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Description of "the problem." Sewer systems,
overflows, and floatables
Q. What is a combined sewer system and how does it affect
water quality?
A. A combined sewer system (CSS) carries both sewage and
stormwater runoff. Normally, its entire flow goes to a waste treatment
plant, but during a heavy storm, the stormwater volume may be so
great as to cause overflows. CSSs are designed to allow overflows
(a Combined Sewer Overflow or CSO) under wet weather conditions
when system capacity is exceeded. Storm water flows are greater
than the sanitary sewage flow; therefore small rainfall events can
cause the system to exceed its capacity and overflow. When this
happens, untreated mixtures of stormwater and sewage may flow into
surface waters. Control of this unsanitary water pollution is a
national environmental priority.
Q. What are floatables?
A. Floatables are street litter and other trash that enter
the storm sewer systems when it rains. In a combined sewer system,
raw sanitary sewage is also present. Floatables can also result
from a dry weather overflow of a sanitary sewer (a Sanitary Sewer
Overflow or SSO). In several highly publicized instances, medical
waste has represented a particularly dangerous form of floatable.
This unsanitary and unsightly mix is discharged into receiving
waters in an overflow condition. Floatables can cause injury
to bathers and boaters; choke, strangle, and suffocate freshwater
and marine fish, animals and birds; and impair the aquaculture
in the sea, lake or river where their discharge occurs. Floatables
can lead to beach and shellfish bed contamination and closings,
and restricted use of recreational waters and beaches. Control
of floatables is an important element of the national CSO control
policy.
Q. What is a stormwater outfall and how does it affect water
quality?
A. Stormwater is runoff water from rain or snow. Stormwater
flows into underground sewers (typically a Municipal Separate Storm
Sewer System, or MS4) that outfall into a waterway (e.g. river,
stream) or in certain cases to a sewage treatment plant. In a wet
weather event, when flow volume exceeds the capacity of the treatment
plant, the stormwater overflows into the receiving water. While
stormwater overflows do not include sanitary waste, they carry high
levels of floatables and can also include pollutants such as pathogens,
nitrogen, phosphorus, sediment, heavy metals, pesticides, herbicides,
and synthetic organic compounds. Stormwater discharges also pose
a threat to aquatic ecosystems by physically altering the receiving
waterbody.
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