FAQ

What does “CSO” stand for?
What type of sewer system(s) does Philadelphia have?
Where does the water from my house go?
What is the difference between combined sewers and separate sewers?
During dry weather, where does the water in separate sewers and combined sewers go?
During wet weather, where does the water in separate sewers and combined sewers go?

What causes combined sewer overflows? Is it the capacity of the treatment plant or the interceptor pipes?
Why do Combined Sewer Overflows (CSOs) exist?
Approximately how many times per year do CSO overflows occur?
Are overflows more frequent in the summer or winter?
What is extent of the local CSO problem? Is there degradation of the local ecosystem?
What is PWD’s approach to manage the CSOs?
What are some of the main components of PWD’s Collection System?
What is a CSO regulating chamber?
What is the amount of impervious cover in Philadelphia?
What happened to most of our creeks that historically existed in Philadelphia?
Where can I find information on tides?

What does “CSO” stand for?

The acronym “CSO” stands for Combined Sewer Overflow.

What type of sewer system(s) does Philadelphia have?

Philadelphia is served by two types of sewer systems. About 60% of the City’s sewered area is served by combined sewers, typically the older sections of the city. The other 40% of the sewered area is served by separate sewers.

Where does the water from my house go?

The Philadelphia collection system deals with two types of water from your house: your sanitary sewage and your stormwater. Your sanitary sewage is the wastewater from your toilets, shower, washing machine, dishwasher, etc. Your stormwater is the rainwater that is collected from your rooftop and transported via your gutters and downspouts. Additional stormwater is collected in the streets via inlets or stormwater basins. Where the water goes once it leaves your house/property depends on which type of sewer system you are connected to – separate sanitary system or combined sewer system.

What is the difference between combined sewers and separate sewers?
During dry weather, where does the water in separate sewers and combined sewers go?
During wet weather, where does the water in separate sewers and combined sewers go?

Combined Sewer Systems

In combined sewered areas, traditionally the older parts of the city, there is one sewer system that collects both stormwater runoff and wastewater from homes, businesses and industries. In dry weather and during smaller storms, these combined sewers bring wastewater or wastewater/stormwater mixture to wastewater treatment plants for treatment prior to discharge of treated effluent into the waterbodies.

However, during most large storms, when the sewer is inundated with large quantities of water, stormwater runoff, mixed with wastewater, is collected at a rate beyond the capacity of the treatment plant or interceptor sewer. As much of this water as possible is transmitted to the treatment plants, and the remainder is released to a nearby receiving stream without treatment and is referred to as a Combined Sewer Overflow (CSO). The water reaches the stream through a combined sewer outfall. There are 164 combined sewer outfalls in Philadelphia that may discharge to the Cobbs Creek, Tacony Creek, Delaware River, Schuylkill River or tidal Pennypack Creek.

Separate Sewer Systems

A separate sewer system consists of two different sewer pipes running one on top of the other, or “piggyback.” In most instances, the sanitary pipe is below the storm pipe. The sanitary sewer pipe transports the sanitary sewage collected from the laterals of the homes, businesses, and industry to the treatment plant. The stormwater sewer pipe carries stormwater flow collected from the street inlets, building downspouts, and other storm sewer lines to the nearby receiving stream and is discharged through a stormwater outfall.

There are stormwater outfalls in every watershed in Philadelphia - Schuylkill River, Delaware River, Cobbs Creek, Tacony Creek, Wissahickon Creek, Pennypack Creek, and Poquessing Creek. Philadelphia has approximately 455 stormwater outfalls.

What causes combined sewer overflows? Is it the capacity of the treatment plant or the interceptor pipes?

Depending on where the rain falls within the City, either treatment plant capacity, interceptor capacity, or the CSO regulating structure could be the limiting factor. If Philadelphia receives evenly spread, low-intensity rainfall, the treatment plants may be at their capacity without any interceptors at capacity. However, in the scenario that an inch of rain falls mostly within several sewersheds, the interceptor may be at capacity but not the treatment plant. Additionally, most regulating structures are designed with a static dam, meaning if the water level rises above the dam height, it overflows. Although the dam heights are designed to overflow only when the interceptor has reached its capacity, you could theoretically overflow before that time. Philadelphia has several more advanced regulators with computer controlled gates that can better regulate the contribution each sewershed makes to its respective interceptor. For regulators that are tidally influenced, there can be no overflow even when the treatment plant, the interceptor, and the regulator are at capacity if the tide is holding the tide gate shut. This situation could result in street flooding.

Why do Combined Sewer Overflows (CSOs) exist?

