Co-Thickening and Cogeneration in the Electric City

This paper focuses on sludge thickening improvements, anaerobic digester upgrades, biogas conditioning and a reciprocating engine installed at the City of Schenectady Water Pollution Control Plant (WPCP). New gravity belt thickeners (GBTs) were installed to co-thicken primary and waste activated sludge (WAS). This has increased the digester feed solids from 3% to over 6% total solids. Improved sludge thickening has increased the digester solids retention time (SRT) and volatile solids reduction (VSR), and doubled anaerobic digester gas flows for producing electricity and heat from a reciprocating engine. Electrical usage has decreased by almost 30% (kWh) and the City is saving over $30,000 per month in electricity. Sludge heating needs have been met with the waste heat alone and no supplemental fuel has been needed. Hot water recovered from the engine is also sent to hydronic unit heaters in two buildings.In addition, power is being saved by more energy efficient gravity belt thickeners than the existing dissolved air flotation thickeners (DAFTs). Less primary sludge is being bypassed around the digesters, and the quantity of sludge to be dewatered and disposed of has been reduced by ∼25% because of a longer digester SRT. Instead of continuously running the dewatering centrifuge 4 days per week, the plant has run 3 days per week.