Few industrial wastewater treatment plants are required to disinfect plant effluents, although they may be required to disinfect sanitary discharges prior to mingling with industrial wastes. All municipal wastewater treatment plants are required to disinfect plant effluents, and this is usually the last or next-to-last treatment step prior to discharge to the receiving stream.

Chlorine gas is the most widely used disinfecting agent for large volumes of water or wastewater. Calcium hypochlorite and sodium hypochlorite pellets are forms used most often by small package plants, mainly for ease of handling and safety. Some cities have switched from 90 ton railroad tank cars or one ton chlorine containers to sodium hypochlorite (i.e., bleach) purely for safety reasons. More and more regulatory agencies are requiring dechlorination of treatment plant effluents prior to discharge. Dechlorination is accomplished via aeration and/or sulfur dioxide addition.

Because of the formation of chlorination byproducts (i.e. chloroform, chloramines, etc.) chlorination as a disinfectant is being replaced in some cases by UV and ozone.

Chlorination is conducted in a chlorine contact chamber built in a serpentine fashion to assure that chlorinated water moves as a discrete unit (a.k.a., " plug flow") and adequate contact time is achieved. The two factors affecting the effectiveness of chlorine disinfection are contact time and chlorine concentration. Contact time is usually between 15 and 45 minutes. Chlorine dosage can range between 2 and 10 mg/l with chlorine residuals between 0.5 and 1.0 mg/l. The degree of pathogen destruction is a function...