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What is a Combined Sewer Overflow?
The majority of New York City’s sewer system is combined, which means it is used
to convey both sanitary and storm flows. Sometimes, during heavy rain and snow
storms, combined sewers receive higher than normal flows. Treatment plants are
unable to handle flows that are more than twice their design capacity and when this
occurs, a mix of excess stormwater and untreated wastewater discharges directly
into the city’s waterways at certain outfalls to prevent upstream flooding. This is
called a combined sewer overflow (CSO). CSOs are a concern because of their
effect on water quality and recreational uses in local waterways.
Combined Sewer Outfall(Normally flowing only during wet weather)
Combined Sewer
Dry weather flow
Wet weather flow
Flow to the treatment plant
Tide gate to keep tidal inflow out of the sewer system
During wet weather , combined sewage may over flow through the combined sewer outfall
Hood to prevent floatables from entering the sewer system
Dr y weather flow channel
Sanitary Sewer
Roof Drain
CatchBasinOverflow
Combined Sewer System
Combined Sewer Overflowsin New York City
Man
hatt
an
Bronx
Queens
Brooklyn
Staten Island
Signs are placed near every CSO outfall location
Combined Sewer Overflow (CSO) VolumesTier 1
Tier 2Tier 3
= 50% of Total CSO Volume
= 20% of Total CSO Volume
= 10% of Total CSO Volume
Paerdegat Basin CSO Retention Facility Gowanus Canal Pump Station & Flushing Tunnel
Flushing Bay CSO Retention Facility English Kills In-Stream Aeration
Various Ongoing & Completed CSO Projects
Current Water Quality Standards
New York StateSaline Surface Water Quality Standards
Bacteria (when disinfection is practiced)Class
Total Coliform Fecal Coliform Enterococci
Dissolved Oxygen
Median< 70 MPN/10 mL
__ Geometric mean< 35/100 mL
> 4.8 mg/l (daily avg)> 3.0 mg/l
Monthly median< 2,400/100 mL80% < 5,000/100 mL
Monthly geometric mean< 200/100 mL
Geometric mean< 35/100 mL
> 4.8 mg/l (daily avg)> 3.0 mg/l
Monthly median< 2,400/100 mL80% < 5,000/100 mL
Monthly geometric mean< 200/100 mL
Geometric mean< 35/100 mL
> 4.8 mg/l (daily avg)> 3.0 mg/l
Monthly geometric mean< 10,000/100 mL
Monthly geometric mean< 2,000/100 mL
__ > 4.0 mg/l
_
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__ __ __ > 3.0 mg/l
SA
SB
SC
I
SD
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Public participation Consideration of sensitive areas
Evaluation of alternatives to meet CWA requirements
Cost/performance considerations
Operational Plan Maximizing treatment at the existing wastewater
treatment plant
* As defined by EPA CSO Control Policy
Implementation schedule
Post-construction compliance
monitoring program
Characterization, monitoring, and modeling of the combined
sewer system
Nine Elements of a Long Term Control Plan*
N .Y.C . S E W E R
$
NYC's Combined Sewer Areas and LTCP Waterbodies
Coney Island Creek
HutchinsonRiver
Alley Creek
Newtown Creek
Gowanus Canal Jamaica Bay
and Tributaries
Flushing Creek
WestchesterCreek
Flushing Bay
Bronx River
East River and
Open Waters
Combined Sewer WatershedsOther (Separate Sewers, Direct Drainage, Unsewered Areas)