The feasibility of using spent sulfidic caustic as alternative sulfur and alkalinity sources in autotrophic denitrification

Batch experiments using acclimated sludge to sulfur utilizing autotrophic dentrification were performed to determine the applicability of spent sulfide caustic in autotrophic dentrification as alternative sulfur and alkalinity sources. Fluorescence in situ hybridization (FISH) analysis showed that the microbial community of beta-proteobacteria/ Eubacteria increased from 45% to 69% during enrichment period and nitrate removal reached up to 84% under this enriched sludge condition. In thiosulfate utilizing autotrophic dentrification, the initial condition at a sulfur/nitrate (S/N) ratio of 1.5 showed higher nitrate removal with 95.9%, and nitrate removal could be expressed by a first-order function of biomass concentration if all parameters such as pH, alkalinity and S/N ratio were in the optimum range. In spent sulfidic caustic utilizing autotrophic dentrification, the sulfate formation ratios to nitrate reduction were lower than those in thiosulfate utilizing autotrophic denitrification with a range of 2.65 to 2.78, and nitrate removal was over 95% at 1.0 and 1.5 S/N ratios. For S/N ratios of 1.0 and 1.5 initial alkalinities were sufficient to maintain optimum pH range of autotrophic denitrification. Furthermore, well enriched seeding sludge showed good activity of autotrophic denitrification at pH over 10. Therefore, spent sulfidic caustic could be effectively applied to autotrophic denitrification as an alternative sulfur source and an alkalinity source.