Zoology - Microbes in Human Welfare Question with Solution | TestHub

Sewage treatment plants (STPs) - There are two stages of treatment :

A. Primary treatment

-It involves the physical removal of the small and large sized particles from the sewage.

-Processes involved :

1. Filtration 2. Sedimentation

B. Secondary treatment or Biological treatment

-It involves the physical removal of the small and large sized particles from the sewage.

-Mechanism

Step 1 - Primary effluent is passed into large aeration tanks in which constant agitation and pumping of air is there$\to$Growth of microorganisms (specially aerobic) is promoted → Flocs (large asses of bacteria along with fungal filaments to appear like a mesh like network) are formed

These microbes take oxygen during their growth (thus increasing the BOD) and decompose or consume major organic part of the effluent.

Step 2- Once the BOD is significantly decreased, effluent is transferred to a large settling tank$\to$Bacterial flocs settle here, which are termed as activated sludge.

Step 3 - A small part of the activated sludge is transferred back into the aeration tank to act as inoculum.

Step 4 - Major part of the activated sludge is pumped into the anaerobic sludge digesters which are large tanks having anaerobic bacteria$\to$These anaerobic bacteria digest the bacteria and fungi present in the sludge$\to$In this process of digestion, various gases are produced in the form of a mixture - Methane, hydrogen sulphide and carbon dioxide. These gases are collectively termed as biogas.

Flocs contain aerobic bacteria and fungus to reduce BOD of${\mathrm{H}}_2\mathrm{O}$. Aerobic bacteria uses oxygen to decompose organic matter.