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Membrane Bioreactor Process Model for the Removal of Biodegradable Organic Matter and Disinfection Byproduct Precursors from Water Supplies

Mark D. Williams1, Varadarajan Ravindran2, and Massoud Pirbazari2. (1) Metropolitan Water District of Southern California, 700 Moreno Avenue, La Verne, CA 91750, (2) University of Southern California, Department of Civil and Environmental Engineering, Los Angeles, CA 90089-2531

This research discusses the use of membrane bioreactor (MBR) technology for the removal of biodegradable organic matter (BOM) and disinfection byproduct (DBP) precursors from potable water sources after ozonation. It is indeed well known that ozonation of natural organic matter (NOM) in water supplies results in the cleaving of the organic macromolecules into more easily biodegradable organic matter including aledehydes and ketones. The BOM essentially consists of smaller molecules that can serve as organic substrates for microbial growth, and can constitute organic molecules that can generate DBPs such as trihalometahnes and haloacetic acids. The purpose of BOM removal using the MBR technology was twofold: preventing the growth or regrowth of bacteria in the water distribution system; and reducing the potential for forming DBPs such as trihalometahnes and haloacetic acids.

The removal of BOM was measured in terms of three variables, namlye: dissolved organic carbon (DOC), assimilable organic carbon (AOC), and total aledehydes. These parameters were measured after ozonation, and biological treatment in the MBR system. Besides these variables, the DBP formation potentials were also measured in terms of trihalomethane formation potential (THMFP). The AOC was determined using Pseudomonas flourescens P-17 and Aquaspirillum NOX bacterial strains as bioassay organisms. The total aledehydes denoted the total concentration of formaldehyde, acetaldehyde, glyoxal, and methyl glyoxal.

A predictive model was developed for performance forecasting and design of the MBR process for BOM removal. A lumped parameter approach was used in using the three variables for representing BOM (DOC, AOC and total aledehydes). The model involved the phenomenological aspects pertaining to pollutant transport, sorption equilibrium, and biochemical reactions. The model considered film transfer from bulk liquid phase to the biofilm-liquid interface, biofilm diffusion, and diffusion into the adsorbent particle, besides biochemical reaction in the biofilm and in the suspension phase. The adsorption equilibrium and kinetic relationships controlled the sorption of BOM from the aqueous phase. Therefore, the BOM removal mechanisms involved a combination of biofilm degradation, suspended phase biodegradation, and adsorption.

The MBR model parameters were determined from independent laboratory-scale experiments and correlation techniques for each of the three BOM variables of interest (DOC, AOC and total aledehydes). The adsorption equilibrium and rate parameters were obtained from batch reactor studies, while the biokinetic parameters were estimated from chemostat studies. The MBR experimental data provided model verification under different process conditions. Simulation studies provided a qualitative evaluation of the parameters influencing the MBR process dynamics under different conditions. These parameters involved liquid film mass transfer, biofilm transport, biological degradation kinetics, influent concentration, and reactor hydraulic retention time.

These experiments established the effectiveness of the MBR technology in achieving removals of BOM and DBP precursors denoted by THMFP. Both AOC and total aldehydes were completely removed under optimal operating conditions, while the DOC removal was 60 percent. The studies further demonstrated showed that PAC application not only enhanced DOC removal but also greatly increased the membrane permeate flux, improving the overall economic viability of the process.