Sewage Overflow


1. Combined Sewage Overflow Background (CSO)
2. Treatment Facilities
3. NYC Plan
4. CSO During Sandy
5. Prevention Methods
6. Global Examples
7. Citations

CSO Overview
A combined sewer is a sewage system that collects and disposes of both sanitary sewage and storm water runoff into sewage treatment works. It is called combined because in some more modern sewage systems, sanitary sewage and storm water runoff are not handled by one system but rather by two distinct pipes. The purpose of this system is twofold. One, it safely disposes of pollutants swept by a storm that are found on the ground, like pesticides, oil, and grease into treatment facilities. This system is also responsible for collecting domestic and industrial sewage. Under normal circumstances, the sewage is moved to a sewage plant and then safely disposed in a body of water.
Summary of what causes CSOs

Older cities, like New York, have a combined system. combined sewage overflow occurs when there is an overflow in the sewage system, causing the sewage to be disposed in a river stream, lake, or ocean without going to a treatment plant first. This overflow is experienced in the New York system practically every time it rains, but the problem is exacerbated with increased sea level rise and increased rainfall. Increasing these pollutants in these bodies of water can lead to water contamination and loss of wild life. Furthermore, combined sewage overflow can overwhelm domestic sewage pipelines, resulting in sinks and bathrooms not working properly or at all in the affected areas. Lastly, reduced water quality can lead to less swimming and boating space for residents.


Comined Sewage Overflow caused by increased rain

Interestingly, sometimes better infrastructure, such as paved roads, can exacerbate combined sewage overflow. While normally, excess water would be absorbed by the ground and not moved to bodies of water, paved streets insure that water will easily flow, and therefore the pollutants transported by the runoff will end up in a gutter and then enter the combined sewage system, causing a buildup. For instance, from 1984 to 2002, 9,000 acres of trees, bushes and vegetative were paved over in New York City . According to a government estimate, this land land could have absorbed 243 million gallons of water.

In New York City, there has been a movement towards containing combined sewage overflow with a detail plan published by the city in 2011.The 1972 Clean Water Act established pollution quotas  and provided grants for helping reach those quotas.

Screen Shot 2012-12-06 at 11.06.48 PM

Quality of water in NYC today  (PlaNYC2011,

NYC Treatment plants

Primary Treatment

The New York City Department of Environmental Protection has published an extensive document concerning sewage plants and how they function. First the incoming water, or influent, is passed through  preliminary treatment. The influent passes through upright bars, like screens to remove any large debris.This cleans the water and protects the equipment. Next, water stops in pools, allowing heavy material to sink to the bottom. Floating trash is skimmed from the top. The sludge from the bottom is removed and moved to landfills via trucks.


Secondary Treatment

Air and seed sludge is added to the water allowing for the growth of bacteria. These microorganisms consume most organic material and produces heavier particles that will settle later. Again, water is moved to tanks where residue is removed from both the top and the bottom.The water is then disinfected in chlorine tanks. Next, Oxygen is removed from the water, and the water is heated, allowing anerobic bacteria to grow and digest more of the harmful microorganisms in the water. Methane gas is used as an energy source in these plants. The water is then “dewatered” by entering spinning factories. This removes most of the water from the sludge. After the water is deemed pure, it is released into the waterways through ducts. The debris and pollutants removed in each of these processes are again moved to landfills via trucks.

Map of drainage areas among 14 treatment plants throughout New York City. This interactive map titled “Where Does My Toilet Flush To?” details both water decontamination at one the facilities as well as discharge of untreated sewage into NYC waterways.


The city published a detailed plan to deal with sewage overflow:


1.Upgrade waste water treatment plants to achieve secondary treatment standards:

In this initiative, the city pledged over 5 billion dollars to upgrade the capacity of the Newtown Creek Wastewater Treatment Plant. This upgrade will enable the plant to treat 700 million gallons per day, up from 620 gallons per day. This will ensure effluent discharge
requirements of the Clean Water Act. Furthermore, increasing the capacity of water handling will prevent overflow in times of storms.

