A Review of Features, Benefits and Costs of Tunnel Composting Systems in Europe and in the USA

Odour control is identified as the critical technical issue for the success of composting projects. Tunnel composting has the advantages that the compost can be kept aerobic, all the excess air is collected for treatment and that building corrosion, that leads to fugitive emissions, is prevented. Data is given for the number of installations in Holland and USA. Rough costs indicate that tunnel composting has comparable costs to other s mechanised systems. Two types of tunnel system are described: batch and plug flow and their features compared.All compost projects are subject to the same pressures to a lesser or greater degree depending on the difficulty of composting the source material and local sensitivities. Compost projects fail for 3 major reasons: lack of proper project finance; poor community acceptance normally due to poor odour control and third, lack of product market. The first and the last come down to good management, but some systems are more prone to odour problems than others. Table 1 is a summary of technical factors to consider when choosing a composting system.There has been a gradual evolution and a wider range of wastes composted. In the US there was a large increase in composting following the USDA work at Beltsville in the late 70s. There was a dramatic increase in aerated static pile systems. Many of these early systems have had to be retrofitted with expensive leachate collection and odour control systems. The researchers did not take into account community acceptance as a key issue. This then led to upsurge so called invessel systems in the mid 80s. Some being true invessel built around a silo configuration and others being based on shed or hangar systems where aerated static pile, windrows or a combination has been carried out in a building. Many of the early silo systems had problems, primarily due to poor porosity and mechanical failure. The longitudinal or agitated bin system has proved to be the most robust system of the shed systems and has been widely adopted. There are still some inherent weaknesses due to the possibility of fugitive emissions and building corrosion for the more putrescible wastes which is coming to light for some of the earlier systems.