Methanogenic population dynamics in full-scale UASB reactors

Abstract

A prerequisite for stable performance in an anaerobic treatment system is maintenance of active methanogenic populations in the system. However, there are few published studies available that assess performance of a full-scale anaerobic reactor in relation to the make up of reactor biomass in terms of qualitative and quantitative measures of methanogenic species and their activities. This study was undertaken to assess changes in performance of three full-scale upflow anaerobic sludge blanket (UASB) reactors, namely IUASB, TUASB and CUASB, at wastewater treatment plants of Istanbul Alcohol (Raki), Tekirdag Alcohol (Raki) and Canakkale Alcohol (Cognac) distilleries in relation to qualitative and quantitative measures of the relevant archaeal methanogenic populations. Composition and quantity of archaeal methanogens were determined using fluorescent in situ hybridization (FISH) combined with epifluorescence microscopy. Specific methanogenic activity (SMA) test was used to determine the potential methane production (PMP) rates of the anaerobic sludges. The IUASB reactor performed well achieving COD removal efficiencies of no lower than 80% at a range of OLRs from 6-11 kg COD $m^{-3} day^{-1}$ in years between between 2001 and 2004. The TUASB reactor also performed well achieving COD removal efficiencies between 70% and 85% at OLRs in a range of 2-12 kg COD $m^{-3} day^{-1}$ between 2001 and 2004. COD removal efficiency of the TUASB reactor varied between 60% and 80% at OLRs in a range of 2.5-12 kg COD $m^{-3} day^{-1}$ between 2002 and 2004. According to the SMA tests results PMP rates of the IUASB, TUASB and CUASB sludges were 321, 344 and 256 mL $CH_4 gVSS^{-1} day^{-1}$ respectively in the year 2002 and decreased to 133, 109 and 108 $CH_4 gVSS^{-1} day^{-1}$ respectively after two years of operation. When the PMP rates were compared with actual methane production (AMP) rates obtained from the three UASB reactors, AMP/PMP ratios were evaluated to be between 0.1 and 0.4. These results can be interpreted that the three UASB reactors were under loaded compared to their potential acetoclastic methanogenic capacities. This could have been due to retaining high amount of granular sludges within the UASB reactors resulting in F/M (food to microorganisms) ratios in a range of 0.02-0.07 gCOD $gTVS^{-1} day^{-1}$ which is much lower than the typical values reported for similar reactors in literature. Since all other operational parameters such as pH, temperature, alkalinity, nutrients etc. have been maintained within their typical ranges, it was concluded that the prolonged operation of the three UASB reactors under very low F/M ratios might have lead to the significant decreases in the PMP rates of the reactor sludges. FISH results revealed that the relative abundance of archaeal cells within the IUASB, TUASB and CUASB sludges were in range of 14-18%, 15-17% and 14-15% respectively. Methanosaeta spp. were the predominant methanogen in all of the anaerobic sludges. However, over time the relative abundance of acetoclastic Methanosaeta in the IUASB and TUASB reactors reduced from 83% $pm$ 1.6 and 90% $pm$ 1.2 to 58% $pm$ 2.1 and 79% $pm$ 1.4 of the archaeal population respectively. These decreases coincided with 59% and 68% decreases in the acetoclastic methanogenic capacities and 20% and 24% increases in the relative abundance of hydrogenotrophic methanogens in the archaeal population of the IUASB and TUASB sludges respectively. Although a 58% decrease in the acetoclastic methanogenic capacity of the CUASB sludge was detected, the reactor sludge had stable archaeal community structure. Among the hydrogenotrophic methanogens, Methanococcales followed by Methanobacteriales were dominant methanogens within the IUASB reactor. This study is the first that reports dominance of Methanococcales among the hydrogenotrophic methanogens within UASB reactors. Methanobacteriales was the predominant hydrogenotrophic methanogen within the both TUASB and CUASB reactors. In this study the significant decreases in the activity of methanogens was detected at an early stage by using the SMA test so that process conditions can be changed, and collapse of the reactors can be avoided. The SMA test and FISH results showed that the reduction of acetoclastic methanogenic activity in IUASB and TUASB reactors is related to loss of Methanosaeta spp. However, acetoclastic methanogenic activity losses in the CUASB reactor were not reflected in the archaeal community structure of the reactor sludge.

Özet

Bu çalışmada, alkol distilasyon atıksularını arıtan, kısaca IUASB, TUASB ve CUASB olarak adlandırılan üç farklı gerçek ölçekli yukarı akışlı anaerobik çamur yatağı (UASB) reaktörünün 2002-2004 yılları arasındaki işletme performansları, Metan Arke komunite yapıları ve Potansiyel Metan Üretim (PMÜ) hızları tartışılmıştır. Aynı süreçte UASB reaktörlerinden 2-12 kg KOİ/m3.gün aralığındaki organik yükleme hızlarında, %60-95 aralığında KOİ giderim verimleri elde edilmiştir. Spesifik Metan Aktivite (SMA) test sonuçları IUASB, TUASB ve CUASB reaktörlerinin PMÜ hızlarının 2002 yılında sırasıyla 321, 344 ve 256 mL CH4/gUAKM.gün iken, 2004 yılında sırasıyla 133, 109 ve 108 mL CH4/gUAKM.gün değerlerine düştüğünü göstermiştir. Reaktörlerden elde edilen gerçek metan üretim (GMÜ) hızları, reaktör çamurlarının PMÜ hızları ile oranlandığında elde edilen değerler, 0.1-0.4, reaktörlerin maksimum kapasitelerinin çok altında yüklendiklerini göstermektedir. Floresanlı yerinde hibritleşme (FISH) sonuçları her üç reaktörde de baskın metanojenlerin, asetoklastik bir cins olan Methanasaeta’ya ait olduğunu göstermiştir. Hidrojen kullanan metan arkelerinden Methanobacteriales TUASB ve CUASB reaktörlerinde, Methanococcales ise IUASB reaktöründe baskın halde bulunmaktadır. IUASB ve TUASB reaktörlerinin asetoklastik metan üretim kapasitelerinde meydana gelen, sırasıyla %59 ve %68’lik kayıplara pararel olarak, reaktörlerde rastlanan tek asetoklastik cins olan Methanosaeta’nın rölatif miktarında sırasıyla %25 ve %11’lik kayıplar, hidrojen kullanan metanojenlerin rölatif miktarlarında ise sırasıyla %20 ve %24’lük bir artış meydana gelmiştir. CUASB reaktöründeki metanojenik komünite yapısı zaman içerisinde stabil kalmıştır