Dry fractionation of olive pomace for biogas production: milling, sieving and electrostaic separation

Olive oil extraction processes are worldwide extended especially in Mediterranean regions, but they generate huge quantities of olive wastes which show environmental risks [1]. In particular, solid residues, olive pomaces (OP), are rich in organic matter and lipids, and could be valorized by anaerobic digestion [2]. The aim of this study was to investigate the effect of dry fractionation (milling, sieving and electrostatic separation) for OP anaerobic digestion. Milling at 4 mm and 1 mm, was carried out using knife miller, while smaller sizes (200 μm and 50 μm) were obtained after ball milling. Samples at 4 mm were sieved using a 0.9 mm screen. Ultrafine samples (50 μm) were subjected to an electrostatic separation (ES) process [3]. Batch experiments were conducted in the biochemical methane potential assays, at different particle sizes. Effects of pretreatments were evaluated by biochemical composition of OP fractions. Milling at 50 μm increased both soluble chemical oxygen demand and methane yield by 36% and 53% respectively, compared to OP at 4 mm which was due to the increase of surface area and accessibility to microorganisms [4]. Similarly, samples having size higher than 0.9 mm contained 2 fold lesser lipids which decreased methane production by 19% compared to raw OP. The negatively charged-ES fraction of milled samples at 50 μm represented the highest polyphenols content (13 mg equilavent galic acid/gTS) and produced the highest cumulative methane volume (157 ml CH4/gVS), which was 56% higher than the raw OP. In conclusion, milling, sieving and separations process are both efficient and clean pretreatments enhancing methane production from olive pomace without the addition of neither water nor chemical reagents