Sanitary sewer overflows (SSOs) are releases of raw sewage from a separate sanitary sewer system before it has reached a treatment facility. Raw sewage contains bacteria and nutrients that endanger both human health and the environment. SSOs occur when the flow into the system exceeds the design capacity of the conveyance system, resulting in discharges into basements, streets, and streams. Sewage overflowing from a manhole is a common SSO, and it frequently results in untreated sewage flowing into a stream. While SSOs can occur in any system due to flooding or temporary blockages, chronic overflows indicate a deteriorating system or a system where supply has exceeded capacity. For every 1,000 miles of sanitary sewer lines, an estimated 140 overflows occur annually. (AMSA, 1994). An Association of Metropolitan Sewage Agencies survey also found that 15 to 35 percent of all sewer lines were over-capacity and could potentially overflow during a storm.
Sanitary sewer overflows occur in urbanized areas where a separate sanitary sewer system has been created to move wastewater from households and businesses to treatment plants. It is important to detect and eliminate SSOs because sanitary sewer collection systems represent a significant investment for urban municipalities.
Depending on their size, sanitary sewer systems can cost billions of dollars. Therefore, they require not only programs to identify and eliminate overflows, but programs for preventative maintenance.
A number of factors contribute to a sanitary system's vulnerability to failure and overflows. The age of the pipe system is an important factor. In older systems, deteriorating main and lateral pipes can cause laterals to protrude and lines to sag, crack, and develop holes. The type of material used in the pipe system can cause this deterioration, as can the failure of the material used to seal pipe joints.
Poor siting or inferior installation techniques are other contributors to sanitary system failure. Poorly placed sewer lines may depend on support from the surrounding earth. When the earth surrounding these lines moves, cracks or misaligned and open joints result.
The inadequate size of existing sewer pipe may be another factor. New sewer hook-ups, underground water infiltration and inflow, and inputs from roof and yard drain connections may combine to cause increases in wet weather discharges. This in turn can cause an undersized sewer system to overload.
Other factors, both man-made and natural, may also contribute to SSOs. Roots can create stoppages and damage the structural integrity of the sewer line. Grease from residential and commercial sources can clog sewer lines. Ground water influences and temperature fluctuations may contribute to sanitary sewer system failure. Equipment failure and power outages affecting pumping stations and sewage treatment plant operations also contribute to overflows.
Inadequate operation and maintenance, improper design and construction, and planning that has failed to consider the effects of new development on system capacity all contribute to overflows. A number of practices can often reduce or eliminate sanitary sewer overflows, including the following:
- Cleaning and maintaining the sewer system
- Reducing infiltration and inflow through rehabilitation and repair of broken or leaking sewer lines
- Enlarging or upgrading the capacity of sewer lines, pump stations, or sewage treatment plants
- Constructing wet weather storage and treatment facilities to treat excess flows
- Addressing SSOs during sewer system master planning and facilities planning
A number of key elements should be included in SSO control programs. Guidance on structuring and organizing operation, maintenance, and remediation of sanitary sewer collection systems suggests that the following measures be incorporated by sewer authorities (USEPA, 1998):
Many of the same monitoring techniques used to identify other illicit connection sources are also used in sewer system evaluation surveys. These include the following:
- Identification and tracking of sanitary sewer discharges
- Identification of the causes of any overflow through monitoring and field screening
Physical inspection. This involves examining the physical condition of manholes and other sewer structures to determine their structural integrity and to identify possible sources of infiltration and inflow.
Flow monitoring/flow isolation. Rainfall gauges are installed to monitor subbasins with overflow problems by collecting and analyzing flow data during normal and storm-related weather events.
Smoke testing. Smoke testing is used to locate defects in sewer mains and laterals that contribute infiltration or inflow to the sewer system. Smoke testing involves injecting a non-toxic vapor (smoke) into the manholes and following its path of travel in the mains and laterals.
Dye water flooding. Colored dye is added to the storm drain water. Dyed water appearing in the sanitary sewer system indicates an existing connection between the sewer and storm drain system.
Closed-circuit television inspection. This useful tool can locate specific sources of infiltration and determine the structural condition of the sewer system. This information is necessary for the design of sewer replacement and rehabilitation projects.
Sewer maintenance records. Reviewing records helps identify areas with frequent maintenance problems, and it can indicate potential locations of system failure.
- Implementation of both short-and long-term remediation actions and the modification of operation and maintenance measures to mitigate the impacts of overflows
- Public notification of overflow events and impacts
- Provision of adequate maintenance, both preventative and routine, and the updating of procedures as problems arise
- Measures to ensure adequate maintenance facilities, equipment, and inventory
- Implementation and enforcement of sewer-use ordinances or other legal documents that prohibit new connections from inflow sources, guarantees of the testing and inspection of all portions of the collection system that handle discharge (including new collector sewers and service laterals which may be owned by another entity), and the regulation of the discharges of toxics and pollutants that may endanger public safety or the physical integrity of the system or cause the municipality to violate water quality limitations
- The development and the tracking of system performance indicators, including hydraulic performance, during wet weather flows.
