| With the aggravation of global energy shortage and environmental pollution, anaerobic digestion was considered to be a desirable method for food waste treatment owing to the advantages of combination of waste disposal, energy recovery and ecologically benign cycle. However, in practice, long-term anaerobic digestion system of food waste was often unstable, and even worse, the mechanism of process imbalance was not clear. Meanwhile, the efficient control strategy for improving anaerobic process of food waste was also deficient. These factors greatly limited the widespread application of anaerobic digestion of food waste.In order to solve these problems, a series researches were carried out. Firstly, process performance of batch and semi-continuous anaerobic digestion of food waste were investigated. The relationship of process performance changes and methanogens community and diversity dynamics during long-term operation was also analyzed to explore the interaction of running status of anaerobic system and ecological functions of methanogens community. The results showed that the semi-continuous anaerobic system was stable under a low loading condition (OLR:4 g VS/L-day, HRT:20 days), but after system loading was increased (OLR:6 g VS/L·day, HRT:15 days) it became unstable and failed, i.e. methane yield declined to 0, and the VS removal dropped to below 40%, when pH reached below 4.0 due to of VFA accumulation (26310.8 mg/L). When the anaerobic system was out of balance, the rapidly increased concentration of propionate (from 0 to above 10000 mg/L) suggested that propionate played an important role in VFA inhibition. The running status of anaerobic system was proved to be closely related to methanogens community and diversity dynamics. During the long-term operation of anaerobic system, the methanogens community changed a lot especially when the system was out of balance (Day 129-155). The contents of hydrogenotrophic Methanoculleus and Methanobacterium greatly decreased from 16.5% and 10.7% to 0.2% and 2.4%, respectively, which blocked the CO2 reduction pathway for methane production. As a result, the hydrogen partial pressure in anaerobic system increased. Then, the feedback inhibition of hydrogen led to the decline of propionate degradation rate, which resulted in propionate accumulation. Meanwhile, the content of Methanosarcina dropped from 73.5% to 4.7%. The concent of Methanosaeta increased from 9.7% to 80.3% and Methanosaeta became the dominant methanogen. The diversity of methanogens also decreased. Due to above factors, the ecological function of anaerobic system gradually degraded, and finally led to the failure of anaerobic process.In order to examine the feasibility of improving anaerobic digestion of food waste by co-digesting with MSW incineration plant waste leachate, the process performance and stability of batch and semi-continuous anaerobic co-digestion system were investigated. The results indicated that when the initial concentration of substrate of batch anaerobic system was 20-20.5 g VS/L, compared to batch anaerobic mono-digestion of food waste, the methane yield was increased by 1%,10%, and 16%, the time to reach the peak methane yield was reduced by 1,2, and 4 days, and the VS removal was increased from 72.8% to 74.1%,75.9%, 77.6% with the addition of waste leachate (5.6%,11.2%, and 22.1%, respectively). When the proportion of waste leachate was 22.1%, the process performance was best. The poor stability of long-term anaerobic digestion of food waste could be reversed by co-digestion strategy. During a long-term operation period (218 days), contrary to the failure of mono-digestion of food waste, semi-continuous anaerobic co-digestion with waste leachate exhibited a much better performance and stability under the low (OLR:4-4.1 g VS/L·day, HRT:20 days), moderate (OLR:6.1-6.2 g VS/L·day, HRT:15 days), and high (OLR:8.1-8.3 g VS/L·day, HRT:10 days) loading condition. When the proportion of waste leachate was 22.7%, the system performance was best as indicated by the highest methane yield (452.2-506.3 mL/g VSadded), the greatest methane content (64.2-65.5%), and the most VS destruction (71.6-78.0%). The unstable mono-digestion of food waste could be recovered by metal elements supplementation (Fe:100 mg/L, Co:2.0 mg/L, Mo:5.0 mg/L, Ni:10.0 mg/L), which indicated that trace metal element was the key factor for maintaining process stability of anaerobic system. These results were in line with our analytical results that the food waste was deficient in trace metal elements, and waste leachate was rich in them. Co-digestion strategy provided abundant trace elements for anaerobic process and exhibited the better process performance. This research provides a more technically and economically feasible approach to co-treating and co-utilizing food waste and waste leachate from MSW incineration plant and a method for real-time recovery of unstable anaerobic system of food waste by supplementing trace metal elements.In consideration of the significant effects of trace metal elements on anaerobic digestion, the mechanism of stimulative effects of metals on anaerobic digestion