Page 478 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch12_398-456.qxd 8/4/10 9:37 PM Page 436

436 Chapter 12 Urban Runoff and Combined Sewer Overflow Management
12.6 COMBINED SEWER OVERFLOW CONTROL PRACTICES

Some of the urban runoff BMPs discussed above are applicable to combined sewer over-
flow (CSO) control. Additional control practices commonly used for CSO control are de-
scribed in this section, including a general discussion of each practice’s applicability, its
pollutant removal effectiveness, and its maintenance requirements. Because CSOs contain
sanitary wastewater and other waste streams, the primary pollutants of concern in CSO
control are suspended solids, biochemical oxygen demand, and pathogens. CSOs, how-
ever, also contain nutrients, metals, and other toxic substances.

12.6.1 Source Controls
12.6.1.1 Water Conservation Programs One way to reduce the amount of sanitary
wastewater in a combined system is to attempt to control the amount of water used by
homes and businesses that is then converted to wastewater. Typical programs and practices
for control include the following:
• Plumbing retrofit: Using low-flush toilets, flush dams, faucet aerators, and other
water-saving devices
• Plumbing code changes: Requiring implementation of water-saving devices in new
construction or as plumbing is replaced
• Education programs: Encouraging water conservation in businesses and homes by
providing information on its benefits
• Technical assistance: Providing water-use audits or case studies demonstrating po-
tential savings to businesses
• Rate system modifications: Adjusting rate systems to promote or reward water savings.

Although these programs might require minor changes in personal habits, they can be
cost effective compared to end-of-pipe treatment. There are limits, however, to the reduc-
tions in water use that can be achieved reasonably.

12.6.1.2 Pretreatment Programs Pretreatment programs are implemented at the local level
to control industrial and commercial sources of wastewater discharging to a municipal sewer
system. The goals of a local pretreatment program are to stop or prevent industrial and com-
mercial pollutants from passing through a municipal wastewater treatment plant, thereby vio-
lating state water quality standards; to stop or prevent disruption of treatment plant operations
caused by industrial and commercial pollutants, including the contamination of municipal
treatment plant residuals; and to ensure the safety of municipal sewer system and treatment
plant workers by minimizing their exposure to potentially dangerous or toxic pollutants.

12.6.2 Collection System Controls

Many collection system controls exist for addressing pollution from CSO discharges.
These controls focus on modifying the sewer system to reduce CSO flow, volume, and
contaminant load.

12.6.2.1 Sewer Separation One method for addressing CSO pollution is to convert the
combined collection system to separate stormwater and sanitary sewer systems by construct-
ing a new separate sanitary sewer. Sewer laterals from homes and businesses are then con-
nected into the new system. Inappropriate connections to the old system from buildings are
plugged. This conversion eliminates the possibility of sanitary wastes entering the drainage