Salt Inhibition Effects on Simultaneous Heterotrophic/Autotrophic Denitrification of High Nitrate Wastewater

Abstract

Denitrification of high- nitrate high- salinity wastewater is difficult due to plasmolysis and inactivation of denitrifiers at high salinity conditions. In this study, the effects of salinity and empty bed contact time (EBCT) on simultaneous heterotrophic and sulfur based autotrophic denitrification of synthetic wastewater were evaluated in an up flow packed bed reactor .The reactor was filled with granular elemental sulfur particles with diameters of 2.8-5.6 mm and porosity of 40%. The initial culture was prepared from sludge of Shahrak-e- ghods domestic wastewater treatment plant. The influent nitrate concentration and EBCT were 600 mg NO3-N/lit and 16 h respectively. First, the stoichiometric fraction of nitrate removed by heterotrophic denitrification (with methanol as organic carbon source) supplied enough alkalinity to compensate the autotrophic alkalinity consumption, was determined 60%. Then, salt concentration was gradually increased with NaCl from 0% in the feed. The Process kept high nitrate removal efficiency (>99%) even at 3.5 % NaCl. During these changes the alkalinity variations were insignificant which showed the microbial population ratio of acclimated autotrophic to heterotrophic denitrifiers had no any significant changes with NaCl concentrations up to 3.5% in the feed. At 4 and 5% NaCl, the efficiency drastically decreased to 78% and 48%, respectively. Similar behavior was also observed for methanol removal efficiency, effluent turbidity as an indirect determinant of biological mass and sulfate production. The effects of flow rates on denitrification of synthetic high nitrate high salinity wastewater with 3.5 %NaCl under mixotrophic condition were also investigated by increasing the flow rate from 7.06 lit/day to 70.6 lit/day with corresponding EBCT 20 to 2 h. Denitrification efficiency was close to 100% at EBCT of 20 to 8 hr, but decreased to 79% and 39% when the EBCT was 4 and 2 h, respectively. The decrease in effluent sulfate concentration (as an indicator for autotrophic denitrification) and the increase in effluent alkalinity (as an indicator for heterotrophic denitrification) and pH at EBCT of 4 and 2 h were considerable correspondingly. These results imply that the population ratio of autotrophic to heterotrophic denitrifiers depends on EBCT.