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Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →The difference is usually the sewer design. In U.S. EPA terminology, a storm overflow generally means a combined sewer overflow (CSO): one pipe carries sanitary sewage and stormwater, and rain or snowmelt pushes more flow through the system than it can convey or treat. A sewage spill from a separate sanitary sewer is generally a sanitary sewer overflow (SSO): sewage has escaped a wastewater collection system that is normally separate from storm drainage.
Neither label means the discharge is harmless. Both can contain untreated or partly treated sewage, pathogens and other pollutants. The event notice from the local utility or public-health authority—not the label alone—determines what happened at a particular location and what precautions apply.
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The short answer: one is a combined-system overflow, the other is a sewage release
A combined sewer carries wastewater from homes and businesses and stormwater runoff in the same pipe. During dry weather, that flow normally goes to a treatment plant. During heavy rain or snowmelt, the combined volume can exceed the capacity of the pipes, pumps or treatment plant. The system then relieves excess flow through an outfall to a nearby waterway. EPA calls that discharge a combined sewer overflow (CSO). Many U.S. notices and news reports call it a “storm overflow.”
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A sanitary sewer is intended primarily for wastewater, while stormwater is collected in a separate drainage system. When sewage escapes from that sanitary system, EPA calls it a sanitary sewer overflow (SSO). A blockage, broken pipe, root intrusion, pump or power failure, poor maintenance, undersized equipment, or water entering through defects and improper connections can all cause an SSO. Wet weather can contribute when infiltration or inflow overwhelms the sanitary network, but rain is not required.
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The distinction is therefore about the collection system and the mechanism of release—not a promise that a CSO contains only rainwater or that an SSO always follows a storm. In a combined system, the overflow is an engineered relief response to excess wet-weather flow; in a sanitary system, an overflow means sewage escaped where it was supposed to remain contained.
CSO and SSO compared
| Question | Combined sewer overflow (CSO, often “storm overflow”) | Sanitary sewer overflow (SSO, a type of sewage spill) |
|---|---|---|
| Pipe layout | Stormwater runoff and sanitary wastewater share one pipe. | Wastewater travels in a sanitary sewer; stormwater is normally routed separately, although groundwater, rainwater or improper connections can enter. |
| Typical trigger | Rainfall or snowmelt adds enough flow to exceed the combined system or treatment capacity. | Sewage escapes because of a blockage, break, defect, inflow or infiltration, pump or power failure, design or maintenance problem, vandalism, or equipment failure. Wet weather is only one possible contributor. |
| What may discharge | A mixture of stormwater and untreated or partly treated wastewater, potentially containing debris and other pollutants. | Untreated or partly treated sewage, with possible bacterial, viral, protozoan and other contamination. |
| Where it may go | Through a relief outfall to a stream, river, lake, estuary or other receiving water. | To a waterway, street, yard, park, basement, building or other surface, depending on where the containment failed. |
| Meaning of the event | A wet-weather relief event associated with an older combined system. It remains a pollution and public-health concern and is subject to controls. | A release from a sanitary collection system. Repeated events can point to operational, maintenance or capacity problems. |
EPA explains the basic combined-sewer arrangement in Combined Sewer Overflow Basics and describes sanitary overflows in its Combined Sewer Overflows (CSOs) and Sanitary Sewer Overflows (SSOs) materials.
What exactly is a combined sewer overflow?
How the combined system works in dry weather
In a combined sewer area, household and commercial wastewater enters the same underground network that receives runoff from streets, roofs and other paved surfaces. Under ordinary, dry-weather conditions, the combined flow is sent to a wastewater treatment plant. The system was built as a single network, a design common in many older U.S. communities.
Why rain or snowmelt causes the discharge
Rain and melting snow add a large, rapid volume to the sanitary flow. When pipes, storage structures, pumps or the treatment plant cannot accept the combined rate, excess water is diverted through a relief outfall. The resulting CSO can contain stormwater, untreated sewage, partly treated wastewater, floating debris, bacteria and other pollutants. The discharge may occur during a short, intense storm or during a longer wet period; the local permit and utility notice identify the event and its timing.
“Designed to overflow” does not mean “safe”
A combined system’s relief point is an intentional part of its historical engineering: it prevents the entire network from backing up when wet-weather volume exceeds capacity. That design fact does not mean the outfall is a treatment process or that contact is safe. EPA identifies CSOs as potential sources of bacteria, debris and hazardous substances, and says they can contribute to beach or shellfish-bed closures and other water-quality impacts. Those are potential outcomes, not consequences that occur after every overflow.
