Chlorine Gas vs. Sodium Hypochlorite: What's the Best Option?

Abstract

This article discusses a study by the city of Lakeville, Minnesota, to evaluate alternative disinfection technologies that was driven by U.S. Department of Homeland Security concerns about risks associated with handling, storing, and using toxic chlorine gas. The study evaluated alternative disinfection systems for the city's water treatment plant, considering the plant's existing system, safety, security, cost‐efficiency, and finished water quality. The Lakeville study did not evaluate ultraviolet, ozone, chloramine, or chlorine dioxide systems because of site and cost constraints. The study focused on sodium hypochlorite (NaOCl), that provides residual protection but is safer than chlorine gas and less expensive than other alternative disinfection systems. Installing a NaOCl system would require the city of Lakeville to make few modifications to its existing system, and finished water quality would be similar to what is presently produced. The article discusses the advantages and limitations of using sodium hypochlorite, and provides comparisons of capital costs and annual O&M costs of chlorine gas, commercial sodium hypochlorite, and on‐site sodium hypochlorite generation. The article includes a total present worth analysis for a 20‐mgd sodium hypochlorite disinfection system.

Biofouling of Polyamide Membranes: Fouling Mechanisms, Current Mitigation and Cleaning Strategies, and Future Prospects

“…Gaseous chlorine is more toxic than hypochlorite and requires careful handling. It is, however, more economical than hypochlorite, special handling not withstanding [100]. Chlorine gas does not remain in gaseous form in water, which inhibits its ability to penetrate biofilm; it immediately hydrolyzes in water (Equation (1)) to form aqueous hypochlorous acid, HOCl: Liquid sodium hypochlorite also hydrolyzes in water (Equation (2)) to form hypochlorous acid:Cl 2 + H 2 O ← → HOCl + HCl NaOCl + H 2 O → HOCl + NaOH…”

“…On a universal level, chlorination forms THMs and HAAs, both of which are carcinogenic species. Hypochlorous acid can also form disinfection by-products (DBPs), such as bromate (in seawater—maximum contaminant level, MCL, of 0.01 ppm) and chlorate (no official MCL, but California has a notification level of 0.8 ppm) [100,103]. Cryptosporidium parvum and Mycobacterium avium, which are pervasive in water system biofilms, are not well controlled using chlorine; endospores and protozoa are also not well controlled with chlorine [104,105].…”

“…Bromamine has several times the oxidation strength of chloramine [96,148], and has as much biocidal activity as hypobromous acid [148]. Severe membrane damage was incurred at West Basin Municipal Water District (Carson, CA, USA), during a pilot study on seawater intake due to the formation of bromamines via chloramination [100].…”

Comparison of disinfectants for drinking water: chlorine gas vs. on-site generated chlorine

“…Chlorine has been widely used for drinking water disinfection since the early 20 th century because it is cost-effective, and it can inactivate a range of pathogenic microorganisms and maintain disinfection properties in the distribution system [1,2]. A couple of different chemicals are used for chlorine disinfection, e.g., chlorine gas (liquid chlorine), commercial sodium hypochlorite (NaOCl) solution, and calcium hypochlorite (Ca(ClO) 2 ) tablets [1][2][3]. When all of these chemicals are added to water, they form free available chlorine (FAC), i.e., hypochlorous acid (HOCl) and hypochlorite ion (OCl -), which can inactivate microorganisms and oxidize organic matter [1,3].…”

“…A couple of different chemicals are used for chlorine disinfection, e.g., chlorine gas (liquid chlorine), commercial sodium hypochlorite (NaOCl) solution, and calcium hypochlorite (Ca(ClO) 2 ) tablets [1][2][3]. When all of these chemicals are added to water, they form free available chlorine (FAC), i.e., hypochlorous acid (HOCl) and hypochlorite ion (OCl -), which can inactivate microorganisms and oxidize organic matter [1,3].…”