Fundamentals of Microbiology

4.4.4 Secondary Treatment

Secondary treatment is the biological degradation or organic matter in sewage after primary treatment. In this process, the sewage is aerated vigorously to encourage growth of aerobic bacteria and other microorganisms that oxidize the dissolved organic matter to CO₂ and water. Two commonly used methods of secondary treatment are activated sludge systems and trickling filters.

Activated sludge system consists of aeration tank where air or pure oxygen is mixed with the effluent from primary treatment and some of the sludge from a previous batch is added to the incoming water. Sludge inoculum contains large number of metabolizing bacteria, together with yeasts, molds and protozoa. Species of Zoogloea bacteria are important as they form flocculant masses in the tank and oxidizes the organic matter. As these flocs settles out organic matter is removed along. Most of the settled sludge is removed for treatment in an anaerobic sludge digester. Then the effluent is sent on for final treatment. Activated sludge systems remove 75% to 95% of the BOD from sewage.