(27e) Biological Sulfate Reduction in Fluidized Bed Reactors: Byproduct Control and Recovery

Industrial wastewaters with high sulfate concentrations ranging from 1,500 to 20,000 mg/L are produced by a large spectrum of industries of economic significance, namely, paper and pulp, sea-food processing, fermentation for starch production, fertilizer production, leather tanning, textile processing, electroplating, and mineral processing. These wastewaters with high sulfate concentrations cannot be directly discharged into water bodies under the present environmental regulations of the National Pollution Discharge Elimination Systems (NPDES) program because of the limits on the chemical oxygen demand (COD) for surface water discharges. Industrial wastewaters rich in sulfate pose a major problem for industries because they increase the total dissolved solids content, and interfere with methanogenic processes during biological treatment. The interference of sulfate with methanogenic processes results in poor production of methane in anaerobic processes, a major setback from an energy recovery standpoint.

Microbial sulfate reduction has certain advantages over other technologies besides high efficiency and potential for energy recovery. The process is effective in handling industrial wastewaters containing heavy metals because it facilitates the precipitation of metals as sulfides. Furthermore, sulfate reducing bacteria have adaptability and can acclimate themselves to different environmental conditions such as pH, temperature and toxic metals. Nevertheless, the process has certain disadvantages that must be overcome before it can be used on a variety of industrial applications. It produces hydrogen sulfide that is toxic and is inhibitive to microbial sulfate reduction and reduces the process efficiency. Additionally, hydrogen sulfide is undesirable owing to its toxicity to humans and animals, and its bad odor.

Our previous study investigated the increase in the effectiveness of microbial sulfate reduction in anaerobic fluidized bed reactor (FBR) systems for the treatment of industrial wastewaters of high sulfate content. 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. Lactate and acetate were identified as favorable electron donors for microbial sulfate reduction from thermodynamic considerations. The FBR studies were conducted for these electron donors to evaluate the biological sulfate reduction efficiency.

The present study was directed at for recovery of sulfur, methane and hydrogen as by-products of the FBR system. Furthermore, the study was intended to examine methods for hydrogen sulfide reduction in the liquid phase and the gaseous phase. An anaerobic biofiltration system (ABS) was employed as an approach for hydrogen sulfide control within the FBR system, and simultaneous recovery of elemental sulfur. Two groups of anoxygenic photolithotrophic autotrophs were identified as the major bacterial strains responsible for the transformation of hydrogen sulfide to elemental sulfur.

The ABS was evaluated for hydrogen sulfide conversion with the idea of increasing methanogenic activity of microorganisms for the production of methane or hydrogen in the FBR system. The study also examined the mass-transfer of hydrogen sulfide produced in the FBR from the liquid phase to the gaseous phase, and subsequent conversion of hydrogen sulfide to elemental sulfur. The increase in methanogenic activity potentially improved methane or hydrogen production for energy recovery. A sulfur mass balance analysis scheme was employed for the FBR system using lactate and acetate as electron donors so as to measure the quantity of hydrogen sulfide, and the amount of elemental sulfur recoverable. This approach of using an ABS conferred on FBR microbial sulfate reduction process a dual advantage of sulfur recovery and energy production.