(364c) Design of Anaerobic Fluidized Bed Reactor Technology for Microbial Sulfate Reduction in Surface and Ground Water Sources

Sulfate contamination in surface waters and ground waters is a significant  problem in the United States.  The major sources of sulfate contamination are rock weathering, volcanoes, and biochemical processes.  Human activities such as mining, waste discharge and fossil fuel combustion were also important sources. Sulfate is often used as a means to kill those noxious algal blooms due to the presence of excess nutrients, nitrogen and phosphorus.

                The wide use of sulfate in agricultural areas has created serious environmental problems in rivers and lakes. Similar problems have been caused across the nation due to mining of sulfide and subsequent oxidation f sulfide to sulfate The Upper Potomac River in the Maryland-District of Columbia region was identified as a major area affected by sulfate contamination due to agricultural runoff of sulfate fertilizers. In the lakes of Minnesota, sulfate contamination arises from mining activities associated with sulfide ores.  In the Florida Everglades covering the neighborhoods of Miami and Fort Lauderdale, the sulfate added to control algal blooms in wetland marshes has led to sulfate contamination.  The reactions of mercury (from rainfall) and sulfate results in a “cocktail” of Methyl-mercury, a highly toxic substance causing severe environmental and ecological problems.   Similar problems have been experienced in the Missouri and Mississippi regions due to agricultural and industrial sulfate discharges.

            Microbial sulfate reduction is recognized as an effective technology for removing sulfate from sulfate contaminated water sources.   The sulfate reducing bacteria have adaptability and can acclimate themselves  to different environmental conditions such as pH, temperature and toxic metals.  Microbial sulfate reduction can be economically and efficiently accomplished using anaerobic fluidized bed reactors with recycling.  The sulfate contamination of surface water sources  in  the United States was in the range of 400-1500 mg/L.           

            The present  research is directed at  evaluation and design of anaerobic fluidized bed reactor (FBR) process for  the removal of sulfate present in surface waters and potable water sources.  The FBR process employs granular activated carbon (GAC) or sand as the packing medium for microbial support.  The GAC was chosen owing to its ability to promote microbial growth and to resist shock to biologically toxic and inhibitive constituents. However, sand is sometimes preferred as a medium for biofilm support owing to its low cost and easy availability. One important aspect of the technology was the choice of appropriate electron donor as several electron donors are currently available including hydrogen, acetate, lactate, methanol and ethanol.  The process was based on a preliminary thermodynamic and kinetic approach for feasibility evaluation of each electron donor.  Initially, completely mixed batch reactor (CMBR) studies were conducted to investigate the process feasibility with each electron donor and to determine the biokinetic parameters for sulfate reduction.  These included  the Monod maximum substrate utilization and half-saturation coefficients, the decay coefficient, and the yield coefficient. The effects of various biological process variables including pH, temperature, and carbon-to-sulfur ratio on the extents and rates of overall sulfate conversion were evaluated.  After a preliminary screening, lactate and acetate were tested as potential electron donors. The estimates of biokinetic coefficients and other biological parameters such as biomass concentrations, and sulfate loading provided sufficient information for modeling and designing FBR systems. 

            The FBR model that we have developed was tested for performance prediction and forecasting of the microbial sulfate reduction process.  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 FBR 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. The model was tested under two different scenarios:  (1) employing GAC as an adsorbing medium, and (2) using sand as a non-adsorbing medium for biofilm formation.   Chemostat studies were employed in an earlier investigation to determine the Monod coefficients and other biological parameters. They also provided useful information for FBR design such as biomass concentrations, biomass loading, sulfate loading and electron donor levels for the model. 

            The FBR studies were conducted under different experimental and operating conditions to determine the process efficiency as a function of time.  The FBR 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 FBR model successfully predicted the process dynamics with reference to sulfate removal and carbon source utilization, and was validated by experimental results. 

            The results of this study demonstrated that the FBR system represents a reliable, efficient, and cost effective technology for removing sulfate from potable water supplies. It was found that the FBR system using GAC was significantly more efficient than the FBR using sand in sulfate reduction.  However, the FBR using sand had certain advantages in so far as requiring lower hydraulic retention time, and therefore entailing smaller reactor sizes and lower energy costs. The FBR model successfully predicted the process dynamics with reference to sulfate removal and carbon source utilization.  Furthermore, it was found useful in the performance prediction of laboratory-scale FBR systems and provided the means for process upscaling to pilot-scale systems using dimensional analysis and similitude techniques.