Utilization Statusand Prospect of Anaerobic Digestion Residueof Food Waste

The residual organic matter in digestate contains nutrients such as nitrogen, phosphorus, potassium, and humic acid needed for plant growth, making it a good soil conditioner with the potential to develop into organic fertilizer and form green soil. Research has shown that anaerobic digestion of kitchen waste to produce organic fertilizer from digestate is feasible and safe. Furukawa et al. found that digestate has good soil utilization value, with nitrogen content mainly affecting soil quality. Wang Aiying et al. studied the improvement effect of digestate as a soil conditioner on soil acidification, and the results showed that the application of digestate increased soil pH, porosity, and water retention capacity, effectively regulating acidic soils. Digestate should not be directly applied to soil due to its stimulating odor, which can easily cause secondary environmental pollution. Currently, aerobic composting is a high degree of resource utilization technology for digestate treatment. Anaerobically digested digestate from kitchen waste can be converted into organic fertilizer substrates or garden soil through aerobic composting. However, the quality of digestate-made fertilizer is poor and may contain excessive heavy metals, making it more difficult to sell compared to fertilizers made from other types of organic waste. Wang et al. believe that aerobic composting of digestate can produce higher-quality organic fertilizer and should not be directly used as fertilizer. Due to the high oil and salt content of kitchen waste itself, digestate still contains a certain amount of oil and salt, which need to be treated and further determined for agricultural use.

With the development of biogas projects, the application of digestate in the livestock industry has also progressed, primarily using digestate as feed or feed additives. Digestate can also be processed into high-value-added products such as high-protein feed and biochar. Zhang Ji et al. invented a method for producing feed using cockroaches to digest kitchen waste digestate. The digestate from anaerobic digestion of kitchen waste can be transformed into feed with the digestive and reproductive abilities of cockroaches. Although this method solves the problem of homogeneity pollution, the issue of limited processing scale remains unresolved. Researchers have conducted experimental studies on the pyrolysis of kitchen waste digestate to produce biomass carbon. Liu et al. used pyrolysis gasification technology to produce biomass carbon from dried digestate, while Zheng Yangqing conducted experiments on the pyrolysis of kitchen waste digestate combined with surfactants and fly ash. The results showed that the process of producing biomass carbon from digestate is environmentally friendly. However, there are drawbacks to using digestate for feed, such as slow processing speed and the risk of spreading homologous diseases. Digestate contains a large number of pathogenic microorganisms and parasite eggs. If these pathogens and parasite eggs are not effectively eliminated, it can not only cause infectious diseases but also endanger human health. Therefore, digestate should be carefully treated for harmless disposal before being applied in the livestock industry.

3.2 Current Utilization Status of Biogas Slurry

The biogas slurry produced after the anaerobic digestion of kitchen waste contains high levels of total phosphorus and total nitrogen, which can lead to eutrophication of water bodies, and high Na+ content can cause land salinization. After treatment, it can be used as fertilizer in agriculture. The disposal options for biogas slurry include incineration and landfill, wastewater treatment plants, use as fertilizer (such as application to fields or foliar spraying), seed soaking in biogas slurry, and pest and disease control. In the livestock industry, it can be used as feed for livestock and poultry, thereby realizing environmental protection while fully utilizing its value. A comparison of the advantages and disadvantages of different utilization methods of biogas slurry is shown in Table 4. Currently, biogas slurry application to fields is the primary method. Studies by Sun Fangfang and others have found nitrogen leaching risks associated with biogas slurry irrigation. Considering nitrogen leaching risks, rapeseed agronomic traits, and biogas slurry assimilation demands, the recommended nitrogen application rate for biogas slurry application to fields should be within 240 kgN·hm^-2. Research by Lu Xinmiao and others investigated the effects of different rates of biogas slurry application on watermelon yield and soil environment, finding that biogas slurry application could increase crop yield. Studies by Sun Tianzi and colleagues have shown that the application of liquid bacterial fertilizer prepared from anaerobically digested kitchen waste biogas slurry can promote the nutrient content of farmland soil physicochemical properties, enhancing soil organic components and improving soil fertility. Research by Hao Minjie and others on the utilization of biogas slurry in earthworm breeding has shown that adding biogas slurry to breeding beds can increase the weight of earthworms by 12% compared to blank beds, and the morphology of earthworms becomes visibly coarser. However, biogas slurry cannot be directly applied, especially for seedlings, as it may cause seedling scorching. Peng Liyu used the O/ABR process to degrade anaerobically digested biogas slurry to achieve harmless treatment of biogas slurry. The nutrient content in biogas slurry determines the environmental and economic benefits of anaerobic digestion of kitchen waste.

In engineering applications, due to the complex composition and relatively low stability of residual materials from kitchen waste digestion, it is often necessary to effectively treat the digestate before reuse.

4.Conclusion and Outlook

In China, the large volume of kitchen waste generated possesses significant potential for utilization. When treated using anaerobic digestion technology, the production of biogas is accompanied by the generation of a substantial amount of digestate and biogas slurry.

Analysis of the characteristics of digestate and biogas slurry confirms their potential for utilization. They are rich in nutrients, making them suitable for secondary use as fertilizers, soil amendments, agricultural irrigation, and pest control. However, it is essential to ensure the stability and compliance of digestate and biogas slurry before application to prevent secondary pollution.

Several prospects for the utilization of digestate and biogas slurry are proposed:

Avoiding heavy metal contamination to prevent soil, groundwater, and crop pollution.

Utilizing digestate and biogas slurry as soil amendments holds promising prospects. However, careful evaluation of their fertility is necessary when used as fertilizers to prevent adverse effects on crops.

Preventing the formation of saline-alkaline land due to the high salt content of kitchen waste.

Strengthening research on the resource utilization, reduction, and harmless utilization of kitchen waste, especially through anaerobic digestion technology, and developing research on the utilization of its residues, can yield significant economic, environmental, and social benefits.

If you're interested, you can follow along for even more excitement in the next edition.

Information sourced from China biogas