A number of excellent resources in the reference section below explain in greater detail the monitoring techniques and reporting requirements for sewer collection systems and the operation and maintenance procedures for correcting system problems.
As with most illicit connection detection, identifying exact causes of sanitary sewer overflow can be time-consuming and difficult. The biggest obstacle to identifying and correcting sanitary sewer overflows is often public access to private property. In some areas, improper connections from private sources may contribute significant inflows to the system. An ordinance to ensure the authority to inspect may be necessary to correct these connections. St. Louis, Missouri's sewer-use ordinance is available for review at the City of St. Louis' website. Some municipalities have taken the opposite approach and instituted programs that provide homeowners with cash incentives or financial assistance to correct improper connections.
The cost of equipment and staff time for SSO correction may present a burden for some municipalities. Inspection equipment, replacing undersized sewer lines, and upgrading treatment plants or pumping stations would be included in those costs. These system repairs and the materials required could be expensive, and homeowners may be reluctant to pay for a service that they perceive as having no benefit to them.
A good way to avoid system failure and expensive repairs is to regularly maintain the sanitary sewer collection system. Preventative maintenance through scheduled inspections and routine cleaning of the sewer system can identify and help eliminate many of the causes of SSOs.
Eliminating SSO sources can significantly improve water quarter. Blockages, breaks, and infiltration and inflow in municipal sewer systems create overflows that represent significant risk to humans and the environment. SSOs discharge raw sewage containing microorganisms that affect the health of the urban population. Untreated sewage enters streams and other waterbodies and affects aquatic organisms and their habitat. Raw sewage often contains pollutants and toxins that limit dissolved oxygen and promote algal blooms, endangering the aquatic environment.
Sanitary sewer collection systems are a valuable part of a municipality's infrastructure. EPA estimates that our nation's sewers are worth more than $1 trillion (USEPA, 1996). The collection system of a single large municipality is worth billions of dollars, and a smaller city's could cost many millions. Reducing or eliminating SSOs can be expensive, but this cost must be weighed against the value of the collection system and the cost of replacing if it is allowed to deteriorate. Ongoing maintenance and rehabilitation add value to the system by maintaining capacity and extending its life. The costs of correcting SSOs can vary according to community size and sewer system type. Costs will often be highest, and ratepayers will pay more, in communities lacking preventive maintenance or remediation programs to handle system failures. Table 1 gives examples of the cost associated with sanitary sewer remediation to both homeowners and the agency responsible for management of the sanitary sewer collection system.
Table 1. Three case studies of SSO costs.
Cost to Agency or Municipality
Cost to Homeowner
Washington Suburban Sanitary Commission, Maryland
From 1990 to 1994, SSO-related basement backups totaled 2,690, with an average cleanup cost of $700 each
Upgrades at pumping stations and sewage treatment plants: $38 million
Collection system improvements: $22 million
Sewer reconstruction: $6 million (annual)
Maintenance program: $10 million (annual)
$50 per household per year
$10 per household per year
Louisville/Jefferson County, Kentucky
Long-term budget plan for corrective actions totaled $14.6 million
$40 per household per year
SSOs can also result in significant economic losses. Shellfish bed closures and bans on fish consumption create economic hardships for associated industries. Waterbody closures can affect tourism and lower property values. Basement cleanups due to sewage backup must be done at homeowner and municipal expense.
Arbour, Rick and Kerri, Ken. 1998. Collection Systems: Methods for Evaluating and Improving Performance. Sacramento Foundation, California State University, Sacramento, CA.
Massachusetts Department of Environmental Protection. 1993. Guidelines on Performing Infiltration/Inflow Analyses and Sewer System Evaluation Survey. Massachusetts DEP, Boston, MA.
Massachusetts Department of Environmental Protection. 1989. Guidelines on Performing Operation & Maintenance on Collection Systems. Massachusetts DEP, Boston, MA.
USEPA. 2000. Wet Weather. U.S. Environmental Protection Agency, Office of Wastewater Management. [http://cfpub1.epa.gov/npdes/home.cfm?program_id=4].
USEPA. 1998. Key Components of Operational, Maintenance and Remediation Programs for Municipal Sanitary Sewer Collection Systems, Draft. U.S. Environmental Protection Agency, Office of Wastewater Management, Washington, DC.
USEPA. 1996. National Conference on Sanitary Sewer Overflows. U.S. Environmental Protection Agency, Office of Research and Development, Washington, DC.
USEPA. 1996. Sanitary Sewer Overflows, What are They and How Can We Reduce Them? U.S. Environmental Protection Agency, Office of Wastewater Management, Washington, DC.