Publication Date
4-8-2008
Abstract
In the U.S. the organic chemical industry is ranked third in volatile organic compound (VOC) emissions, after the petroleum refining and ground transportation. VOCs are a significant health concern because of atmospheric reactions to form ozone. Many VOCs are also air toxics regulated under the USEPA Clean Air Act Amendments. This research investigated the performance of a membrane bioreactor (MBR) for treatment of wastewater containing VOCs from synthetic resin manufacturing. In Phase I, a mathematical model was developed to simulate volatilization, adsorption and biodegradation of three VOCs, acetaldehyde, butyraldehyde and vinyl acetate, at varying solids residence times (SRT), organic loading rates (OLR) and dissolved oxygen (DO) concentrations. Model results showed that biodegradation is maximized and volatilization minimized for suspended growth systems operated at high SRT and OLR, typical MBR operating conditions. A bench scale MBR was operated with synthetic chemical industry wastewater at varying OLRs (1.1 to 2.0 kg COD m-3 d-1). Greater than 99 % BOD5 and COD removal was achieved at all OLRs. Biodegradation was the main removal mechanism, with <12.5% acetaldehyde, <2.0% butyraldehyde and <14.0% vinyl acetate removal due to volatilization. The model predicted greater volatilization than the bench scale results, possibly due to the use of literature values for biodegradation kinetic parameters, rather than site specific measurements. The effect of DO concentration on membrane filtering resistance, soluble organic matter (SOM) and extracellular polymeric substance (EPS) characteristics was investigated in Phase II of this study. The bench scale MBR was operated and under DO limited (0.2 mg L-1) and fully aerobic (3.7 and 5.4 mg L-1) conditions. Membrane filtering resistance was determined for the MLSS and resuspended microbial biomass after removing the SOM. Regardless of DO concentration, the cake resistance (Rc) was approximately 95% of the total resistance (Rt) and cake resistance decreased significantly after removing the SOM. Under DO limited conditions, SOM contained a larger amount of high molecular weight compounds, leading to higher cake resistance than under fully aerobic conditions. Organic carbon and protein concentrations in the bound EPS were linearly correlated with total membrane resistance (Rt) of the resuspended microbial biomass.