Continuing from the previous segment.
Anaerobic digestion of kitchen waste and livestock manure yields similar properties in biogas slurry, yet the "four highs" characteristic of kitchen waste results in higher levels of potassium dichromate, ammonia nitrogen, and salt content, thereby increasing pollution levels and treatment difficulty. Due to the large quantity and complex composition of biogas slurry and digestate, which contain rich nutritional elements alongside higher concentrations of heavy metals, improper utilization may lead to secondary pollution.
2.2 Physicochemical Characteristics 2.2.1 pH Value In the anaerobic digestion process of kitchen waste, the pH value typically exhibits an initial decrease followed by an increase trend. This is due to the accumulation of volatile fatty acids in the early stages of digestion, leading to acidification of the digestion fluid, which is later utilized by methane-producing bacteria. Additionally, ammonia nitrogen produced by various microbial metabolisms can also influence the pH value of the anaerobic digestion system. The pH value of digestate mostly falls within the range of 6.54 to 8.41, while the pH value of biogas slurry after anaerobic digestion of kitchen waste ranges from 6 to 9, indicating a slightly neutral acidity.
2.2.2 Salt Content The physicochemical properties of kitchen waste can be described as "four highs," with high salt content mainly originating from the addition of table salt and seasonings in daily meals. This results in the salt content of the residual material after anaerobic digestion of kitchen waste being essentially consistent with the initial substrate. Typically, the salt content of kitchen waste ranges from 1.26 to 2.62 wt%. Studies by Huang Junhan et al. have shown that biogas production is influenced by salt content, with excessive salt content exhibiting inhibitory effects.
2.2.3 Heavy Metal Content Sewage irrigation leads to the accumulation of heavy metals in plant crops, while the widespread use of hormones, growth-promoting drugs, and additives in modern crops results in a significant presence of heavy metals and harmful substances in animal feces. Anaerobic digestion of materials such as straw, feces, fruits and vegetables, and kitchen waste may yield biogas slurry and digestate containing substantial amounts of heavy metals. Research by Wang Qiaoling on the influencing factors of anaerobic fermentation of kitchen waste indicates that kitchen waste contains heavy metal components such as Cu, Zn, and Hg, without other heavy metals. Therefore, some heavy metals may also remain in the fermentation residue, which, if directly discharged, could cause environmental pollution.
3 Current Utilization Status of Anaerobic Digestion Residues from Kitchen Waste 3.1 Utilization Status of Digestate Based on the characteristics analysis of anaerobic digestion residues from kitchen waste, the characteristics of digestate render it feasible for resource utilization. Currently, digestate can be utilized for heat generation through incineration, agricultural land application, as a substrate for cultivation, and for promotion in the livestock industry. The resource utilization of digestate has become one of the crucial developing industries of anaerobic digestion of kitchen waste. A comparison of the advantages and disadvantages of digestate utilization methods is shown in Table 3.
We studied the feasibility of using biogas slurry as fuel. The research findings indicate that biogas slurry is not suitable for direct combustion applications. However, after high-temperature drying at 150°C, its calorific value reaches 16.6 MJ/kg, making it suitable for use as fuel. This suggests that the organic matter present in anaerobically digested biogas slurry provides the feasibility for its use as fuel. However, the high moisture content of biogas slurry increases the cost of combustion.
For more exciting content, please refer to the next article.
Data comes from the China biogas.
