Water Quality Management - River Dynamics and Integrated River Management - page 605

Chlorination —Done properly, chlorination following secondary treatment of wastewater will inactivate
more than 99% of the pathogenic bacteria in the effluent. The chief elements in the use of chlorination
have been its efficiency and economy compared to other means of disinfection. However, viruses and
parasites found in municipal wastewater, whether primary or secondary, are characterized as being much
more resistant and have different sensitivities to chlorination. Chlorine disinfection can inactivate some
viruses in wastewater, but not as effectively as it does in drinking water because of interference by dissolved
organics and suspended particles. Currently no data are available to demonstrate that Giardia cysts are
inactivated during chlorine-based disinfection of secondary effluents (Geosyntec, 2008). Studies on infectivity
of Cryptosporidium have found no inactivation due to chlorination of even highly treated wastewaters
(WERF, 2005).
One of the primary problems with chlorination of wastewater effluent is that unless ammonia-nitrogen
is removed from wastewater, the predominant form of chlorine will be chloramines, which are generally
regarded as being less effective against viruses and parasites than free chlorine (WERF, 2005; USEPA,
1999b). There is very little inactivation of viruses with chloramines. Further, chlorination of secondary
effluents produces insufficient virus reduction because suspended solids and turbidity in secondary effluents
furnish a means by which particles may be mechanically protected from the inactivating effects of chlorine
(Lue-Hing et al., 1976).
Other key problems with chlorine include (Lue-Hing et al., 1976):
(1) Residual chlorine and certain chlorine-based compounds formed as a result of the chlorination of
domestic wastewater can be toxic to aquatic life in low concentrations.
(2) Chlorination may from carcinogenic compounds in secondary effluents.
Both the free and combined chlorine (chlorine available as chloramines) are toxic to fish and other
aquatic organisms at concentrations down to 0.002 mg/L. Available scientific evidence indicates that 0.05
to 1.2 mg/L of chlorine as chloramines is toxic to aquatic organisms under average water quality conditions
(Lue-Hing et al., 1976). In addition to toxicity, chlorine compounds can repel and deny spawning grounds
to anadromous fish (Sedita et al., 1987). Studies have shown that dechlorination is capable of removing
87% to 98% of residual chlorine, but the remainder, which may exceed regulatory limits, was very slowly
reduced (Geosyntec, 2008). Thus, in effluent dominated streams where little flow is available to dilute
chlorine compounds in wastewater treatment plant effluent toxic conditions for fish may exist.
Sedita et al. (1987) documented large changes in fish communities after chlorination was ended at the
wastewater treatment plants discharging to the Chicago Waterway System on April 1, 1984. For example,
at Touhy Avenue (1.45 km downstream of the North Side Wastewater Treatment Plant) a grand total of
only six species (20 individual fish) were collected in six collections during 1974 to 1980, whereas during
one collection on November 5, 1984, nine species of fish and 115 individuals were collected. Further at
Peterson Avenue (3.86 km downstream) only two fish species and nine individual fish were collected in
six collections during 1974 to 1980, whereas during one collection on October 30, 1984, 11 species of
fish and 366 individual fish were collected. Lue-Hing et al. (1976) noted the major wastewater treatment
plants (Stickney, Calumet, and North Side) discharged, during dry weather, over 5.3 million m 3 /day of
chlorinated secondary effluent to the Chicago Waterway System, which at the state permissible level of 1
ppm residual chlorine represent a discharge of nearly 5 t of chlorine and chlorine compounds daily. The
removal of this chlorine load was considered to be the reason for the recovery of the fish population.
Trihalomethanes (THM), mainly chloroform (CHCl 3 ), bromodichloromethane (CHBrCl 2 ), dibromo-
chloromethane (CHBr 2 Cl), and carbon tribromide (CHBr 3 ) account for the majority of chlorine DBPs on
a weight basis. Most of these THMs are suspected carcinogens. Haloacetic acids are the next most significant
fraction, accounting for about 25% of chlorine DBPs. Aldehydes account for about 7% of chlorine DBPs
(USEPA, 1999b). More than 500 DBPs have been reported in the technical literature, but only a limited
number of them have been studied for adverse health effects (Geosyntec, 2008).