Industrial wastewaters rich in sulfate pose a major problem for the industries because they increase the total dissolved solids content, and interfere with methanogenic processes during biological treatment. The interference with methanogenic processes results in poor production of methane in anaerobic processes, a major setback from an energy recovery standpoint. Sulfate-rich wastewaters are produced by a large spectrum of industries of economic significance, including paper and pulp production, sea-food processing, potato-starch production, tanneries, fermentation, ore processing, fertilizers, and mining. These wastewaters have such high concentrations of sulfate in the range of 1,500-20,000 mg/L, and therefore they cannot be discharged into water bodies without sulfate removal. Microbial sulfate reduction is recognized as an effective technology for removing sulfate from sulfate-rich industrial wastewaters. The sulfate reducing bacteria have adaptability and can acclimate themselves to different environmental conditions such as pH, temperature and toxic metals. Microbial sulfate reduction is particularly attractive for the treatment of industrial wastewaters containing heavy metals owing to the ability of the process to precipitate out these metals as sulfides.

The present research investigates the effectiveness of microbial sulfate reduction for the removal of sulfate present in sulfate-rich industrial wastewaters. Anaerobic fluidized bed reactor (FBR) with recycling was chosen as a favored technology for achieving the microbial sulfate reduction for wastewaters with sulfate concentrations of over 2000 mg/L. The FBR system employed granular activated carbon (GAC) as the packing medium for microbial support and its choice was attributed to its ability to promote the growth of microbial biofilms, and to remove biologically toxic and inhibitive constituents in wastewaters. 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 present approach employed a preliminary thermodynamic and kinetic approach for feasibility evaluation of each electron donor. The thermodynamic analysis involves free energy evaluations of biochemical reactions to determine a list of favorable electron donors, and to perform energy analysis of the reaction system. Initially, completely mixed batch reactor (CMBR) studies were conducted to investigate the process feasibility with each electron donor. Additionally, the effects of various biological process variables including pH, temperature, and carbon-to-sulfur ratio on the overall microbial sulfate conversion as well as reaction rates. After a preliminary screening, lactate and acetate were tested as potential electron donors. The results from CMBR studies provided the biokinetic parameters including the Monod coefficients, decay coefficient, and the microbial yield coefficient, so that the best electron donor could be determined. The estimates of biokinetic parameters and other biological parameters such as biomass concentrations, and the sulfate loading provided sufficient information for the design and operation of FBR systems for sulfate reduction in sulfate-rich wastewaters.

The FBR studies were conducted to evaluate the biological sulfate reduction efficiency under different electron donor concentrations. Furthermore, the FBR system was checked for the possibility of methane formation, and was analyzed from a mass-balance standpoint to identify and account for the fate of sulfur in the liquid and gas phases. The operational and design problems associated with the FBR biological sulfate reduction system were identified. An economic analysis was performed to identify the best electron donor under the process conditions. The FBR studies showed that high sulfate reduction efficiencies of over 85-95 percent could be achieved at high sulfate concentrations.

An important aspect of the work was the prevention of biological reaction inhibition in FBR systems due to hydrogen sulfide production. A biofiltration system was employed for the conversion of hydrogen sulfide to sulfate to prevent its accumulation within the bioactive FBR system, solve any odor problems, and significantly improve the overall process efficiency. The hydrogen sulfide was partly oxidized to elemental sulfur, a desirable aspect from the stand-point of process economics.

Key words: sulfur-rich industrial wastewaters, microbial sulfate reduction, completely, batch reactor, granular activated carbon, fluidized bed reactor, electron donors, thermodynamic and energy analysis.