A. Pretreatment: The sewage undergoes a preliminary treatment to remove large debris and objects such as sticks, stones, rags, and plastics. Bar screens and grit chambers are used to trap and remove these materials.

B. Primary Treatment: The sewage enters primary settling tanks, also known as sedimentation tanks or clarifiers. Here, the heavier solids settle to the bottom as sludge, while lighter materials like oils and grease float to the surface and are skimmed off. The settled sludge is pumped to sludge treatment facilities for further processing.

C. Secondary Treatment: The partially treated wastewater from the primary settling tanks is then subjected to biological treatment in order to remove dissolved and suspended organic matter. The most common method for secondary treatment is the activated sludge process, where air is continuously pumped into aeration tanks to promote the growth of aerobic bacteria. These bacteria consume organic matter and convert it into carbon dioxide, water, and more bacterial cells. The mixture of wastewater, bacteria, and suspended solids (activated sludge) is then sent to secondary settling tanks, where the sludge settles to the bottom and is recycled back into the aeration tanks, while the clarified water flows to the next stage.

D. Tertiary Treatment: Sometimes, a tertiary treatment stage is included to further improve the quality of the treated wastewater. This stage involves additional processes to remove remaining contaminants, including nutrients (such as nitrogen and phosphorus) and fine suspended solids. Common tertiary treatment methods include filtration (sand filters, microfilters, or ultrafilters), disinfection (chlorination, UV radiation, or ozone treatment), and chemical processes (such as coagulation and flocculation).

E. Sludge Treatment: The sludge collected from primary and secondary settling tanks, as well as excess activated sludge, undergoes further treatment. This includes processes like thickening (removal of water to reduce volume), digestion (anaerobic decomposition of organic matter to reduce its volume and stabilize it), and dewatering (removal of more water to produce sludge cake with higher solids content). The treated sludge can be further processed for reuse (e.g., as fertilizer) or disposed of in an environmentally safe manner.