Composting of municipal green waste using a hybrid Mode of the aerated static pile
The aerated static pile (ASP) system developed in Rutgers University (Finstein et al., 1980) is based on a deep understanding of composting as a microbial process. It allows for a good degree of process control and high organic matter stabilisation rates, with relatively low labour and cost. For these reasons, the ASP system has been intensively studied with, among others, the following aims: evaluation of the system's efficiency, in terms of organic matter degradation and pathogens' inactivation, for a wide range of wastes (Finstein et al., 1986; Lopez- Real and Baptista, 1996; Pereira-Neto et al., 1986; Sesay et al., 1997; Stentiford et al., 1985); improvement of the aeration conditions and moisture control (Robinson and Stentiford, 1993); introduction of more advanced control technologies through the use of computerised systems (Eccles and Stentiford, 1987); comparison of different aeration modes (suction, blowing or both) (Stentiford et al., 1996).
The production of green waste from municipal tree pruning and yard trimmings in the area of Cascais, Oeiras and Sintra (Lisbon surroundings) adds up to 8000 tons / year. The waste has been disposed in a landfill, after a comminution step. This is a valuable and relatively clean source of organic matter and nutrients, and therefore should be recycled. Furthermore, there is a potentially good market for soil improvers and growing media. In order to assess the possibility of valorisation of these residues by the ASP method, a hybrid ventilation (suction and blowing) ASP of about 18 m3 (3,6 ton) was studied. The system had automatic temperature monitoring. The pile's inner gas composition (CO2 and O2) was also monitored, as well as the most relevant physical, chemical and microbiological parameters. The suitability of the compost produced for use as a growing media was studied.