A neutralisation step takes place because of wide pH range in the wastewater.

The biological oxidation needs a neutral, or slightly alkaline pH value. Neutralisation is carried out automatically with soda or acid, depending on the characteristics of the wastewater, by dosing pumps and it is controlled by a pH meter equipped with alarm signal. The pumps take the reagents from storage tanks of suitable capacity. Their start is automatic and such to provide for a pH value between 7 and 8. The pH measurement is carried out by a glass made probe, equipped with potentiometer, easy to change in case of failure or wear.

Sometimes, when the inlet pH value is higher than 11, it may be economically convenient to use smokes coming from boilers for discharged water neutralization. For that, it will be necessary to install a system to redirect and distribute gases from chimneys into equalization tank.