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A comparison study of agricultural materials as carbon sources for sulphate reducing bacteria in passive treatment of high sulphate water Brown, Amber

Abstract

Several natural and agricultural organic materials were assessed as carbon sources for a potential passive sulphate reduction treatment system to treat lake and pond water containing high concentrations of sulphate for the purpose of creating drinking water for cattle. Of primary concern was that the system be low cost and simple to operate, since the end users would be farmers. Sediment removed from a low sulphate lake located near Kamloops, which contained a potentially highly active sulphate reducing bacteria (SRB) population was used in a mesocosm experiment to determine its effectiveness at supporting sulphate reduction. Originally operated as a continuous system, a flowrate of 6 mL min⁻¹ flushed the nutrients out of the system. While operating in batch mode, sulphate reduction was observed within the sediment, but not above in the water layer due to mass transfer limitations. Thus, recirculation of water through the sediment was required to increase the overall rate of sulphate-reduction. Four different agricultural materials; barley, molasses, hay, and silage, were supplied to a SRB inoculum and concentrated SO₄²⁻ water to test their effectiveness as nutrient sources. A 1:1 (wt) mixture of hay and silage showed the highest drop in SO₄²⁻ and achieved a maximum SO₄²⁻ reduction rate of 14.4 mg SO₄²⁻ L⁻¹d⁻¹. A second study, which tested the leachates of barley and orchard grass silages, found similar rates (19.4 ± 1.6 mg SO₄²⁻ L⁻¹d⁻¹). Based on this work, a pair of mesocosms were built and supplied with a mixture of silage and hay. These systems achieved initial maximum rates of 33.8 mg SO₄²⁻ L⁻¹d⁻¹. After a second sulphate addition the maximum rate increased to 132.4 mg SO₄²⁻ L⁻¹d⁻¹. The ΔSO₄²⁻ / ΔsCOD ratios were determined to be 1.10 and 0.91 g g⁻¹ for mesocosms A and B. A halt in sulphate reduction was concomitant with an increase in sulphide concentration to 323.5 mg L⁻¹. It is hypothesized that sulphide inhibition of the SRB occurred. Recycling the water through an aerobic treatment system containing sulphur-oxidizing bacteria, to oxidize the sulphide to sulphur, is one possible method to remediate these high sulphide levels.

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