TS, it is for the chemical treatment to convert the polluted water that you discharge from your unit, treated it before the water is being release into the river / sea.

If you think it is expensive, go search oversea sewerage water treatment charges from the developed nations. You see the bill, you will keep silent after this. The sewerage fees is the 2nd most expensive after the utilities like electricity / gas. at UK, it was at £34 in 2021/22.