The Philadelphia Water Department needed more than $25 million in capital repairs and improvements to its water pollution control plant’s secondary treatment system.

Malcolm Pirnie collaborated with the water department to investigate alternative treatment processes capable of meeting discharge requirements at lower capital and operating cost. After analysis, the Pirnie team recommended a sludge-reaeration, step-feed process that employed selector zones at the aeration tank’s two primary effluent feed points.

By modifying the secondary treatment process, the Philadelphia Water Department earned a cost savings of $25 million. Over the next two years, the team developed operating procedures which improved efficiency and addressed peak wet-weather flows and loadings. This allowed final settling tanks to operate well above typical hydraulic loading rates.