Sanitary Sewer Overflows
Sanitary sewer overflows (SSOs) are releases of untreated or partially treated sewage from a municipal sanitary sewer system. These systems are designed to collect wastewater from homes, businesses, and industrial facilities and transport it to a treatment plant. When an overflow occurs, this raw sewage is discharged into the environment before it can be treated, posing significant risks to public health and water quality. The U.S. Environmental Protection Agency (EPA) estimates that between 23,000 and 75,000 SSOs occur each year in the United States, releasing billions of gallons of raw sewage.
The primary water quality concern with SSOs is the introduction of dangerous pathogens into surface waters. Raw sewage contains high concentrations of bacteria (such as E. coli and Enterococcus), viruses, and protozoa that can cause diseases in humans who come into contact with the contaminated water through recreation. Beyond pathogens, SSOs deliver high loads of nutrients like nitrogen and phosphorus, which can trigger algal blooms and lead to low-oxygen “dead zones” that harm fish and other aquatic life. The organic matter in sewage also exerts a high biochemical oxygen demand (BOD), further depleting dissolved oxygen as it decomposes. Other pollutants include solids, trash, and various household and industrial chemicals. Preventing SSOs is a critical component of any municipal pollution prevention program.
How the pollution pathway works
The pollution pathway for sanitary sewer overflows begins when the capacity of a sewer pipe or pump station is exceeded, or a blockage prevents the normal flow of wastewater. Unlike older combined sewer systems, which were designed to overflow during heavy rain, modern sanitary sewers are supposed to be a closed system, completely separate from the storm drain network. When an SSO occurs, raw sewage surcharges and escapes the system, typically through a manhole cover, a broken pipe, or a cleanout port.
Once released, the sewage flows over land surfaces like streets, lawns, and parking lots. It follows the topography, picking up other surface pollutants as it travels, and enters the storm drain system through inlets and catch basins. The storm drain system is designed to convey rainwater quickly and efficiently to the nearest water body—a stream, river, or lake—with no treatment. Therefore, any raw sewage that enters a storm drain is discharged directly into receiving waters. This creates an acute pollution event, leading to immediate beach or waterway closures, public health advisories, and severe stress on the local aquatic ecosystem.
Recommended practices
A comprehensive strategy to prevent SSOs revolves around a framework known as Capacity, Management, Operations, and Maintenance (CMOM). This proactive approach focuses on properly managing and maintaining all parts of the collection system to prevent failures before they occur.
Capacity, Management, Operations, and Maintenance (CMOM)
A CMOM program is the foundation of SSO prevention. It is an asset management system that requires a municipality to have a complete understanding of its sewer system and a detailed plan for its operation and maintenance. Key elements include:
- Sewer System Mapping: Maintaining an accurate, up-to-date map of all sewer mains, manholes, pump stations, and laterals.
- Asset Evaluation: Regularly assessing the condition of pipes and equipment to identify areas at high risk of failure.
- Capital Improvement Planning: Budgeting for the repair, rehabilitation, and replacement of aging infrastructure before it fails.
- Staff Training: Ensuring that collection system operators are properly trained in maintenance procedures, safety protocols, and emergency response.
Blockage Prevention
Blockages are a leading cause of SSOs, and most are preventable. Municipal programs should target the two most common sources: fats, oils, and grease (FOG), and tree roots.
- FOG Programs: Food service establishments are a major source of FOG. An effective program requires an ordinance that mandates the installation and regular maintenance of grease traps or interceptors. Municipal staff should conduct regular inspections to ensure compliance, provide educational materials, and enforce penalties for non-compliance. Disposing of grease down manholes is an issue that effective illegal dumping control ordinances can also address.
- Root Control: Tree roots naturally seek out sources of water and can infiltrate sewer pipes through cracks and joints. A root control program involves mechanically cutting roots from inside the pipe with hydraulic or robotic cutters, followed by chemical treatment with a root-inhibiting foam to slow regrowth. Municipalities often have ordinances regarding planting trees near sewer lines.
- Public Outreach: Educating residents not to flush items that can cause clogs, such as paper towels, feminine hygiene products, and so-called “flushable” wipes, is crucial for preventing blockages in smaller residential lines.
Infiltration and Inflow (I/I) Reduction
Infiltration is groundwater that enters the sewer system through cracks and holes in pipes. Inflow is stormwater that enters through direct connections like downspouts or leaky manhole covers. Excessive I/I can overwhelm the system’s capacity during wet weather, causing overflows.
