Effects of olive mill wastewater addition in composting of agroindustrial and urban wastes

In order to study the suitability of olive mill wastewater (OMW) for compos ting, this liquid waste was added to two different mixtures of agroindustri al and urban wastes and the composting process was compared with two other piles of similar composition, but without OMW. These four piles were studie d in a pilot plant using the Rutgers static pile system. The addition of OM W produced a greater proportion of degradable organic matter or a higher de gradation rate, higher electrical conductivity values, greater losses of to tal N and lower nitrification than in piles without OMW. Its addition also restricted the increase of the cation exchange capacity and provoked the ap pearance of phytotoxicity or a longer persistence of phytotoxicity. However , in general, all the composts showed increases in the cation exchange capa city, the percentage of humic acid-like carbon, the polymerisation ratio of these humic substances (which revealed that the organic matter had been hu mified during composting) and the germination index, the latter indicating the reduction of phytotoxicity during the process.