Effect of pH and external electron donor on mesophilic sulfate reduction during start-up of anaerobic digesters treating skim latex wastewater

Abstract

Effluent of skim latex industry contains high concentrations of sulfate, together with ammonia, protein and organic matter. This research study investigates the influence of pH and addition of external electron donor on improvement of sulfate reduction in anaerobic digestion under high ammonia concentrations, using Skim Latex Wastewater (SLW) during initial start-up period. Experiment was conducted using two 3- liter reactors fed semi continuously at 35 ±1 ̊C. The influent COD/SO4-2 ratio and the pH of real skim latex wastewater was 2.8 and 5.9 respectively. The experiment was conducted in three phases. During phase I and II, the pH of the reactors was not controlled whereas the influent pH of the sample was adjusted to 7, and influent pH was adjusted to 3 in phase III. The influent COD/SO4-2 ratio was 2.8 in phase I, while in phase II and III it was increased to 10 using acetate. In phase I, average sulfate reduction per cycle was only 33 ± 2 %. When the influent COD/SO4-2 ratio was increased to 10 using acetic acid during phase II, average sulfate reduction per cycle increased to 64%, but it gradually decreased to 30%, 8% and 0% within next three feeding cycles, this is because the pH of the anaerobic digesters increased steeply to 8.85 ± 0.3. Although sulfate reduction increased with addition of external electron donors such as acetate, according to results, the sulfate reduction was adversely affected due to free ammonia formation at the end of the phase II. Sulfur reducing bacteria (SRB) withstand high pH values as 8.8. Even though the anaerobic reactors inhibited, they did not completely fail. But when the Free Ammoniacal Nitrogen (FAN) of the system reached 679.5 ± 12.2 mg/l, the system completely inhibited. The completely inhibited sulfate reduction process was recovered by decreasing the pH of the digester by decreasing the pH of the influent feed sample to 3, while influent COD/SO4-2 ratio was at 10. Thus, sulfate reduction again increased in phase III. In the first cycle of phase III, after the low pH influent fed to the reactors, percentage sulfate reduction per cycle increased to 76%, decreasing the accumulated sulfate in the reactors. But within next three cycles of phase III, average sulfate reduction was 58 ± 3%. When the pH of the digester was controlled at 7.5, the FAN concentration was able to be maintained 92% lower. Thus, the system inhibition could be avoided by controlling the digester pH at 7.5 by controlling influent pH and enhancing sulfate reduction with increasing influent COD/SO4-2 ratio to 10.