Lesson 6 of 11 · 6 min
Primary sewage treatment
NCERT §8.3
Friday: the class visits the city sewage treatment plant. The smell at the gate is strong, and the guide says the plant's real workers are too small to see.
The lesson in notes
In short
Towns and cities produce huge volumes of waste water every day, much of it human excreta. This municipal waste water is sewage.
Sewage carries a lot of organic matter and microbes, many of them pathogenic, so it cannot be let straight into rivers and streams.
So it first passes through sewage treatment plants (STPs), which cut its polluting load. The work is done by heterotrophic microbes already present in the sewage.
Treatment has two stages: primary and secondary.
Primary treatment is physical: particles large and small are removed by filtration and sedimentation, in stages.
Floating debris is removed first by sequential filtration; grit (soil and small pebbles) is then removed by sedimentation.
All the solids that settle make up the primary sludge; the liquid above (supernatant) is the effluent, which goes on to secondary treatment.