Acid/Gas digestion (A/G), often called acid phase digestion or two-phase digestion, is a staged digestion process. The purpose of A/G is to provide separate environments for the different groups of microorganisms involved in anaerobic digestion. The first stage of an A/G system is the acid phase, which is characterized by low hydraulic retention time (HRT), acidic pH, and high volatile acids concentration. Volatile acid producing organisms (acidogenic and acetogenic bacteria), sulfate reducing bacteria, and hydrogen-consuming methanogens are capable of thriving under the conditions prevalent in the acid phase. The second stage is the gas phase, which is similar to a conventional anaerobic digester with near neutral pH and low volatile acid concentrations. The environmental conditions in the gas phase favor the growth of volatile fatty acid consuming bacteria (propionate and butyrate degrading syntrophs) and methane producing organisms. In theory, the low pH in the acid phase of the A/G process results in accelerated hydrolysis. The HRT in the gas phase is substantially less than the 15 to 25 days needed for conventional digesters because the A/G process can operate at higher loading rates. The level of volitile solids reduction and biogas production achieved in an A/G system is comparable to that of a conventional digestion system operated with a considerably larger HRT. This chapter provides process information and guidance for the design of A/G digestion systems.
Parry, D.L., C. Clark, D. Craig, R. Kilian, C. A. McPherson, P. Schafer, T. Shimada, A. Travaglia, J. Treadway, S. Vandenburgh, J. Willis, T.E. Wilson. (2012). Acid/Gas Phased Digestion. Solids Process Design and Management (WEF). Alexandria, VA, McGraw-Hill: 389-384