. Within these plants, the separation of solids from liquids occurs through physical processes, while liquids undergo purification through biological and chemical processes in three distinct stages: primary, secondary, and tertiary.
(i) Primary Treatment: Physical removal of particles like debris, soil, sand, and silt is executed through sequential filtration and sedimentation. The settling substances form primary sludge, and the resulting liquid is termed primary effluent.
(ii) Secondary Treatment: This involves biological treatment, where the primary effluent is introduced to large aeration tanks constantly agitated and supplied with air/O2. This induces the creation of flocs, meshlike structures composed of beneficial aerobic bacteria and fungal filaments. Microbes in this process reduce the Biological Oxygen Demand (BOD), representing the amount of oxygen needed to oxidize all organic matter in 1 liter of polluted water by microbes. Once BOD is adequately reduced, the effluent proceeds to a settling tank where flocs settle as activated sludge. A portion of this sludge is returned to the aeration tank for inoculation, while the majority undergoes anaerobic digestion in a sludge digester, releasing a mixture of gases such as CH4, CO2, H2, H2S (biogas). The resulting water is purified and can be discharged into water bodies.
This treatment is crucial to:
(i) Avoid pollution of natural water bodies:Preventing excessive accumulation of harmful chemicals, organic matter, and nutrients that may lead to eutrophication.
(ii) Prevent the spread of infectious waterborne diseases: Caused by deadly pathogens present in polluted water.