The Production of Solid Wastes in CELSS and the Difficulties in Recycling by Composting: A Review

During space missions, various waste products are produced. Recyclable waste can be treated by aerobic composting to achieve in situ recycling. Considering the limited logistics supply in the Controlled Ecological Life Support System (CELSS), waste recycling should reduce energy consumption and material carrying pressure, increase recycling efficiency, and improve the quality of the daily life of astronauts. However, current composting technology often has polluting gas emissions in the case of limited oxygen (O₂) consumption. Therefore, this review aims to distill available information about aerospace mission waste production and its treatment methods, analyze the influencing factors of composting, and summarize optimal O₂ demand of aerobic composting of aerospace biomass waste. It also aims to define optimal O₂ demand of each stage of aerobic composting of organic waste in order to achieve a low O₂ consumption composting technology for the extraterrestrial planet base. The main recyclable wastes (feces, food residue, and non-edible parts of plants or crops) generated in CELSS are expected to be 0.13 kg, 0.31 kg, and 2.12 kg/person/day, which requires 0.173 to 0.692 kg of O₂ for the normal composting process. Due to the change of the degradation rate at different stages, the O₂ demand of each stage is different. In detail, the different stages for varying O₂ requirements are the heating period: 0.022 to 0.032 L^.kg^−1. dry matter (DM)^.min⁻¹, the thermophilic period: 0.027 to 0.05 L^.kg^−1.DM^.min⁻¹, and the cooling period: 0.006 to 0.02 L^.kg^−1.DM^.min⁻¹. The O₂ demand of aerospace biomass waste compost could be 0.022 to 0.153 L^.kg^−1.DM^.min⁻¹. Adequate O₂ supply according to these requirements could reduce O₂ consumption and potential pollutant emission.