Sulfur:limestone autotrophic denitrification processes for treatment of nitrate-contaminated water: batch experiments

In this study, an innovative process of using sulfur:limestone autotrophic denitrification (SLAD) for treatment of nitrate-contaminated surface or wastewater was put forward. The feasibility of this SLAD process was evaluated using lab-scale batch reactors operated under both aerobic and anaerobic conditions. Autotrophic denitrification occurred in batch reactors spiked with sulfur:limestone (S:L) under either aerobic or anaerobic conditions at both high (ca. 300–500 mg NO3−–N/l) and low (ca. 30 mg NO3−–N/l) initial nitrate concentrations. Nitrate–nitrogen removal increased with the addition of granular sulfur and limestone, while the addition of a seed of autotrophic denitrifiers into the batch reactors accelerated nitrate removal. Limestone was necessary to control the pH within the reactors. The optimum sulfur:limestone ratio was 3:1 (v/v) and the extent and rate of nitrate removal depended on the alkalinity within SLAD batch reactors. Nitrate removal efficiency, sulfate production and biomass accumulation were usually higher under aerobic conditions than under anaerobic conditions. Bacterial counts signified that both autotrophic denitrificans and nondenitrifying bacteria such as Thiobacillus thiooxidans were involved in the process under aerobic conditions. The SLAD process may be a replacement for heterotrophic denitrification in pond systems such as constructed wetlands or stabilization ponds due to the fact that no organic carbon source is needed in the SLAD process and that autotrophic denitrificans exist widely in natural sediments or soil.