Case Study on Anaerobic Biogas Residue Treatment of Kitchen Waste

With the implementation of the domestic waste classification policy, Shanghai, Beijing, Hangzhou, Ningbo, Fuzhou, Hefei, Chongqing and other cities have successively landed kitchen waste treatment facilities, which mainly adopt the resource utilization scheme of dry anaerobic biogas production, but such projects will produce a large number of digestive residues. In engineering, the three-stage solid-liquid separation method of spiral extrusion dehydration+vibration screening sand removal+high-speed centrifugal dehydration is generally used for advanced treatment, so as to realize solid-liquid separation and obtain dehydrated biogas residue.

The important way to recycle anaerobic biogAs residue is to produce organic fertilizer by composting. At present, the anaerobic biogas residue composting effect of organic wastes such as crop straw, livestock manure, kitchen waste and municipal sludge is studied. The main problems in the application of biogas residue composting of livestock manure are that the salt content is as high as 1%, and the frequency of heavy metals Cu, Zn and As exceeding the standard is high; The main problems in the application of kitchen waste biogas residue compost are that the salt content is as high as 2%; The main problem in the application of municipal sludge biogas residue compost is that As, Cr, Cu and Zn generally exceed the standard. However, kitchen waste is a classified product of domestic waste, and the impurity content is an important factor affecting the application of biogas residue composting, which is currently lacking in research. In addition, compared with organic solid wastes such as crop straw, livestock manure, kitchen waste and municipal sludge, kitchen waste has higher impurity content and more kinds, so it is necessary to post-treat compost, but there is no relevant report on the post-treatment effect.

Therefore, this study investigates the case of kitchen waste treatment project in a typical city in China, analyzes the physical composition characteristics of feed, biogas residue and compost, clarifies the removal efficiency of impurities, and studies the characteristics of phytotoxicity, biological stability and dissolved substances before and after composting.

In terms of physical composition, the classification of kitchen waste in China is in the initial stage. Except for a few cities such as Shanghai, the correct rate is high, and the impurity content is only 10%. At present, the impurity content of kitchen waste collected by classification in most other cities is still high, generally about 25%. The primary biogas residue is about 25% of the total amount of digestive residues, and the secondary biogas residue is about 10% of the total amount of digestive residues. According to the proportion of various materials, rubber, plastics, metals and textiles were largely removed after pretreatment, but hard and fragile materials (glass, stone, shellfish bones) and long fibrous materials (wood and bamboo) were enriched after pretreatment and anaerobic fermentation, and the proportion of dry basis in the residue increased.

Biological stability mainly considers the degree of maturity of materials to avoid the bad environmental risks of degradation and odor and leachate during land application, so the biological stability test of original kitchen waste is not carried out. After 20 days of aerobic composting, the AT4 of the first-class biogas residue decreased significantly, and the AT4 (on a dry basis) was ≤ 35 mg/g. Phytotoxicity mainly considers the effect on plants after being applied to soil, so the original kitchen waste is not tested for phytotoxicity. Both primary biogas residue and secondary biogas residue have great phytotoxicity, and GI is basically 0. The research results of biogas residue GI in literature are generally 55%~75%. This is mainly because the leaching solution measured by GI in the literature is prepared with the fresh mass ratio of 1∶10, but in this study, according to the requirements of CJJ52—2014, the leaching solution measured by GI is prepared with the solid-liquid ratio of 1∶10 on a dry basis, which makes the concentration of leaching solution higher than other studies, thus reducing GI. In addition, the kitchen waste is digested by dry anaerobic digestion, and its degradation time should theoretically be longer than that by wet anaerobic digestion because of its high concentration. However, because the current dry anaerobic device basically relies on imports, the investment is much higher than that by wet anaerobic digestion. In order to save investment, the current dry anaerobic residence time is shorter than that by wet anaerobic digestion, which leads to further instability of discharge and high plant toxicity.

At present, the residue of anaerobic digestion of kitchen waste in China is often treated by dehydration+composting+screening process, and the products basically meet the requirements of organic fertilizer and organic substrate for greening. After 20 days aerobic composting, the biological stability of the first-class biogas residue can be enhanced, and the AT4 is reduced to about 20. Reduce plant toxicity and increase GI to more than 85%.

However, it should be noted that the first-class biogas residue must be screened after composting, otherwise the impurity content will seriously exceed the standard. The second-grade biogas residue has low impurity content and high nitrogen content, which is more suitable for composting and applying to soil than the first-grade biogas residue, but straw needs to be added as a conditioner during composting.

Source：https://mp.weixin.qq.com/s/7PwvySmhJvK9_58Hj3J8TA