Q.What is biological oxygen demand?

Sewage is treated in an STP by primary (physical settling), secondary (microbial/biological BOD removal) and tertiary (disinfection) stages; microbes digest the organic waste and the sludge yields biogas.

Waste water (sewage) is the used water from homes, industries and streets; it contains suspended solids, dissolved organic matter and disease-causing microbes. If released untreated it pollutes rivers and spreads disease, so it is cleaned in a Sewage Treatment Plant (STP), largely with the help of micro-organisms.

Stages of treatment:

1. Primary (physical) treatment:

• The sewage is passed through screens/grit chambers to remove large floating debris and grit.

• It is then let to stand in settling/sedimentation tanks, where heavier solids sink as primary sludge and lighter matter (oil, grease) floats and is skimmed off.

• This removes suspended solids physically.

2. Secondary (biological) treatment — the key microbial step:

• The primary effluent is passed into large aeration tanks where it is vigorously aerated. This encourages the growth of aerobic bacteria and other micro-organisms, which form masses called flocs.

• These microbes consume (oxidise) most of the dissolved organic matter in the sewage, sharply reducing its Biochemical Oxygen Demand (BOD). A fall in BOD indicates the sewage has been cleaned.

• The effluent then passes to a settling tank where the flocs settle out as activated sludge; a part is recycled as inoculum and the rest is sent to anaerobic sludge digesters.

• In the anaerobic digesters, anaerobic bacteria break down the sludge, producing a mixture of gases (methane, CO2, H2S) — biogas — which is a usable fuel.

3. Tertiary treatment / disinfection: …