Effects of Mixing on Volatile Fatty Acid Production in Anaerobic Digestion

Anaerobic digestion(AD) is a common method of waste treatment where organic waste is recycled into a variety of products using microorganisms in the absence of oxygen. Contemporary AD processes are typically optimized for Methane for use in combustion reactions. Other value-added products of anaerobic digestion include VFAs (Volatile Fatty Acids), which can be extracted and used in biofuels. This investigation focused on determining the effects of agitation on VFA production in AD. Possible effects include suspending solids in the reactor, enzyme-solid attachment, and acetogen-methanogen syntrophy. Results from experiments with four different agitation rates suggested that “middle ground” agitation promoted VFA production. The lowest (50RPM) and highest (800 RPM) settings showed slower VFA production in the first few days than did the moderate (70RPM and 400RPM) settings, suggesting that the hydrolysis step in AD was limited. It is speculated that lower RPM agitation contributes to solid layer formations, leading to limited surface area for food waste hydrolysis. However, too high an agitation rate may inhibit enzymes from attaching to food waste solids, limiting hydrolysis. A promising area for future research could include isolating each step of the anaerobic digestion process to further quantify the effects of agitation on specific stages of AD.