What sort of wwtp do you ...

What sort of wwtp do you have?  What are the mal-odours which are occurring?

There are several mal-odours associated with WWTP.  The most common ones are those associated with sulphur compounds.  These can range from Hydrogen sulphide, which is bad egg, through mercaptans, which is rotting cabbage, to rotten feet, carbon disulphide.  These occur under anaerobic conditions.  There are rotting fish odours, which are amines and these may be a mixture of yeast and or Pseudomonads.  Different parts of a wwtp are more likely to generate the odours.  So sludge processes are more likely to generate mal odours than an aerated system.  The other area would be poor mixing and aeration. 

Without further information it is difficult to advise.