Experimental Study Of An Enhanced Two-Phase Anaerobic Digestion Of Vegetable Waste

As the world's largest agricultual country, China is encouraging the famers to build more centralized vegetable processing facilities to prepare semi-finished or "cleaned" products for city residents. In year 2009, there were approxiamately 100 million tons of vegetable waste produced in this process. With the characteristics of high moisture content and degradability, vegetable waste gives a lot of pressure to the waste disposal unit. However, this large amount of waste has the potential to be used as the feedstock of industrial anaerobic digestion because the volatile solids content is relatively high. Moreover, the digestate also could be used as soil amendment because of the low toxicity. Anaerobic digestion is a kind of environment-friendly, economic-viable and highly effective biological treatment technology. It has proved an effective way to carry out reduction, recycling and harmles sness of fruit and vegetable waste.Funded by the China National "863" Project "the key technology of the anaerobic digestion of biomass (case number:2008AA062401)", the study discussed in this thesis conducted lab scale batch and sequence-batch anaerobic digestion on vegetable wastes and investigated the optimal paramenters for digestion. The feedstock was collected from the dinning house in Beijing University of Chemical Technology and grinded to get the solid fraction (residue) and liquid fraction (juice) separately. As the vegetable waste was heterogeneous, a new method, called two-step strengthened hydrolysis anaerobic digestion, was developed to enhance the degradation of the solid fraction in vegetable. Under mesophilic condition(35℃), batch and sequence-batch anaerobic digestion were carried out to determine the effect of food-microorganism (F/M) ratio on hydrolysis and acidification of vegetable residue and the performance of anaerobic digestion of vegetable juice, respectively. The organic loading rates used in the batch digestion were 4gVS·L-1,6 gVS·L-1 and 8 gVS·L-1, respectively. Six kinds of F/M ratios were 0.5,1.0,1.5,2.0,2.5 and 3.0, respectively. Also, a blank trial was carried out without inoculums to test the self-degradation or the original microorganisms in the feedstock. As the result, methanogenesis was the main reaction in the system when F/M≤1.0. A significant step of hydrolysis reaction was not observed in the digestion system; when F/M> 1.0, various degrees of hydrolysis reaction were observed in the different digestion systems, for example the gas production reduced and effluent pH decreased. Although the effluent pH of blank trial was the lowest, the concentration of the volatile fatty acid was not as high as that of the other trials. The above results proved that the effect of F/M on the acidification of vegetable residues were significant. The process of anaerobic digestion could be affected by controlling the F/M of the system. At the organic loading rate of 80 gVS·L-1, four F/M ratios as of 3.0,6.0,9.0 and blank were investigated to find out the effect of inoculum on hydrolysis and acidification of vegetable residue at high organic loading rate. The results showed that when F/M was in the range of 3.0-9.0, the effect of F/M on hydrolysis and acidification of vegetable residue was not significant. The concentration of volatile fatty acid of these three conditions with inoculum were 13457,11624 and 11480 mg·L-1, respectively, higher than the condition without inoculum.Besides the study of the effect of F/M on hydrolysis and acidification, sequence-batch digestion experiments were carried out to determine the performance of anaerobic digestion of vegetable juice and the optimal process parameters. Anaerobic digestion of vegetable juice through an 150 days experiment showed that anaerobic digestion of vegetable juice has good performance, The optimal organic load rate was 8.0 gVS·L-1·d-1, the biogas yield was as high as 860 mL·gVS-1, the methane content was around 65%, the COD concentration of the effluent were lower than 2800 mg·L-1, COD removal efficency remained at about 95%, organic nitrogen removal efficency kept more than 85%. A higher organic loading rate of 8.5 gVS·L-1·d-1 lead to an overload. The substance was not adequently degraded and the biogas yield and COD removal efficiency decreased respectively. The concentration of ammonia in reactor was in the range of 1348-2383 mg·L-1. No inhibition was observed. The difference between the biogas yield of various vegetable juice samples and HRTs at the same OLR was small. As a conclusion, the VS of the feedstock could be used as a performance indicator of the biogas potential of certain materials.This study was carried out to determining the optimal process parameters of strengthened hydrolysis and anaerobic digestion of vegetable residues and juices. It has provided an experimental basis for an enhanced two-phase anaerobic digestion of vegetable waste.