2.Upgrade treatment plants to reduce nitrogen discharges

Nitrogen discharge is not directly harmful to humans. However, nitrogen causes Algae to boom, which in turn, causes oxygen to be depleted from the water. This can cause an array of different living species to be negatively affected. Therefore, the city has pledged $770 million worth of upgrades at the Bowery Bay, Tallman Island, and Wards
Island waste water treatment plants to reduce nitrogen discharges into the East River by more than 50%. These changes are planned for over the next ten years (from 2011, when the plan was published). The city has paired up with different agencies, such as the New York State Department of Environmental Conservation (State DEC)and the Natural Resources Defense Council, to implement new nitrogen removing tools in the city’s sewage treatment facilities.

3.Complete cost-effective grey infrastructure projects to reduce CSOs and improve water quality:

Another method of prevention relies on large dams to temporarily hold the water and pump the water back to treatment centers once the storm has passed. The city plans on investing $2.9 billion over the next year in building these kind of dams. A notable accomplishment of this initiative is  a  a facility built at Paerdegat Basin that retains  1.7 billion gallons of sewage overflow per year.  Paerdegat Basin now meets dissolved oxygen standards by 90%   and pathogen standards by 100%.

CSOs will be reduced by more than 8.2 blillion gallons per year. This will occur by increasing the pumping capabilities of pump stations in coney Island and Alley Creek in Queens.

The Gowanus Canal Flushing Tunnel has been out of of full capacity since the 1960s. $50 million will be invested to bring reactivate the tunnel ,and 3 pumps will be constructed to further increase the water quality. The pumps and tunnel will bring oxygen-rich water from the Buttermilk Channel into the canal to dilute the CSOs and increase water into the canal by 40%.
4. Expand the sewer network

Expanding the Sewer network allows the sewer system to have more space for water before the need for overflow. Since 2002, the city has spent over $55 million to construct a separate sewer for water for  the Rockaway  Peninsula.This is indicative of a more modern sewage system, moving away from the combined model.High Level Storm Sewers
(HLSS) partially separate the flow in combined sewers and captures 50% of rainfall and diverts it to water ways .
5. Optimize the existing sewer system

Ideally, new sewers that separate rain sewage should be made; However, this proves to be extremely costly. Therefore, this initiative tries to optimize the current system.

Catch basins are structures that help prevent street debris from reaching the city’s sewers. This serves the dual purpose of preventing the blockadge of the sewer system as well as keeping debris away from the city’s beaches. Out of the cities 144, 000 catch basins, 2,350 are in need of repair. The city hopes to have these repaired by 2014.

Tide gates cover the CSO exit points, and are in need of repair. Damage to these gates can allow extra waste to leak out and salt water to leak in. To fix this problem, the city implemented a group that inspects 25 of these tide gates per month.

Furthermore, in 2010, the city began a program to clean all sewer interceptors within two years. Sonar and video recordings will be used to further asses such damage. Once the initial cleaning of the sewer is completed, their will be an attempt to create a permanent solution to prevent blockage.

6. Several initiatives in PlaNYC target green infrastructure as a mode to alleviate some of the pressure placed on the waterways and plumbing system. The Bluebelt system is a cost-effective sustainable storm water management to improve water quality by absorbing storm water runoff. This eliminates costly sewer systems as the natural system replace traditional infrastructure saving taxpayers close to $80 million.  Over 14,000 acres of Staten Island are covered by the program currently and will expand to other areas as well.

7. Green infrastructure projects must be increased to reduce runoff and absorb water before it enters the sewage system. In addition vegetation and trees will absorb excess CO2 in the atmosphere to leave less of the anthropogenic emissions in the air. To control runoff, several projects are underway including tree pits to hold water in pools underground. Blue roofs filter the water slower so it doesn’t overwhelm the sewers. Additionally, permeable pavement allows water to seep through the ground rather than flowing as runoff.

8. Local communities are encouraged to support green infrastructure solutions and grants were given (in 2009) in the amount of $2.6 million in the Flushing and Gowanus areas to fund green infrastructure construction. In 2011, an additional $3 million was given to private property owners, businesses and not-for-profit organizations for the green infrastructure cause. Funds were allocated towards water source control and sewer drainage areas.

9. Code changes and zoning amendments also contribute to green infrastructure. By changing guidelines for parking lots and buildings the hope is that less storm water will enter the sewers.

10.Incentives also are in place to partner with the city in reduction of CSO via a credit system to reduce storm water entrance into the sewers. Increased costs also serve as a disincentive for parking lots which stand-alone and are accountable for an increased burden on the system. Instead of being paid by others, the cost is factored into a charge per square foot. The private industries must team up with the public sector to attend to the disrupted sewer system.

