Study On Characteristics And Mechanism Of Biochar-mediated Anaerobic Digestion Of Chicken Manure

The excessive exploitation and consumption of fossil fuel cause more and more negative impacts on environment and energy utilization. The rapid development of livestock industry cause increasingly outstanding prominent environmental contradictions. Anaerobic digestion(AD) is a complex bioconversion process that can treat organic waste and generate biogas with a high methane content which can be used as renewable energy, reducing the consumption of fossil fuel and curtailing greenhouse gas emissions. AD can treat the environmental pollutation which cause by livestock industry and brings a great opportunity to solve the growing serious energy and environmental problems at the same time. Moreover, policy support has created unprecedented opportunities for application of anaerobic digestion technology in engineering. Most excrement from livestock is rich in nitrogen and with high content of organic matter. Excessive amount of organic acids, ammonia and ammonium ions derived from livestock manure can inhibit the AD process by exerting a toxic effect on microbial activity. Chicken manure(CM) is typical of excrements from livestock for its high nitrogen and organic contents. AD of CM is vulnerably inhibited by both ammonia and organic acids. Previous studies have shown that biochar could enhance direct interspecies electron transfer(DIET). Few studies have been conducted on the effect of bichar on improvement of methane production, anaerobic digestion efficiency and microbals by using imitated wastewater as substrates. Characteristics and mechanism of biochar-mediated anaerobic digestion of chicken manure which was supported by general program of the National Natural Science Foundation of China(Grant NO. 51576167) were investigated. Anaerobic batch digestion was conducted to determine the preferable kind and dosage of biochar. Orthogonal design and quadratic orthogonal rotation regression combination design were employed to obtain optimum parameters for methane production. The effect of biochar dosage(wt. %, on a dry weight basis), biochar particle size and C/N ratio on anaerobic batch digestion were also investigated. On basis of obtained optimum parameters, semi-continous AD of CM was conducted to verify the optimum parameters under different organic loading rate(OLR). OLR was increased gradatim. Characteristic of biogas production, anaerobic digestion liquid and microbial communities were compared between biochar-mediated anaerobic digestion of CM and anaerobic digestion of CM alone. Possible reasons of biochar could increase methane production; methane content and anaerobic digestion efficiency were proposed. Optimum parameters and theoretical basis for engineering application of biochar-mediated AD of CM were also obtained. The results of this study were as follows.(1) Different biochars were made from wheat straw, waste wood chips of apple trees and dry CM under different temperatures. Different chemical elements of biochars were also analyzed. Anaerobic batch digestions were conducted to determine the effect of different biochars on characteristics of AD of CM. Highest SCMP was got from biochar which made from waste wood chips of apple trees mediated AD. Compare to AD of CM alone, SCMP was increased by 68.97%. Microstructures of wheat straw biochars and wood chip biochars were better than chicken manure chars. Microstructures and major elements of biochars varied with different materials and temperature.(2) Anaerobic batch digestion of CM with different biochar dosages was conducted to determine the effect of biochar on AD of CM. Methane production per volatile solid(VS) and contents of biogas change were compared among AD of CM with different biochar dosages and AD of CM alone. The microstructure of biochar and the morphology of predominant microorganisms associated with biochar were also observed in this study using a scanning electron microscopy(SEM). Specific cumulative methane production(SCMP, mLg-1VS-1 chicken manure) could be obviously increased with external biochar addition. The SCMP of biochar-mediated AD of CM was increased and higher than AD of CM alone. Moreover, biochar addition could improve the biogas quality by increasing its methane content and decreasing its carbon dioxide and hydrogen sulfide contents. The preferable biochar dosage for biochar-mediated anaerobic digestion of chicken manure was around 5%. The SEM observations revealed that abundant microbes attached to the biochar.(3) Orthogonal design and quadratic orthogonal rotation regression combination design were employed on basis of obtained preferable kind of biochar and biochar dosage to obtain the optimum paracters for biochar-mediated AD of CM. On basis of obtained optimum parameters, semi-continous AD of chicken manure was conducted to verify the optimum parameters under different organic loading rate(OLR). OLR was increased gradatim. Characteristic of biogas production, anaerobic digestion liquid and microbial communities were compared between biochar-mediated anaerobic digestion of CM and anaerobic digestion of CM alone. The optimum parameters of biochar-mediated AD of CM were 4.97%(wt. %, on a dry weight basis) biochar dosage, 3.125 g·L-1·d-1 OLR and a hydraulic retention time(HRT) of 20 d. 1.46 L·L-1·d-1 and 0.94 L·L-1·d-1 average volume biogas and methane production of biochar-mediated AD of CM were observed which were 10.61% and 13.25% higher than AD of CM alone. SCMP and specific cumulative biogas production(SCBP, mLg-1VS-1 chicken manure) were observed 468 mL·g-1 and 301 mL·g-1 respectively which were 11.16% and 13.16% higher than AD of CM alone. Biochar-mediated semi-continous AD of CM could decrease the concentration of total ammonia nitrogen(TAN), free ammonia nitrogen(FAN) and ions to enhance stability of AD system. Biochar could enhance the conversation of insoluble substrate into soluble substrate and soluble substrate into methane during AD process.(4) Metagenomes obtained from different AD liquid via different time were sequenced. Characteristics of microbial communities on phylum, class and genus levels were compared between biochar-mediated AD of CM and AD of CM alone via different time. Microbial commucities from AD of CM alone were much more abundant than from biochar-mediated AD of CM under all phylum, class and genus levels. Biochar could partily decrease redundancy functions among microbal communities and enhance stability of microbal community structures. Morever, biochar-mediated AD could promote metaboly among interspecies. Biochar-mediated AD also increased relative aboundance of Methanospirillum and Methanobacterium which were possible reasons of biochar-mediated AD could increase methane production and methane content.