Study On The Start-up Environment And Digestion Process Optimization In Two-phase Anaerobic Digestion System Of Food Waste

Annually, approximate 160,000 tons of food waste (FW) are generated every day in China, and the amount of FW is increasing by 10% per year.Without proper disposal, FW can cause severe environmental pollution and human health problems. Due to its inherent high biodegradability, FW is considered to be one of the most promising energy sources for renewable energy yields. Compared with other treatments, the anaerobic digestion (AD)is considered as the best treatment option for organic wastes such as FW to realize energy recovery. However, if the reactor is overloaded, there will be excess volatile fatty acids (VFAs) by acid producing bacteria, followed by low alkalinity and pH, which are unsuitable to methanogen and result in toxic effects on methanogenic activity to lower biogas yield and methane content,which becomes the bottleneck in restricting its development. This study mainly focused on optimization of the two-phase anaerobic digestion (TPAD)to achieve operational stability of the digester and perfect performance of acid and methane yields with high concentration of FW. The main research contents were as followed:(1) Three indexes (Initial pH, hydraulic retention time (HRT) and food-microorganism ratio (F/M)) of operational parameters were optimized with orthogonal data analyses to maintain an acidogenesis phase so that maximum amounts of ethanol (ETH), acetic acid (HAc) can be easily produced for the next step of methanogenesis phase (MP). The results showed that the optimal parameters in acidogenesis phase (AP) were the initial pH,9.00; HRT, 4 days; F/M, 2.5:1, and that priorities of influencing factors was the initial pH > HRT > F/M. Under the optimal conditions, it obtained the maximal levels of indicator (1) Ethanol and acetic acid concentration of unit Volatile solid (ETH-HAc-VS), mg·g-1VS·L-1,and (2) Ethanol and VFAs concentration of unit VS (ETH-VFAs-VS), mg·g-1VS·L-1, (1840 and 3116 mg·g-1VS·L-1 respectively), and the type of fermentation was butyrate type fermentation. Moreover, the biogas yields rates reached the maximum (564 mL) in first day and Total Solid Removal rate (TS-R)?Volatile Solid Removal rate (VS-R) were 14.02% and 20.60%, respectively.(2) Under the optimal conditions from batch tests (initial pH, 9.00; HRT,4 days; F/M, 2.5:1), the methane and biogas yields rates (DMY-VS and DBY-VS) of the TPAD with continuous stirred tank reactors (CSTR) were more than single-phase anaerobic digestion (SPAD) (471, 751 and 416, 662 mL·gVS-1·d-1) and raised 13.22% and 13.44%, respectively. Compared with SPAD (37.10%?47.677%), the TS-R and VS-R of TPAD were 44.25% and 56.36%, respectively. Moreover, there was more operational stability of the digester in TPAD than SPAD with high concentration of FW. With the OLR increasing, DMY-VS and DBY-VS raised of both TPAD and SPAD, but in a few days after lifting OLR, gas yields performance of both decreased and then recovered. The descent range of SPAD was higher than TPAD. By the way of sludge recirculation, there was 4.10%, 3.87% higher the TS-R and VS-R respectively than samples without recycling and reached 46.07% and 58.54%,respectively. Besides, it was helpful to improve the anaerobic digestion ability of TPAD by shortening HRT of methanogenic phase, to some extent. When HRT was 8 d, the methane and biogas yields rates was 687 and 1052 mL·gVS-1·d-1, which were 20.49%, 23.31% and 278.51%, 153.22%,respectively higher than 12 d and 4 d and there were highest the TS-R and VS-R(55.84%?68.41%).(3) The acid yields performance of simple substrates, including glucose,protein and oil, was studied to explore the effects of different components of FW on acid yields. The results showed that priorities of influencing factors in acidogenesis phase was the protein > glucose > oil. Under the content of each component (glucose 36.4%, protein 22.7% and oil 40.9%) unchanged, there was a significant effect of organic loading (OL) on total acid yields, but little on acid concentration of unit VS.Above all, TPAD was more suitable for FW treatment than SPAD under the condition of high concentration of FW. Through the screening of HRT,initial pH and F/M, the acid yields of acidogenic phase improved. And then, it explored the effects of different components of FW on acid yields by simple substrates. Finally, the way of shortening HRT of MP and MP was used to optimize TPAD to improve the digestion performance.