Performance of Pilot-Scale Sulfate-Reducing Bioreactors Treating Acidic Saline Water Under Semi-Arid Conditions
2012
Degens, Brad P.
Groundwater drains used to manage saline watertables in the semi-arid zone of south-western Australia can discharge acidic saline water with high concentrations of metals to waterways. Mitigating the acidity impacts of the waters requires sulfate-reducing bioreactors capable of functioning under semi-arid conditions with limited source materials. Two simple pilot-scale bioreactor designs using straw and sheep manure mixtures were evaluated over several years. The bioreactors increased pH from <3.5 to >5.5 for 125–260Â days, with concurrent evidence of sulfate reduction, >85% reductions in net acidity and >90% reductions in Al and most trace elements (e.g. Pb, Cu, Ni, Zn, Ce and La). When outflow pHâ<â5.5 (remaining greater than inflows), reduction in net acidity was 10–80% but concentrations of Pb, Cu and Ni remained >80% reduced over periods of 250 to >700Â days. Rates of alkalinity generation initially exceeded 10Â g CaCO3/m2/day in both bioreactors thereafter decreasing to >1–2Â CaCO3/m2/day. Al and Fe retention was implicated in trace metal removal when pHâ<â5.5, mediated by biological alkalinity generation. High evaporation rates limited bioreactor function by restricting outflows with no benefits to alkalinity generation rates. This experiment showed that simple bioreactors can neutralise acidic waters and remove metals for short durations and show capacity for sustained reduction in acidity and metal concentrations over several years despite low alkalinity generation.
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