Evaluating co-composting of kitchens, greening wastes, and exhausted tea on a pilot scale

Abstract: Effective disposal of organic wastes has been one of the most important issues for the resource utilization of solid wastes. The degradation rate of organic components as a key factor can pose a serious impact on the composting process. In this study, a series of experiments were performed on the microbial agents and various ventilation systems, in order to evaluate the co-composting performance of kitchens, green wastes, and exhausted tea on a pilot scale. An optimal compound ratio was also achieved to verify the strengthening effect of co-composting. The specific organic components included the fat crude, fibers, and protein in the mixed ingredients during composting. Specifically, kitchen waste, green waste, and exhausted tea were selected as the main raw materials. An aerobic composting was then carried out to control the ventilation and the type of microbial agents in a pilot composting plant. Some parameters were selected to fully evaluate the technical feasibility and economic viability of the co-composting system, including the temperature, water content, the carbon to nitrogen ratio, total nutrients, organic matter content, seed germination index, and hygienic characteristics. The results showed that the optimal combination was the self-developed microbial agents under the intermittent ventilation pattern, where the ratio of compound agents for the co-composting was that m(Aspergillus oryzae): m(Bacillus licheniformis): m(Candida lipolytica): m(Trichoderma viride): m(Azotobacter chroococcum) = 1.5:1:1.2:2:1. Meanwhile, the microbial agents presented a stronger removal effect of the specific organic component in the mixed ingredients, when the inoculation amount was 1‰. Three classic phases were obtained in the temperature of the mixed ingredients during 10 days of drum composting, including heating, thermophilic, and cooling phases. Furthermore, the maximum temperature was up to 68.6℃, while the high-temperature period (65℃ or higher) remained for 7 days. The ratio of water content and carbon to nitrogen declined from 63.5% to 30.1%, and 31.5 to 9.6, respectively, during the entire composting. The total nutrient (9.8%) and organic content (43%) were superior to the Agricultural Industry Standard NY 525-2021. Meanwhile, the seed germination index of the composting product reached up to 88.3%, indicating the perfect hygienic characteristics of composting (3 or fewer fecal coliforms per grams, no ova of roundworm was found). For economic feasibility, the co-composting system was applied for the domestic waste treatment in a district with a population of 30 000 in the Hengqin area of Zhuhai, Guangdong Province, China. A systematic analysis was made to consider the revenues (including waste disposal, organic fertilizer products, and scrap recycling revenue) and expenses (including utilities, salary, transportation, landfill disposal, venue rental, machinery and equipment depreciation, and unforeseeable expenses) during practical application. The average disposal cost was required as low as (248.67±19.89) yuan/t. As such, a process control strategy can be recommended to recycle the kitchen wastes, green wastes, and exhausted tea in the ecosystem, instead of being discarded as solid wastes. The short processing time can be widely expected to accelerate the degradation rate of organic matters for better compost quality during resource utilization. Meanwhile, an improved process control system can be used for the organic waste composting in a rotary drum reactor. The finding can provide a strong reference to effectively treat the environmental pollution of organic wastes in the large-scale kitchen waste, green waste, and exhausted tea at low cost. The recycling of organic waste resources can greatly contribute to reducing landfill disposal for less environmental pollution.