Sewer System Types: Sanitary, Storm, and Combined

The United States operates three structurally distinct sewer system classifications — sanitary, storm, and combined — each governed by separate regulatory frameworks and designed to manage different waste streams. These classifications determine how municipalities handle wastewater, surface runoff, and their intersection, with direct implications for infrastructure planning, environmental compliance, and public health protection. The Sewer Listings resource catalogs service providers operating across these system types at the regional and national level.

Definition and scope

A sanitary sewer system collects and conveys wastewater from residential, commercial, and industrial sources — toilets, sinks, showers, and process drains — to a treatment facility before discharge. These systems carry no stormwater by design and operate under the authority of the Clean Water Act (CWA), 33 U.S.C. § 1251 et seq., which is administered by the U.S. Environmental Protection Agency (EPA).

A storm sewer system is a separate network designed exclusively to collect and convey surface runoff — rain, snowmelt, and irrigation drainage — away from streets, parking lots, and developed land. In most jurisdictions, storm systems discharge directly to waterways without treatment, which is why they are regulated under the EPA's National Pollutant Discharge Elimination System (NPDES) stormwater program.

A combined sewer system (CSS) is a single pipe network that carries both sanitary sewage and stormwater in the same conduit. Combined systems were the dominant design in US urban infrastructure before the early 20th century. The EPA estimates that approximately 860 communities in the United States rely on combined sewer systems (EPA Combined Sewer Overflows), concentrated largely in the Northeast and Great Lakes regions.

The scope of each system type extends into federal, state, and local regulatory layers. The EPA sets baseline standards; state environmental agencies issue permits and enforce compliance; and local public works or utilities departments operate the physical infrastructure.

How it works

Sanitary sewer systems function through gravity flow and, where elevation requires it, lift stations equipped with pumps. Wastewater moves from individual building laterals into collector mains, then into trunk sewers, and ultimately to a wastewater treatment plant (WWTP). Treatment processes — primary, secondary, and in some cases tertiary — are defined under the CWA's technology-based effluent limits.

Storm sewer systems collect runoff through inlet grates, catch basins, and curb openings. Flow moves through underground pipes or open channels to outfalls at rivers, streams, lakes, or coastal waters. Under NPDES Phase I and Phase II rules, municipalities operating separate storm sewer systems (known as MS4s — Municipal Separate Storm Sewer Systems) must obtain NPDES permits and implement stormwater management programs (EPA MS4 Program).

Combined sewer systems use one pipe for both flows. Under normal dry-weather conditions, all flow reaches the treatment plant. During heavy precipitation, the combined volume can exceed treatment capacity. When this occurs, raw or minimally treated sewage mixes with stormwater and is discharged through Combined Sewer Overflow (CSO) outfalls to surface waters. The EPA's CSO Control Policy (published in the Federal Register in 1994) requires municipalities with CSOs to implement Long Term Control Plans (LTCPs) that demonstrate compliance with water quality standards.

The structural distinction between these three systems dictates permitting pathways, inspection protocols, and construction standards. The Uniform Plumbing Code (UPC) and the International Plumbing Code (IPC) both address building drain connections to public sewers, including provisions that separate sanitary and storm connections at the property line.

Common scenarios

The following scenarios represent the operational conditions under which each sewer system type is typically engaged:

1. New residential development in a municipality with separate systems — The builder connects sanitary laterals to the sanitary sewer and installs independent drainage to the storm system. Cross-connections between the two are prohibited and are subject to inspection before occupancy permits are issued.

2. Urban infill construction in a city with a combined system — A single lateral connection serves both wastewater and roof drainage. Local public works departments in these jurisdictions often require downspout disconnection programs to reduce wet-weather CSO volumes.

3. Industrial site discharge — Facilities generating process wastewater beyond domestic-strength sewage must obtain an Industrial Pretreatment permit under 40 CFR Part 403, administered through local pretreatment programs, before discharging to a sanitary or combined sewer.

4. Post-storm flooding of sanitary system — Inflow and infiltration (I/I) occurs when stormwater enters sanitary sewer pipes through cracks, faulty manhole covers, or illicit connections. Municipalities are required to investigate and remediate I/I under their NPDES permits to prevent sanitary sewer overflows (SSOs).

5. CSO community under a consent decree — A municipality operating a combined system under EPA or Department of Justice enforcement may be required to separate combined sewers over a 20-to-30-year capital improvement schedule, representing infrastructure investments that frequently exceed $1 billion in large cities.

Decision boundaries

The distinction between system types governs how infrastructure professionals, regulators, and property owners navigate permitting, inspection, and remediation decisions.

Sanitary vs. storm connection is the most consequential classification boundary at the property level. An illicit discharge of sanitary sewage to a storm system — or the reverse connection — constitutes a CWA violation. Local inspectors performing sewer lateral inspections use smoke testing, dye testing, and CCTV camera inspection to identify cross-connections.

Separate vs. combined jurisdiction determines which regulatory pathway applies. A contractor performing sewer work must verify with the local authority having jurisdiction (AHJ) whether the street infrastructure is a separate or combined system before specifying pipe sizing, materials, or connection methods.

CSO long-term control obligations depend on whether the receiving water body has impaired water quality designations under CWA Section 303(d). Communities discharging CSOs to impaired waterbodies face more stringent LTCP requirements than those discharging to waters meeting designated uses.

Safety standards relevant to sewer work fall under OSHA 29 CFR Part 1926 Subpart P (excavation and trenching) and Subpart AA (confined space entry), which classifies sewer manholes and wet wells as permit-required confined spaces due to atmospheric hazards including hydrogen sulfide (H₂S) and oxygen deficiency.

Permit and inspection requirements for sewer construction and rehabilitation are set at the state and local level, consistent with EPA baseline standards. The purpose and scope of the sewer reference infrastructure outlines how service categories within this sector are organized for professional reference use.

References

• U.S. Environmental Protection Agency — NPDES Program
• EPA — Combined Sewer Overflows (CSOs)
• EPA — Municipal Separate Storm Sewer Systems (MS4)
• EPA — Industrial Pretreatment Program, 40 CFR Part 403
• Clean Water Act, 33 U.S.C. § 1251 et seq.
• OSHA 29 CFR Part 1926 Subpart P — Excavations
• International Code Council — International Plumbing Code (IPC)
• IAPMO — Uniform Plumbing Code (UPC)