射流搅拌提高牛粪中温厌氧发酵产沼气性能

Jet mixing improving biogas production performance of mesophilic anaerobic fermentation with cow manure

现有中温沼气工程的搅拌方式以机械搅拌和沼液回流搅拌为主,存在产气率低、设备难以维护、长期运行不稳定等问题。该研究设计了一种射流搅拌技术,集沼液回流搅拌和沼气搅拌于一体,试验用于牛粪中温厌氧发酵产沼气。通过沼气产率、营养基质消耗、微生物种类和数量的变化等方面分析射流搅拌强化牛粪中温厌氧发酵产气的效果及机理。结果表明:发酵期30 d内,无搅拌、机械搅拌、沼液回流搅拌和射流搅拌的总产气量分别为52.34、64.30、61.97和71.22 L;原料产气量分别为0.238、0.292、0.282和0.324 L/g,射流搅拌厌氧反应器的原料产气量比无搅拌、机械搅拌、沼液回流搅拌分别提高了36.1%、22.9%、18.4%;射流搅拌方式中化学需氧量(chemical oxygen demand,COD)降解率最高达到60.8%;在产甲烷高峰期发酵第10天左右,射流搅拌反应器内产甲烷菌最大值达2.0×108个/m L;挥发性脂肪酸(volatile fatty acid,VFA)浓度的降速较其他有搅拌方式快,VFA无积累,不受高浓度VFA的产物反馈抑制或底物抑制。

Livestock manure was the main pollution source to lake basin. Anaerobic digestion process would be a promising alternative to control the pollution from livestock manure. The prevalent mixing method of mesophilic biogas engineering was mechanical mixing or single slurry recirculation. There were many defects in the biogas plants, such as low gas production, unstable and difficult to repair. To solve these problems, the jet mixing equipment was designed as a mixing strategy, which simultaneously carried out the slurry and biogas recirculation. The fermentation experiment was carried out with cow manure slurry feed with 5.5% total solids（TS） concentration and controlled temperature of 35℃, which was intermittently stirred by different mixing modes for 10 minutes per hour. The methane and carbon dioxide content of the biogas, the microbe species and the colony forming units, the variation of volatile fat acids（VFAs） and the reduction of chemical oxygen demand（COD） in the biogas production process were investigated in the experimental bioreactors with jet mixing. Besides, the efficiency and the mechanism of enhancement on anaerobic fermentation of cow manure were evaluated. The experimental results showed that the total biogas production of non-mixing, impeller mixing, slurry recirculation and jet mixing strategies in 30 days was 52.34, 64.30, 61.97 and 71.22 L, respectively. The biogas production was 0.324 L/g in the jet mixing bioreactor. The jet mixing method improved biogas production by 36.1% compared to the method of non-mixing, and by 13.2% and 17.7% compared to the method of mechanical mixing and single slurry recirculation respectively. The daily biogas production efficiency in the jet mixing bioreactor ascended to the peak quickly on the 6th day, and the peak value was 6.53 L. The highest COD removal efficiency was 60.8% with the jet mixing, about 20% more than the other methods. What was more, the mass transfer was enhanced and the microbial activity was increased in the bioreactor with the jet mixing. The microbial activity would have contributed to the vigorous degradation of organic matter. VFAs were utilized by methanogens for biogas production, which were unable to accumulate during the anaerobic digestion process with the jet mixing, and thus the product inhibition or substrate inhibition was eliminated. The methanogens in the jet mixing bioreactor grew fast since the 10 th day, and the peak value was 2.0×108 m L-1. During fermentation process in the jet mixing bioreactor, the amount and the activity of hydrolytic bacteria, acetogenic bacteria and methanogens remained at a high level. Meanwhile, the methane content of the biogas increased drastically from about 20% to over 55% before the 10 th day, and it ranged between 55% and 65% from the 10 th to the 30 th day. The methane content variation was less than 10% in approximately 20 days. Carbon dioxide in biogas could be absorbed by the jet mixing, and therefore it was reused to generate methane with hydrogen as an electron donor. Moreover, the acetate could be formed from hydrogen and carbon dioxide by hydrogen-oxidizing acetogenic bacteria. These reactions should improve the performance of methanogenesis process. To sum up, the jet mixing method had a higher methane content and lower carbon dioxide content. Based on the results presented in this article, the jet mixing system shows a higher efficiency than the mechanical mixer and single slurry recirculation.