H2S is only controlled in ...

H2S is only controlled in certain areas of the plant, and will be higher in other areas.

It is controlled by a system with good, constant flow, not in upset, plenty of aeration in the aerobic digester areas, and by steadfast prayers and concerns of the operators.

H2S is responsible for most of the corrosion in a WWTP where exposed copper wiring (not sealed in silicone rubber) can corrode away in only a few days.

This gas is very treacherous, because if you cannot smell it (after perhaps you did smell it), you are in extreme danger of unconsciousness and death.