Sewage Treatment: A Summary

Review
Primary treatment : Physically separate out the large solids, sludge produced.
Secondary treatment: Biologically degrades the dissolved organic compounds with the help of micro-organisms and oxygen, disinfects the effluent.
Tertiary treatment: Removes nutrients from effluent

Role of Decomposers in Sewage Treatment: Sewage is decomposed by saprophytic microorganisms.
In the Primary Treatment:
• Solids waste sink to the bottom and are sent to the anaerobic digester. Aerobic bacteria are killed, anaerobic bacteria digest the sludge producing methane gas which can be used as fuel in the boiler house.

In the Secondary Treatment:
• At the aeration tank, aerobic bacteria present in the sewage secrete enzymes to digest the remaining organic matter into soluble, harmless substances.
• At the settling tank, microorganisms from the aeration tank sink to the bottom to form the activated sludge. It is then piped back to the aeration tank to aid in digesting the organic matter.

Goals of Sewage Treatment
1. To remove organic matter from the water ( via Decomposition)
2. Removal of water-soluble nitrates and phosphates from the effluent (prevent eutrophication)
3. Eliminate pathogenic micro-organisms from the sludge and effluent

Products of Sewage treatment
o Microbe-free effluent – returned to the rivers or the sea
o Nutrient-rich sludge – for use as fertilizer to improve soil structure and provide vital missing nutrients
o Methane gas produced during decomposition of sludge- to generate electricity