| China ranks first in the world in vegetable production because of the characteristics of the population and their diet. The amount of agricultural waste produced is high because of difficulties with transportation, processing, customs and other problems. Disposing of this solid waste in a traditional landfill causes secondary environmental pollution and fails to generate economic benefits. Vegetable waste contains highly degradable organic matter. Therefore, the use of anaerobic digestion (AD) technology for processing vegetable waste could be considered as an environmental friendly and cheap alternative to the landfill with both social and economic benefits. AD technology could turn agricultural waste into a resource. However, problems with current AD technology, including the types of devices used, operation process, inefficiency of the current systems and poor stability of the current anaerobic fermentation technology, need to be addressed before it can be utilized for agricultural waste.The aim of this study was to determine how AD technology could be efficiently applied to agricultural waste using a two-phase anaerobic fermentation system based on the high water content of vegetable waste. The waste from the most abundant vegetables of the northern areas in China was used as the raw material for fermentation. Studies with single and mixed substrates were performed to address how the characteristics of the acids produced and the overall methane levels of vegetables waste. The emerging high-throughput sequencing method, MiSeq, was used to analyze microbial species and determine their abundance and function in this two-phase fermentation system. Finally, two types of methane reflux were investigated to determine their effect on gas production.The celluloses of Chinese cabbage (low solid content) and starch of potato (high solid content) were selected as the single substrates for this research because of their regional and seasonal characteristics and their abundant supply. The fermentation mechanism was discussed in detail was performed at 35℃ with a two-phase digestor. When cabbage waste was used as a single substrate, the fermentation products were mainly four types of short chain volatile acid, acetic acid, propionic acid, butyric acid and isobutyric acid at the end of the fermentation. The total concentrations of these volatile acids increase gradually with the prolongation of the acidification time and reached 8058 mg·L-1 in the first 5 days. Acetic acid reached a peak value of 4289 mg·L-1 on the fourth day. It accounted for 53.2% of total volatile fatty acids. The optimal organic loading rate for the methane phase was 2.9 kgVS. m-3 · d-1,the maximum daily biogas production and biogas yield was 169L and 0.57 m3 ·(kgVS)-1 and highest methane content for the fermentation process was 68%. When the potato waste was used as the single substrate, the four main volatile acids were acetic acid, propionic acid, butyric acid and butyric acid in the acidification phase. The total concentration of VFAs in acidification reached 12625 mg·L-1 on the third day, and the acetic acid content reached a maximum value of 7364 mg·L-1 with an output of 58.4%. In contrast to cabbage, the acidification speed of potato waste was faster and the volatile acid content was higher, with an optimal organic loading rate of 3.6 kgVS · m-3 · d-1 in the methane phase. The maximum daily biogas production and biogas yield were 213 L and 0.62 m3·(kgVS)-1, respectively. Daily biogas production and methane content decreased gradually with OLR. The methane content was above 50% when OLR was between 2.9 and 4.3 kgVS · d-1，and the highest value was 72%.Next, different proportions of the cabbage and potato waste were studied. Volatile acids increased with the proportion of potato waste when acidified at 35℃. When the proportion of cabbage to potato waste was 3:7, the methane phase reached the maximum load of 3.9 kgVS · m-3 · d-1.When other mixed vegetable waste was used as the substrate, mixtures with TS of 8% or 12% had the highest acid production with total volatile acids of 3215 mg/L and 5697 mg/L, respectively, on the third day. Firmicutes, Proteobacteria, Acidobacteria and Bacteroidete accounted for 93.2% of bacterial phyla in the acidification phase. According to the classification of the genus, it was mainly Syntrophomonas and Clostridium, accounted for the ratio of 29.16% and 23.41%, respectively. These are common hydrolysis and acid producing bacteria in the anaerobic fermentation process. Lactobacillus accounted for 19.23% and Acidobacterium accounted for 12.15% of the bacteria in the acidification phase.As expected from the methane phase, gas production was fast and stable with the optimal organic load of 3.9-4.3 kgVS-m-3·d-1. The corresponding maximum daily biogas and methane production was 0.69 m3/kgVS and 0.48 m3/kgVS, respectively. The highest volume gas production rate was 2.9 m3·d1-·m-3. Mixed vegetable waste fermentation has a better gas yield and methane content compared to single substrate fermentation.The dominant bacteria in the methanogenic phase included the obligate-acidophile anaerobic bacteria, Methanococcus and the non-specific substrates, facultative anaerobic bacteria Methanosarcina. A small amount of the obligate-hydrogen anaerobic bacteria, Methanoculleus, Proteobacteria, Acidobacteria and Firmicutes was also present. Taken together, these bacteria accounted for 72.95% of the non-methane bacteria. Therefore, these genera play an important ecological role in the methane phase.Finally, the effects of OLR and the ratio of the reflux to the methane phase were studied. When OLR during the methane phase was kept constant at 2 kgVS/m3/d, the highest gas production and most effective separation of the phases was seen with a reflux ratio of 25% return to the acidification phase. Under these conditions, hydrolysis of the acidification phase had the highest levels of methane gas production at 119.6 L, with a methane content in the range of 65%-72%. With the reflux ratio of 55% return to the methane phase, the maximum methane gas produced was 109 L and the methane content in the range of 60%-65%. When the methane phase under different OLRs, the reflux ratio of 55% return to the acidification phase, the methane phase was optimal with a maximum gas production of 186.4 L of the OLR of 3.0 kgVS/m3/d. The reflux to the acidification phase alleviated the acid inhibition to bacteria and promoted substrate hydrolysis in the acidification phase at high load. When the reflux ratio of 55% return to the methane phase, with an increase in organic loading, reflux could promote methane phase gas production, but it had little effect on methane content. When the methanogenic phase load rate was 3.0 kg VS/m3/d, the highest biogas production attained 181.6 L after reflux. |
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