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EPA’s Combined Sewer Overflow Basics states: “CSOs are a major water pollution and public health concern for approximately 700 communities in the United States.” The page does not date that approximately 700 figure, so it should be read as the agency’s statement on that page rather than as a dated census.
What is a sanitary sewer overflow?
The sanitary system and the point of failure
In a separate sanitary sewer, storm drains are normally distinct from the pipes carrying wastewater. An SSO occurs when sewage leaves that sanitary collection system before it reaches the treatment plant. It can emerge from a manhole, a broken line, a pump station, a cleanout, a building connection or another failure point.
Common physical and operational causes
- Blockages: Fats, oils and grease can harden inside a pipe. Wipes and other materials that should not be flushed can form obstructions.
- Roots and structural defects: Tree roots can enter cracks or joints; a broken, collapsed or badly aligned line can restrict flow.
- Inflow and infiltration: Rainwater or groundwater can enter through defective pipes, manholes, sump pumps, roof leaders and other improper connections. During wet weather, that extra water can overwhelm a sanitary system.
- Pumps and power: A failed pump, loss of electricity, inadequate backup power or equipment malfunction can stop sewage from moving.
- Capacity, design and maintenance: An undersized sewer, poor design, deferred maintenance or an operational mistake can leave insufficient capacity.
- Damage or vandalism: Physical damage and deliberate interference can also interrupt collection.
Because these causes are varied, it is inaccurate to call every SSO a “storm overflow.” A sanitary sewer can overflow on a dry day after a grease blockage or pump failure, and it can overflow in wet weather when infiltration and inflow add to the normal sewage load.
Is a storm overflow raw sewage?
It can contain raw or partly treated sewage, but the exact mixture depends on the system, the treatment process operating at the time, the amount of stormwater and the event’s location. A CSO is not simply clean rainwater: in a combined pipe, runoff mixes with sanitary wastewater before the relief discharge. An SSO can likewise release untreated sewage directly from the sanitary network.
EPA’s descriptions use “untreated or partially treated” language because treatment and dilution vary by event. The safest interpretation is to treat either type as potentially contaminated until the responsible utility or health authority says otherwise. Do not infer safety from clear-looking water, a short event, or the fact that the notice uses the word “storm.”
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Health and environmental concerns
How people can be exposed
EPA says sewer overflows can contain microorganisms such as bacteria, viruses and intestinal parasites that may cause illness. Exposure can occur where sewage reaches a waterbody or land surface, including recreational streams, beaches, streets, parks, backyards and basements. People can be exposed through direct skin contact, accidental swallowing, contaminated objects or contact with polluted sediment and standing water.
The event label does not establish the risk at a particular site. A short discharge into a fast-flowing waterway and a backup onto a playground are different exposure situations, even if both are reported as overflows. Follow the dated advisory from the utility, local health department or water-quality authority for the affected place. If an advisory says to avoid contact, keep people and pets out of the water or area until the authority lifts it; do not substitute visual inspection for the official notice.
Potential effects on water quality
CSOs and SSOs can introduce pathogens, nutrients, solids, debris, oils and other contaminants. EPA lists potential CSO effects that include beach closures, shellfish-bed closures, algae growth, reduced dissolved oxygen and visible debris or oil slicks. These are possible impacts, not a checklist that every event will produce. The receiving water, discharge volume, weather and follow-up sampling determine what actually occurs.
How to interpret a local spill or overflow notice
Utilities may use “sewage spill,” “sewer overflow,” “storm overflow” or another plain-language label. The wording can be broader than EPA’s technical categories. For a real incident, preserve the utility or regulator’s classification instead of re-labeling it based only on rainfall.
- Identify the named system or event type. Look for “CSO,” “SSO,” “combined sewer,” “sanitary sewer,” or a description of the failure.
- Check the date and time range. A notice may cover a discharge that has ended, is ongoing or is under investigation.
- Locate the affected area and waterbody. Read the street, outfall, stream, river, lake, beach, park or neighborhood description carefully.
- Read the advisory language. Note whether the authority says to avoid swimming, fishing, boating, wading or contact with land surfaces, and whether a reopening or cleanup update has been issued.
