| During the process of dry anaerobic digestion, volatile fatty acids(VFAs) are important intermediates for the methane generation, and thus it could be the key factor of the fermentation. In this paper, we studied the dry anaerobic digestion process with the addition of different external organic acid such as acetic acid, propionic acid, butyric acid and different ratio of kitchen wastes, analyzed the effect of different VFAs accumulation on the abundance of all the microbiologic populations as well as the production of methane. Furthermore, the relation between VFAs and corresponding microflora was investigated, and the dynamic models of microbial growth and methane generation were established.The initial and operating conditions were determined including: the total solid was 20%, the inoculum rate was 10%, and the mixing frequency was once every two day. The trace elements(Fe 、 Co 、 Ni) were optimized using response surface methodology, and the optimizing concentration of Fe、Co、Ni added in anaerobic digestion were 15.0 mg/L, 1.0 mg/L and 1.2 mg/L, respectively.During the process of dry anaerobic digestion, the optimal amount(21 g) of acetic acid was added in R2 ace reactor every other day. After six times of the addition, the R2 ace system reached at the stability stage. The abundance and activity of methanogens in R2 ace was the highest of all reactor systems. The accumulated biogas and methane were 157.97 L and 73.46 L, respectively. The yield of methane in R2 ace was 5.27 m L CH4/g VS/d, which outclassed that of the control and other reactors with adding acetic acid at different concentration.The optimal added amount of propionic acid was 4 g in R2 pro reactor and 8 g in R3 pro reactor with adding every other day for six times. The yield of methane of R2 pro was similar with that of R3 pro, and it was much higher than that of other reactors. The yield of methane in R3 pro reactor was the highest with value of 4.17 m L CH4/g VS/d. When the methanogenic activity was calculated by the ratio of daily methane generation to the abundance of Archaea, it was higher in R3 pro than that in other reactors, while the activity in R2 pro was the highest if it was calculated by the ratio of daily methane generation to total volatile solid.The optimal added amount of butyric acid was 7.2 g in R6 but reactor with adding every other day for six times. The yield of biogas and methane in R6 but were the highest with the value of 10.62 m L/g VS/d and 5.11 m L CH4/g VS/d, respectively. When 4.8 g and 2.4 g butyric acid were added separately in R5 but and R4 but, the methanogenic activity of R5 but and R4 but were similar, while it was much higher than that of other reactors. It was shown that the abundance of microbiology, the yield of biogas and methane in R4 but, R5 but and R6 but were much higher than those obtained in the control and other reactors with adding butyric acid at different concentration.It was known that concentration of VFAs was correlated with the corresponding microorganism. At the start-stage, the results showed that the concentration of acetic acid was negative linear correlated with the population of H2-utilizing methanogen(relative coefficient R2=0.8658), while the concentration of acetic acid was positive linear correlated with the population of acetate-utilizing methanogen(relative coefficient R2=0.8578). Once the concentration of acetic acid was over 100000 mg/L, the relationship between acetic acid and population of acetate-utilizing methanogen was negative linear correlated with the coefficient of 0.8984.The population of Archaea and acetate-utilizing methanogen were positive linear correlated with the yield of methane, and syntrophic propionate-oxidizing was also positive linear correlated with propionic acid which was under 40 g/L. However, once the concentration of propionic acid was higher than 40 g/L, there was a negative linear correlation. The population of butyrate-degrading acetogenic hydrogenogen was positive linear correlated with acetic acid as well as butyric acid.In the proceeding of dry anaerobic digestion, different VFAs utilized by sulfate reducing bacteria(SRB) were propionic acid, butyric acid and acetic acid in turn. It was shown that moderate accumulation amount of VFAs benefited to the synergy of SRB and methanogens in anaerobic fermentation.With the addition of 20% kitchen waste(the total solid of added kitchen waste relative to that of cattle manure, that is, the ratio of cattle manure to kitchen waste was 5:1), the produced VFAs were mainly acetic acid and butyric acid in the dry anaerobic digestion. The total VFA concentration was under 100 g/L and the yield of biogas in the reactor was the highest with the value of 9.64 m L/g VS/d. With the addition of 14% kitchen waste(the ratio of cattle manure to kitchen waste was 7:1), the yield of methane(5.09 m L CH4/g VS/d) in the reactor was higher than that of other reactors, and the methanogenic activity was also the highest. When the added content of kitchen waste was controlled at 9%~20%, the yields of biogas and methane were much higher than the control and other reactors.The kinetics of methane generation at the start-up and stable stages of the anaerobic digestion process was analyzed using the formula of ln(V_m)/(V_m-V)＝kt and the kinetic constant of k was obtained. The kinetic of acetate-utilizing methanogens was simulated using the Logistic model which was applied for the reactor with the highest k at the start-up stage with the addition of different acetic acid, propionic acid, butyric acid and kitchen waste, respectively. By comparison of different kinetic constant of k, we concluded that the accumulation of acetic and butyric acid benefited to the acetate-utilizing methanogens to use the substrate, while the propionic acid accumulation was harmful to this process. Additionally, the effect of the addition of moderate kitchen waste on the improvement of acetate-utilizing methanogens to use the substrate was better than the direct addition of acetic acid. The Gompertz model was used to simulate the kinetics of methanogens growth and methane generation. The results showed that appropriate acetic acid accumulation was good for methanogens growth and methane generation, while propionic acid accumulation did not benefit this process. Furthermore, the obtained simulated parameters of dry anaerobic digestion with the addition of 9% kitchen waste were similar with the process with the addition of acetic acid. |
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