Fundamentals of Microbiology

4.4.6 Final Treatment

Usually the effluent from secondary treatment is disinfected and discharged on to land or into water. Sometimes, it is subjected to tertiary treatment. Tertiary treatment uses physical filtration and chemical precipitation to remove all the BOD, nitrogen and phosphorus from water. Tertiary treatment provides drinkable water, when as secondary treatment provides water for irrigation.

The sludge is further digested in an anaerobic sludge digester. Bacteria degrade organic matter and produce methane and CO₂. In the first stage of anaerobic sludge digestion, there is production of CO₂ and organic acids by anaerobic fermentation. In the next stage organic acids are metabolized to form hydrogen and CO₂ and some organic acids like acetic acid. In the third stage there are converted into methane by methane producing bacteria

CO₂ + 4H₂ — CH₄ + 2H₂O

CH₃ COOH — CH₄ + CO₂

(acetic acid)