Comparison of anaerobic sludge granules from different wastewater plants with respect to granule size, substrate degradation and methanogenic community

Abstract: Within the objective to replace fossil fuel with sustainable sources there is a growing pressure to advance such development. Anaerobic digestion (AD) is beneficial not only as a waste management technology, but also to recycle organic waste into methane which can be used for heating, production of electricity or vehicle fuel. Although the anaerobic digestion for biogas production has been used and refined at a larger scale for several decades the system is susceptible to perturbations due to sensitivity of the degradation process. The process of degrading organic waste anaerobically is a complex process involving many trophic groups of microorganisms in a series of degradation steps, each step decomposing the substrate further. The most sensitive group are the methanogens, which come exclusively from the domain Archaea and produce methane. Because they are easily inhibited they are often rate limiting in the degradation process. Common digesters for AD of sludge from wastewater treatment plants are the Up-flow Anaerobic Sludge Blanket (UASB) digester and the Expanded Granule Sludge Bed (EGSB) digester, which both utilize granular suspended solids that form a thick sludge blanket at the bottom of the digester tank. The major difference between the digesters is that the EGSB operates with higher up-flow velocity, enabling treatment of higher organic load. The size and shape and the microbial community compo-sition in the granules impact on the operational performance of the digester. The focus of this project was to investigate granules in respect to size distribution, their fraction of volatile solids, monitoring the consumption hydrogen, acetate, propionate and n-butyrate in respect to gas production, gas composition and volatile fatty acid (VFA) consumption. The methanogenic community of granules was measured using Terminal Restriction Fragment Length Polymorphism (T-RFLP) fingerprinting with the restriction endonucleases BstNI and MwoI. Granules was sampled from mesophilic UASB and EGSB digesters feed with wastewater from sugar industry, paper industry and methylcellulose. The dominating methanogens found in the granules studied, ranked in order of prevalence include: Methanosaeta (6-63%), Methanobacterium (0-45%), Methanosarcinaceae (0-42%), Methanocaldo-coccus (0-17%), Methanomicrobiales (0-19%), Methanomassiliicoccaceae (0-13%) and Meth-anoculleus (0-29%). The results show that there can be a significant difference in the consumption of acetate, propionate and n-butyrate between reactor granules. Granules dominated by Methanosaeta had high VFA degradation rates, while granules dominated by Methanosarcinaceae had greater hy-drogenotrophic activity. Granules from investigated paper waste reactors were predominantly 3 mm or larger, whereas reactors treating methylcellulose had granules that were more evenly distributed. In general, larger granules had slightly higher VS content than average. The methanogenic commu-nity varied between granules of different size but seems to be more influenced by reactor parameters such as high salt concentration in methylcellulose waste, which favoured Methanosarcinaceae. While other reactor granules treating paper and sugar waste was dominated by Methanosaeta.