Anaerobic Acidification Of Vegetable Waste And Recirculation Control

China is a large vegetable producing country. However, some parts of produced vegetable are lost as waste throughout the vegetable supply chain, which includes collection, storage and transportation. Vegetable waste (VW) is often a source of nuisance as leachate when it is disposed in landfills because of its high moisture and organic contents. Anaerobic digestion is an alternative technology for treatment in consideration of solids reduction and energy recovery.A two-stage anaerobic digestion system consisting of CSTR and fixed-bed biofilm reactor was used in the experiments. To investigate effects of OLRs on two-stage anaerobic digestion, the systems were operated at four different OLRs of1.3,1.7,2.1g VS/L/d and2.6g VS/L/d. The results showed that with increasing OLRs to2.6g VS/L/d, total volatile fatty acid concentration and CODcr concentration in acidogenic reactor was maintained at about8500mg/L and20000mg/L, whereas VS content sharply increased to20g/L. It has been revealed that inhibition of hydrolysis in acidogenic reactor occured.The effluent recirculation from methanogenic reactor to acidogenic reactor was introduced to improve hydrolysis. Effluent recirculation showed a considerable positive effect on alleviating VFA inhibition and improving biogas production in acidogenic reactor because of the effect of dilution and pH adjustment, particularly at high OLRs. The effects of recirculation rate (RR) on the performance of two-stage VW anaerobic digestion were also investigated. The system was continuously operated at an organic loading rate of1.7g VS/L/d with varying RRs (0,0.6,1, and1.4). Results demonstrated that biogas production in acidogenic reactor increased, when pH value is increased from approximately5.1to6.7. This increase indicates that recirculation of alkaline effluent from the methanogenic reactor helps create a favorable condition for biogas production in the acidogenic reactor. The decrease in chemical oxygen demand (COD) concentrations was also observed in the acidogenic reactor. This condition may be attributed to dilution under recirculation. Meanwhile, the process of acidogenesis was stimulated by an RR of0.6. The dynamics between hydrolysis and methanogenesis under recirculation indicated that the mass transfer capacity between the two-stage reactors improved. It was note that a relatively high recirculation rate would cause disproportionate increase in the effective loading rate of the methanogenic reactor and interactively give rise to a progressive increase in the organic output concentration and decline in the performance.To improve the performance of VW in two-stage anaerobic systems under high organic loading rate, the acidogenic reactors were accompanied by a serial methanogenic reactors configuration with volume distribution ratio of4L and7L, and recirculation rate of0.8and1.6. The results showed that recirculation improved performance of VW anaerobic digestion under OLR of2.6g VS/L/d. The organic load pulses were introduced to compare stability of the processes and to study behavior of the serial systems. The VS content was sharply increased to50g/L in the acidogenic reactor of system control without recirculation. The output of the acidogenic reactor was fed directly to the methanogenic reactor, which led to acidification and overall process failure. TVFA/TIC of methanogenic reactor in3 L/4L configuration with RR of1.6increased to about0.5, which indicated a potential imminent failure of the methanogenic process. The methanogenic reactor in3L/7L configuration helped utilizing VFA produced from overloading in the acidogenic reactor, which improved CH4production and conversion efficiency of the system.The anaerobic digestion of vegetable waste was performed at various OLRs in single-stage with food waste (FW) addition. Changing the feed to a higher FW content in a ratio of VW to FW resulted in an increase in biogas yield and methane yield. The single-and two-stage anaerobic digestion of vegetable waste was compared, which indicated that two-stage digestion presented more stable operation and higher OLR treatment capacity.