Degrading malonate waste in a membrane bioreactor : modeling and experiments



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Membrane bioreactors (MBRs) combine the advantages of biological treatment and microfiltration processes. MBRs have been successfully tested for many purposes including municipal and industrial wastewater. Although numerous research efforts involving MBRs have been performed, most have been focused on the removal efficiency of the wastes being treated, rather than developing models that describe the biological and physical processes that occur in MBRs. The main objective of this research was to apply the standard biokinetic model used for conventional activated sludge to a MBR treating malonate, an organic acid contaminant found in industrial wastewater. Consequently, the biokinetic parameters synthesis yield coefficient (Y), endogenous decay coefficient (Kd), half velocity constant (Ks), and maximum specific substrate rate utilization (k) were determined. The experimental results were obtained in an 8.1 L laboratory-scale MBR fed with malonic acid, essential nutrients, and a buffer. The submerged polyethylene flat sheet microfiltration membrane (0.4 μm pore size) was obtained from Enviroquip, Inc, Austin, TX, and air was input continuously directly under the membrane to provide scouring. Monitoring included influent and effluent pH and malonic acid concentration, and the total suspended solid concentration in the reactor. The experimental values were fit to the standard biokinetic model for conventional activated sludge process to obtain values for the biokinetic parameters: Y = 0.297 g active biomass/g malonic acid, kd = 0.331 d⁻¹, Ks = 76.1 mg malonic acid / L, k = 16.8 g malonic acid / g active biomass -- d. The last step of the experimental program was to validate the model at conditions different from those used to calibrate the model parameters. The agreement between the model and experimental results suggests that the model (with its assumptions) is a reasonable description of the phenomena associated with a submerged MBR treating wastewater contaminated with malonic acid.