Depending on whether you are ...

Depending on whether you are talking about an aerobic plant or anaerobic plant, the answers are different.  With an aerobic plant, the objective is to keep it aerobic because the bacteria, which produce the sulphide smells do not like oxygen.  There are two things which help, fully mixing plant using air to prevent dead areas where anaerobic bacteria grow and use fine bubble aeration to diffuse the air into the water and drive off carbon dioxide.  The latter produces better growth.  It must be remembered that there are other parts of a plant where sulphide odours are generated.  These are the sludge handling and wasting and any balance tanks especially if the liquor from sludge wasting is returned to the tank.  Whereas ferric chloride may be added in some plants, in others it is self defeating because bacteria with ferric chloride release sulphide after mixing.

In the case of anaerobic digestion, sulphides will always be produced at some level, but if the methane produced and is burnt then the waste sulphide will oxidise, but this by itself causes problems because condensed water combines and generates sulphur acids, which are corrosive.