Influences of temperature (25–35 °C) and substrate particulate content (3.0–9.4 g total suspended solids (TSS)/L) on granular sludge bed anaerobic digestion (AD) were analyzed in lab-scale reactors using manure as a substrate and through modeling. Two particle levels were tested using raw (RF) and centrifuged (CF) swine manure slurries, fed into a 1.3-L lab-scale up-flow anaerobic sludge bed reactor (UASB) at temperatures of 25 °C and 35 °C. Biogas production increased with temperature in both high- and low-particle-content substrates; however, the temperature effect was stronger on high-particle-content substrate. RF and CF produced a comparable amount of biogas at 25 °C, suggesting that biogas at this temperature came mainly from the digestion of small particles and soluble components present in similar quantities in both substrates. At 35 °C, RF showed significantly higher biogas production than CF, which was attributed to increased (temperature-dependent) disintegration of larger solid particulates. Anaerobic Digestion Model No.1 (ADM1) based modeling was carried out by separating particulates into fast and slow disintegrating fractions and introducing temperature-dependent disintegration constants. Simulations gave a better fit for the experimental data than the conventional ADM1 model.

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An investigation of particle disintegration was carried out using batch anaerobic reactors and a particle-rich substrate from pig manure supernatant. Two types of samples were applied, one high in suspended particles (raw feed) and another low in suspended particle content (centrifuged feed). Both feeds were digested with and without cellulase enzyme addition to obtain a better understanding of particle degradation mechanisms. An automatic methane potential test system (AMPTS) was used to carry out batch reactions at 35 °C. The raw feed with high-suspended solids had higher biomethane potential than the centrifuged feed but the conversion rate and methane yield was lower. The addition of cellulase increased biomethane production rates in both high- and low-particle content samples enhancing yield by 54% and 40%, respectively and converting 69% and 87% of feed chemical oxygen demand (COD), respectively. This implies that the feed particles have high contents of cellulose. This is also the case for the smaller particles remaining after centrifugation. Comparisons of anaerobic digestion model no. 1 (ADM1) simulations with experimental data reveal that classifying substrate particles into a fast and a slow degrading fraction with separate disintegration kinetics fit the experimental data better than lumping all particles into one parameter.

Granular sludge bed (GSB) anaerobic digestion (AD) is a well-established method for efficient wastewater treatment, limited, however, by the wastewater particle content. This review is carried out to investigate how and to what extent feed particles influence GSB to evaluate the applicability of GSB to various types of slurries that are abundantly available. Sludge bed microorganisms evidently have mechanisms to retain feed particles for digestion. Disintegration and hydrolysis of such particulates are often the rate-limiting steps in AD. GSB running on particle-rich substrates and factors that affect these processes are stdied especially. Disintegration and hydrolysis models are therefore reviewed. How particles may influence other key processes within GSB is also discussed. Based on this, limitations and strategies for effective digestion of particle-rich substrates in high-rate AD reactors are evaluated. some of the main challenges associated with high-rate anaerobic digestion of particle-rich substrates with special emphasis on manure as a substrate, due to its abundance. Most high-rate AD processes in operation depend on granular sludge to retain sufficient active biomass. Granules are formed by the aggregation of microorganisms that develop into dense masses with sedimentation velocity high enough to avoid washout even under high hydraulic load [7]. It is observed that UASB reactors treating particle-rich manure slurries also accumulates suspended solids from the feed, forming an additional suspended fraction together with the granules [8,9]. The influence of such solids on AD is not understood well, leading some experts to claim that granular sludge bed (GSB) processes may not be appropriate for particle-rich substrates. [10]. It appears, however, that a significant fraction of feed particles can be digested and enhance methane production [11]. This review was undertaken to investigate to what extent and how sludge bed high-rate AD can be used to treat particle-rich substrates. We aim to find more evidence for particle digestion in granular sludge beds, identify process limitations, and find appropriate kinetic models in order to establish design criteria for such processes. There is little directly relevant literature on the topic, limiting this review to mainly indirectly relevant literature. The review covers particle-rich substrates characteristics; particle disintegration and hydrolysis, including models for such; physical characteristics of granular sludge; sludge bed reactor designs and observations of particle effects.