Waste Water Engineering
Pick up the incorrect statement from the following for allowing workers to enter sewers

Proper tests for the presence of poisonous gases must be carried out
Warning signals should be erected
The men entering the manhole should be advised to smoke in the sewer
The particular manhole and one manhole on upstream and one manhole on downstream should remain open for 30 minutes

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Waste Water Engineering
In the activated sludge process

Sludge is activated by constant stirring
Aeration is continued till stability
Water is removed by centrifugal action
Aeration is done with an admixture of previously aerated sludge

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