High Sulphide Content in WWTP Effluent

We recently realized that one of our municipal wastewater treatment plants has effluent with high sulphide content of about 20 mg/l. With 2 million cubic meters the plant treats wastewater from a mall. The treatment process is designed as flows: Two screen systems Grit chamber Flow equalization tank Aeration( MBBR) Clarifier Discharge We suspect that the detergents used in cleaning of the mall might be cause for increased sulphide. What do you think are the possible causes for increased sulphide levels and what can we do to reduce the sulphide content to the permissible level of 2 mg/l before discharge?