Residual disinfection effect

Other photocatalytic disinfection experiments (not shown here) were carried out and in all cases EDT24 was reached before 3h of phototreatment (Rincon and Pulgarin 2004). In preceding works, a residual disinfection effect was observed at laboratory and field scale after solar photocatalytic treatment (Rincon and Pulgarin 2004a, b, 2005, 2007b, c). 

Alternative water disinfection techniques use chlorine dioxide, ultraviolet radiation, ozone and ultrafiltration. Normally these techniques form less hazardous by-products, but the technologies are more complex and the total costs are higher. Little is known about the nature and possible toxicity of the other disinfectants and their by-products and they do not have the ability of the residual disinfection effect, so the treated water may be protected from recontamination by a small dose of chlorine. The quality of the raw water and the concentration and the nature of the contaminants may justify the decision for an alternative disinfection technique. 

The two chemical species formed by chlorine in water, HOCl and OCl , are known as free available chlorine and are very effective in kilting bacteria and other pathogens. In the presence of ammonia, HOCl reacts with ammonium ion to produce monochloramine (NH2CI), dichloramine (NHCI2), and trichloramine (NCI3), three species collectively called combined available chlorine. Although weaker disinfectants than chlorine and hypochlorite, the chloramines persist in water distribution systems to provide residual disinfection. 

Oxidation of manganese is one of the main uses of chlorine dioxide. Another advantage of chlorine dioxide is its effectiveness in inactivating Cryptosporidium. Although it is sometimes used as a residual disinfectant in the distribution system, it is more common to use chlorine dioxide only as a primary disinfectant and then use chlorine or chloramines to provide a distribution system residual. 

Water chlorination is carried out by using both free and combined residuals. The latter involves chlorine application to produce chloramine with natural or added ammonia. Anhydrous ammonia is used if insufficient natural ammonia is present in the wastewater. Although the combined residual is less effective than free chlorine as a disinfectant, its most common application is as a post-treatment following free residual chlorination to provide initial disinfection. 

Ozone is more effective than chlorine in deactivating poliovirus, Cryptosporidium parvum, Giardia lamblia, and other protozoa. It also improves the color, taste, and odor of water dramatically. However, since no residual amount remains, it is always necessary to add a small amount of a more stable disinfectant as well (sodium hypochlorite, chlorine dioxide, etc.). 

Studies have demonstrated that one such method is to examine the effects of disinfectants on endogenous RNA-dependent DNA polymerase (i.e. reverse transcriptase) activity. In essence, HIV is an RNA virus after it enters a cell the RNA is converted to DNA under the influence of reverse transcriptase. The virus induces a cytopathic effect on T lymphocytes, and in the assay reverse transcriptase activity is determined after exposure to different concentrations of various disinfectants. However, it has been suggested that monitoring residual viral reverse transcriptase activity is not a satisfactory alternative to tests whereby infectious HIV can be detected in systems employing fresh human peripheral blood mononuclear cells. 

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