In the Bar Rack coarse solids are removed, such as sticks, rags, and other debris in untreated wastewater by interception. By use of fine screening even floatable matter and algae are removed

In the Grit Chamber grit is removed consisting of sand, gravel, cinders, or other heavy solid materials that have subsiding velocities or specific gravities substantially greater than those of the organic putrescible solids in wastewater.

The Primary Clarifier is a basin where water has a certain retention time where the heavy organic solids can sediment (suspended solids).

Efficiently designed and operated primary sedimentation tanks should remove from 50 to 70 percent of the suspended solids and 25 to 40 percent of the BOD. The influent of the aeration tank is mixed with activated sludge and in the

Aeration Tank the mixed liquor is aerated. By aerating the mixed liquor the aerobic processes will be stimulated, the growth rate of bacteria will be must faster. Because the bacteria deplete the substrate, flocculation takes place . The soluble substrate becomes a solid biomass. These flocks of biomass will sediment in the

Secondary Clarifier. At the end of the process the effluent water is treated to

Disinfect it and make it free of disease-causing organisms.

The picture below shows a simplified flow diagram for biological processes used for wastewater treatment. The influent wastewater (e.g. municipal wastewater) goes through several stages in which different compound are removed out of the wastewater.

ACTIVATED SLUDGE PROCESS

In the initial stages of wastewater treatment, coarse solids such as plastics, rags, and wood are intercepted in the Bar Rack, preventing damage to downstream equipment. This is followed by fine screening, which removes smaller floatable materials and algae, ensuring smoother processing ahead. The wastewater then flows into the Grit Chamber, where heavy inorganic particles like sand, gravel, and cinders settle due to their higher specific gravity. This process effectively separates non-organic debris, reducing wear and tear on pumps and mechanical systems. From there, the partially cleaned water enters the Primary Clarifier, where it is held for a set retention time. During this time, heavier organic solids settle to the bottom, forming sludge, while lighter materials float and are skimmed off. Properly functioning clarifiers can eliminate up to 70% of suspended solids and significantly reduce biological oxygen demand (BOD).

ACTIVATED SLUDGE PROCESS

Following primary treatment, the wastewater moves into the Aeration Tank, where it is combined with activated sludge—biological material rich in bacteria. Through mechanical or diffused aeration, oxygen is introduced into the tank, stimulating aerobic bacterial activity. These microorganisms rapidly consume the organic matter in the wastewater, transforming it into biomass. As these biomass particles clump together through flocculation, they form heavier flocs that are easier to separate. This mixture, known as mixed liquor, is then transferred to the Secondary Clarifier, where the solid biomass settles and is either recycled or removed. Finally, the treated effluent undergoes Disinfection—typically through chlorination or UV treatment—to eliminate any remaining pathogens, making the water safe for discharge or reuse. This multi-stage process ensures high-quality effluent that meets stringent environmental and health standards.

ACTIVATED SLUDGE PROCESS