of food waste was investigated. The results suggested that trace metal elements had a "Low-dose stimulation and high-dose inhibition" effect on batch anaerobic digestion of food waste. Trace metals supplemented under moderate concentrations greatly enhanced the process performance. The strengthening effect of trace metals would weaken, when the supplemental concentrations exceeded the optimal dosage. However, the excessive supplementation of trace metals exhibited the obvious toxicity to methanogens, such as Fe (1000 mg/L) and Ni (50 mg/L). The optimal dosages of Fe, Co, Mo, Ni elements were 100,1,5,5 mg/L, and the increments of methane yield were 11.3%,12.4%,11.6%,14.0%, respectively. The combinations of trace metals exhibited remarkable synergistic effects. The supplementation of Fe (100 mg/L)+Co (1 mg/L)+Mo (5 mg/L)+Ni (5 mg/L) obtained best performance in terms of the greatest increments of coenzyme F42o (52.8%) and F430 (85.0%) concentrations and methane yield (35.5%). Trace metals could greatly enhance the performance of semi-continuous anaerobic system of food waste. Firstly, abundant trace metals were beneficial to maintain the stability of methanogens community and keep the satisfactory ecological functions of methanogens. Trace metals ensured the preferential growth of Methanosarcina (between 67.2-87.5%) and kept the quantity of hydrogenotrophic methanogens at a stable level (about 10%). In addition, trace metals supplementation greatly imporved the ability of anaerobic system to adapt environmental changes by improving the diversity of methanogens. Due to above factors, trace metals greatly improved the performance and stability of anaerobic digestion of food waste.According to above results, the mechanism of process imbalance of long-term anaerobic digestion of food waste was proposed. In semi-continuous or continuous reactors, the initial trace metal elements obtained from metal-rich seed sludge were enough for maintaining the growth and metabolism of methanogens, so the anaerobic system remained stable in the initial stage. However, due to the low content of trace metals in food waste, trace metals in anaerobic system could not be supplemented by food waste substrate and therefore would be lost with the regular substrate feeding and digestate discharge. As a result, trace metals concentrations reduced gradually and might not meet the requirement of metalloenzyme synthesis of hydrogenotrophic methanogens and the needs of the growth of Methanosarcina. On one hand, the CO2 reduction pathway for methane production would be blocked. Consequently, both the activity and quantity of hydrogenotrophic methanogens decreased, which resulted in the increase of hydrogen partial pressure in anaerobic system. The feedback inhibition of hydrogen would lead to the decline of propionate degradation rate, which resulted in the accumulation of propionate. On the other hand, the growth of Methanosarcina was also limited and the dominant methanogen changed to Methanosaeta. The diversity of methanogens also decreased. Due to these two factors, the ecological function of anaerobic system gradually degraded, and finally the reaction failed.The changes of metals speciation showed the reduction of metals bioavailability during anaerobic digestion, which might weaken the stimulative effects of trace metals. The initial soluble fractions of Fe, Co, Mo, Ni were 69.6%,40.0%,88.8%, and 85.2%, respectively. However, at the end (Day 20), only 12.3%,10.8%,26.0%, and 35.5% of total amounts of Fe, Co, Mo, Ni were soluble fractions, respectively. Compared to the initial state, the acid soluble and the reducible fractions of Fe, Co, Ni increased greatly after the reaction of 20 days. Whereas the oxidizable fraction of Mo greatly increased from 3.4% to 62.9%. Thus, in order to strengthen the stimulative effect of metal elements on anaerobic digestion of food waste, the feasibility of improving metals bioavailability and reducing metals dosage by ethylenediamine-N,N'-disuccinic acid (EDDS) was investigated. The results showed that EDDS could not reverse the tendency of the reduction of metals bioavailability, but it could reduce the conversion rate of soluble fractions of metal elements. In the middle phase of the reaction, the soluble fractions of Fe, Co, Mo, Ni elements were increased by 143.2%,36.4%, 23.8%,42.4% by the addition of EDDS. At the end of the reaction, the corresponding increments reached 96.6%,52.9%,18.2%,69.6%, respectively. The addition of EDDS improved metals bioavailability for microbial uptake and stimulated the activity of methanogens, and therefore, the concentrations of coenzyme F420 and F430 were increased by 12.3% and 34.2%, respectively. The batch and semi-continuous experiments confirmed that the addition of EDDS (20 mg/L) bonded to trace metals prior to their supplementation could obtain a 50% reduction of optimal metals dosage without the degeneration of process performance of anaerobic system. This study provided a feasible method to reduce trace metals dosage by green chelating agent EDDS via improving metals bioavailability. |
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