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.
