How Does Combined Sewer Separation Work in Illinois Cities?

The engineer begins with field truth. Crews survey rim and invert elevations, CCTV-inspect the pipes for structural condition, and flow-monitor the system through wet weather to calibrate the model. The hydraulic model simulates the combined system under design storms, locating the regulators, overflow points, and bottlenecks. That model becomes the baseline against which the separated design is tested: the engineer must prove the new sanitary sewer handles peak wastewater flows without surcharging and the new storm sewer handles its design storm without flooding streets. Pipe design follows the verified concept. The engineer selects the conversion direction, usually keeping the old pipe sanitary and building new storm, because the old pipe's depth and alignment already serve the buildings. New storm sewers get inlets spaced for the street's spread criteria, manholes at required intervals, and outfalls with energy dissipation and erosion protection at the waterway. The engineer checks the downstream receiving system for the added stormwater, designs any needed detention or green infrastructure to offset peak flows, and details the lateral reconnections property by property. Structural condition of the retained pipe is assessed, with lining or replacement where CCTV shows failure.

Construction sequencing is designed like a second project. The engineer divides the sewershed into phases that keep every resident on working sewer service throughout, designs bypass pumping with redundant pumps and monitored alarms for each phase, and specifies traffic control that keeps streets functional. Work is sequenced with other utilities so water main, gas, and telecom conflicts are resolved before the sewer trench opens. The engineer also plans for the surprises old streets hold: unknown utilities, contaminated soil, and timber piles under the pavement all get contingency details. Private property disconnection is where separations succeed or stall. The engineer designs the inflow and infiltration program: dye testing and inspections that find downspouts, sump pumps, and foundation drains connected to the sanitary side, then the corrective work that reroutes them to storm or daylight. This requires public outreach the engineer supports with clear exhibits, because homeowners must grant access and sometimes pay for their side of the fix. The design includes post-construction flow monitoring to prove the separation actually removed the stormwater from the sanitary system, which is the metric regulators and funders judge the project by.