Quality assessment of a simplified static method for olive mill waste composting

The activity deals with a novel static composting procedure set-up at farm scale by ISAFOM-CNR which aims at Olive Mill Waste (OMW) bioremediation. OMW was thoroughly mixed with hygroscopic additives (bulking agents), such as waste wool and straw, to avoid leaching and provide optimal moisture and mass porosity; fresh mixture was immediately packaged in gas-permeable 1 m3 polyethylene bags (Big Bag, BB), where the Active Thermal Composting (ATC) phase naturally occurred. The use of BB during the ACT phase has not required forced aeration nor watering and turning of the mixture thus allowing a naturally aerated static composting. After the ATC phase, BB have been opened, thoroughly mixed, slightly watered and allowed to age naturally in the successive "curing" phase conducted in windrow. During the ACT phase, temperature were taken every 15 minutes through an experimental tailor-made ArduinoTM based device with 45 thermistors arranged in a vertical section mesh placed into the BB; in order to confirm the aerobic nature of the ongoing maturation process, oxygen was also monitored in the exhausted air periodically extracted from the BB. The oxygen and temperature trends of BB showed patterns typical of a common composting system. Respiration and phytotoxicity standard tests stated high biological stability and maturity of the end product (compost). The maturation process caused drastic degradation of the labile organic matter and polyphenols while the stable carbon (humic substances) still remain at a high level in the end compost which therefore showed to be an excellent carbon sink. Chemical-physical and microbiological properties of the OMW compost obtained at the end of the static composting procedure above described fully satisfied the standard requirements set by EU regulators for compost.