Removal of molecular weight fractions of COD and phenolic compounds in an integrated treatment of olive oil mill effluents

Previous works (Beccari et al. 1999b; Beccari et al. 2001a; Beccari et al. 2001b) on the anaerobic treatment of olive oil mill effluents (OME) have shown: (a) a pre-treatment based on the addition of Ca(OH)(2) and bentonite was able to remove lipids (i.e. the most inhibiting substances present in OME) almost quantitatively; (b) the mixture OME-Ca(OH)(2)-bentonite, fed to a methanogenic reactor without providing an intermediate phase separation, gave way to high biogas production even at very low dilution ratios; ( c) the effluent from the methanogenic reactor still contained significant concentrations of residual phenolic compounds (i.e. the most biorecalcitrant substances present in OME). Consequently, this paper was aimed at evaluating the fate of the phenolic fractions with different molecular weights during the sequence of operations (adsorption on bentonite, methanogenic digestion, activated sludge post-treatment). The results show that a very high percentage ( above 80%) of the phenolic fraction below 500 D is removed by the methanogenic process whereas the phenolic fractions above 1,000 D are significantly adsorbed on bentonite; the 8-day activated sludge post-treatment allows an additional removal of about 40% of total filtered phenolic compounds. The complete sequence of treatments was able to remove more than the 96% of the phenolic fraction below 500 D (i.e. the most toxic fraction towards plant germination). Preliminary respirometric tests show low level of inhibition exerted by the effluent from the methanogenic reactor on aerobic activated sludges taken from full-scale municipal wastewater plants.