Disease-producing microorganisms are potentially present in all wastewaters. These microorganisms must be removed or killed before treated wastewater can be discharged to the receiving waters. The purpose of disinfection is to destroy pathogenic microorganisms and thus prevent the spread of water borne diseases.

Although pathogenic microorganisms are reduced in number by the various treatment processes and by natural die-off in unfavorable environments, many microorganisms still remain. To ensure that essentially all pathogenic microorganisms are destroyed in the effluents of wastewater treatment plants, disinfection is practiced. Since chlorine is the most widely used chemical for disinfection, this chapter will deal primarily with the principles and practices of chlorine disinfection and dechlorination. At the end of this chapter another method of disinfection that is increasingly being used in wastewater treatment plants ultraviolet (UV) light systems is discussed.

Two terms you should understand are “disinfection” and “sterilization.” Disinfection is the destruction of all pathogenic microorganisms, while sterilization is the destruction of all microorganisms.

The main objective of disinfection is to prevent the spread of disease by protecting:
1. Public water supplies
2. Receiving waters for recreational uses, and
3. Shellfish growing areas.

Disinfection is effective because pathogenic microorganisms are more sensitive to destruction by chlorination than nonpathogens. Chlorination for disinfection purposes results in killing essentially all of the pathogens in the plant effluent. No attempt is made to sterilize wastewater because it is unnecessary and impractical.

Chlorine is the most widely used disinfectant because it is readily available, easily applied, and cheaper than other oxidizing agents such as potassium permanganate (KMnO4), chlorine dioxide (ClO2), or ozone (O3). Chlorine is applied in one of three forms; chlorine gas, chlorine powder (HTH), or an aqueous solution like chlorine bleach. Even at relatively low dosages, chlorine is extremely effective.

Chlorine is applied to wastewater as free chlorine (Cl2) Hypochlorite ion (OCL>), or as chlorine dioxide (ClO2). In either the free chlorine or Hypochlorite ion form, chlorine is an extremely active chemical and acts as a potent oxidizing agent. Since chlorine is very reactive, it is often used up by side reactions before disinfection takes place. These side reactions can be with such substances as organic material, hydrogen sulfide, phenols, thiosulfate, and ferrous iron. These side reactions occur first and use up a major portion of the chlorine necessary to meet the chlorine demand for a wastewater.