Normally sulphide enters the ...

Normally sulphide enters the water termed as MIC (Microbe Induced Corrosion). Bacteria are always looking for a food source and this is how sulphate reducing bacteria enter the water source. 

The best way to treat sulphides is to carry out final polishing of the wastewater with chlorine dioxide. As regards to the dosage to be applied you can start at 3 ppm, that amounts to nearly 19 cubic meters of chlorine dioxide every day.

Please confirm whether the volume of wastewater is 2 million cubic meters per day or 2000 cubic meters per day. As treating the wastewater with so much of chlorine dioxide would be expensive. 

In case the volume is indeed 2 million cubic meters then lower the chlorine dioxide dosage to 1 ppm, which would work out to 6 cubic meters of chlorine dioxide every day. 

As chlorine dioxide is very effective you should be able to get down to 2 mg/l before discharge.