挥发性脂肪酸对厌氧干式发酵产甲烷的影响

为了提高中温干式厌氧间歇发酵效率,研究了发酵过程中间产物———挥发性脂肪酸对产甲烷的影响。实验分2批进行,第1批在牛粪发酵过程中分别添加乙酸、丙酸和丁酸,第2批发酵添加易产生挥发酸的厨余垃圾混合发酵。结果显示,添加单一挥发酸的发酵过程中,添加丙酸的产甲烷速度较慢,因为丙酸降解生成乙酸的速度较慢,减慢了甲烷的形成;混合发酵过程厨余垃圾产甲烷速度比牛粪快,发酵过程产生2个产气高峰;牛粪和厨余垃圾固体物质含量比在11∶1到5∶1范围内较好,比牛粪单独发酵产气多,产酸高但不酸败,产生的挥发酸主要是乙酸和丙酸,其中比例为7∶1混合发酵的产甲烷速率最大,为4.89 mL/(g VS·d)。实验表明,牛粪厌氧干式发酵过程添加一定量的厨余垃圾可加快挥发酸的产生并提高挥发酸产量,从而提高甲烷的产量,但是总挥发酸长时间超过10 000 mg/L,pH降到不适于产甲烷菌生长的范围时,将抑制甲烷的生成,挥发酸积累导致厌氧发酵酸败。

Effect of volatile fatty acids on methane production of dry anaerobic digestion

In order to improve the efficiency of dry anaerobic digestion process, the effect of volatile fatty acids (VFAs) on dry mesophilic anaerobic batch digestion of cattle manure was investigated in this study. The whole experiment concluded two groups. In the first group, additional acetic, propionic and butyric acid, were separately added in different cattle manure-applied anaerobic reactor, while in the second group, kitchen wastes mixed with cattle manure at different ratios since kitchen wastes could produce volatile fatty acids easily. The results of the first group experiment indicated that the rate of methane generation in the reactor with addition of propionic acid was more slowly than that in the reactors with addition of acetic and butyric acid, which was probably due to the low produced rate of acetic acid by degradation of propionic acid. The biogas production rate in the second group with kitchen waste was higher than that in the first group. Two biogas production peaks existed during the co-digestion with kitchen waste. The suitable total solid ratio of cattle manure to kitchen waste was in the range of 11:1 to 5:1. Under this range, the volatile fatty acid and biogas generation rate were higher than that of the reactor with only addition of cattle manure, and the mainly produced VFAs were acetic and propionic acid. Furthermore, the highest methane yield of 4.89 mL CH₄/(g VS·d) obtained when the ratio of cattle manure to kitchen wastes was 7:1. The results concluded that the addition of certain amount of kitchen waste could accelerate and improve the digestion process for enhancing VFAs production within 10 000 mg/L and thus increasing the methane yield. However,once the ratio exceeded the safe range, the VFA would accumulate and pH rapidly decrease, which would inhibit the activity of methanogen and finally block the fermentation.