The elimination of OMW is of prime environment importance because this liquid represents the main environmental threat of the Mediterranean olive oil industry, where production of both oil and waste is very high. Several methods have been proposed for OMW disposal, based on evaporation ponds, thermal concentration and different physico-chemical and biological treatments, as well as its direct application to agricultural soils as an organic fertiliser (Martínez Nieto and Garrido Hoyos, 1994). Nevertheless, few studies have looked into the possibility of its composting (Cabrera et al., 1990; Tomati et al., 1995).
Composting is a controlled bio-oxidative process that involves a heterogeneous organic substrate in the solid state, which evolves through a thermophilic stage and the temporary release of phytotoxins to produce carbon dioxide, water, mineral salts and a stabilised organic matter which contains humic-like substances. One of the most frequently used composting systems is the Rutgers static pile system, which maintains a temperature ceiling in the pile, providing a high decomposition rate through the on-demand removal of heat by ventilation, since high temperatures inhibit and slow down decomposition due to the reduction of microbial activity (Finstein et al., 1985). This system has been shown to be a good method for reducing N-losses through NH3-volatilization and, hence, for producing a N rich compost (Sánchez-Monedero et al., 1996).
Since OMW is a liquid waste that has to be absorbed on solid lignocellulosic wastes before it can be composted, the two above mentioned agro-industrial wastes, CW and OW, were mixed, and two piles of the mixture were composted. In order to explore its suitability for composting, OMW pollutant liquid waste was added to one of the piles and the effects on the composting process and the characteristics of the resulting compost were studied in comparison to those of the other pile, where no addition of OMW was performed.