In waste water treatment, after chemical treatment why stagnant semi treated water became black colour?

Question

what happening in the waste water? how to solve the water for prevent become black?

Answer 1

In chemical treatment of wastewater, the pollutants which are chemically oxidizable are reduced by chemical oxidation. However, there are certain other bacterial pollutants which can only be removed by biological treatment. Generally, sewage is termed as 'black water' which is mainly composed of bacterial contamination due to human excreta/urine. Hence, the semi treated wastewater may be subjected to undergo biological treatment to produce acceptable colorless final effluent.

Answer 2

After chemical treatmant, all bacteria in the water die. The bacteria come from natural sources- the waste itself. It works to break down the waste. The purpose of chemically treating wastewater is to kill harmful bacteria (cholera, staph, etc.) The good bacteria, which makes up virtually all of the bacteria in the water, dies too. When all this bacteria dies, decomposition of the bacteria (oxidation, in this case) AND the waste creates a low-to-no oxygen environment in the water. Oxidation uses up the oxygen. Without the oxygen new bacteria will begin to colonize the water- these anaerobic bacteria work by using sulphur instead of oxygen as they feed on the waste. Anaerobic digestion creates nasty sulphur compounds in the water, mostly hydrogen sulfide (H2S), which is very nasty stuff.

    Thing is, it does prevent disease-causing bacteria from proliferating... but it's nasty in its' own right.