Exploring the Impact of Diffusion and Substrate Structure on Hydrolysis in Mesophilic Anaerobic Digestion Open Access
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Anaerobic digestion (AD) batch tests were conducted on-site at DC Water’s Blue Plains Advanced Wastewater Treatment Plant to investigate the impact of various physical parameters on hydrolysis efficiency within digestion. First, the distance between substrate and microbes, the viscosity, and the floc size were changed to determine the impact of bulk diffusion limitations within hydrolysis. The results from these experiments indicated that these bulk diffusion limitations did not significantly affect hydrolysis. Second, the physical characteristics of two model hydrolysis substrates were changed by subjecting them to increased temperatures. Both of the model compounds, bovine serum albumin (BSA) and amylopectin, underwent conflicting changes on their molecular-scale versus macro-scale structures: with increasing temperatures, as their molecular structures became denatured, their macro-structures formed intermolecular bonds, converting from their liquid forms into gels and eventually into solids. Results obtained through using the model compounds in their different forms as the feed for AD batch tests showed that hydrolysis was impacted by the molecular and macro-scale structural changes of the substrates. They also indicated that the molecular-scale and macro-scale structures impacted the hydrolysis of each substrate differently. Third, the impact of the thermal hydrolysis process (THP) on the model compounds (BSA and amylopectin) was explored. When the substrates were combined with low solids sludge (BT2, 4.4% TS) and fed into a pilot-scale THP reactor, changes in their biogas yield indicated that THP was causing the substrates to denature and act in a similar manner as seen in the AD batch tests conducted when the substrates were heated alone. However, when BSA was combined with high solids sludge (concentrate, 9.2% TS) and fed into a pilot-scale THP reactor, conflicting results were obtained. These results indicated that THP had the potential to impact the structures and biodegradability of hydrolysis substrates.