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%.ii
Process ChemicalsCaustic soda, sulphuric and phosphoric acid,
MicroSludge ® is a patented chemical and pressure pre-treatment process that liquefies waste activated sludge (WAS) to increase both the rate and extent that WAS is degraded in an anaerobic digester. The process uses chemical pre-treatment to weaken cell membranes and then a high-pressure homogenizer, or "cell disrupter", to provide an enormous and sudden pressure change to burst the cells in WAS. The resulting solubilized WAS is then more readily converted to biogas. At the Los Angeles County Sanitation Districts' (LACSD's) Joint Water Pollution Control Plant (JWPCP), a MicroSludge System with two 4,000 L/h cell disruption modules was continuously operated for 24 hours per day from October 2005 to October 2006 to process approximately 192 m 3 /day (50,000 GPD) of thickened WAS at 5 to 6% total solids (TS) concentration. The processed WAS was co-digested with primary sludge (PS) in a 75:25 by volume (approximately 68:32 by mass) PS:WAS mix at a nominal HRT of 19 days. The MicroSludge fed digester performance was compared to that of one of the other 23 digesters operated under similar conditions at the JWPCP, except that the WAS feed was not pre-treated.
Recently several technologies and process innovations have been proposed to reduce or eliminate the waste activated sludge (WAS) fraction resulting from treatment, or to render it more amenable to anaerobic digestion, a common sludge stabilization step, in order to maximize biogas production and the corresponding potential for energy cogeneration. Under WERF's 05-CTS-3 Evaluation of Processes to Reduce Activated Sludge Solids Generation and Disposal, data collected from the full-scale demonstration of MicroSludge TM , a WAS reduction technology, at the Des Moines Water Reclamation Facility was independently evaluated. Results indicate that sludge pre-treatment ahead of mesophilic anaerobic potential did improve biogas production rates and yields as well as volatile solids reduction under most of the tested conditions. Results also indicated lower odor generation potential for the MicroSludge TM pretreated sludge before co-digestion of primary and WAS, and an ability to destroy filamentous organisms that could be precursors to foaming problems in anaerobic digesters. However this technology did not have a significant impact on pathogen reduction ahead of the anaerobic digestion process
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