Bioreactor applications utilizing mesophilic sulfate-reducing bacteria for treatment of mine wastewaters at 9-35 oC

Abstract

The exploitation of low-grade ores, treatment of wastewaters of mining activities, hydrometallurgical recovery processes and bioremediation of metal contaminated environment require novel and economical bioprocesses. Biotechnology has recently been introduced to mining technology, including for example bioleaching and biological metal recovery processes. The biological processes are a low cost option to traditional mining and metallurgical processes. The exploitation of metals is being focused to low-grade ores and to deposits located at high altitudes and northern regions having demanding environmental conditions. The mining operations are expensive, and introduction of bioprocess technology to mining processes may increase the profits of the operation. On the other hand, the mine wastewaters and metallurgical effluents produced in active mines also at low ambient temperatures have to be treated, and there is limited information on the bioprocess operation at sub-optimal temperatures. Acid mine drainage (AMD) is continuously being produced in old mines, also in those located at cold regions. The quantity of AMD production may be large, although the temperature may affect on the rate of the AMD formation. Heating of a bioreactor and wastewater stream or AMD to the optimal temperature of the biological treatment process may not be feasible, thus the low temperature biological mine wastewater treatment is a compromise between the microbial activity, temperature and reactor size. Biological sulfate reduction provides simultaneous treatment of the major pollutants of acid mine drainage and mine wastewater: sulfate and metal concentrations are decreased, metals are precipitated as low soluble sulfides and the acidity of the solution is neutralized by biologically generated alkalinity. In a chemical process all these steps would require several unit processes and careful control of pH. The biological mine wastewater treatment with sulfate reduction has several benefits when compared to chemical precipitation with lime: the metal and sulfate concentrations in the biologically treated effluent are lower, and the produced sludge is more stable, dense and has high re-use potential. The objective of the present study was to develop sulfate reducing bioprocess technology for mine wastewater and AMD treatment. The limiting factors in the use of sulfate reducing bioreactors for mine wastewater treatment can be divided to two categories: 1) the costs of the bioreactor operation due to electron donor and heating and 2) the limitations of the sulfate reducing bacteria (SRB), which do not tolerate high metal concentrations and acidity. Because the tolerance of SRB for mine wastewater treatment can be resolved with reactor technological solutions, e.g. dilution and solution recycling in the process, the aim was to focus on studying the electron donors and activity of SRB at sub-optimal temperatures. There is limited number of publications describing low temperature sulfate reducing bioprocesses. In present study, a low temperature formate-fed sulfate reducing fluidized-bed bioreactor (FBR) treated synthetic and real mine wastewater at 9°C with stable sulfate reduction rate of 8-14 mmol SO42- L-1 d-1, and high metal precipitation, 5.4 mmol Fe L-1 d-1 (99% precipitation), was achieved. The microbial community and the active species of the low temperature sulfidogenic FBR were analyzed with denaturing gel gradient electrophoresis (DGGE). The results showed that this reactor was dominated by a mesophilic SRB Desulfomicrobium sp., which was also the active species in the reactor. Therefore, the long-time operation at low temperature resulted in enrichment of psychrotolerant mesophilic SRB. Since formate is not a commercially feasible electron donor, further experiments were made with hydrogen-fed membrane bioreactors (MBR) and gas-lift bioreactors (GLB) at 9°C, resulting in sulfate reduction rates of 6.9 and 6.2 mmol SO42- L-1 d-1 in these reactors, respectively. The specific sulfidogenic activities in these bioreactors were 1.6-33 mmol SO42- g VSS-1 d-1, demonstrating that high biomass activity can be achieved in low temperature. Sulfate reduction consumed majority of the electrons in these reactors, while acetate production from homoacetogenesis consumed a minor part of the electrons when the temperature was low and the reactor retention time was long. The temperature dependency of the sulfate reduction of the enrichment culture used in the low temperature bioreactor was analyzed, and the optimal temperature was 31°C, demonstrating that this was a psychrotolerant mesophilic enrichment culture. Therefore, the following membrane bioreactor experiments included also operation at 15 and 30-35°C with a reference mesophilic enrichment culture. The operation at these temperatures showed that mesophilic SRB processes can be operated at sub-optimal temperatures, but the activity is decreased by 10-40 % at 15°C when compared to optimal temperature. The activity of SRB at sub-optimal temperature is limited by transport and oxidation rate of the electron donor, because the electron flow to sulfate reduction and specific sulfidogenic activity decrease with the temperature. Mine wastewater treatment at 35°C was studied using fluidized-bed bioreactor fed with ethanol-lactate mixture. The sulfate reduction rate was high and stable, being 62-100 mmol SO42- L-1 d-1, and the metal precipitation rates were 11 mmol Fe L-1 d-1 (99% precipitation) and 1 mmol Zn L-1 d-1 (99% precipitation). This experiment included also biological hydrogen sulfide production experiment, where sulfide production rate of 73 mmol H2S L-1 d-1 was obtained. The sulfate reduction rate in this FBR was limited by the acetate oxidation rate, which was at maximum 50 mmol acetate L-1 d-1. Therefore, the acetate oxidation kinetics of this reactor process was studied, and kinetic constants for acetate oxidation were defined. The Km, affinity for acetate was 63 µmol, indicating high affinity for acetate. The maximum acetate oxidation rate, Vmax, was 0.76 µmol g VSS-1 min-1. These results demonstrate that although the enrichment of acetate oxidizing SRB is slow. The acetate oxidation rate controls the treatment capacity of the bioreactor fed with an organic electron donor. Pure electron donors, such as ethanol and hydrogen are expensive. Therefore, low-cost options as electron donors are needed. Therefore, the amenability of reed Canary grass (Phalaris arundieace) plant material hydrolyzate as electron donor for mine wastewater treatment was studied. The experiments were performed with a fluidized-bed bioreactor, and sulfate reduction rate of 21-34 mmol SO42- L-1 d-1 and metal precipitation of was 15 mmol Fe L-1 d-1 (99% precipitation) were achieved, although the acetate oxidation rate limited the process. Also the suitability of the dry reed Canary grass plant material as substrate for sulfate reduction was demonstrated in batch assays with H2S yield of 0.8 mmol H2S g-1 plant material. For comparison, the H2S yield with the hydrolyzate was 6.2 mmol H2S g-1 plant material. In summary, the experiments conducted for this thesis increased the knowledge on the achievable sulfate reduction rates and treatment capacity of mesophilic SRB at sub-optimal temperatures with several bioreactor types. It was demonstrated that mesophilic SRB could be enriched and long-time maintained in active state at low temperature sulfidogenic bioreactors. The microbiology and metabolic capacities of mesophilic SRB at decreased temperatures were studied. The suitability of sulfidogenic fluidized-bed bioreactors for mine wastewater treatment and biological hydrogen sulfide production was demonstrated with a number of electron donors, including also a potential low-cost electron donor, the plant material hydrolyzate.