(521f) Membrane Bioreactor Process for the Removal of BDOC, AOC, Aldehydes and THMFP From Water Supplies: Biokinetic Considerations

The membrane bioreactor (MBR) process is an efficient 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. Subsequent to ozonation, the BOM essentially consists of smaller molecules that can serve as organic substrates for microbial growth, and can constitute organic generate DBPs such as trihalomethanes and haloacetic acids. The purpose of BOM removal after ozonation using the MBR technology was twofold: preventing the growth or regrowth of pathogenic microorganisms 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 four variables, namely, biodegradable dissolved organic carbon (BDOC), assimilable organic carbon (AOC), total aledehydes, and trihalomethane formation potential (THMFP). The total aledehydes denoted the total concentration of formaldehyde, acetaldehyde, glyoxal, and methyl glyoxal.

The present study is part of the development of a predictive model for performance forecasting ting and design of the MBR process. This approach was intended to be used for the design of pilot-scale or full-scale process from the results obtained from bench-scale or laboratory-scale experiments for the removal of contaminants at low concentrations such as present and emerging endocrine disrupting chemicals (EDCs) including pesticides, pharmaceuticals, and personal care products. The same approach was employed for the design of the process for the removal of BOM constituents. The model involved the phenomenological aspects pertaining to pollutant transport, sorption equilibrium, and biochemical reaction. The model parameters were determined from independent laboratory-scale experiments and correlation techniques for each of the BOM variables of interest (DOC, AOC and total aledehydes). The adsorption equilibrium and rate parameters were obtained from batch reactor adsorption studies, while the biokinetic parameters were estimated from batch reactor biokinetic studies.

The present study constitutes the use of biokinetic studies for the determination of biokinetic parameters for BDOC, AOC, total aldehydes, and THMFP (THM precursors) at low concentrations, a considerable challenge in view of the analytical complexities of these measurements . The biokinetic parameters included the Monod maximum utilization rate coefficient, Monod half-saturation coefficient, microbial decay coefficient, and the microbial yield coefficient. In these studies, batch reactor systems were preferred to chemostat systems for several reasons. Firstly, in the removals of trace contaminants (as in this case), the biomass concentrations were typically low and the use of a chemostat system was not feasible. Secondly, BOM constituents such as BDOC and AOC were generally comprised of complex and heterogeneous mixtures, so that increasing their for chemostat studies was not practical.

These studies were conducted using high-DOC and low-DOC California State Project Waters (SPW) for determining the BDOC biokinetic parameters. In the case of AOC and THMFP the experiments were conducted for a low-DOC water (low DOC SPW), while in the case of total aldehydes a synthetic water was used that was spiked with known concentrations of total aldehydes (typically found in ozonated waters). In all these experiments, nutrients were sufficiently added so that the biochemical reactions and microbial growth were limited by the organic substrate (organic carbon) and not by nitrate or phosphate. The biokinetic data were carefully processed to estimate the biokinetic parameters using search routines and least square fits based on the Monod kinetic model.

Under conditions of high substrate and nutrient concentrations, all the four characteristic phases, namely, the initial lag phase, the growth phase (log-growth phase), the stationary phase, and the endogenous decay phase, are experienced. However, when the substrate(s) are present in low concentrations, depending upon the characteristics of the substrate(s), variations in the phases are manifested, requiring careful analysis of data for estimation of biokinetic parameters. For instance, the microbial yield coefficients were substantially different for the high-BDOC and the low-BDOC waters owing to variations in the BDOC compositions for the two cases. The biokinetic studies for total aldehydes exhibited certain important characteristics. In the case of aldehyde mixtures, it was observed that acetaldehyde was consumed first, followed by formaldehyde. Glyoxal and methyl glyoxal were utilized after the other aldehydes were depleted. Therefore, formaldehyde and acetaldehyde could be considered as surrogates for rapidly degradable BOM, while glyoxal and methyl glyoxal could be regarded as surrogates for slowly degradable BOM. The biomass growth rates and yields were generally low for aldehydes as compared to those for BDOC. The THMFP compounds (THM precursors) were not significantly degraded in these tests. Possibly these compounds could only be degraded by mature biofilm communities in full-scale biofilter systems. Biodegradation of THMFP compounds was an important aspect regarding DBP control in water treatment.