- Look for the cause and status. Blockage, equipment failure, rainfall, power loss and infiltration imply different corrective work and different likelihood of recurrence.
- Use the issuing authority for updates. Federal definitions provide context, but the operator and local public-health or water-quality agency control event-specific information.
When reporting an incident, include the utility or regulator’s exact event label, the location, the date, the affected waterway or land area, the current advisory and any stated cause. Do not infer that an event was a CSO merely because it followed rain, or that it was an SSO merely because sewage was visible.
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Federal permits and public notification
NPDES coverage
EPA regulates discharges through the National Pollutant Discharge Elimination System (NPDES). CSOs are subject to NPDES permitting. EPA says an SSO that reaches waters of the United States is a point-source discharge and is prohibited unless authorized by an NPDES permit. An overflow that does not reach those waters can still signal improper operation or maintenance and may violate permit conditions.
Permits can require monitoring, corrective measures, reporting and controls. EPA’s management material identifies reducing discharges and providing timely public notification as goals, but there is no single nationwide notification deadline that applies identically to every event. The actual timing, method and wording depend on the permit and the jurisdiction.
For background on management approaches, see EPA’s Combined Sewer Overflow Solutions – Management Approaches. For an individual event, the local permit, utility report and health advisory are more specific than a general federal summary.
How common are these events?
EPA’s SSO Frequent Questions page gives an estimate of at least 23,000 to 75,000 sanitary sewer overflows per year in the United States, excluding sewage backups into buildings. EPA does not state the estimate’s original year on the current page, so it is not a verified 2026 count or a complete live tally. The range is useful for showing the scale of the issue, but it should be quoted with both the exclusion and the uncertainty about its date.
The reviewed EPA materials do not provide a comparable current national annual CSO event count or volume. Historic totals should not be presented as current without checking their date, geographic coverage and measurement method. EPA’s NPDES Sewer Overflow and Bypass Event Download Summary provides event-data context, while local reporting systems may contain the most relevant information for a particular community.
Why the difference matters in practical terms
For residents and recreation users
Knowing whether the system is combined or separate helps explain the likely trigger, but it does not replace an advisory. A CSO notice usually points to wet-weather flow through a combined network. An SSO notice may require investigating a clog, broken line, pump station or unauthorized inflow, and it may describe contamination on land as well as in water. In either case, rely on the authority’s location-specific instructions.
For city planning and infrastructure
CSO control focuses on managing wet-weather volume in a combined network: storage, treatment capacity, source controls, separation or other measures may be part of a permitted program. SSO prevention focuses on keeping sanitary pipes and equipment functional, removing blockages, controlling roots and inflow, maintaining pumps and providing adequate capacity and backup power. The remedies differ because the failure modes differ.
For journalists and technical writers
Use “CSO” when the source identifies a combined sewer and a wet-weather discharge through a relief outfall. Use “SSO” when sewage escaped a sanitary sewer, regardless of whether rain contributed. If the notice is vague, quote the issuing agency’s wording and explain what is known without assigning a technical category that the source did not establish.
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Common misunderstandings to avoid
- “Storm” means only rainwater. In a combined sewer, stormwater and sewage share the pipe; a CSO may include untreated or partly treated wastewater.
- Every overflow after rain is a CSO. Rain can overload a separate sanitary sewer through inflow and infiltration, producing an SSO.
- Every SSO is caused by weather. Blockages, roots, breaks, pumps, power, maintenance and vandalism can cause dry-weather events.
- Designed relief means no pollution. A CSO outfall is an engineered overflow point, not a guarantee of treatment or safety.
- The label alone gives the exposure risk. Site, volume, duration, waterbody, land contact and the current advisory matter.
- A national estimate is a current count. The EPA range of 23,000–75,000 SSOs excludes building backups and has no publication year stated on the cited page.
- One federal notice rule applies everywhere. NPDES permits and local reporting procedures determine the timing and content of public notification.
Bottom line
In U.S. usage, a “storm overflow” generally means a combined sewer overflow: stormwater and sewage in one pipe exceed wet-weather capacity and discharge through a relief outfall. A “sewage spill” from a separate sanitary network is generally a sanitary sewer overflow, caused by a blockage, break, inflow, pump or power problem, design or maintenance failure, or another escape of sewage. Both can carry pathogens and pollutants. To understand a specific event, check the utility or regulator’s classification, date, location and current public-health advisory rather than relying on the shorthand label.
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