Question

What is the key difference between primary and secondary sewage treatment?

Answer 1

Sewage is treated in a sewage treatment plant so that it can be free from pollution. This treatment is completed in two stages,

1. Primary treatment: In primary treatment, mainly large and small particles are separated by physical processes such as sedimentation, filtration, flotation, etc. First of all, floating garbage is removed by filtration. After this, grit soil and small particles are separated by sedimentation. The fine particles settle down as primary sludge and supernatant effluent is formed. The effluent is taken from the primary treatment tank for secondary treatment.
2. Secondary treatment: Microorganisms are used in secondary treatment. For example, an oxidation pool is a shallow reservoir collecting sewage. Due to the high amount of organic matter in it, algae and bacteria start growing well. The bacteria decompose and the algae use the carbon dioxide they produce in photosynthesis. The oxygen released in photosynthesis protects the water from getting contaminated. Thus, an oxidation pool is an example of symbiosis between algae and bacteria. Infectious bacteria are destroyed by the processes taking place in the oxygen pool, and only harmless substances remain after the decomposition of organic matter. After secondary treatment, the effluent from the plant is usually released into natural sources of water, such as rivers and springs, etc.; or for tertiary treatment, after separating nitrogen and phosphorus salts from it through chemical processes, the effluent is released into reservoirs.

Answer 2

Sr No.	Aspect	Primary Sewage Treatment	Secondary Sewage Treatment
1.	Nature of Process	Physical removal of suspended particles.	Biological treatment using microbes.
2.	Method	Filtration to remove floating debris and sedimentation to remove grit (soil, small pebbles).	Aeration tanks with mechanical agitation and air pumping to promote growth of aerobic microbes forming flocs (bacteria + fungal mycelium).
3.	Outcome	Solids settle to form primary sludge; effluent is the supernatant liquid.	Microbes consume a major part of organic matter, reducing BOD; activated sludge settles out.
4.	Effect on BOD	Little to moderate reduction.	Significant reduction of biochemical oxygen demand (BOD).
5.	Further Treatment	Effluent from primary treatment moves to secondary treatment.	Activated sludge is digested anaerobically, producing biogas; effluent is released into natural water bodies like rivers and streams.