Competition between SRB and MPB according to temperature change in the anaerobic treatment of tannery wastes containing high sulfate

UASB reactors were operated for treating tannery wastewater containing high sulfate to assess their performance, competition between sulfate-reducing bacteria (SRB) and methane-producing bacteria (MPB), and the change in the characteristics of microbial granules according to the temperature change in the reactor. The reactor was fed with a physicochemically treated tannery wastewater to decrease the inhibition caused by high suspended solid. The volumetric loading rates (VLR) were between 0.8 and 2.0 gCOD.l(-1).d(-1) as hydraulic retention times (HRT) maintained 18 to 30 hrs at 35 degrees C. At the highest VLR, COD removal efficiency was more than 70%. COD removal efficiency and gas production rate decreased rapidly when the temperature dropped to 25 degrees C. When the reactor temperature increased from 25 to 35 degrees C, COD oxidation and sulfate reduction rates increased and methane production rate decreased. This indicated that SRB was much less sensitive to the temperature change than MPB. In the competition between SRB and MPB, about 15% of the removed COD was utilized by SRB at HRT of 30 hrs, while about 40% was used by SRB at HRT of 18 hrs at 35 degrees C. Whereas, after temperature exposure to 25 degrees C about 67% was utilized by SRB at HRT of 24hrs. When temperature was again increased from 25 to 35 degrees C, about 80% was used at HRT of 28hrs. It indicated that MPB was strongly suppressed by the occurrence of significant sulfate reduction since a large electron flow was distributed to the SRB. In addition, the granulation in the presence of high sulfate was unfavoured because of substrate transport limitations into MPB like Methanothrix spp. which was an important factor in the composition of the granules. Accordingly, granule size of the sludge in the UASB reactor decreased with time due to weak network frame of granules by the activity decrease of MPB.