Table olives are produced from processing fresh olive drupes that typically involves de-bittering, together with other things, to hydrolyse some secoiridoids particularly oleuropein (a phenolic component with bitter characteristics), making it more palatable/acceptable for consumers. The industrial activity of table olives generates a significant amount of olive residues/by-products, among them includes wastewater residue (the liquid effluent) which contains various organic/chemical compounds, including phenolics. The presence of these compounds in the effluent, upon exposure to the environment may cause toxicity, contamination, and pollution. In this regard, numerous treatment approaches have been developed; some have shown great potential for efficient removal/degradation of phenolics. The following methods are typical examples being applied to separate phenolics from effluents before their discharge to the reservoirs.
Membrane separation typically uses a membrane (with semi-permeability) that allows a selective separation driven by pressure gradient, chemical, and electrical potential. The pressure-driven types are particularly more effective than conventional treatments in respect of using i) no additives, ii) less energy use, and iii) no phase transition. The membrane through which water is passed through entraps the selected pollutants which relatively have larger particle size and/or greater molecular weights compared to the smaller particles that pass through the pores. The variations of membrane techniques, depending on the selected driving force, include ultrafiltration, microfiltration, nanofiltration, and reverse osmosis.
The main disadvantages of membrane methods include i) occurrence of membrane fouling that may render the permeability difficult and entails cleaning and regeneration expenditures, ii) overall equipment cost, and iii) potential deterioration/weakening of membrane structure that contains polymeric compounds.
Ozonation involves the inclusion of ozone into water (an oxidant) through which the produced free radicals react with phenolics, giving rise to their removal from the water. Among the improved technologies includes an integration of ozonation with UV radiation (in presence/absence of hydrogen peroxide) that may be applied, as an effective pre-treatment, in advance of further treatment (biological process) to degrade phenolics. The incorporation of ozonation into the pre-treatment helps complement the efficiency of UV photolysis in phenolic reduction/degradation, and thus reduces the toxicity of the wastewater.
Ozonation process is potentially effective in i) removing phenolics, ii) reducing toxicity, iii) increasing biodegradability, and iv) producing no sludge/chemical waste. The main downside associated with ozonation process is inefficiency/inability to eliminate chemical oxygen demand (COD), together with other things.
The biological treatment may follow microbial or enzymatic method. The former, making use of microorganisms (such as bacteria, fungi, and yeast), enables decomposition and separation of phenolics. The treatment system is performed aerobically or anaerobically. The aerobic system is potentially capable of decreasing COD, relatively degrading phenolics and aromatic constituents.
The enzymatic approach makes use of enzymes that act as biological catalysts to separate the pollutants efficiently/rapidly, usually through moderate pH and temperature.
Photocatalytic decomposition, using photocatalyst nanoparticles such as Titanium dioxide (TiO2), when activated through the absorption of energy photons, enables acceleration of the reaction (degradation of organic compounds present in water). This method may offer numerous advantages including inexpensiveness/stability of the selected photocatalyst, and the ability to the use of solar irradiation. The rate of efficacy highly relies on the amount of catalyst, intensity of light, and duration of exposure.
Application of suitable treatment methods to remove phenolics from wastewater may optimally address the waste disposal problem. In addition to the waste disposal management, it is possible to sustainably re-utilize the extracted/removed phenolics (due to their potential bio-functionalities), through using appropriate/effective valorization techniques to deliver value-added bio-materials across different industrial applications including food, feed, nutraceuticals, cosmetics, and renewable energy.
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