Historically, the combined sewer systems were designed and sized to carry both wastewater and stormwater flow in the same pipe to the treatment plant. As the amount of impervious cover in Philadelphia increased, the amount of stormwater being captured by the collection system also increased. Due to the increase in stormwater runoff from development and poor land use management, combined with population (sanitary) increases, the capacity to carry all the flow in the existing sewer system can be exceeded. To avoid basement flooding, house fixture back-ups, and street flooding, the sewer system was designed to provide for overflow points that will send any flow in excess of treatment or carrying capacity to a nearby waterbody.

Approximately how many times per year do CSO overflows occur?

Philadelphia experiences, on average, 65 rainfall events per year. This is based on analysis of historical rainfall records from 1902 to 2005. Combined sewer overflow frequency depends on numerous conditions, including the size of the sewer and drainage area, precipitation, land use, antecedent moisture conditions, etc. Therefore, some CSOs may discharge every storm, while some discharge only several times a year. As for CSO outfall size, these may range anywhere from 2 ft. in diameter to 21 ft. x 24 ft. rectangular. The quantity, frequency, and impact are unique at each CSO, making it difficult to generalize. The Office of Watersheds' hydraulic and hydrologic models estimate annual CSO discharge at about 16 billion gallons from our 164 CSO point sources.

View CSOcast - our CSO Public Notification System.

Are overflows more frequent in the summer or winter?

In analyzing historical rainfall records, precipitation is found to be most frequent between April and September. In the summer, all precipitation is rainfall. In the winter, we can expect a mix of rain and snow. In some instances, a rain event occurs when there is snow pack present from previous snowstorms. When this occurs, the sewer system can simultaneously experience runoff from both the precipitation and from snowmelt. In addition, warm days when snow melts actually generate enough runoff to trigger an overflow event.

What is extent of the local CSO problem? Is there degradation of the local ecosystem?

It is difficult to quantify the extent of the local CSO problem and its relationship to degradation on the ecosystem. CSOs are permitted sites that are necessary for preventing health and human safety issues that may result from localized flooding. PWD is spending tens of millions of dollars per year to mitigate, abate, and/or eliminate CSOs.

Water quality issues vary, depending on CSO volume and frequency, land use in the drainage area, existing receiving stream water conditions, the stream's physical dimensions, season, antecedent weather conditions, future weather conditions, impervious percentage, tide cycles, etc. There are also erosion and sedimentation issues within the stream system that may cause habitat problems for fish and macroinvertebrate communities. Aesthetics, odor, and safety issues have also been linked to uncontrolled stormwater runoff.

What is PWD’s approach to manage the CSOs?

The Philadelphia Water Department's approach to long-term management and control of combined sewer overflows has been integrated with a comprehensive watershed management program. Through a watershed-based planning and management effort, the PWD has developed a control program with three broad goals:
LAND - Managing wet-weather flows at the source through improved land management practices
WATER - Direct restoration of the aquatic habitat to support living resources
INFRASTRUCTURE - The upgrade and expansion of infrastructure to further reduce CSO frequency and volume

These programs are integrated to provide multiple ecological, aesthetic, and social benefits.

What are some of the main components of PWD’s Collection System?

PWD has approximately 3,000 miles of sewer, 79,000 stormwater inlets, three drinking water treatment plants, three sewage treatment plants, more than 25 pump stations, 175 CSO regulating chambers, 164 CSO outfalls, more than 3,000 miles of water mains, and more than 450 stormwater outfalls.

What is a CSO regulating chamber?

A control structure, such as Brown and Brown regulator, side overflow weir, slot, dam, etc., contained within an underground vault, chamber, or manhole, that prevents excessive pipe surcharging within the combined sewer and prevents both street surface and basement flooding.

What is the amount of impervious cover in Philadelphia?

Roughly 54% of Philadelphia is considered impervious. Impervious is defined as a surface that water cannot penetrate. Examples include traditional street paving material, roofs, sidewalks and parking lots.

What happened to most of our creeks that historically existed in Philadelphia?

Historically, creeks were present throughout most areas of Philadelphia. The creeks enabled various industries to flourish on near its streambanks. But as Philadelphia grew, so did the pollution. Waste and trash was disposed of directly into the creeks – waste from slaughterhouses, used dye, trash, and sewage (sanitary waste). It was standard practice in the 18th and 19th centuries to use creeks as open sewers. At one point, the rivers and creeks were so filthy that they created a health hazard resulting in the deaths of thousands of Philadelphians. Creeks were also looked upon as obstacles to development, since it is impossible to construct houses, businesses and industries atop flowing streams and creeks. Disease epidemics and economics stemming from development drove many miles of creeks underground into pipes.

Where can I find information on tides?

Tide data can be found at:
http://tidesandcurrents.noaa.gov/station_retrieve.shtml?type=Historic%20...