Extraction of antioxidants from olive mill wastewater and electro-coagulation of exhausted fraction to reduce its toxicity on anaerobic digestion

Liquid-liquid extraction was used in order to recover phenolic compounds from centrifuged olive mill wastewater (OMW), a polluting by-product of olive oil production process, and to reduce their toxicity for a subsequent aerobic or anaerobic digestion. Phenolic compounds were identified in untreated and treated OMW by gas chromatography coupled to mass spectrometry (GC-MS). The experimental results of ethyl acetate extraction showed that the monomers recovery efficiency was over 90%. This pre-treatment resulted in the removal of the major LMM phenolic compounds and a small part of HMM polyphenols. The aerobic treatment of the exhausted OMW fraction removed 78.7% of the soluble COD. In the case of anaerobic digestion at OLR ranged from 1 to 3.5 g COD l−1 day−1, methanisation process exhibited high methane yield as 0.3 l CH4 produced per g COD introduced and high COD removal (80%). However, a disruption of the process was observed when the OLR was increased to 4.5 g COD l−1 day−1. A pre-treatment by electro-coagulation resulted in decreasing the toxicity and enhancing the performance of methanisation operated at higher OLR from 4 to 7.5 g COD l−1 day−1.