- System Inspection: Regular inspection is key to finding sources of I/I. Common methods include smoke testing (blowing non-toxic smoke into the sewer to see where it escapes), dye testing (flushing water-soluble dye into storm drains to see if it appears in the sanitary sewer), and Closed-Circuit Television (CCTV) inspection, where a robotic camera is sent through the pipe to record its condition.
- Pipe Rehabilitation: Once I/I sources are identified, pipes can be repaired. Trenchless technologies like cured-in-place pipe (CIPP) lining, where a resin-saturated liner is inserted into the old pipe and cured to form a new, seamless pipe within the old one, are often more cost-effective than full excavation and replacement.
- Private Lateral Programs: A significant portion of I/I (often estimated at 40-60%) originates from private sewer laterals—the pipe connecting a home or business to the public main. Municipal programs may offer financial incentives or require inspections at the time of property sale to encourage homeowners to repair faulty laterals, a responsibility similar to private septic system care.
During smoke testing, field crews force a non-toxic, odorless smoke into sewer lines. Residents may be alarmed to see this smoke emerging from roof vents, storm drain inlets, or even cracks in the ground or pavement. This is the intended result, as it reveals connections and leaks between the sanitary sewer and the storm drain system or the surface. Public notification before testing is essential to prevent unnecessary calls to the fire department and to explain to residents that the smoke is harmless and a vital part of keeping the sewer system functioning properly.
Pump Station and Force Main Maintenance
Pump stations (or lift stations) are critical components that lift wastewater to higher elevations. Their failure guarantees an SSO.
- Preventive Maintenance: Regular inspection and servicing of pumps, motors, and electrical controls.
- Redundancy and Alarms: Stations should have redundant pumps, reliable backup power (e.g., permanent generators or quick-connect ports for mobile units), and a telemetry alarm system that notifies staff immediately of a power failure or high water level.
- Force Main Assessment: Force mains (pressurized pipes leading from a pump station) must be periodically assessed for corrosion and leaks.
Building a municipal program
Establishing an effective SSO prevention program requires a long-term commitment of resources, staff, and political will. A sewer use ordinance is the legal backbone, granting the municipality authority to set standards for discharges, require grease interceptors, conduct inspections, and levy fines for non-compliance.
Funding is typically managed through an enterprise fund, where revenue generated from sewer user fees is dedicated solely to the operation, maintenance, and capital improvement of the wastewater collection and treatment system. This ensures a stable funding source insulated from other municipal budget pressures. Staffing needs are significant and specialized, requiring trained collection system operators for jetting and CCTV work, pump station mechanics, FOG inspectors, and engineering staff for capital planning.
A critical component is a formal SSO Emergency Response Plan. This plan should detail procedures for receiving and logging overflow reports 24/7, dispatching crews, containing and cleaning up the spill, disinfecting affected areas, collecting water samples, and notifying the public and regulatory agencies (such as state environmental departments and the EPA) within required timeframes.
Effectiveness
The effectiveness of SSO prevention programs is well-established. While completely eliminating all overflows is a difficult goal, a well-run CMOM program can dramatically reduce their frequency, volume, and severity. Proactive maintenance is significantly more cost-effective than emergency response and cleanup.
Targeted programs show high rates of success. For example, a well-enforced FOG program can reduce grease-related blockages by over 80-90% in targeted areas. Systematic I/I reduction programs that combine inspection with rehabilitation can remove millions of gallons of extraneous flow per day from the system, freeing up critical capacity during rain events. The true measure of effectiveness is a year-over-year reduction in the number and volume of SSOs, improved compliance with environmental regulations, and the protection of public health and local water resources.
| Attribute | Sanitary Sewer Overflow (SSO) | Combined Sewer Overflow (CSO) |
|---|---|---|
| System Type | Occurs in separate sanitary sewer systems, which are not designed to carry stormwater. | Occurs in combined sewer systems, which are designed to carry both sewage and stormwater in the same pipe. |
| Cause of Discharge | System failure: blockages, pipe collapses, pump failures, or excessive infiltration/inflow. Unintentional and illegal. | System design: occurs during heavy rain when the combined flow of stormwater and sewage exceeds the system’s capacity. An intentional design feature of older systems. |
| Discharge Content | Undiluted raw sewage. | A diluted mixture of raw sewage and stormwater. |
| Regulatory Status | Prohibited under the Clean Water Act. Any occurrence is a permit violation that must be reported. | Permitted under the Clean Water Act, but municipalities are required to implement long-term control plans to reduce or eliminate them. |
Frequently Asked Questions
What is the difference between a sanitary sewer overflow (SSO) and a combined sewer overflow (CSO)?
