Abstract
Olive oil extraction processes generate three phases: Olive oil, solid residue (pomace) and aqueous liquor Olive Mill Wastewater (OMW). OMW causes serious environmental deteriorations such as coloring of natural waters, alteration of soil quality, phytotoxicity and odor nuisance. The direct discharge of this effluent into the soil bring various negative effects on the environment, due to its high load of organic matter and phenolic compounds. Coagulation, flocculation and their combination processes are among the most applied processes for wastewater treatment. The application of coagulation technique using mineral and inorganic polymeric coagulants and acid cracking is not always effective to reduce COD and phenolic compounds from OMW except lime. Although direct flocculation is a low treatment cost, use less chemical and produce less sludge, the efficiency of COD and phenolic compounds removal remain very low. Coagulation-flocculation process is developed in order to enhance the removal efficiency of organic matter from OMW by optimizing the operating conditions. However, many studies showed that the combination of various cationic organic coagulant and flocculant agents did not show any significant decrease of COD and phenolic compounds except for lime combination. In the present review, recent research studies dealing with the application of coagulation/flocculation for removal of phenolic compounds and COD from OMW are presented and compared. Furthermore, the main factors that influence the selection between coagulation, direct flocculation and coagulation-flocculation are presented and the principal removal mechanisms are discussed.
Highlights
The production of olive oil is not restricted to the Mediterranean Basin
The results showed that the use of Olive Stone (OS) filter leads to a higher removal of Total Suspended Solids (TSS) (82.5%) and fatty matter (73.8%) from the raw Olive Mill Wastewater (OMW) and a depletion of total phenolic compounds (11.3%) and Chemical Oxygen Demand (COD) (23.2%)
The results demonstrated that the use of combination of Poly-Aluminum Chloride (PAC)/FLOCAN 23-20 is advantageous over ASM coagulation with anionic polyelectrolyte Praestol 2240
Summary
The production of olive oil is not restricted to the Mediterranean Basin. Lately, it is increasing in other countries like France, Serbia, Montenegro, Macedonia, Cyprus, Turkey, Israel, Jordan, USA, Australia and the Middle East. As far as phenolic compounds concerned and considering the olive oil extraction processes and origin of OMW, as well the structural characteristics of polyphenols carbon skeletons in term of number and position of acid functions (COOH and/or OH) on the aromatic cycle, the combined removal mechanisms will substantially differ (Duan and Gregory, 2003; Crittenden et al, 2005). Many researchers report that the efficiency of coagulation process to remove organic matter and phenolic compounds from OMW, depends mainly to coagulant's type and dosage, pH, mixing condition and temperature on the one hand and phenolic compounds properties including particle size, charge and hydrophobicity (Korshin et al, 2009; Iakovides et al, 2014) and the presence of anionic (PO43-, SO43-, Cl-) and cationic (Ca2+, Mg2+, Na2+) minerals (Lin and Lee, 2013; Hecini and Achour, 2014), on the other hand.
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