Residual Disinfectant in Water

Residual Disinfectant in Water

Residual disinfectant in water will be present in raw water supplies from a surface source or groundwater source influenced by a surface source. The U.S. EPA Ground Water Rule (GWR) (U.S. EPA, 2006a) requires community water systems with potential introduction of viral and bacterial pathogens to conduct a risk-targeting approach that consists of four components, one of which is treatment to provide at least a 4-log inactivation or removal of viruses as well as compliance with the Total Coliform Rule (U.S. EPA 815-F-06-003).

While raw water supplies with a groundwater source may not exhibit bacteria at the treatment facility, it is highly likely that bacteria will be introduced during distribution. Subsequently, raw water supplies from a surface source, a groundwater source influenced by a surface source, and a groundwater source will be treated with residual disinfectant to destroy bacteria.

Chlorination of municipal water supplies began around 1908. Chlorine is an extremely effective disinfecting agent. When added to water, chlorine produces a mixture of hypochlorous acid and hypochloric acid, which then produces hypochlorite ion, as outlined by the following reactions:

Cl2 + 2H2O ↔ HOCl + H3O+ + Cl-

HOCl + H2O ↔  H3O+ + OCl-

The ratio of hypochlorite ion to hypochlorous acid is a direct function of pH. For example, at a pH of 7.5, equal concentrations of hypochloric acid and hypochlorite ions will be present. However, at a pH of 9, approximately 95% of the chlorine “residual” occurs as hypochlorite ion, a very powerful oxidant. According to research, it is indicated that chlorination of municipal water supplies containing NOM and certain inorganic matter resulted in the production of disinfection by-products—trihalomethanes (THMs) and haloacetic acids (HAA5). Research identified that the humic acid fraction of NOM, the trihalomethane “precursor,” is the primary mechanism for production of the THMs.

In November 1979, the U.S. EPA established a maximum total THM level of 100 mg/L. The Stage 1 Disinfection Byproducts Rule reduced the maximum average total trihalomethane limit to 80mg/L. The Stage 2 Disinfection Byproduct Rule provides a phased distribution system monitoring program outlining enforcement dates based on the number of individual served by the municipal treatment facility.

In an attempt to provide excellent disinfecting properties while minimizing the production of THMs, many treatment facilities employ primary disinfection with chlorine and final disinfection, prior to distribution, with a mixture of chlorine and ammonia, producing an alternate disinfecting agent, chloramines. Hypochlorous acid produced by the reaction of chlorine with water will react with ammonia to form multiple species of chloramines. The disinfecting properties of trichloramines and dichloramines are poor when compared with monochloramines. As a result, the pH of systems using chloramines for disinfection is generally maintained at a value greater than 7–8. In general, residual chloramines at a concentration of approximately 3.0 mg/L will produce the same disinfecting properties as residual chlorine at a concentration of 0.5 to 1.0 mg/L.

The transition of residual disinfecting agents from chlorine to chloramines (monochloramine) significantly affects pharmaceutical water purification systems. This is particularly true for systems using thin-film composite polyamide RO membranes that are incapable of tolerating trace concentrations of residual disinfectant; the most popular membranes used in pharmaceutical water purification systems.

For these systems, a conservative design of activated carbon units for residual disinfectant removal must be maintained. These design features are discussed in detail in future post. In addition, raw water supplies containing residual chloramines will also require more frequent replacement of activated carbon media. Finally, for raw water supplies using chloramines as a residual disinfectant, the ability of conventional double-pass RO to produce USP Purified Water (conductivity specification) is extremely poor.

A limited number of treatment facilities employ chlorine dioxide or ozone for primary disinfection. These agents produce limited disinfection by-products when compared to chlorine and are subsequently attractive primary disinfecting agents.

It would be inappropriate to conclude any discussion of residual disinfectant without discussing general operating trends associated with seasonal and climatic changes. For systems using residual chlorine, THM production will continue to occur in distribution piping from the municipal treatment facility. Further, bacteria will also deplete the residual disinfectant at an increased rate as the temperature of the surface water increases with seasonal fluctuations, or if the water contains a higher degree of bacteria associated with severe climatic conditions. Some treatment facilities may use residual chlorine concentrations to ensure that the total THM level is not exceeded. Under these conditions, particularly during summer months, it is possible that extremely low residual disinfectant concentrations may be present in the feed water to a facility. This situation may require initial injection of residual disinfectant to ensure that the feed water supply to the pharmaceutical water purification system meets the U.S. EPA’s NPDWR and the 500 cfu/mL total viable bacteria level historically indicated in the USP General Information section. Well water supplies influenced by a surface source treated with chloramines will also exhibit seasonal and climatic changes. In general, however, since the production of THMs is not a controlling factor, the increased demand for residual disinfectant is generally addressed by increasing the concentration of residual chloramines. Again, for systems using RO as the ion removal technique, a decrease in product water quality should be anticipated, since higher concentrations of ammonia associated with the increased chloramine concentration will affect the RO system product water quality.

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