Question

Why do we prefer to call secondary wastewater treatment as biological treatment?

Answer 1

Sewage treatment:

Sewage treatment refers to the processes involving the treatment of sewage wastes in order to transform them into forms fit for drainage into water bodies. The treatment generates by-products which have economic utilities. Sewage treatment is carried out by two main steps:-

• Primary treatment
• Secondary treatment

Secondary treatment:

During the secondary treatment of sewage, the primary effluent goes through the following stages:-

• Aeration tank: The following steps take place in the aeration tank: ○ The primary effluent is aerated allowing the growth of flocs.
  • The flocs consist of mainly aerobic heterotrophic bacteria associated with filaments of aerobic fungi.
  • These microbial flocs oxidise the organic matter present in the primary effluent, reducing the BOD (Biochemical Oxygen Demand).
• Settling tank: The treated effluent is then pumped into the settling tanks where the flocs settle down. This sediment is called activated sludge. Most of the sludge is made to flow into anaerobic digesters while some amount is pumped back into the aeration tank as an inoculum.
• Anaerobic digesters: The environment in the digester is anaerobic, hence. most aerobic bacteria die. Anaerobic bacteria digest the bacteria and fungi (comprising the flocs), thus producing a mixture of gases that can be used as biogas.

Secondary treatment:

Biological treatment Microbes, both aerobic and anaerobic, are used in the secondary treatment of sewage. Since biological agents are utilised during secondary sewage treatment in order to treat the remnants of the primary treatment, hence, it can also be called biological treatment.