Investigating the methane production efficiency of single- and two-phase anaerobic digestion and population dynamics of a second-phase anaerobic digestion

Koç, Engin
The aim of this thesis study was to investigate the methane production efficiency of single-phase and two-phase anaerobic digestion (AD) in batch reactors and anaerobic sequencing batch reactor (ASBR). It was also aimed to monitor the population dynamics and change in the quantity of aceticlastic methane-producers of the second-phase of a two-phase ASBR. Two sets of batch reactors were conducted, namely, Batch Reactor Set-1 and Batch Reactor Set-2. The aim of Batch Reactor Set-1 was to investigate the effect of initial substrate concentration to initial microorganism concentration ratio (S/X0) on methane yield. To this purpose, batch reactors were conducted at different initial S/X0 ratios of 0.5, 1, 2, 3 and 4 g sCOD/g VSS. Results revealed that the highest methane yield (267±5 mL CH4/g CODadded) and anaerobic treatability were observed at S/X0 ratio of 1 g COD/g VSS. The results also indicated the applicability of the anaerobic seed sludge and 1 g COD/g VSS as the optimum S/X0 ratio for the following experiments. Batch Reactor Set-2 was conducted to investigate and compare the methane production efficiency of a single-phase AD and the second phase of a two-phase AD system. To this purpose, sucrose and effluent of a dark fermentative sequencing batch reactor (DF-SBR) operated with sucrose (Tunçay, 2015) were used as substrates. The highest vi methane yield and content in biogas were observed as 344±20 mL CH4/g CODadded and 83%, respectively, in the second phase of the two-phase AD. Two-phase AD resulted in 39% increase in methane yield compared to its single-phase counterpart. ASBR study was conducted to investigate the effect of hydraulic retention time (HRT) and solid retention time (SRT) on methane production, and archaeal and bacterial population dynamics, the latter performed with denaturating gradient gel electrophoresis (DGGE). The change in the quantity of aceticlastic methane-producers, namely, Methanosaeta and Methanosarcina sp. with respect to the changing SRT and HRT conditions was also investigated with quantitative polymerase chain reaction (qPCR) analysis. Thus, an ASBR fed with the effluent of the DF-SBR was operated. Results showed that the highest average methane yield was achieved at 20 days of SRT and 6 days of HRT as 343±17 mL CH4/g CODadded. The highest average methane productivity was observed as 1794±279 mL CH4/L/day at SRT and HRT values of 10 and 0.7 days, respectively. Methanosaeta sp. was found to be the dominant specie among the archaeal group during all HRT and SRT combinations (10-20 days of SRT and 0.7 to 6 days of HRT). Although Methanosarcina species was not found in sequence analysis, qPCR results revealed the existence of Methanosarcina species through the whole operation period. Yet, Methanosaeta dominated the ASBR for all HRT and SRT combinations studied. Gradual decrease in HRT from 6 days to 0.7 days resulted in three fold decrease in Methanosaeta (9.5×1014 to 3.1×1014 gene copy/g VSS) and nearly five fold increase in Methanosarcina concentration (9.4×1012 to 5.1×1013 gene copy/g VSS) at 20 days of SRT. On the other hand, decrease in SRT from 20 days to 10 days did not lead any significant change in the concentration of Methanosaeta and Methanosarcina species in the ASBR.