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12.6 Combined Sewer Overflow Control Practices 439
suspended solids. Chemical precipitation generates more sludge buildup and han-
dling can become a major problem.
5. Biological treatment. Although biological treatment processes have the potential to
provide high-quality effluent, the disadvantages of biological treatment of CSOs
include the following:
• The biomass used to break down the organic material and assimilate nutrients in
the combined sewage must be kept alive during dry weather, which can be diffi-
cult except at an existing treatment plant; biological processes are subject to
upset when exposed to intermittent and highly variable loading conditions.
• The land requirements for these types of processes can preclude their use in
urban areas.
• Operation and maintenance can be costly and the process requires highly skilled
operators.
6. Disinfection. Because pathogens are the primary pollutant of concern in CSO con-
trol, practices focusing on disinfection are commonly used:
• Chlorination. Combined flows can be treated with dissolved or gaseous chlorine
to reduce the level of pathogens in the flow. Chlorination is typically used in
conjunction with upstream solids removal. Dissolved chlorine (hypochlorite) is
currently more commonly used than gaseous chlorine because the equipment is
more reliable and storage of the chemicals is safer. Dechlorination might be nec-
essary to minimize the adverse effects of chlorine on aquatic life. With sufficient
dosage and mixing, close to 100% destruction of pathogens is possible.
• UV radiation. Introduction of ultraviolet radiation to combined wastewater is de-
signed to provide disinfection without the addition of harmful chemicals. This
practice uses an ultraviolet lamp submerged in a baffled channel located down-
stream of an effective solids removal process.

CASE STUDY 12.1 CITY OF AUSTIN, TEXAS, LOCAL WATERSHEDS ORDINANCES
Austin, a highly urbanized city bisected by the Colorado River, contains a number of high-quality
lakes, aquifers, and streams. The major water resources in the area include three lakes—Lake
Travis, Lake Austin, and Town Lake—which form a major drinking water reservoir acting as the
main water supply for the city; Edwards Aquifer and Barton Springs are the area’s other major
water resources. These water resources are potentially threatened by urban runoff pollution from
urbanized areas; Town Lake has already been affected significantly. To reduce and prevent urban
runoff pollution problems in these resources, Austin has developed and passed three major water-
shed ordinances: the Comprehensive Watersheds Ordinance in 1986, the Urban Watersheds
Ordinance in 1991, and the Barton Springs Ordinance in 1992.
The primary goal of these ordinances is to protect the water resources of the Austin area from
degradation from nonpoint source pollution. Other goals include preventing the loss of recharge to
the Edwards Aquifer, preventing adverse impacts from wastewater discharges, and protecting the
natural and traditional character of the water resources in the Austin area. In addition, the city has
implemented other ordinances that control nonpoint source (NPS) pollution.
Water pollution problems in the Austin area have been extensively studied since the mid-
1970s. In 1981, the city participated in the Nationwide Urban Runoff Program (NURP) and began
implementing and monitoring the effectiveness of urban runoff structural controls. The city has
been a leader in developing and implementing NPS regulatory controls. The city’s first NPS control
ordinance, the Lake Austin Watershed Ordinance in 1978, was followed by other watershed ordi-
nances in 1981 and 1984 designed to protect additional sensitive watersheds and upgrade the level
of protection. The experience and data gathered as a result of these ordinances led the city to