AUTOTROPHIC DENITRIFICATION OF NITRATE AND NITRITE USING THIOSULFATE AS ELECTRON DONOR

Abstract

Nitrogen is principle constituent found in wastewater, landfill leachate and many industrial wastewaters. Nitrogen is found in the form of ammonia, nitrate and nitrite. Biological nitrification and denitrification is considered to be the most economical way to remove nitrogen from the environment. Denitrification is a process of removal of nitrate and nitrite to nitrogen gas. It can be carried out by either heterotrophic bacteria or autotrophic bacteria. Aged Landfill leachate and industrial wastewater, low in carbon source and organics, need external input of carbon source to carryout denitrification, which adds lot of cost. Also, during autotrophic denitrification of nitrate, alkalinity is consumed and external input of alkalinity source is required to maintain pH in optimal range, for maximum denitrification activity. This research was carried out to take advantage of shortcut biological nitrification process in which nitrite is accumulated, to find the possibility of autotrophic denitrification of nitrite using thiosulfate as an electron donor, comparing the kinetics of autotrophic denitrification of nitrate and nitrite and to check the effect of pH and the S/N ratio on the denitrification rate of nitrite. Aerobic sludge was acclimated for nitrate and nitrite in separate reactors, by gradually increasing the substrate concentration from 20 mg N/L to 500 mg N/L and wasting small amount of sludge at every feed change. All batch experiments were performed 3 month after complete acclimation. Feed was changed every time substrate concentration went below 20 mg N/L. Complete removals up to 1000 mg N/L of nitrate and 800 mg N/L of nitrite was observed with the S/N ratio of 5.1 and 2.5, respectively. The Nitrate denitrification up to 500 mg N/L was carried out in two way process. First, nitrate reduction to nitrite, which resulted in nitrite accumulation and then nitrite reduction to nitrogen gas. At higher concentration of nitrate i.e 1000 mg N/L, less nitrite accumulation was observed at low initial MLSS. High amount of nitrite accumulation during nitrate denitrification process showed inhibitory behavior to nitrate reduction thus, reducing overall denitrification rate of nitrate. During nitrate denitrification pH decreased, while during nitrite denitrification pH increased, indicating gain in alkalinity. Sulfate production rate for nitrate denitrification was 11.95 mg SO42-/mg N removed and for nitrite was 5.4 mg SO42-/mg N removed, showing more than twice sulfate production during nitrate denitrification as compare to nitrite denitrification. It was observed that maximum denitrification activity of nitrite was observed at the pH value of 8.0, denitrification activity reduced above and below pH 8, but the effect was more detrimental at pH 7 than at pH 9. Denitrification rate reduced to 30% & 39 %, respectively, at pH 7 and 9. Results also indicated that this detrimental effect at pH 7 was not due to FNA, but due to pH itself and pH 7 was not favorable for nitrite denitrifying cells. Effect of S/N ratio showed that the S/N ratio did not have any impact on denitrification activity of nitrite, but thiosulfate concentration above 1000 mg S2O32- -S/L was inhibitory and at concentration of 2250 mg S2O32- -S /L of thiosulfate, 10% inhibition to denitrifying activity was observed. However, at S/N ratio less than 2.5, complete denitrification was not achieved and nitrite remained as end product. Kinetic study showed that highest specific substrate utilization rate of nitrate and nitrite was 3.26 mg NO3 ?N/mg MLSS-d and 3.34 mg NO2 ?N/mg MLSS-d, at concentration of 166 mg N/l and 336 mg N/L, respectively, indicating that nitrite denitrification was slightly faster than nitrate denitrification. The Nitrate inhibition was observed at concentration above 366 mg N/l and nitrite inhibition was observed at concentration above 336 mg N/L, but inhibition due to nitrite was more severe than due to nitrate. Specific substrate utilization rate of nitrite and nitrate reduced to 38% & 48% of highest observed value, respectively, at the substrate concentration of 800 mg N/L. Results also indicated that nitrite was inhibitory to nitrate reduction, and presence of 200 mg N/L of nitrite reduced the denitrification rate of nitrate to 45%. Monod kinetics was applied to inhibition free data of nitrite and nitrate. Maximum specific substrate utilization rate for nitrate and nitrite was observed as 3.61 mg NO3 ?N/mg MLSS-d and 3.70 mg NO2 ?N/mg MLSS-d, respectively. Observed Ks value for nitrate and nitrite were 23.9 and 35 mg/L, respectively. Biomass yield for nitrate denitrification was 0.53 mg VSS/ mg N and for nitrite denitrification was 0.34 mg VSS/ mg N, showing less sludge production during nitrite denitrification. Results also confirmed that, this process can be applied to short cut biological nitrification process, as nitrogen concentration up to 630 mg N/l containing 60 % nitrite and 40 % nitrate combined in same reactor, was removed successfully without any net consumption of alkalinity.