|M.Sc Student||Hindawi Ahmad|
|Subject||Removal of Phenolic Compounds from Oil Mill Wastewater by|
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Emeritus Carlos Dosoretz|
|Full Thesis text - in Hebrew|
Olive oil production consumes large volumes of water generating substantial volumes of wastewater, known as olive mill wastewater (OMW). The seasonality of the OMW production and its composition (high COD and phenols content) make the olive oil industry an environmentally problematic food industry. The toxicity of OMW is caused primarily by the presence of monomeric phenolic compounds.
The oxidization-polymerization-settling enzymatic reaction of phenols was studied as a mean for their removal. Three OMW types: raw, after flocculation and settling, and after anaerobic digestion, were tested, in which organic matter that can interfere in the enzymatic oxidization were gradually removed. The phenol oxidizing enzymes laccase and horseradish peroxidase (HRP) were utilized. First, the oxidization-polymerization reactions of three model compounds, tyrosol (monophenol), catechol (diphenol) and hydroxtyrosol (diphenol) were characterized.
Laccase exhibited a higher affinity for catechol oxidation than HRP but at a lower reaction rate. HRP displayed a twofold higher catalytic coefficient for tyrosol than for catechol oxidation. The optimal molar ratio of H2O2:substrate for maximal oxidation and sedimentation for HRP was 1:1 for tyrosol and 3:1 for catechol (the theoretical ratio is 1:2), indicating H2O2 consumption in side oxidation reactions, i.e., dimers and recycled radical monomers.
Regarding the extent of total organic carbon (TOC) removal, i.e. polymerization and sedimentation, laccase removed catechol more efficiently than tyrosol (92% vs. 55%, respectively), in line with the higher reactivity of the catechol, while removal by HRP was almost similar for both substrates (~70%). The addition of catechol to tyrosol reactions improved removal with both enzymes, whereas addition of tyrosol to catechol reactions lowered the extent of TOC removal by laccase. A higher termination rate of the reactions was observed for catechol, which led to lower molecular weight (1650-2100 Da) products than tyrosol oxidation (3000 Da). In all cases, 75-80% of the enzymes were removed to the sediment as a result of hydrophobic interaction and/or covalent binding to the polymerization products.
Both enzymes displayed a more efficient removal of total phenols for raw OMW than for OMW after flocculation and settling or anaerobic treatment (the least efficient). It seems that there is no absolute preference regarding the ability of either enzyme to remove total phenols (removal of specific phenols is more efficient with HRP). Lacasse might display an advantage when working with raw OMW without dilution because the pH is in the optimal working range of the enzyme. Excess H2O2 consumption appears to be an additional obstacle for the application of peroxidase.