Performance of tunnel composters fitted with heated walls

Two composting tunnels, whose unique feature was heated sidewalls, were designed to take batches of around 200 wet tonnes typically consisting of mixtures of green waste and kitchen waste. Compliance with the Animal By-Products Regulations (ABPR) required the material in each stage to reach 60°C for 2 days. The focus of this paper is materials balances, energy balances, ABPR compliance and the impact of heated walls on performance. Each batch studied gave compost output in the range 22·1–28·6% of the input by wet weight using a 10 mm final screen. The final rejects to landfill were around 2 t for 200 t of input. The energy input requirement ranged from 52 to 92 kWh/t of input depending on whether the walls were heated and what system was used for post-treatment of the compost. Temperature loggers within the compost mass showed that the system routinely achieved the designed Animal By-Products Regulations compliance conditions. Loggers fixed to the sidewalls showed that there were some cold spots, but it was considered that adjustment of the control system could eliminate these. The monitored trial runs did not demonstrate that heating the walls brought any discernible benefit to the process.