St. Louis carries some of the oldest sewer infrastructure in the Midwest. Sections of the collection system predate the 1904 World’s Fair, and decades of Mississippi and Missouri River flooding have left a mark on nearly every pipe segment near the floodplain.
For engineers and contractors managing sewer rehabilitation across St. Louis City and County, inflow and infiltration (I&I) is not a one-time repair. MSD’s long-running consent decree work, Project Clear, has kept I&I reduction at the center of a multi-decade capital program, and recent heavy rain events have kept the pressure on.
A Combined System, Two Sources of Excess Flow
Infiltration shows up in St. Louis the way it does everywhere else: groundwater working through cracked pipes, failed joints, and deteriorated manholes. Inflow arrives more directly, through roof drains, foundation drains, and other connections wired straight into the sanitary line.
Both draw down sewer capacity fast in a city with a large, combined sewer footprint. St. Louis’s older neighborhoods route stormwater and sewage through the same pipes, so every gallon of excess groundwater competes directly with the sewage that has to reach treatment.
What a Century-Old System Hides From a Fresh Liner
St. Louis crews turn to CIPP to renew mains running beneath historic neighborhoods without disturbing the streets above, and structurally, it delivers. The century-old infrastructure beyond the mainline, though, has its own set of problems that a liner can’t reach.
A newly lined St. Louis main doesn’t always translate into lower infiltration numbers. Century-old clay tile laterals and river-adjacent groundwater conditions stress the system on their own timeline. A connection that held steady for decades can start leaking the moment river levels rise, with no connection at all to the mainline’s condition.
Common infiltration sources across St. Louis’s neighborhoods include:
- Deteriorated laterals beneath older South City and Central West End properties
- Failed main-to-lateral connections stressed by clay soil shifting near the river corridors
- Cracked host pipes weakened by more than a century of service
- Manhole joint failures allowing groundwater intrusion near flood-prone low-lying areas
- Root intrusion into aging clay tile pipe common throughout St. Louis’s historic districts
Tracking the Leak Before Committing to Repair
Because river levels here rise and fall on their own schedule, engineers combine CCTV footage with flow monitoring and smoke testing across different river stages to confirm whether a leak actually sits at the main, the lateral, the connection, or the structure.
The fix should follow the defect: mainline CIPP for structural trunk-line deterioration; full lateral lining for a compromised private or public service line; targeted connection rehabilitation where soil movement has separated a joint; structural manhole rehab where river-driven groundwater pressure has cracked a structure; and injection grouting for a small, active, localized leak that doesn’t warrant a full reline.
What’s at Stake for Engineers and Contractors
MSD works under a rate structure built to fund its long-term Project Clear commitments, including the ongoing cost of pumping and treating groundwater that shouldn’t be in the system. Sewer overflows during heavy rain carry real regulatory consequences under the region’s consent decree obligations.
A system this exposed to century-old pipe and recurring river flooding needs to be managed as one connected network, mains, laterals, connections, and manholes together. Lining the main satisfies the structural side of the equation; the river-adjacent laterals and connections need their own pass before the program can claim a real infiltration reduction.
Find a WRT-certified installer serving St. Louis City and County to assess your collection system’s full scope of I&I sources.
