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Hi

If you are looking at large volumes, your first option would be to consider burying the composting unit below the frost line thereby insulating the unit and reducing heat loss. A second option would be to provide an energy input to maintain temperatures. This could potentially be achieved by maintaining an anaerobic digester & using the methane produced to provide heat to the digester, potentially using a heating coil within the unit. Another alternative is, as Harriet suggests, to incorporate Fourier's law into your design and construction). All of the above obviously come at a cost, both in terms of process management and capital costs. Unfortunately there is also a trade off in your objective as operating a digester to produce methane reduces the quality of the solids produced as a fertiliser. You would also need to provide a source of digestible carbon (e.g. food wastes), and potentially animal manure ( cattle or poultry) to your input stream to maintain optimal conditions and an effective microbial community within the digester. You also need to consider disposal of the effluent from the digester, which would be highly polluting within the aquatic environment. Potentially this effluent could be used directly as a liquid fertiliser, following settlement and/or screening. The likelihood is that the effluent would need to be treated aerobically, to reduce Biological Oxygen Demand (BOD) & Chemical Oxygen demand (COD) before discharge. An alternative to an aerobic digester would be to construct an artificial wetland/reed bed to remove nutrients, BOD and COD from the effluent. These however require management and a large land surface. See https://americanbiogascouncil.org/pdf/paulgreene.pdf for an outline. For household volumes you could use dry toilets, see https://wikiwater.fr/a10-ecosan-ecological-compost as a starting point. These could provide a solution to the sanitation issue, but not your composting temperature. You could review the WHO guidelines on the safe reuse of human faeces and gray water in agriculture to determine if it is feasible to provide sufficient retention time within your system to allow safe reuse as a fertilizer. See https://www.who.int/water_sanitation_health/publications/gsuweg4/en/.

Hope this helps.