Water Quality Management - River Dynamics and Integrated River Management - page 604

to better represent the variability and uncertainty that exists for each input parameter (USEPA, 1997a).
As is discussed in more detail in Section 9.5.5, the major wastewater treatment plants in the Chicago area
currently do not disinfect their effluent so a microbial human health risk assessment for recreational use
of the Chicago Waterway System was recently completed by Geosyntec (2008). For the Chicago Waterway
System, 1,000,000 combinations of weather condition (wet or dry), pathogen concentration, recreation
type, duration, ingestion rate, and dose-response were considered for each pathogen to determine health
risks (Geosyntec, 2008). Table 9.11 lists the number of illnesses per 1000 exposures for combined wet
and dry weather samples for different recreational uses downstream from the major wastewater treatment
plants. For all designated recreational uses evaluated, the risks of developing illness were less than the
proposed USEPA (2003a) limit of 14 illnesses per 1000 exposure events for freshwater recreational use
including primary contact. This example shows the great potential value of microbial risk assessment in
determining the true human health risks posed by pathogens in water bodies.

Table 9.11 Estimated illness rates for combined wet and dry weather samples assuming single recreational use with
no effluent disinfection downstream from each major wastewater treatment plant in the Chicago Waterway System
( after Geosyntec, 2008)

Illnesses per 1000 exposures

Recreational use North Side Stickney Calumet

Canoeing 2.45 3.19 0.52

Fishing 1.42 1.90 0.31

Pleasure boating 0.66 1.05 0.14

9.4.3 Disinfection Methods
Pathogen sources include: faulty sewage disposal systems, combined and sanitary sewer overflows, wild
and domestic animal waste, illegal discharges to drains and sewers, storm water runoff, and treated, but
non-disinfected wastewater effluent. Among these the primary sources of human pathogens are those
containing human waste that reaches rivers through urban sanitary sewers, combined sewers, groundwater
leaching from septic fields, among other pathways. Therefore, the primary means of controlling pathogens
is disinfection of wastewater. For example, once water disinfection by compounds of chlorine (1908 in
the U.S.) and by chlorine itself (1911 in the U.S.) was added to wastewater treatment the incidence of
waterborne typhoid fever was driven substantially to the vanishing point at less than 1 per million in
communities of the U.S. and Europe (Fair et al., 1971, p. 283). While chlorination of effluent has greatly
contributed to public health around the world it also has many potentially negative side effects, which led
the USEPA in 1976 to delete the fecal coliform standard from its definition of secondary treatment,
stating that the benefits achieved by disinfection by chlorine should be weighed against the environmental
risks and costs. Further, other methods of disinfection—ultraviolet (UV) radiation and ozonation—were
developed in the latter half of the 20 th Century that have benefits over chlorine and should be considered
for possible application to treated wastewater effluent.
The Water Environment Research Foundation (WERF, 2005) found because of post-disinfection regrowth
of bacteria, relatively poor virucidal behavior, and generation of persistent disinfection by-products
(DBPs), it is not clear that wastewater disinfection always yields improved effluent or receiving water
quality. Most disinfectants are strong oxidants, and can generate oxidants, such as hydroxyl free radicals,
as by-products that react with organic and inorganic compounds in water to produce DBPs. In applying
any disinfectant, it is important to strike a balance between risks associated with microbial pathogens and
those associated with DBPs. The following subsections describe the advantages, disadvantages, problems,
and effectiveness of the various disinfection methods.