Combined Sewer: Definition & Professional Guide
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A combined sewer is a municipal wastewater collection system that carries both sanitary sewage and stormwater runoff through a single pipe network to a treatment plant. Combined sewers serve over 700 communities across the United States and represent legacy infrastructure that creates unique flooding, backup, and contamination risks for homeowners connected to these systems.
Overflow Events, Surcharge Pressure & Residential Backup Risk
Combined sewer systems function adequately during dry weather, when only sanitary waste flows through the pipes. During heavy rainfall, however, stormwater volume can overwhelm the system’s capacity by a factor of 10 or more. When flow exceeds what the treatment plant can process, the excess mixture of raw sewage and stormwater discharges directly into rivers, lakes, or coastal waters through designed relief points called combined sewer overflows.
The overflow problem also affects individual properties. When the main sewer surcharges during a storm, hydraulic pressure can force sewage backward through residential laterals and into basement floor drains. This backup occurs even when the homeowner’s lateral is completely clear, because the problem originates in the overwhelmed municipal main rather than in any private-side blockage. A backwater valve installed on the lateral is the most effective protection against this type of backup, typically costing $500 to $1,500 installed.
Many municipalities operating under EPA consent decrees are now mandating sewer separation projects that require property owners to disconnect roof downspouts, foundation drains, and other stormwater sources from the sanitary lateral. These separation projects can cost homeowners $3,000 to $15,000 depending on property configuration and pipe routing.
Fully Combined, Partially Separated & Separated System Types
Fully combined systems carry all sanitary waste and stormwater through a single pipe with no separation at any point. These are the oldest configurations, common in cities built before 1950.
Partially separated systems have undergone incremental upgrades where some stormwater sources have been redirected to separate storm sewers while the remaining connections feed the combined main. These transitional systems still experience overflow events during major storms.
Separated systems maintain completely independent sanitary and storm sewer networks. Most new development since the 1970s uses separated infrastructure, eliminating the overflow contamination risk entirely.
Lateral Repair Permitting, Backwater Valves & Flood Prevention Services
Combined sewer connections directly affect the scope of Bonded Plumbworks’ drain and sewer services work. Before quoting a sewer lateral repair on a property connected to a combined system, the plumber must verify whether the municipality requires separation as a condition of the repair permit, which can change a straightforward repair into a major project.
Backwater valve installation is a priority recommendation for any home on a combined sewer. Bonded Plumbworks’ technicians install check-style and gate-style backwater valves as part of sump pump and flood prevention services, protecting basements from the surcharge-driven backups that combined systems produce during storms.
EPA CSO Control Policy, Clean Water Act Section 402 & Local Ordinances
The EPA Combined Sewer Overflow Control Policy, first issued in 1994 and updated through subsequent enforcement actions, requires municipalities with combined systems to develop Long-Term Control Plans that reduce overflow frequency and volume. Clean Water Act Section 402 regulates CSO discharges through the National Pollutant Discharge Elimination System permit program. Local sewer use ordinances vary significantly by municipality and may impose separation requirements, downspout disconnection mandates, or stormwater fees on properties connected to combined systems.
Mainline Backwater Valves & Zoeller Ejector Pump Systems
Mainline manufactures backwater valves specifically engineered for sewer lateral protection in combined system areas, featuring transparent bodies that allow visual confirmation of valve operation. Zoeller produces ejector pump and sump pump systems designed for basement protection in neighborhoods prone to combined sewer surcharging, with battery backup options that maintain protection during the power outages that often accompany severe storms.