Page 480 - 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 438

438 Chapter 12 Urban Runoff and Combined Sewer Overflow Management
a necessary control alternative because of the high volume and variability associated with
storm and combined sewer overflows. Storage facilities are frequently used to attenuate
peak flows, thereby reducing the size of facilities required for further treatment. Storage
techniques include in-line and off-line storage.
In-line storage uses existing capacity in major combined sewer trunk lines or intercep-
tors to store combined flows. During storms, regulators are used to cause flow to back up
in the system allowing it to be stored there. Although not all flow can be stored in the sewer
system, this practice can reduce overflow volumes during large storms and eliminate over-
flow volumes during small storms. Care must be taken to ensure that flows do not back up
onto streets or into homes.
Off-line storage consists of constructed near-surface or deep tunnel detention facili-
ties. Near-surface facilities usually consist of concrete tanks or, in some cases, large con-
duits that convey flow to a treatment facility. Tunnels can provide large storage volumes
with relatively minimal disturbance to the ground surface, which can be very beneficial in
congested urban areas. Overflows are directed to the storage facility, held during the storm,
and pumped to the publicly owned treatment works (POTW) after the storm, thus reducing
the overflow quantity and frequency.

12.6.4 Treatment
Treatment of urban runoff water includes preliminary treatment, flotation, filtration,
chemical precipitation, biological treatment and disinfection.
1. Preliminary treatment. Most of the urban runoff BMPs previously discussed em-
ploy physical processes to reduce pollution. Physical treatment practices can also
be used to reduce pollutant discharges from CSOs. These practices use screening
technologies to reduce the flow of solids in combined systems. They are typically
used as a preliminary treatment step to remove floatables upstream of other
processes. Different screens have different size openings to provide various levels
of solids removal. Bar racks have the largest openings (typically 1 inch or more)
and microstrainers have the smallest openings (typically as small as 15 microns).
All of these practices require periodic and regular cleaning to prevent the accumu-
lation of solids.
2. Dissolved air flotation. Dissolved air flotation (DAF) removes solids from waste-
water by introducing fine air bubbles, which attach to solid particles suspended in the
liquid, causing the solids to float to the surface where they can be skimmed off.
Because of its relatively high overflow rate and short detention time, DAF does not
require as large a facility as conventional sedimentation. Oil and grease are also more
readily removed by dissolved air flotation. The high operating costs for DAF are due
to large energy demand; in addition, skilled operators are required for its operation.
3. Filtration. Dual-media high-rate filtration has been used for treatment of CSO
flows using a two-layer bed, consisting of coarse anthracite particles on top of less
coarse sand. After backwash, the less dense anthracite remains on top of the sand.
2
2
Filtration rates of 8 gal/ft /min (325 L/m /min) or more result in substantially
smaller area requirements compared with sedimentation.
4. Chemical precipitation. Chemical precipitation facilities store and use polymer,
alum, or ferric chloride to cause solids to precipitate. Chemical precipitation can
increase the pollution removal that generally occurs from other settling practices,
thereby allowing for the design of smaller sedimentation tanks. Removal rates for
these practices are up to 70% for biochemical oxygen demand (BOD) and 85% for