Limitations

Although chlorine disinfection is a largely reliable and effective process, it has certain limitations. For examples, chlorine reacts with certain chemicals in the wastewater, leaving only the residual for disinfection. Wastewater components that readily combine with chlorine include reduced iron and sulfur compounds, ammoniated‐nitrogen, organic nitrogen, tannins, uric and humic acid, cyanides, phenols, and unsaturated organics. Cysts of Entamoeba histolytica, Giardia lamblia, and Mycobacterium tuberculosis, some viruses, and eggs of parasitic worms show resistance to chlorine. Consistent disinfection in effluents containing organic nitrogen may pose problems, even when a measured free chlorine residual is present.

Human Health and Environmental Impact

Chlorine is toxic to aquatic, estuarine, and marine organisms. An additional hazard is the carcinogenic potential of chloro‐organic compounds. Chlorine gas is potentially toxic when inhaled, and chlorine transport poses a risk (see Section 5.10.5and Tables 5.8 and 5.9). Special handling is required and emergency response plans are required under right‐to‐know regulations for on‐site storage of gaseous chlorine. Chlorine gas and the hypochlorites are also highly corrosive. Chlorine gas concentrations of 15–20 ppm for 30–60 minutes are dangerous; higher concentrations for very brief periods can be fatal. Chlorination can result in the formation of carcinogenic chloro‐organics.

The EPA has established toxicity criteria for TRC in receiving waters. In freshwaters the acute level is 19 mg/l (1‐hour average) and the chronic level is 11 mg/l (4‐day average). The saltwater acute and chronic criteria are 13 mg/l (1‐hour average) and 7.5 mg/l (4‐day average), respectively. Due to the toxicity of chlorine residuals at such low concentrations and the high limit of analytical detection (50–100 mg/l), chlorine induced toxicity in the receiving stream is difficult to control.