Removal Mechanisms Involved in the Petroleum Refinery Wastewater Treatment by MBBR System

Effluents bearing petroleum hydrocarbons must be treated prior to release into the environment due to the high toxicity of these compounds and the consequent hazards they pose to the receiving environment. The moving bed biofilm reactor (MBBR) is one of the several biofilm systems used in treating different types of wastewater that offers a good resistance to both toxic and hydraulic shocks. This study investigates the roles of the three air stripping, adsorption, and biodegradation mechanisms in the biofilm reactor using polyethylene sponge strips used as the biomass to treat the effluent from Tehran Refinery. Based on the results obtained, the optimum values of loading rate, retention time, and media filling ratio were equal to TPH=278 mg/L (COD=1000 mg/L), 22 hours, and 50 percent, respectively. Laboratory studies confirmed that each of the three mechanisms of air stripping, adsorption, and biodegradation may be involved in the removal of petroleum hydrocarbons by the MBBR system depending on hydrocarbon concentrations. More specifically, biological degradation was the dominant removal mechanism with lower loading rates (TPH=57 mg/l with a COD equal to 200 mg/l) while stripping became the dominant mechanism at higher loading rates (TPH=278 mg/l with a COD equal to 1,000 mg/l). It was found that under optimum conditions and at TPH=278 mg/L (and COD=1,000 mg/l), stripping removed 58%; biodegradation, 29%; and adsorption, 13% of the petroleum hydrocarbon content. It was observed that the removal of petroleum hydrocarbons by stripping and adsorption processes took place within the first process hour at the latest.