(330f) Modeling and Design of Anaerobic Fluidized Bed Reactor Process for Sulfate Reduction in High-Strength Industrial Wastewaters

Sulfate-rich wastewaters are produced by a large spectrum of industries including paper and pulp production, sea-food processing, potato-starch production, tanneries, fermentation, ore processing, fertilizers, and mining. Biological sulfate reduction is recognized as an effective technology for removing sulfate from such industrial wastewaters with sulfate concentrations in the range of 1,500-20,000 mg/L, and can be economically and efficiently accomplished using an anaerobic fluidized bed reactor with recycling (FBRR). The sulfate reducing bacteria in the FBRR process have the adaptability to acclimate themselves to different environmental conditions such as pH, temperature and toxic metals. The technology is particularly attractive for the treatment of industrial wastewaters containing heavy metals owing to its ability to precipitate out metals as sulfides. The FBRR system has several advantages over conventional fixed film or suspended growth technologies. It can provide 5 to 10 times the microorganism concentrations as compared to suspended growth processes. Additionally, it can easily handle shock loads, and can recover quickly after shutdowns. Furthermore, it requires relatively smaller reactor size and is amenable to easy operation and maintenance, experiencing fewer hydraulic problems. The present research was directed at the modeling and design of anaerobic FBRR process for sulfate removal of sulfate present in sulfate-rich industrial wastewaters. The process employed granular activated carbon (GAC) as packing medium owing to its ability to promote microbial growth and to resist shock to biologically toxic and inhibitive constituents. Lactate and acetate were chosen as electron donors based on thermodynamic, economic and biokinetic considerations. The FBRR studies were conducted to evaluate the process performance as a function of several variables including influent sulfate concentration, electron donor, carbon-to-sulfur ratio, and pH.

The FBRR system proved a reliable, efficient, and cost effective technology for removing sulfate from sulfate-rich wastewaters. The experiments demonstrated that sulfate reduction efficiencies of over 85-95 percent were achievable at high influent sulfate concentrations. A mathematical model was therefore employed for predicting/simulating the FBRR process dynamics under different process or operating conditions. The FBBR model incorporated the following fundamental mechanisms: (a) substrate transport from bulk liquid to biofilm through an external liquid film, (b) mass transfer and degradation within the biofilm, (c) growth of biofilm, and (d) suspended biofilm in bulk liquid solution. Some important assumptions of the FBBR model are discussed below. The GAC particles are spherical and homogeneous; and the biofilm developed on GAC is also homogeneous and grows with time. The substrate uptake by GAC particle involves a one-step mass transport mechanism of liquid film mass transfer followed by biofilm diffusion. The substrate concentration profile across the biofilm is in pseudo-state, although the biofilm thickness varies with time. Biodegradation and substrate utilization, biomass growth and biofilm development are represented by Monod kinetics. Biodegradation occurs in both the biofilm layer and bulk liquid phase, but not within the GAC particle. The biofilm growth does not affect the porosity of the fluidized bed or associated fluid flow patterns. The biomass loss due to fluid shear from high superficial velocities in the bed is negligible during the initial stages when the biofilm is thin. However, biomass loss due to shear and decay balances the new biomass formed, so that a steady-state maximum biomass concentration is reached within the bed. The mixing and fluidization in the FBRR is achieved by high recycle ratios and upflow velocities.

The FBBR model equations were solved using the MATLAB partial differential equation solver in conjunction with the Runge-Kutta method of order-five. The important parameters in the model were as follows: (1) bioreactor size, (2) biomass concentration and biofilm thickness, (3) bed expansion due to hydraulic flow and biofilm growth, (4) expansion index and expanded bed height, (5) type, size and density of support media, and (6) superficial fluid velocity in the reactor. The most important FBRR design parameters were substrate per unit of biomass, and biomass per unit mass of support medium. Model calibration was based on biological and transport parameters determined from independent laboratory experiments and/or correlation techniques. Chemostat studies were employed to determine the Monod coefficients and other biological parameters. They also provided useful information for FBRR design such as biomass concentrations, biomass loading, sulfate loading and electron donor levels. The FBRR studies were conducted under different experimental and operating conditions to determine the process efficiency as a function of time. The FBRR model was tested and validated for different process operating conditions. Process design and upscaling strategies were developed, and non-dimensional groups relevant to process dynamics were identified. The FBRR model successfully predicted the process dynamics with reference to sulfate removal and carbon source utilization, and was validated by experimental results. Anaerobic biofiltration studies demonstrated that the technology was the preferred treatment strategy for handling the hydrogen sulfide product stream from the FBBR system. The recovery of elemental sulfur from the biofiltration system proved important from an economic standpoint.