Innovative DBO Two-Stage Activated Sludge Upgrade at the Woonsocket WWTF to Achieve 3 mg/L TN and 0.1 mg/L TP

In June 2012, the City of Woonsocket selected CH2M to assume operations of the existing facility, perform design-build upgrades, followed by long-term operations of the improved wastewater treatment facility. A key objective is to reduce nitrogen and phosphorus loads to the Blackstone River and thereby make a positive impact on water quality. Another significant consideration at this facility is that it also has a regional sludge receiving/incineration facility. Sludge loads to this on-site sludge management facility are above five times the load from the Regional Woonsocket Wastewater Treatment Facility itself, and the return streams from thickening, dewatering, and incineration are high in nutrients and organics.The operational goal for the facility is to achieve a maximum monthly average effluent of 3.0 mg N/L total nitrogen and 0.1 mg P/L total phosphorus during the summer nutrient removal season. Until May 1st, 2017, the plant was only required to achieve a 10 mg/L TN and 1 mg/L TP limit during the nutrient removal season. It was decided to convert the facility to a two-stage activated sludge facility to take best advantage of the wastewater characteristics and the existing equipment at the plant. The first stage activated sludge system was designed to do the bulk of the nutrient removal, and the second stage was designed to polish the first stage effluent to achieve these very low nutrient levels.The upgrade of the Woonsocket WWTF through the DBO process, provided the City of Woonsocket significant capital and operating cost savings. The innovative use of North America’s first new two stage activated sludge system in many years has exceeded performance expectations in both effluent quality and ability to deal with high nutrient loads. Additionally, the collaboration between the City of Woonsocket, Weston & Sampson, and CH2M realized additional savings through process innovation and managing risks, while still reliably producing some of the lowest levels of effluent nutrients in the world.