Dynamics And Process Optimization Of Biogas Production Through Anaerobic Fermentation Of Waste Mixture

The environmental problem has been getting more and more serious for the increase of various organic wastes (including the waste of kitchen, fruit, vegetable, livestock and poultry) with the rapid development of economy and urbanization progress. The large amount of improperly handled organic waste polluted soil, atmosphere and water resource and it decreases the environment health and threats people’s health. Biogas fermentation technology can handle the organic waste, reduce pollution to the environment. Municipal wastes were employed in this study to overcome the defect of insufficient feeding, high requirement of pre-processing, inadequate gasification and poor structure of unitary fermentation raw materials. It can also improve the environment and resourcelize the waste. The study may provide technical support and application instruction for biogas utilization of municiple organic biomass wastes.This paper studied the biogas production properties of fermentation materials such as pig manure, kitchen waste, civil sludge and corn stalk and it’s effect on anaerobic fermentation; the dynamic model of biogas production by anaerobic fermentation was set up; the experimental study on dry fermentation with corn stalk with different preprocessing was carried out; the anaerobic fermentation process of municipal wastes was optimized based on the effect of mixture of kitchen waste with pig manure, civil sludge and corn straw on biogas productivity and production rate respectively. The mathematical model was established, analyzed and the reliability was verified.It can be concluded from above studies that:(1) There are many elements in kitchen waste and pig manure, and the element in civil sludge is unitary, but it has the potential for fermentation; during fermentation, the PH values of the four kinds of raw materials decreased to minimum in 2-6d. The minimum PH value for kitchen waste during fermentation appeared on the sixth day, and the PH value increased a little bit but not to the normal fermentation level for biogas production; the PH value of kitchen waste and pig manure decreases to 6.30 then the fermentation for ethane production continues; the pH value of civil sludge decreases to 5.38 then it restore quickly. The variation trend of VFAs and ORP of the four wastes are similar, and the maximum of VFAs is 12～17 g/L, the ORP of subsequent ethane production is between -300～-460mV. From the standpoint of biogas production, the four materials all have the stages of primary hydrolytic acidification massive gas production and the subsequent stable gas production, but the accumulate yield of ethane differs considerably, it’s 1418mL for corn stalk,2050mL for kitchen waste,1927mL for civil sludge and 2130mL for pig manure. Analysis of gas production potential of the mixing raw materials may provide application basis for the study of fermentation process and theoretical guarantee for the stable operation of biogas fermentation technology of municipal wastes.(2)The anaerobic experiments were carried out to the corn stalk preprocessed with dilute sulphuric acid, dilute sodium hydroxide and puffing and the results showed that the gas production rate and ethane content of acid treated stalk are all higher than that of alkali treated stalk. The highest gas production rate of corn stalk treated with dilute acid at 80℃ for 3 hours was 0.83 m3·m-3·d-1, the total yield of 60 days was 4.47 m3·m-3, and the highest ethane content was up to 74.9%; the highest gas production rate of corn stalk treated with dilute alkali at 80 ℃ for 3 hours was 0.71 m3·m-3·d-1, the total yield of 60 days was 3.46m3·m-3, and the highest ethane content was up to 70.1%; the biogas production of puffing treated stalk was stable than that of alkali treated one, and the highest gas production rate was 0.63 m3·m-3·d-1 and the total yield of 60 days was 5.89m3·m-3, the highest ethane content was up to 82.9%.(3) The dynamics studies of biomass ethane production with kitchen waste, corn stalk, pig manure according to the dynamic equations of ethane production by biomass anaerobic fermentation of Cheynoweth were conducted and the results showed that the fitting of anaerobic fermentation process can be performed at two stages(0～20 days and 21～40 days), and the relative coefficient of the models are all greater than 0.9, and the result is close to the experimental data. It indicated that the rule of biomass anaerobic fermentation biogas production can be expressed with Cheynoweth equation, and it can provide instruction for the optimization of municipal wastes fermentation process.(4) The optimization to the process of municipal wastes fermentation of mixtures of kitchen waste with pig manure and civil sludge were conducted separately and the results showed that, through comprehensive comparison and analysis with Box-Benhken Design model, the optimal value of pig manure to kitchen waste ratio, pH value and TS were 1:1, 7.11 and 10.08% respectively and the predicted biogas yield was 30 459.5mL under this condition; the optimal value of civil sludge to kitchen waste ratio, pH value and TS were 4.2:1, 7.19 and 17.67% respectively and the predicted biogas productivity was 618.60ml/g under this condition, the optimal value of corn straw to kitchen waste ratio, pH value and TS were 3.16:1,7.43 and 17.44% respectively and the predicted biogas yield was 20623.3mL under this condition;The model verification experiment showed that the relative error of predicted and actual values of biogas productivity was less than 5%, It indicates that the models are well fitted and of high reliability.