Nitrate Removal in Sulfur: Limestone Pond Reactors

Abstract

The feasibility of using sulfur:limestone autotrophic denitrification (SLAD) pond reactors to treat nitrate-contaminated water or wastewater after secondary treatment was investigated with four lab-scale continuously fed SLAD ponds. The start-up period, temperature effects, and effects of different feed solutions were evaluated. With an influent concentration of 30 mg NO3--N/L at an HRT of 30 days, the pond reactors had an overall nitrate removal efficiency of 85–100%. Effluent nitrite concentrations were <0.2 mg N/L in all tests. Aerobic conditions could result in a decrease of the SLAD pH of the pond by 2 to 3 units and a large increase in sulfate production (∼1600-1800 mg-SO42-/L). Under unmixed (anoxic) conditions, the pH and sulfate produced were maintained at approximately 5.5 to 5.6 and 400-600 mg-SO42-/L, respectively, in all the SLAD ponds. Temperature affected the pond reactors adversely. By assuming that a first-order reaction occurred in a SLAD pond reactor, the temperature-activity coefficient, θ was found to be 1.068. Treatment of nitrate-contaminated surface water and wastewater using SLAD pond systems is feasible only if (1) the chemical oxygen demand (COD)/nitrate–N (COD/N) ratio is low (<1.2 with an initial NO3- concentration of 30 mg-N/L), (2) sulfur:limestone granules are not covered by sediment, and (3) sulfur-utilizing but nondenitrifying bacteria (SUNDB) are greatly inhibited due to the lack of DO in the pond systems. The SLAD ponds are not feasible for the treatment of raw wastewater or surface water if they contain high concentrations of organic matters due to the possible inhibition of sulfur-based autotrophic denitrifiers by heterotrophs (including heterotrophic denitrifiers). In addition, a high sulfate and low DO concentration as well as a low pH in the SLAD effluent of the pond (even when the pond is operated in an unmixed mode) also will limit the application of SLAD pond processes.