Theoretical and experimental determination of key operating parameters for composting systems

The combination of increasing quantities of Municipal Solid Waste (MSW) and increased legislation for the disposal of this type of waste have created a need to develop different disposal or treatment routes for waste. Approximately 60% of MSW by mass is biodegradable and many disposal routes for this waste allow energy recovery. However the waste hierarchy presented in the National Waste Strategy for Wales emphasises the importance of materials recovery over energy recovery. It has been shown that the only way to achieve these targets is through the recovery of catering waste, which requires an in-vessel composting facility. In order to gain an insight into the aeration requirements for in-vessel composting, calculations were performed to ascertain the required airflow for the supply of oxygen, the removal of excess moisture and the removal of excess heat. It was found that approximately 450kJ are released for each mole of oxygen utilised whilst 500kJ are released per mole of carbon dioxide evolved. It was found that the air requirement for removal of heat from the process was approximately 100 times greater than the air required to supply oxygen to the system. In order to determine the power of aeration equipment required for composting facilities a static pressure test rig was constructed. From the results gained a model relating the static pressure to the bulk density of compost was developed. Initially a windrow composting system processing green waste at the Carmarthenshire Environmental Resources Trust (CERT) composting facility was studied. A canopy system was developed to monitor the respiration rate of this system and allow comparison between different feedstocks and control strategies. For a green waste only windrow the highest recorded respiration rate was 38gC02kgVS"1day"1. The respiration rate was observed to reduce with temperature above 55 C. In addition to the green waste windrows a temperature managed windrow and a windrow constructed from a mixture of green waste and chicken litter were also tested. The final series of trials involved the testing of the ability of a containerised composting system to meet the Animal By-Products Regulations. The vessel was fed various mixtures of green waste and factory waste. Airflow and insulation within the vessel were analysed and a composting rate of over 45gC02kgVS'1day1 was achieved. If this had been sustained then the vessel may have met the Animal By-Products Regulations