Engineering Critical Upgrades to Boost Treatment Capacity and Reduce Sewer Overflows

Allegheny County Sanitary Authority's (ALCOSAN) long-term Clean Water Plan, established under the 1996 Act 537, aims to reduce combined sewage overflows by 7 billion gallons per year and prevent sanitary sewer overflows into the region’s rivers during rainfall events. A key effort of the plan is expanding wet weather treatment capacity — from 250 million gallons per day (MGD) to 295 MGD by 2025 — to meet a consent decree requirement. To support this goal, HDR was selected in 2018 to design and provide construction management support of the North-End Plant Expansion, which added a new outfall, a disinfection facility, and secondary clarifiers.

The project includes significant secondary treatment improvements such as two 141-foot-diameter final settling tanks, new return activated sludge (RAS) pumping stations, mixed liquor channels and secondary effluent conduit extensions, automated controls, and new electrical power feeders. Disinfection improvements include two new chlorine contact tanks, new sodium hypochlorite and sodium bisulfite storage tanks, a new disinfection building, new sampling systems, a new outfall to the Ohio River, and relocation of a major substation. Our team was also tasked with the challenge of keeping the existing secondary treatment facilities operational throughout construction.

Another major challenge was the constrained site, bounded by railroad tracks and the Ohio River. A river wall was constructed that encloses one acre of usable land from the Ohio River, which provides additional area for the construction of the new north end Chlorine Contact Tanks and Final Settling Tanks. The expansion also prepares ALCOSAN for increased biological loads by staging new RAS piping beneath the plant.

Completed in early 2026, the North-End Plant Expansion is a key milestone toward achieving 295 MGD of secondary treatment capacity and improving regional water quality.