Study On Transformation Of Rural Organic Solid Waste Into Biological Energy In Dry-Anaerobic Digestion Process

The great amount of agricultural organic solid wastes with a wide range of spices in China was produced each year including livestock waste, fruit and vegetable waste and straws etc. Dry anaerobic digestion technology could effectively reuse these kinds of solid wastes, and had it transformed into biogas, which could match with the farmers’ need on fuel energy, in meantime, the residue could be utilized as organic fertilizer. The technique could improve the sanitary conditions in rural areas as well.Based on the success and results made by the early researches in dry anaerobic digestion, this study took the three solid wastes as the materials for the orthogonal tests in which the three factors-temperature, the inoculums concentration and total solid concentrations (TS) were taken into the consideration. In order to obtain the foundation and references for the large dry anaerobic digestion, this study also conducted the practical tests with small scale on ordinary material characteristics and spectroscopic analysis. The main studies were as follows:(1) Taking the temperature(26℃,38℃,50℃), the inoculums concentration10%,20%,30%) and total solid concentration(TS,20%,25%,30%) as the three factors, L9(34) orthogonal test was run in which the materials were three wastes above-mentioned. Then the biogas production efficiency, influential factors and the optimal fermentation parameters were analyzed while a three-factor linear regression model was established in this study too.(2) Dynamic characteristic study on dry anaerobic digestion process including pH, gas production, dissolved organic carbon (DOC), chemical oxygen demand (COD) and ammonia nitrogen (NH4+-N). The L4(23) orthogonal scale experiment was conducted, in which the factors were ranged including materials with different ratio (ratio4:1:1and4:2:0), various TS contents (25%and35%) and different inoculation amount (30%and50%). Finally, the changes of pH, gas production, COD for fermented liquid, DOC and NH4+-N for fermentation material were analyzed, and the influences of three factors on biogas production were determined.(3) Alternatives of spectral characterization for the optimal gas production groups were analyzed, as well as for ultraviolet and fluorescence spectra for dissolved organic matter (DOM) and infrared spectrum for materials. Then the change of internal structure on dry anaerobic digestion process was discussed when the alternative regulations were generally ascertained. According to the experimental results, the following main conclusions could be concluded:(1) Through orthogonal experiment in lab., Influence of three factors on gas production folloed the order as:temperature>inoculation amount>TS. At the high temperature (50℃), fermentation action was significant with high gas production and good quality. The gas production increased obviously if the inoculation amount ranged from10%to20%. However, the highest production could be found at TS25%.(2) Based on orthogonal test analysis, the three-variable linear regression equation with the fermentation materials of1.65kg was obtained and expressed as:y=0.298+1.660x1+123.700x2-90.500x3, Where, temperature as x1∈[26,50,], inoculation amount as x2∈[10%,30%] and TS as x3∈[20%,30%].(3) Alternative trend of pH values in dry anaerobic digestion scale test showed "fell down after it ascended first, and finally tended to be stable" type, which conformed to the three stage model. DOC and COD content presented a negative change at the initial stage. The alternative of NH4+-N had a tendency similar to total gas production.(4) At the initial stage of reactions, fluorescence absorption intensity of protein-like substances in fermentation liquor fermentation increased when there was a large generation of protein-like substances. At the methane-producing stage, simple organic compounds gradually increased with the decrees of protein-like substances, moreover, fluorescence intensity of unsaturated bond also declined with the consumption of protein-like substances. The ultraviolet spectroscopic analysis shows that appeared a conjugate double-type structure at wavelength around280nm increased, laterally the intensity slightly became weaker, but the aromaticity and unsaturation had a little increase. Based on infrared spectral analysis results, it was found that the pretreatment at the composting stage had polysaccharide degraded to micro molecule monosaccharide with the decrease of cellulose crystallinity, in meantime protein increased gradually. In the anaerobic digestion process, monosaccharide was basically degraded to small molecules substrates that could directly be used by methanogens to produce methane when fatty materials and protein of organic matter could continue to be dramatically degraded.