This article takes a look at ways to achieve microbial inactivation in wastewater treatment:
Enteric viruses, (represented by polivirus type) are more resistant to inactivation by antimicrobial chemicals like chlorine than bacteria (represented by e.coli). And protozoan cysts are nearly two orders of magnitude more resistant to wastewater sterilization chemicals than the enteric viruses. Differences in effectiveness of HOCl and OCl- against the viruses and bacteria are also noticed.
Comparison of chloramines with chlorine for disinfection of microorganisms shows that, in general, for all types of microorganisms, CT (chemical treatment) values for chloramines are higher than CT values for free chlorine species. However, CT values for giardia lamblia cysts are lower, in contrast to the result for free chlorine.
3) Chlorine Dioxide
Chlorine dioxide CT values used show that, at pH 7.0, ClO2 is not as strong a bactericide and virucide as HOCl. However, as the pH is increased, the efficiency of ClO2 for chemical inactivation of viruses increases. CT data for protozoan cyst inactivation is not available.
Overall, comparison of CT values for ozone with those for chlorine and ClO indicates that ozone is a much more effective biocide than the other antimicrobial chemicals. Escherichia coli is about 10-fold (1 log 10) more resistant to ozone than poliovirus-type 1. Giardia muris cysts are about 10-fold more resistant to ozone than poliovirus type 1. Since ozone is a powerful oxidant, it reacts rapidly with both microorganisms and organic solutes and is very useful as primary disinfectant.
The order of microbial disinfectant effeciency is O3 (Ozone) > ClO2 (Chlorine Dioxide) > HOCl > OCl- > NH2Cl (monochloramine) > NHCl2 (dichloramine) > rnNHCl (organic chloramines). However, for technical reasons, practical handling considerations, cost and effectiveness, the frequency of use of antimicrobial chemicals by utilities in the United States is generally chlorine > chloramines > O3 (Ozone) > ClO2 (Chlorine Dioxide).
5) Ultraviolet Light
Sensitivity of the various microbial groups of ultraviolet light is similar to that for antimicrobial chemicals. Enteric bacteria are most sensitive, followed by enteric viruses; protozoan cysts are least sensitive. Organisms that are sub-lethally injured by UV light exposure may, under appropriate conditions, be able to repair the damage (i.e., Phyto activation or dark repair).
Ranges or UV dosages required for 99.9% inactivation of microorganisms of concern in drinking water are: bacteria, 1400 -- 12,000 uw.sec/cm2; viruses, 21,000 -- 46,800 uw.sec/cm2. The UV disinfection values given for protozoan cysts are not practical with current UV technology used for water treatment.