11. Actively Participate in  Clean Up Effort – NYC plans to work with the EPA and the state DEC to investigate the cause of the contaminations and find possible solutions. The EPA has designated the Gowanus Canal and Newton Creek as superfund sites in 2010. Even before this, NYC has begun major upgrades to these sites such as expanding the capacity of the Canal’s pumping station and installing equiptment at the Newton Creek to increase oxygen levels in the water. NYC also plans to investigate CSO related sediments at the bottom of many other sites.

Protect and Restore Wetlands, Aquatic Systems and Ecological Habitat

Wetlands are a biologically rich intersection of land and waterways. They act as a natural filtration system by slowing and retaining stormwater that can harm the nearby waterways. They also mitigate the impacts of erosion on the coastline. There has been a significant loss of wetlands and NYC plans to actively restore these natural systems.

12. Enhance Wetlands Protection – In 2005, NYC formed the Wetlands Transfer Task Force to asses  wetlands in City-owned properties. In 2007, they came out with a report that recommended 82 parcels for transfer to the Department of Parks and Recreation, along with 111 for further review. As of 2008, NYC has completed reviews of the 193 parcels, including field inspections. Because wetlands frequently expand and detract, the report showed that accurate mapping of the wetlands is crucial for protecting the most vulnerable wetlands.

13. Restore and Create Wetlands – At Alley Pond Park in Queens, NYC recently completed 16 acres of restoration to revive the local ecosystem and improve water quality. They plan to create 38 acres of new and restored habitat along Paerdegat Basin near Jamaica Bay. NYC work with state and federal partners has resulted in the completion of over 165 acres of restored or enhanced wetlands since 2002. They plan to invest $54 million at 17 sites to enhance 58 acres of wetlands.

14. Improve Wetlands Mitigation – Mitigation in NYC is often not practical because of the lack of space around the mitigation sites and disturbance to the areas in the city. They are also not using their money as efficiently as possible. NYC is now trying new mitigation mechanisms. One alternative is in-lieu fee mitigation, which allows wetlands loss to be mitigated by paying a fee to a fund that then aggregates payments to larger restoration projects. Another option, mitigation banking, uses a similar approach by encouraging large-scale wetland restoration projects to generate “credits” that can be transferred to compensate for wetlands loss within a predetermined area.

15. Improve Habitats for Aquatic Species- The New York Harbor used to be filled with oyster, eelgrass and muscles. NYC has launched a plan to test  if they can reintroduce these three species into the harbor in order to recoup lost water quality benefits that they provided.


Screen Shot 2012-12-06 at 10.14.38 PM

PlaNYC cost projections (PlaNYC 2011,

CSO During Sandy

Treatment facilities during the storm suffered damage and several leaked sewage for many hours. Estimates indicate that 10 out of 14 plants suffered damage. Staten Island’s plant did put forth precautions such as generators to continue running during the storm. Despite being on the coastline, the plant was functional due to its elevation of wires and machinery.  Discussions are ongoing to mandate minimum height levels of electronic equipment as well as waterproofing circuitry. Nassau’s Bay Park facility provides services for 40% of the population in Nassau and was inactive for over 50 hours sending over 200 million gallons of untreated sewage into waterways. Nassau county was fined $1.5 million last year for illegally pumping sewage into  East Rockaway Channel.



A construction worker  assessing damage to the city’s waste infrastructure after Sandy


To improve infrastructure of crippled treatment plants, Governor Cuomo estimated $1.1 billion in repairs. US Senate hearing on water infrastructure as Environment and Public Works Committee advocating for federal funding. U.S. Clean Water Act does require permits to dump sewage in waterways, but it has been violated on many occasions. As a result of a fire in Harlem’s Treatment facility, Governor Cuomo signed into law “Sewage Right To Know” but it doesn’t take effect until May 1, 2013 so it couldn’t help the public during Hurricane Sandy.

