| Περίληψη: | The olive oil extraction industry represents a substantial share of the economies of Mediterranean countries but leads to serious environmental problems by producing huge amounts of wastes (by-products) within a short production period. The production rate of olive oil is about 1.4-1.8 million tonnes per year in the Mediterranean, resulting in 30 million m3 of by-products and 20 million tonnes of olive pomace. A small portion of these wastes can be used as raw materials in different industries as they contain valuable natural resources. Greece has about 2300 small-scale, rural, agro-industrial units that extract olive oil. These are generally three-phase systems and their by-products include olive mill residual solids (olive pomace and leaves) and olive mill waste water. Olive mills produce significant quantities of solid wastes with outputs of 0.35 tonnes of olive pomace and 0.05 tonnes of leaves per tonne of olives. The huge quantities of olive pomace and olive leaves produced within the short oil extraction season cause serious management problems in terms of volume and space. The solid wastes (olive pomace and olive leaves) that are produced contain almost 95% organic matter and although they could be highly beneficial to agricultural soils, it has been shown that they also contain toxic compounds and lipid which increase soil hydrophobicity and decrease water retention and infiltration rate. The soils of most Mediterranean countries have low organic matter contents (<1%) which has negative impacts on agriculture. Frequent application of composted organic residues increases soil fertility, mainly by improving aggregate stability and decreasing soil bulk density. Organic amendments play a positive role in climate change abatement by soil carbon sequestration. Recurrent use of composted materials enhances soil organic nitrogen content by up to 90%. To replenish soil organic matter content and promote eco-friendly crop production, the application of olive pomace compost could be a good solution.
To examine olive mill solid waste composting, four pilot-scale experiments were carried out to produce good quality compost using three phase olive mill solid waste (olive pomace, OP) and different bulking agents such as rice husk (RH), olive leaves (OL) sawdust (SD), wood shavings (WS), and chromium treated reed plants (RP). A series of parallel experiments was carried out to examine the effect final compost quality of: (a) initial moisture content, (b) water addition during the composting process, and (c) material ratios, and to also determine the toxicity level in plants and human blood lymphocytes (genotoxicity and cytotoxicity). For each experiment, six trapezoidal bins were used with dimensions 1.26 m long, 0.68 m wide and 0.73 m deep, and a total volume of 0.62 m3. The study was carried out in the facilities of the Department of Environmental and Natural Resources Management, University of Patras, Agrinio, in a closed area to maintain controlled temperature conditions. To monitor the composting process and evaluate compost quality, physicochemical parameters (temperature, moisture content, pH, electrical conductivity, organic matter, volatile solids, total organic carbon, total nitrogen, total phosphorus, potassium, sodium, and water soluble phenols) were measured at different phases. The respirometric test (O2 uptake) was performed to determine compost stability.
Experimental results showed that even after short composting periods, the quality of the final product remained high. The final product had excellent physicochemical characteristics (C/N: 12.1–17.5, germination index (GI): 88.32–164.43%, Cr: 8–10 mg/kg dry mass, that fulfill1 EU requirements and can be used as a fertilizer in organic farming. To achieve higher quality of the final product, Olive pomace should be used in higher ratios than the other materials (OL, RH, WS, SD and RP). The amount (volume of humidifying agents) and time (frequency) of moisture addition also played an important role during composting.
Based on the experimental results, olive mill wastes can produce a high quality soil amendment which has no phytotoxic, genotoxic or cytotoxic effects. Nevertheless, composting duration and bulking agents and their ratios are crucial factors that determine the quality of the final product. Finally, the revision of EU regulations is proposed to include genotoxic and cytotoxic evaluation of composts that enter the human food chain. A full-scale compost unit was designed based on the experimental results. For a typical small-sized olive mill, processing 30 tonnes of olives per day for a 100-day operation period, a total area of about 850 m2 is needed to compost the mill’s entire annual waste production.
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