What effect does an increase in demand for chlorine have on the ORP in the final effluent?

An increase in demand for chlorine in a wastewater treatment process typically results in a decrease in oxidation-reduction potential (ORP) in the final effluent. This occurs because chlorine acts as an oxidizing agent. When chlorine is introduced into the effluent, it reacts with available reductants (organic matter, bacteria, other contaminants) in the wastewater. As more chlorine is consumed in these reactions, the overall availability of oxidizing species may diminish, leading to a higher concentration of reduced materials in the effluent.

The loss of oxidizing power can shift the ORP to a lower value because ORP is a measure of the overall electron activity in the water; it essentially dictates the balance between oxidizing and reducing conditions. When the demand for chlorine increases, and more of it gets utilized to react with reduced compounds in the effluent, the environment becomes more reducing, thus lowering the ORP.

Hence, the correct understanding is that as the demand for chlorine increases, it leads to greater consumption of oxidizers and contributes to a more reducing environment, which is reflected in a drop in ORP measurements.