Performance enhancement of leaf vegetable waste in two-stage anaerobic systems under high organic loading rate: Role of recirculation and hydraulic retention time

Continuous laboratory-scale experiments were conducted on two-stage anaerobic systems treating vegetable waste (VW) to improve CH4 production. The acidogenic reactors were employed with a serial methanogenic reactor configuration with volume distribution ratios of 3L/4L and 3L/7L (acidogenic reactor/methanogenic reactor), as well as recirculation rates (RRs) of 0.8 and 1.6. Results showed that recirculation improved the performance of VW anaerobic digestion under an organic loading rate (OLR) of 2.6gVS/L/d. The OLR increased from 2.6gVS/L/d to 3.0gVS/L/d to compare the stability of the processes and to study the behavior response of serial systems. System control without recirculation was susceptible to overloading and volatile fatty acids (VFAs) utilization was inhibited in the methanogenic reactor, which was implemented as a fixed-bed biofilm reactor with active carbon fiber textiles. These findings indicated overall process failure. The ratio of total volatile fatty acid (TVFA) and alkalinity gives a good indication of the process stability of anaerobic digestion. The TVFA/alkalinity ratio of the methanogenic reactor in the 3L/4L configuration, with RR of 1.6, increased to approximately 0.5, which indicates potential imminent failure of the methanogenic process. However, the methanogenic reactorin the 3L/7L configuration helped in utilizing the VFA produced by the overloading in the acidogenic reactor, which improved the CH4 production and conversion efficiency of the system.