Quality assessment of composts prepared with olive mill wastewater and agricultural wastes

The co-composting of solid residue from olive oil production process, exhausted olive cake (EOC), with poultry manure (PM) watered with olive mill wastewater (OMW) was considered as an efficient method for the treatment of olive oil extraction effluent having high organic content including phenolic polluting compounds. The process was carried out by using three aerated windrows of variable compositions. OMW was used continuously during the bio-oxidative period, which lasted three months, to replace water for windrow moistening. The main process parameters (temperature, pH, humidity and C/N) were monitored over four months to ascertain the maturity of the compost. The composting process lasted four months during which 26 moistenings of the mixtures were performed with OMW or water to keep moisture within the ideal range of 45–60% (w/w). At the maturity stage, the C/N ratios were less than 16, pH of the resulting products were slightly alkaline (pH = 8) and electrical conductivity was relatively high in the OMW mixtures (5.46–5.48 S m−1) when compared with water application. Nitrates increased (0.16–0.42%) and phenol contents were reduced by more than 49%. Mature composts were then used as an amendment for potato production in a field where no inhibitory effect was observed. Potato productivity increased 10–23% as a result of compost application. No noticeable negative impact of OMW on the soil system was observed. Phenolic compound concentrations in the stabilised composts were comparable in the three studied mixtures (different sites) and averaged 0.24%. Considering previous results and this three year study, it has been observed that the benefit of these composts demonstrated the potential sustainable agronomic production of potato while using locally available recycled organic materials.