Running status of large and medium scale biogas project and physical, chemical and biological characteristics of materials before and after fermentation in winter of Beijing

Abstract: To explore the existing problems of biogas projects operating condition and properties of digestate in winter of Beijing, a field investigation on utilization of fermentation raw material, biogas, residue and slurry of 29 biogas projects were carried out in Beijing. Meanwhile, the operating conditions of biogas projects were also investigated. The samples of raw material, biogas residue and slurry were collected and their physicochemical properties such as nutrient content, number of fecal coliforms, heavy metal content were analyzed. Results showed that livestock and poultry manure were the anaerobic fermentation raw materials of the 29 biogas projects in winter. Continuous stirred tank reactor (CSTR) and upflow solid reactor (USR) were the mainly anaerobic fermentation process. The produced biogas was mainly used as production and living energy for the surrounding farmers. Biogas residue and biogas slurry were used as manure for the farmland round the projects. Only 4 biogas projects produced organic manure for sale using biogas residues. The pH values of biogas residue and biogas slurry were in the range of 6.83-8.41 and 7.01-8.05, respectively. EC value of biogas residue (3.44 mS/cm) increased obviously comparing with EC value of raw material (1.48 mS/cm). With the fermentation process, the average mass fractions of organic and volatile solids in solid components of biogas projects decreased from 57.31% and 71.61% to 45.28% and 51.61%, respectively. The decreasing amplitudes were 20.99% and 27.93%, respectively. The chemical oxygen demand (COD) concentration of biogas slurry ranged from 2 549.21 to 26 815.81 mg/L. The average mass fractions of total nutrients in biogas residue were 7.23%, comparing with the total nutrients content of biogas slurry (4.20 g/L), which indicating better nutrition of the biogas residue. The contents of cuprum (Cu), zinc (Zn), arsenic (As) and plumbum (Pb) in biogas residue were also detected. The contents of As in 6 biogas projects exceeded the fertilizer standards for farmland. The number of fecal coliforms in biogas residue and biogas slurry was on the high level. The results showed that no biogas project could eliminate fecal coliforms completely, which indicating the low anaerobic fermentation efficiency of the biogas projects. Therefore, the biogas residue and biogas slurry produced in winter must be treated by advanced treatment processes such as aerobic fermentation before their application to farmland. The results of clustering analysis indicated that the operating condition of biogas projects in Fangshan district was better than that in the other two districts. According to the field investigation results and physicochemical properties data of raw material, biogas residue and slurry, some relevant suggestions were proposed as follows: 1) the biogas projects operating management should been strengthened to improve anaerobic fermentation efficiency in winter, especially the projects in Shunyi district and Daxing district; 2) the pathogenic bacteria (such as coliform bacteria) and As in the residue should be further eliminated to reduce its environmental risk before its application on farmland; 3) to reduce environmental exposure of the pathogenic bacteria in biogas slurry, the slurry storage tank should be enlarged and multistage treatment technique should be conducted before its being used for irrigation.