Abstract:
Abstract: 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 10th day, and the peak value was 2.0×108 mL-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 10th day, and it ranged between 55% and 65% from the 10th to the 30th 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.