Analysis of archaeal community dynamics in full-scale anaerobic reactors using fluorescent in situ hybridization

Abstract

In order to benefit from great potential of anaerobic reactors in terms of achieving high Chemical Oxygen Demand (COD) removal efficiencies (> %90), careful operation and control monitoring the conventional parameters such as pH, alkalinity, temperature, VFA concentration, etc. are required. These parameters are actually monitored to maintain optimum environmental conditions for a stable and a highly active microbial community in the reactor. However, little attention has been paid to determination of the composition and activity of the microbial community. The maintenance of active methanogenic populations in an anaerobic reactor is especially critical for stable performance. In this study, composition of methanogenic archaeal populations in three full-scale upflow anaerobic sludge blanket (UASB) reactors, namely IUASB (Istanbul), TUASB (Tekirdag) and CUASB (Canakkale) treating alcohol distillery effluents were analyzed using fluorescence in situ hybridization (FISH). The reactors were also investigated in terms of acetoclastic methanogenic capacity using specific methanogenic activity (SMA) test. The results of this study were then compared with the previous studies considering archaeal community dynamics, operating conditions and performance of the reactors in a long term operating period, years between 2001 and 2005. The IUASB and CUASB reactors achieved COD removal efficiencies between 60%- 80% at organic loading rates (OLR) in a range of 3-7 kg COD/m3.day and 2-10 kg COD/m3.day, respectively while the TUASB reactor was operated at OLRs 3-5 kg COD/m3.day achieving COD removal efficiencies between 50%-65% in year 2005. Throughout year 2005, the three UASB reactors had been operated under a food to microorganisms (F:M) ratio of 0.02-0.09 which is much lower than the typical values (0.4- 0.6) reported for similar reactors. Also, temperature of CUASB reactor was maintained in a range of 15-25oC while that of IUASB reactor was 30oC±1oC in 2005, which are lower than the optimum temperature levels for mesophilic reactors (35-37oC). All other operational parameters were maintained within their desired ranges. According SMA tests’ results, the PMP rates of the IUASB, TUASB and CUASB reactors’ sludges were 192 mL CH4/gVSS.day, 132 mL CH4/gVSS.day and 167 mL CH4/gVSS.day, respectively, in year 2005. These values are lower than the PMP rates of anaerobic reactors successfully treating similar wastewaters reported in literature (>300 mL CH4/gVSS.day). According to FISH results, the relative abundance of archaeal cells within the IUASB, TUASB and CUASB reactors’ sludges increased from 14.2%±0.3% to 20.7%±0.9%, 15.0%±0.7% to 23.5%±0.2% and 14.6%±0.7% to 22.3%±0.8%, respectively between years 2004 and 2005. Methanosaeta spp. was the most abundant methanogen in the three UASB reactors’ sludges. The relative abundance of acetoclastic genus Methanosaeta in IUASB and CUASB reactor sludges increased from 58.0%±2.1% to 71.8%±5.5% and 53.0%±0.7% to 55.0%±1.4% of the archaeal subpopulation, respectively. Meanwhile, the relative abundance of acetoclastic genus Methanosaeta in TUASB reactor sludge decreased from 79.0%±1.4% to 60.4%±0.6%. In the same period, the relative abundance of hydrogenotrophic methanogens, Methanobacteriales within the archaeal subpopulation in IUASB, TUASB and CUASB reactor sludges increased from 10.0%±0.7% to 32.7%±3.8%, 24.0%±0.7% to 43.7%±0.3% and 39.0%±0.7% to 43.5%±0.3%, respectively. Other important archaeal groups such as Methanosarcina, Methanococcales and Methanogenium relatives were not observed in the three UASB reactors’ sludges in November 2005.