Approaches to Disinfection

Chlorination is currently the most widely used disinfection method. However, there have been concerns about chlorinated disinfection byproducts that form when chlorine reacts with natural orgnic matter. Furthermore, it has been discovered that certain protozoa are resistant to deactivation by chlorination, namely C. parvum and G. lamblia. For this reason various other methods have been introduced. The table below compares various methods of disinfection currently in use in North America.

Disinfectant Class Chemical Advantages Disadvantages
Free Chlorine Liquified chlorine gas A common well-practiced disinfectant. Easier integration in facility than other approaches. Public concern about toxicity of residuals. Produces THMs and HAAs, and several other disinfection byproducts which may become regulated in the near future. Not highly effective against C. parvum and G. lamblia .
Sodium Hypochlorite Same disadvantages as above, but difficulties with storage and decay complications, and produces chlorate ion, soon to be regulated byproduct.
Calcium Hypochlorite Occasionally used in small facilities. Same as for liquified chlorine gas, but Hardening effect can cause problems with filters.
Combined Chlorine (chloramination) Chlorine plus anhydrous liquified ammonia gas Reduces disinfection byproducts of chlorine-only treatment when added as residual disinfectant. Lower odour threshold than free chlorine. Aqua ammonia is more easily handled and more common. Slower deactivation kinetics. Anhydrous ammonia is the more expensive of the three. Produces cyanogens and NDMA, which may become regulated.
Chlorine plus aqua ammonia (aqueous ammonium hydroxide)
Chlorine plus ammonium sulphate
Chlorine Dioxide Reduced disinfection byproducts relative to free chlorine. Not widespread. Banned in California. Problems with odour under certain conditions. Manufactured on-site using chlorine and sodium chlorite. Produces chlorate byproduct.
Ozone Strongest oxidant and disinfectant in wide use. Effective against C. parvum and G. lamblia . Cannot be stored, must be produced on site from air or O2. Contactors are more complex. Produces bromate a regulated byproduct.
Ultraviolet light Most effective against Oxidation-resistant C. parvum and G. lamblia . Less effective against endospores. Very low contact time. No known disinfection byproducts. Particulate matter can greatly reduce efficiency due to shading/encapsulation. Fast contactors are at risk of developing short-circuiting flow. A less-well disinfection strategy.
C. parvum = Cryptosporidium parvum
G. lamblia = Giardia lamblia
THM = trihalomethanes
HAA = haloacetic acid
NDMA = N-Nitrosodimethylamine

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