(149c) Membrane Bioreactor Process for Purification of Groundwater Contaminated with Petroleum Hydrocarbons: Process Design and Upscaling

The present investigation relates to the application the membrane bioreactor (MBR) technology for the purification of groundwater contaminated with petroleum organics. It represents a continuation of our previous study, where a predictive model was developed and used for performance forecasting and design of the MBR process under a variety of operating and process conditions. The petroleum organics in question included aromatics such as benzene, toluene, ethylbenzene and xylenes (BTEX), and aliphatic oxygenated additives exemplified by methyl-tert-butyl ether (MTBE). The MBR system discussed here employed a powder activated carbon (PAC) slurry with microorganisms acclimated to petroleum hydrocarbons. The process was operated as a closed system to prevent the volatilization of gasoline contaminants from the aqueous phase into the atmosphere.

This is an on-going study using a modeling approach wherein the total concentration of petroleum hydrocarbons was represented by total organic carbon as a lumped parameter for MBR process design and upcaling. The model considered various phenomena pertaining to pollutant transport, sorption and biochemical degradation. With reference to substrate transport, the model considered film transfer from bulk liquid phase to the biofilm-liquid interface, biofilm diffusion, and diffusion into the adsorbent particle. The biochemical reaction occurred in the biofilm immobilized on the PAC adsorbent particle as well as in the aqueous suspended phase. These reactions described biofilm and biomass growth and decay within the MBR system, and were governed by Monod kinetics. The adsorption equilibrium and kinetic relationships controlled the sorption uptake from the aqueous phase, and release of contaminants from the adsorbent to the biofilm immobilized on the PAC as well as to the microorganisms in the suspended phase. Thus, the contaminant removal mechanisms in the process included a combination of biofilm degradation, suspended phase biodegradation, and adsorption. The MBR model parameters were determined from independent laboratory-scale experiments and correlation techniques. The adsorption equilibrium and rate parameters for the model were determined by independent batch reactor studies, while the biokinetic parameters were estimated from chemostat studies. The MBR studies involved laboratory-scale experiments with and without PAC slurry, and organic contaminant removals as well as permeate fluxes were measured in each case. These experiments established the effectiveness of the MBR technology and provided the means for model verification under different process conditions.

The present study focuses on the application of the above model in upscaling the process from laboratory-scale to pilot-scale and eventually to full-scale. Scale-up of biological sorption processes and membrane filtration processes pose considerable challenge in the realm of process design. An integrated hybrid MBR process would therefore pose a bigger challenge owing to the combination of bio-sorption and membrane systems. The modeling and upscaling technique proposed in this presentation is an efficient time-saving and cost-effective strategy for design of real processes from bench-scale or laboratory-scale systems. The scale-up process shall involve some dimensional analysis and similitude techniques, wherein the scale invariance of dimensional groups will be investigated. The process upscaling protocol proposed here takes into consideration several aspects: biodegradation of the target contaminants in the biofilm phase and/or the bulk liquid phase, sorption of the contaminants on the adsorbent particles, and membrane filtration and separation. The dimensionless groups are developed specifically for the adsorption and biodegradation phenomena associated with the MBR process. The upscaling will involve dimensionless groups that compare the relative rates of biofilm kinetics and sorption kinetics. These will involve a combination of Monod kinetics and adsorbent diffusion transfer, and liquid film transport of the target contaminant(s). Various dimensionless numbers will be employed to examine the scale invariance or variance of different phenomenon. For instance, Damkohler numbers would relate the ratio of biochemical reaction rates with the rates of film transfer, adsorbent particle diffusion and biofilm diffusion of the contaminant(s). The Sherwood number would establish the relationship between liquid film transport rate outside the adsorbent, and diffusion rate into the adsorbent (pore and/or surface diffusion). The Schmidt number would denote the ratio of mass diffusive transport to momentum transport relating the diffusion rate of the contaminant(s) to the dynamic viscosity of the fluid. Several other dimensionless groups will be used to describe the relative rates of biochemical reactions and mass-transfer. The fluid dynamic regime will be characterized by the Reynolds number, and this shall be compared to the relative influence of other non-dimensional groups of importance. Another important aspect of upscaling the MBR process shall relate to membrane module design, essentially based on permeate flux, reactor residence time, process throughput, trans-membrane pressure, and contaminant levels.

Model simulations of the MBR process will be employed to predict the dynamics of the upscaled system under different conditions based on flow characteristics, biomass requirements, adsorbent requirements, contaminant removals, and membrane permeate fluxes. The demonstration of this upscaling protocol will be based on real MBR data for laboratory-scale operations, comparison with model simulations, and subsequent upscaling to a hypothetical pilot-scale or full-scale plant.