Conversion to Long Sludge Age Process after 17 Years at Ultra-Low SRT: Cost and Operational Benefits

The 16 million gallon per day (MGD) Fourche Creek Wastewater Treatment Plant, together with the 36 MGD Adams Field WWTP, serve the city of Little Rock, Arkansas. Both plants discharge into the Arkansas River and have discharge permit limits of 30/30 mg/L BOD5/total suspended solids, with no requirement for ammonia or phosphorus removal. Historically, both plants have operated at an ultra-low solids retention time (SRT) to avoid nitrification, and typically SRTs of less than 2 days have been consistently maintained. This operating mode was adopted primarily to save aeration energy, since a longer SRT process would result in nitrification, require a higher biomass inventory, and would consume more aeration energy.In December 2005, the Fourche Creek WWTP was converted to a long SRT operation that allowed nitrification to proceed, even though ammonia removal is not required. The objective of running the activated sludge process at the longer SRT was to simplify operations, eliminate some operational difficulties associated with the short SRT process, and improve the solids concentration of the waste activated sludge (WAS) which would also improve the anaerobic digester operation.As part of the new operating mode, a simple anoxic selector zone was incorporated into the aeration basins. After more than a year of operation, the benefits of the longer SRT together with the anoxic treatment zone have been surprising. The plant is actually consuming slightly less power than was necessary with the ultra-low SRT mode, and the past operational difficulties have been eliminated. This paper compares the process operating details before and after the conversion to a longer SRT operation. Other benefits of the longer SRT operation compared to running at an ultra-low SRT are also described.