Enhancement of anaerobic treatability of olive oil mill effluents by addition of Ca(OH)₂ and bentonite without intermediate solid/liquid separation

Previous work on the anaerobic treatment of olive oil mill effluents (OME) have shown : (a) lipids, even if more easily degraded than phenols, were potentially capable of inhibiting methanogenesis more strongly; (b) a pretreatment based on addition of Ca(OH)₂ and bentonite removed lipids almost quantitatively; (c) preliminary biotreatability tests performed on the pretreated OME showed high bioconversion into methane at very low dilutions ratios, especially when the mixture (OME, Ca(OH)₂ and bentonite) was fed to the biological treatment without providing an intermediate phase separation. This paper was directed towards two main aims: (a) to optimize pretreatment : the best results in terms of methane production were obtained by addition of Ca(OH)₂ up to pH 6.5 and of 10 g L^-1 of bentonite; (b) to evaluate the enhancement of anaerobic treatability of OME pretreated under optimized conditions in a lab-scale continuous methanogenic reactor fed with the substrate without intermediate solid/liquid separation: very satisfactory performances were obtained (at an organic load of 8.2 kg COD m^-3 d^-1 and at a dilution ratio of 1:1.5 total COD removal was 91%, biogas production was 0.80 g CH₄ (as COD)/g tot. COD, lipids removal was 98%, phenols removal was 63%). The results confirm the double role played by bentonite (adsorption of the inhibiting substances and release of the adsorbed biodegradable matter in the methanogenic reactor).