Zoology - Microbes in Human Welfare Question with Solution | TestHub

Domestic sewage rich in organic waste will exert a very high demand of oxygen in order to get biodegraded. Thus depleting the dissolved oxygen below critical level necessary to support aquatic life. Especially fish, which is very sensitive to the dissolved oxygen available in water, starts dying if the level goes below $4\mathrm{mg}/\mathrm{L}$.