Co-composting of kitchen waste with agriculture and forestry residues and characteristics of compost with different particle size: An industrial scale case study

Since the implementation of domestic waste classification in China, the kitchen waste production has increased rapidly. The unique physical and chemical properties of kitchen waste make it impossible for direct composting for composting alone. This study investigated the co-composting of kitchen waste with agriculture and forest residues at an industrial scale at the Nangong Composting Plant (Located in Beijing). Cornstalks, garden waste, and watermelon seedlings were composted with kitchen waste, with the added agriculture and forestry residues comprising 5%, 10% and 20% of the weight. Industrial composting was performed 30 days at a scale of 165-180 tone. The mixed compost products were screened to different particle sizes, and the maturity, humification, and calorific value were analyzed. The kitchen waste mixed with 20% agricultural complementary materials reached hyperthermophilic temperature (82 °C), had reduced moisture content (45%), and resulted in better composting performance at an industrial scale. By adding 20% complementary materials to kitchen waste produced mature compost with a higher germination index (GI) (91%) by adjusting the pH, electrical conductivity (EC), carbon to nitrogen ratio (C/N), and moisture content. The compost in the 5% and 10% complementary materials treatments did not fully mature and had a GI of<10%, influenced by the higher EC and NH4+-N content. The property of final compost with different particle size vary greatly. The small particle size compost (≤45 mm) had higher uniformity, maturity, and humification degree, and it was suitable to use as a fertilizer; the larger particle size (>45 mm) had more material with lower calorific value (8000-10,000 kJ·kg-1), and could be used as refuse-derived fuel. To make better use of kitchen waste compost, 45-mm particle size screening is suggested at an industrial-scale composting plant. These results support industrial-scale kitchen waste composting in China.