Treatment of high sulfite refinery wastewater by conventional activated sludge

Abstract

A clean fuels project at a U.S. refinery included the addition of a gasoline desulfurization unit that generates SO2 offgas. The offgas will be scrubbed in a caustic scrubber before released to the atmosphere. The SO2 will primarily be converted to Na2SO3 (sodium sulfite) and to a lesser extent to NaHSO3 (sodium bilsulfite) and NaSO4 (sodium sulfate). The scrubber blowdown will require treatment to oxidize the sulfites to sulfate. Two options have been identified to treat the scrubber blowdown: (1) Oxidation in the Refinery’s activated sludge wastewater treatment plant and (2) Oxidation in a new air oxidizing system. A preliminary evaluation of (1) indicated that the existing activated sludge system would be capable of treating the scrubber blowdown with minimal impact on its hydraulic and treatment capacity. However, there were some concerns regarding its implementation because of potential inhibitory effects, non compliance with Whole Effluent Toxicity (WET) test, and incomplete oxidation of sulfites to sulfate in the activated sludge system. In order to address these concerns, a bench scale simulation of the activated sludge system was operated for six weeks while being fed with wastewater from the Refinery. Results of the study are presented herein.