An SSO is an unintentional release of raw sewage from a sanitary sewer system, which is designed to carry only wastewater. These are caused by failures like blockages or broken pipes. A CSO is an intentional, permitted discharge from an older combined sewer system, which carries both sewage and stormwater in one pipe. CSOs occur during heavy rain when the combined flow exceeds the system’s capacity, releasing a diluted mix of sewage and stormwater. SSOs are illegal under the Clean Water Act, while CSOs are permitted but heavily regulated and targeted for elimination.
What should I do if I see sewage overflowing from a manhole?
If you see or smell sewage coming from a manhole, a drain, or the ground, you should immediately report it to your local public works department or sewer utility. Most have a 24-hour emergency number. Do not attempt to touch the sewage or clear the blockage yourself, as raw sewage contains harmful pathogens. Keep people and pets away from the affected area until utility crews arrive. Providing a precise location is critical for a rapid response, which can significantly reduce the environmental impact and public health risk.
Are “flushable” wipes really safe for sewer systems?
No. Despite the “flushable” label, most wipes do not break down in the sewer system the way toilet paper does. They are made of synthetic materials that remain intact, accumulating in pipes and tangling with other debris and grease to form massive clogs. These clogs are a primary cause of blockages in both private laterals and public sewer mains, leading to costly repairs and sanitary sewer overflows. Wastewater professionals universally recommend that nothing other than human waste and toilet paper be flushed.
How do tree roots get into sewer pipes and cause blockages?
Tree roots are naturally drawn to the warm, moist, nutrient-rich environment inside a sewer pipe. They can find their way into tiny cracks, loose joints, or connection points in the pipe. Once inside, they grow rapidly, creating a dense root mass that can fill the entire pipe. This mass traps solids, grease, and other debris, eventually forming a complete blockage that causes sewage to back up into homes or overflow from manholes. Older pipes made of clay or concrete with mortar joints are particularly vulnerable to root intrusion.
What is smoke testing and why is it done in my neighborhood?
Smoke testing is a common method used by sewer utilities to find sources of infiltration and inflow (I/I). Crews force a safe, non-toxic, odorless smoke into the sanitary sewer lines. They then watch to see where the smoke escapes. Smoke coming from storm drains, ditch inlets, or downspouts indicates an improper connection that allows rainwater into the sanitary sewer. Smoke emerging from the ground can indicate a broken pipe. It is a highly effective and low-cost way to identify system defects that need repair to prevent wet-weather overflows.
How much does it cost a municipality to manage its sewer system to prevent SSOs?
The costs are substantial and ongoing. They are typically funded by monthly sewer fees paid by residents and businesses. A medium-sized utility might spend hundreds of thousands to millions of dollars annually on routine operations and maintenance, including pipe cleaning, CCTV inspection, and pump station servicing. Capital improvement costs for major rehabilitation or replacement projects can be much higher, often running into tens of millions of dollars over several years. However, these proactive investments are far less expensive than the costs of emergency response, cleanup, regulatory fines, and potential lawsuits resulting from frequent SSOs.
What are the main health risks associated with SSOs?
The primary health risk from SSOs is exposure to waterborne pathogens. Raw sewage contains a high concentration of disease-causing bacteria, viruses, and parasites. Direct contact with contaminated water can lead to gastrointestinal illnesses (like E. coli or norovirus), skin infections, and other diseases. These risks are why public health departments immediately issue no-contact advisories and close beaches or recreational waterways following a known SSO event. Contamination of drinking water sources is also a major concern, although less common for surface water overflows.
Can sanitary sewer overflows happen during dry weather?
Yes, dry-weather SSOs are common and are almost always caused by a system failure rather than excess flow. The most frequent causes are blockages from fats, oils, and grease (FOG), tree roots, or solid objects that should not have been flushed. Other causes include a structural collapse of a sewer pipe or a mechanical or electrical failure at a pump station. While wet-weather overflows are often larger in volume due to stormwater inflow, dry-weather overflows are a clear indicator of a critical maintenance or structural issue that needs immediate attention.