Effects of partial stone removal on the co-composting of olive-oil processing solid residues with poultry manure and the quality of compost

Full scale composting studies were performed with stoned (SEOOPSW) and non stoned exhausted olive-oil processing solid waste (NSEOOPSW) mixed with 20% poultry manure to reach a C/N ratio of around 30 and to improve the chemical and microbiological properties of the end products. The experiment was performed on two windrows (3 m wide, 2.5 m high and 10 tonne weight) to investigate the effects of stone removal and poultry manure addition on composting process efficiency and on the quality of composts. The partially stone removal operation reduced the total composting time by at least one month compared with the untreated waste. The addition of poultry manure improved the chemical quality of the end product, as shown by the doubling of the nitrogen concentration after four months composting together with the increased amount of P and K. Compost from stoned exhausted olive-oil processing solid waste had higher levels of cation exchange capacity (CEC) and humic acid content with a lower [FA/HA] ratio indicating a high polymerisation level. Agronomic field test showed that both composts are not phytotoxic. When a lied at the rate of 40 tonne ha(-1) to a potato culture, they produced approximately the same yield (46 and 48.5 tonne ha(-1)) but they acted differently in terms of plant development Compost produced from the NSEOOPSW acted more positively on the stem length (84.1 against 77.6 cm) and on leaf weight (354 against 238 g), whereas compost from the SEOOPSW more positively affected tuber sizes (76% large grade compared to 55%).