Single-phase vs. Two-phase digestion of fruit and vegetable waste: process stability and efficiency

A two-phase anaerobic system is generally considered as advantageous over a single-phase system as phase separation of the acidogenic and methanogenic processes in the two-phase system can be capitalized for high OLR operation and process efficiency. This can be particularly true for wastes with a large rapidly biodegradable fraction, such as fruit and vegetable waste (FVW). This study was conducted to assess and compare the reactor stability and process performance of a single-phase system with two-phase system for fruit and vegetable waste. A conventional single-phase reactor was used and compared with a two-phase reactor system, which consisted of an acidification reactor and a methanogenic reactor, with solids and liquid recirculation. The operation of acidification reactor was based on decoupling of the solids retention time from the hydraulic retention time and the digestate from acidification reactor was centrifuged, followed by recirculation of solids back to the reactor. The separated liquid was fed to the methanogenic reactor for methane production and the methanogenic effluent was recirculated back to the acidification reactor. This strategy of solids recirculation was to increase the solids retention time and thereby enhance the degradation of organic matter. Recirculation of methanogenic effluent was to provide alkalinity, maintain a constant solids concentration in the reactor and for possible enhancement of solids hydrolysis. The initial inoculation conditions of the single phase and the two-phase acidification reactor were similar. The single-phase reactor was operated at low loading rates of 2 then 3.5 kg/m3 d and the acidification reactor was operated with step-wise increase in the loading rate up to 10 kg/m3 d. A new strategy was developed to convert methanogenic sludge to acidification sludge in a short time at this high OLR and with total recirculation of solids, followed by operation under acidification conditions. The acidification reactor achieved stable performance at 7.0 kg VS/m3 d and an optimum pH of 5.5 - 6.2 with very high substrate solubilization rate. Overall methane yield and VS removal were 0.30 m3 CH4/kg VS fed and 98% in the two-phase system and 0.47 m3 CH4/kg VS fed 83% in the in the single-phase reactor. Material balance and energy yield calculations for the two processes showed that the two-phase system was inferior in terms of volumetric methane production and energy yield by 33%. Loss of energy during hydrolysis in the acidification reactor and lower methane production in the methanogenic reactor contributed to the lower energy yield. The study showed that the two-phase process with solids and liquid recirculation was a feasible option where high OLR operation and solids removal is desired. However, considering the complicated operation procedure (centrifugation process with solids and liquid recirculation) plus the economics makes the process unviable. Single-phase process on the other hand is a simple and straightforward operation and hence could be the preferred system for the digestion of FVW.