Why is water becoming dark black in an aeration tank in a wastewater treatment plant ?

They are facing a problem as the wastewater getting black color after aeration and the sludge is not settling in the settling tank thereafter.Also there is a huge odour occurred in the aeration tank now.

What will be the reason for this and what will be the solution ?

Recommendation

It means you have increased the organic load to your aeration tank as a result there is shortage of oxygen in the aeration tank. This can be checked by measuring DO in the aeration tank and should be about 1.5 to 2 mg/l.

A) Please check for malfunctioning of Aeration system fouled membranes etc.

B) You can improve the situation by cutting down load to the aeration tank by reducing the flow or the inlet BOD concentration or by adding more air to the aeration tank.

C) Ensure that you are not dumping some chemical like food preservative,detergent,oil or off spec product in the waste water system as this can increase the ETP load.

It will take about 4 to 8 hours to restore, if you balance the BOD load and the oxygen supply ( 1.5 kg O2/KG BOD loading) subject to you have not spoiled the biomass otherwise it may take 2 to 3 days to bring the normalcy.

If you need more correct solution you may have to share design and operation details

Note: the black colour is due to shortage of oxygen (less oxygen/more organic load) and will vanish once you restore the balance.Normally the healthy biomass should be brownish in colour and smell like moist soil.

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