Gowanus neighborhood in Brooklyn is a frequent victim of industrial pollution and sewerage discharge.  Neighboring a 1.8 mile canal, Gowanus was inundated with debris, toxic chemicals, and raw sewerage threatening the health of is residents as a result of Hurricane Sandy. When sewers overflow, it normally is discharged into waterways with little human contact, but the waste has found its way back to densely inhabited streets and buildings. Flooded basements were full of a “brown, murky soup” carrying a “putrid” odor. Water levels rose to a record 13 feet during the disaster. Residents were recommended to avoid any contact with the water, especially consumption of the water. Although plumbing filters the sewerage wastewater and rainwater to treatment plants, toxic chemicals continue to be a long-term concern.

As little as a tenth of inch of rain is enough to burden the antiquated plumbing pipes diverting over 20 billion gallons of contaminated water to be poured into designated bodies of water. The pollution and sewerage can settle and remain visibly and by odor. Toxic chemicals carried by the CSO breed pathogenic microorganisms and leave people vulnerable to contamination. New York, as dictated in the PlaNYC initiative, has invested to modernize archaic pipes as well as storage tanks to mitigate the burden placed on the waterways. Green infrastructure is also being used to intercept rainwater before it reaches the plumbing system.

Sewage Works In Osaka Japan — Global examples

In the 1970’s sewage system construction in Osaka city, Japan has been greatly promoted and since then the sewerage works have been dramatically improved. The sewerage system serves almost the entire city area. There are 12 sewage treatment plants that provide secondary treatment. Then BOD (Biochemical Oxygen Demand) concentration is reduced from 130mh/l to 10mg/l. The water is then discharged to public water bodies. However, not all rivers meet Environmental Water Quality Standards and sometimes red tide does occur in Osaka bay.

The Hirano Sewage Treatment Plant uses advanced wastewater treatment methods to improve the quality of the water. Some of these methods include using rapid sand filtration facilities and an anaerobic-aerobic activated sludge process. This process is responsible for the removal of phosphorus in the water.

For a long time, the expansion of storm water drainage has been a major task of the city of Osaka. Osaka adopted a combined sewer system because of its many advantages over a separate sewer system. A combined sewer system is cheaper then a separate sewer system and can be constructed in a shorter period of time because the storm and wastewater are being carried in the same pipes. However, there are disadvantages to combined sewer systems. Such as, storm water, which includes pollutants and debris, is discharged directly into rivers from combined sewage outlets during heavy rains.

From 2002 to 2006 Osaka has launched “Urgent Countermeasure for CSO Control”, which is a plan for the short term and includes some highly effective projects. In order to reduce the discharge of pollution and debris, Osaka is promoting three different projects. The first is the Adoption of Wet Weather Activated sludge process (Three-W Method). Second is the construction of storm water reservoirs. And third is the effective utilization of large-scale trunk sewers from storm water storage.

The Three-W method is a technology capable of secondary-treatment in wet weather over three times of the quantity of wastewater in dry weather.

Reaction take removing sludge  (


Qsh:  Maximum quantity of sewage per hour allowed to flow in during the planned time under fine weather conditions.

This method has attracted the attention of other major cities because it takes very little modification to existing sewage systems and it effectively lessens the pollutants in the water.

Reduction in effluent load (


Construction of storm water reservoirs

Storm water reservoirs reduce the discharge of pollutant to public water bodies. It stores the water that comes down from heavy rains and once it stops raining the stored water is treated before being released to the river.

 Storm water reservoir model (


Large-Scale Trunk Sewers

The role of these trunk sewers is the immediate discharge of storm water inflow using pumps. They can also be used a storage for the water because of their large capacity. Therefore, when it first rains the large-scale trunk sewers store the polluted water and once it stops, the water is treated then released.

Model for large scale trunk sewer (


Thames, London Sewage Overflow

On Sunday, June 9, 2011 nearly 30mm of rain fell in only a few hours in Thames, London. This caused 250,000 tons of raw household sewage from drains and 200,000 tons from the Mogden sewage treatment works in Iselworth to overflow into the river, killing thousands of fish.

In response to this incident they proceeded to pump oxygen peroxide into the water for the next four days to try to build back the waters health. Sewage floods the river around 60 times a year because the Victorian-age drains system is unable to handle the quantity of sewage London now produces.

Thames Water is planning to build the Thames Tideway Super Sewer that would collect sewage before it overflows and send it to treatment a plant. However, this would cost  £3.6billion for the 20-mile long sewer and it is not expected to be operational till 2020.


Leave a Reply

Your email address will not be published. Required fields are marked *