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Removal of oxygen demand and acute toxicity during batch biological treatment of a petroleum refinery effluent Sarathy, Brihas Pathi

Abstract

This study was undertaken in order to investigate the removal of oxygen demand and acute toxicity from a petroleum refinery effluent. The results of this study provide a better understanding of the biodegradation process occurring at the Chevron refinery wastewater treatment plant (WWTP) in Burnaby, BC. The WWTP consists of a deep shaft bioreactor followed by a dissolved air flotation clarifier and effluent polishing biofilters. The treatment plant is able to degrade approximately 75% of the COD and 95% of the BOD present in the wastewater. During the study period, the toxicity of the wastewater before and after treatment was 1.9±0.12 (%v/v) and 25.4±5.9 (%v/v), respectively (as measured by the 5 minute Microtox™ assay in EC₅₀ units). The experimental program utilized two batch bioreactors operated at 35°C to characterize the removal of organic compounds (as measured by BOD and COD), and to deterrnine the capacity of the biomass for acute toxicity removal. A six-litre batch bioreactor was operated for 52.5 hours (Run 1), and a 15 L batch bioreactor was operated for 120 hours (Run 2). In order to assess the abiotic rate of volatilization, a stripping test was also performed using the six-litre batch reactor. In both runs, the BOD and COD reduction occurred over the first 24 hours of treatment. The BOD removal was 92-96%, and the COD removal was 73-75%. These values are similar to the removal levels reported at the Chevron refinery WWTP. Stripping accounted for 3% of the COD removal over 52.5 hours. Since stripping was insignificant, the compounds contributing to BOD and COD were most likely removed via biodegradation. Both runs showed similar patterns of toxicity removal. Rather than being continuously removed, as in the case of BOD and COD, toxicity appeared to be removed in discrete stages. The first stage of toxicity removal corresponded to the degradation of BOD and COD, and the second stage occurred after BOD and COD had been removed. The raw wastewater in Runs 1 and 2 had a 5-minute EC₅₀ of 4.6+0.5% and 4.9±0.4%, respectively. In Run 1, the toxicity was reduced to a 5-rninute EC50 of 7.9±0.7% in 10 hours, and further removal did not occur until after 28 hours. In Run 2, a 5-minute EC₅₀ of 16±3.2% was achieved over the first 10 hours, and the toxicity remained at this level until 48 hours. A second significant toxicity removal step between 48 and 72 hours resulted in a final 5-minute EC₅₀ of 27.8±1.6%.

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