Sulfur-based autotrophic denitrification pond systems for in-situ remediation of nitrate-contaminated surface water

The feasibility of using the sulfur/limestone autotrophic denitrification (SLAD) process as an in-situ method for remediation of nitrate-contaminated surface water was investigated. Four bench-scale pond systems with working volumes of 21.4 litres each and hydraulic retention time (HRT) of 30 days were operated under either mixed conditions or unmixed conditions. Under mixed (aerobic) conditions, with the addition of alkalinity to raise pH, NO₃⁻-N removal in the SLAD ponds were 85-100%, while the control reactor showed negative removal. Sulfate production under mixed conditions, due to the activities of non-denitrifying bacteria such as Thiobacillus thiooxidans, was between 1000-2500 mg/l SO₄²⁻, which makes the application of the SLAD ponds under aerobic conditions questionable. Under unmixed (anoxic) conditions, NO₃⁻-N removal in the SLAD ponds decreased to 75-88%; sulfate production, however, also decreased by more than 50% due to the inhibition of the activity of non-denitrifying bacteria. Additional batch experiments indicated that sulfate production in the SLAD systems was acceptable under anoxic conditions. Therefore, the application of the SLAD ponds under unmixed (anoxic) conditions is feasible for in-situ remediation of nitrate-contaminated surface water.