Water Quality Management - River Dynamics and Integrated River Management - page 607

effluent, the ozone demand, and the transfer efficiency of the ozone system (USEPA, 2002). Ozone has
been found to be very effective at inactivating a wide range of microorganisms and is generally believed
to be more effective than chlorine. Ozone is particularly effective against viruses. For example, a 5 mg/L
dose and a 5 min. contact time were sufficient to achieve a 5-log removal of the highly resistant virus,
MS2 bacteriophage (Geosyntec, 2008). However, the heterogeneous nature of municipal wastewaters and
the relatively high cost of ozone application make it unlikely that organic substances can be completely
degraded (to carbon dioxide and water) by ozone treatment (Geosyntec, 2008). Finally, ozone may be
unreliable when turbidity is high or variable, because viruses are protected in flocculated particles (Health
Canada, 2004).
Because ozone dissipates rapidly and decomposes to oxygen, ozone residuals will normally not be
found in the effluent discharged into the receiving water. However, some researchers have reported that
ozonation can produce some unstable, toxic, mutagenic, and/or carcinogenic compounds (USEPA, 2002).
By-products, such as aldehydes, ketones, acids, and other species, can be formed upon ozonation of
wastewater (Geosyntec, 2008). The scarcity of information concerning the formation of ozonation by-products
in wastewater effluents clearly indicates that further investigations are necessary on this subject (Paraskeva
and Graham, 2002).
Bromate ion formation is an important consideration for waters containing more than 0.1 mg/L bromide
ion. Even small residual ozone concentrations can cause mortality in fish and larvae (Paraskeva and
Graham, 2002). Studies using fish and crustaceans as test organisms did not result in any changes in the
toxicity of a secondary effluent after ozonation (Geosyntec, 2008).
Disinfection effectiveness —The effectiveness of disinfection is a complex function of several variables
including type and dose of disinfectant, type and concentration of microorganisms, contact time, and
water quality characteristics. The effectiveness of the various disinfectant methods for bacteria, protozoa,
and viruses are summarized in the following paragraphs and in Table 9.13.
Bacteria —UV radiation and chlorination/dechlorination, when applied with the goal of complying with
conventional effluent discharge regulations, are similar in terms of their ability to inactivate water-borne
bacteria, although regrowth is more likely in chlorinated effluents than in UV radiated effluents. Bacterial
spores are extremely resistant and many of the chemical disinfectants normally used will have little or no
effect (WERF, 2005). Sporular bacteria forms are always far more resistant to ozone disinfection than
vegetative forms, but all are easily destroyed by relatively low levels of ozone (USEPA, 1999b).
Protozoa —The resistance of Giardia cysts to chlorine has been reported to be two orders of magnitude
higher than that of enteroviruses and more than three orders of magnitude higher than that of enteric bacteria
(USEPA, 1999b). Chang et al. (1985) reported that the UV dose necessary to cause 99% inactivation of
Giardia was within the operating range of many UV systems, but it was beyond the usual operating dose.
According to Chang et al. (1985), the extreme resistance of Giardia makes it unlikely that normal UV
radiation procedures would be sufficient to destroy cysts.
Cryptosporidium oocysts are resistant to chlorine-based disinfectants at the concentrations and contact
times practiced for water treatment (Clancy et al., 2004). Cryptosporidium oocysts are approximately 10
times more resistant to ozone than Giardia . Ozone is very effective towards Pseudomonas aeruginosa ,
moderately effective toward Giardia , and substantially ineffective toward Cryptosporidium (USEPA,
1999b). Several recent studies have shown that UV is highly effective at relatively low doses (10 mJ/cm 2 )
for control of Cryptosporidium (Geosyntec, 2008) .
Reactivation of Giardia and Cryptosporidium after ozonation is unlikely to occur (Paraskeva and
Graham, 2002).
Viruses —Although viruses cannot replicate outside their host's cells and, therefore, cannot multiply in
the environment, they can survive for several months in fresh water and for shorter periods in marine