Use of different microbial sources and S-reduced compounds for high nitrate concentration removal from mining wastewater

Chemolithotrophic denitrification is a promising process for nitrate removal from organic deficient waters such as mining wastewaters. Thiosulfate and elemental sulfur serve as source of energy for chemolithotrophic denitrifiers and are generally available in mining environment. Extremely high concentrations of nitrate can occur in mining and metal processing wastewaters due to the use of ammonium-nitrate fuel oil (ANFO) as blasting agent and other nitrogen-containing reagents during processing. In this study, the potential of four different microbial cultures to remove 1860 mg/L of nitrate using thiosulfate and elemental sulfur as electron donors was investigated in batch bioassays. A pure culture of Thiobacillus denitrificans, ethanol enriched denitrifiers and activated and methanogenic sludge were tested under different initial conditions. T. denitrificans completely reduced the feed nitrate in 8 days using thiosulfate as electron donor, whereas the use of elemental sulfur resulted in slower and incomplete denitrification due to sulfur mass transfer limitation. The initial S/N ratio affected denitrification depending on the microbial culture and the electron donor used. Increase in the initial VSS concentration enhanced denitrification. Sulfur-driven denitrification was stimulated at pH 8 and 30°C. Lower grain sizes and high S/N ratios enhanced sulfur mass transfer and resulted in higher nitrate removal rates.