| With the increasing of municipal solid wastes production, waste incineration power generation, an effective technology of enery recovery, has been appiled more and more widely all over the world. What followed is a large amount of incineration leachate, with extremely high organic concentrations, generated during stored in storage bunkers, which requires specific treatment. Different from aged landfill leachate, incineration leachate is fresh leachate with good degradability (B/C>0.5). Thus, incineration leachate is usually treated by the anaerobic bio-methanogenic technique to remove the high concentration of COD. This leachate contains vast amounts of organic compounds. The typical one group of the dissolved organic matter (DOM) is fulvic acids, which are a kind of refractory organics and will severely affect the treatment efficiency during the leachate bio-treatment processes. Thus, the main research objectives of this study are to investigate the effect of fulvic acids from incineration leachate on anaerobic bio-methanogenic process, and the degradation and transformation of fulvic acids during this treatment process.In this study, a lab-scaled expanded granular sludge bed (EGSB) reactor was set up for treating the fresh leachate from a municipal solid waste incineration plant in north China. The reactor was operated stably and efficiently with an influent COD of 55,000 mg/L and an organic loading rate (OLR) of 23.0 kgCOD/(m3 d). A COD removal of >91% under this condition was achieved and 86% of the removed COD was transformed into methane. Further increasing OLR, the reactor became soured and propionate accumulated with a sharp decreasing of COD removal efficiency, indicating that the maximum OLR for the EGSB reactor was 23.0 kgCOD/(m3 d). Analysis of the DOM in the effluent of reactor showed that, the levels of both humification and aromatization of the residual organics after the bio-treatment were increased significantly, which were mainly composed of fulvic acids, aromatic substances and long-chain alkanes etc. As a main and typical composition of refractory organic in the effluent of the EGSB reactor, the degradation and transformation of fulvic acids needs further demonstration.The effect of fulvic acids on the treatment efficiency of EGSB reactor treating incineration leachate was then investigated. The results showed that fulvic acids would affect the COD removal efficiency and methane production. COD in the influent was kept constantly, and stepwise increased the fulvic acid concentration in the influent from 1,500 mg/L to 8,000 mg/L. The COD removal efficiency was gradually decreased from 96% to 81, and the methane content in the biogas decreased from 72% to 60%. Analysis of methane production, molecular structure of fulvic acid and microbial community of sludge revealed that, the effect of fulvic acids on the treatment efficiency of the bioreactor was conducted by the fact that fulvic acids would compete electrons with the methanogens to result in a decreasing of COD removal and methane production. As a result, the EGSB reactor was operated stably without any risk of souring during the entire operational period. And the reactor could completely recover to the initial status after a not too long time (25 days) when the additional fulvic acids in the influent was removed. In addition, it was observed that when FA concentration in the influent increased to 4,000 mg/L, COD removal efficiency decreased significantly and could not recover at all even after a long-time operation. Thus, it is recommended that FA concentrations in the influents of full-scale incineration leachate treatment process should be controlled under 4,000 mg/L to ensure the treatment efficiency.The degradation of fulvic acids was relative efficient by the EGSB reactor. However, the degradation rate was gradually dcreased from 86% to 72% as the fulvic acids in the influent stepwise increased from 1,500 mg/L to 8,000 mg/L. The residual fulvic acids in the effluents were mainly with a molecule size of over 20 kDa. The degradation rate of the fulvic acids with this molecule size was only 46% when the reactor was fed with 8,000 mg/L fulvic acids. In addition, the non-humic substances of DOM in the effluents were also affect by the concentration of fulvic acids. Microbial community analysis revealed that, the growth of the predominant family of Anaerolinaceae was enriched under high fulvic acid concentration. It was also suggested that Anaerolinaceae might take part in the degradation and transformation of fulvic acids via extracellular electron transfer, by obtaining electrons from the degradation of other substrates and transferring to the fulvic acids. As a large amount of electrons were consumed by the fulvic acids, the methane production rate would have affected by the loss of electrons.The molecular weights of the residual fulvic acids with macromolecules (>20 kDa) in the effluents of the EGSB reactor grew even larger compared with raw fulvic acids from raw incineration leachate, and the levels of both humification and aromatization of these residual fulvic acids all increased significantly. The chemical structure and elemental composition of fulvic acid molecules from various effluents with different fulvic acid concentrations in the influents varied a lot, which was mainly due to the different degradations of the branch carbon-chain on the fulvic acid molecules under different fulvic acid concentrations in the influents. The raw fulvic acids from the leachate can play as an electron shuttle for enhancing the electron transfer between Geobacter sulfurreducens and FeO(OH) or between Geobacter metallireducens and Geobacter sulfurreducens. However, the residual fulvic acids from the effluents of the EGSB reactor lost this electron shuttling capability, and what’s more, even inhibited the growth of the cells. This is because the quinone moieties in the fulvic acids, which is key functional groups for electron shuttling, were completely and irreversibly destroyed during the anaerobic bio-methanogenic process. It might be harmful to the natural environment if these fulvic acids were discarded without any further treatment. |
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