Column studies on nitrate removal from potable water

Biological processes can achieve nitrate removal from groundwater. The sulfur/limestone autotrophic denitrification by Thiobacillus denitrificans was evaluated with three laboratory-scale column reactors. The optimum sulfur/limestone ratio was determined to be 2: 1 (mass/mass). Different hydraulic retention times were used during the column tests to examine nitrate removal efficiencies. Under an HRTs of 13 h, nitrate concentration of 60 mg NO3--N L-1 was reduced to less than 5 mg NO3--N L-1. On a higher HRT of 26 h the nitrate removal efficiency was close to 100% for all nitrate-nitrogen loading rates. Different initial nitrate-nitrogen concentrations (30, 60, and 90 mg NO3--N L-1) were used in the study. Column tests showed that the nitrate-nitrogen loading rate in this study was between 50 to 100 g NO3--N m(-3) d(-1) to obtain a removal efficiency of 80-100%. It was found that approximately 6 mg SO42- was produced for 1 mg NO3--N removed. Nitrite-nitrogen in all cases was less than the maximum allowable concentration of 1 mg NO2--N L-1. Effluent pH was stable in the range of 7 to 8; the effluent dissolved oxygen was less than 0.15 mg L-1 and the oxidation-reduction potential in all columns was in the range of -110 to -250 mV.