Acid-Gas Anaerobic Digestion Applying Biological and Chemical Based Acid-Phased Pretreatment

Parallel anaerobic digestion systems were operated in order to study the importance of biological fermentation in acid-gas anaerobic digestion. One system was operated as a conventional acidgas anaerobic digestion system with a 48-hour SRT thermophilic fermentation reactor. A second system was operated where the pre-fermentation reactor was replaced by the addition of 6N hydrochloric acid to achieve a pH of 4.0 and 24 hours of storage at 20°C. Although the biological reactor produced greater than four times the amount of volatile fatty acids during acid phased digestion, there was an insignificant difference in terms of overall solids destruction or biogas production following the respective gas digestion phases. Both systems significantly outperformed a single-stage mesophilic digestion system operated as a control.Chemical and biological acid phases appeared to act on slightly different fractions of wastewater sludge, based measurements of solubilization/hydrolysis products of proteins and lipids. This factor did not seem to impact the performance of the overall systems, however. During biological fermentation, the pH of the acid phase digester remained near-neutral, possibly due to alkalinity release during solids hydrolysis. While this is believed to be an artifact of the specific sludge used in this study, it served to separate the mechanism of fermentation from that of acid hydrolysis in the biological acid-phase digester. Because both acid-phase treatments enhanced whole-system volatile solids destruction by a similar degree, it is concluded that sludge minimization by acid-gas anaerobic digestion is dependent on acid-phase sludge solubilization, rather being obligatorily tied to acid-phase fermentation.Placing the responsibility for enhanced process performance on sludge solubilization and particulate hydrolysis, suggests that acid-gas anaerobic digestion may be considered and evaluated among the litany of sludge minimization technologies that focus on these same ends.