Treatment technologies of liquid and solid wastes from two-phase olive oil mills

Abstract

Over the last 10 years the manufacture of olive oil has undergone important evolutionary changes in the equipment used for the separation of olive oil from the remaining components. The latest development has been the introduction of a two-phase centrifugation process in which a horizontally-mounted centrifuge is used for a primary separation of the olive oil fraction from the vegetable solid material and vegetation water. Therefore, the new two-phase olive oil mills produce three identifiable and separate waste streams. These are: 1) the wash waters from the initial cleansing of the fruit; 2) the wash waters from the secondary centrifuge and 3) the aqueous solid residues from the primary centrifugation. As well as offering process advantages they also reduce the water consumption of the mill. The introduction of this technology was carried out in 90% of Spanish olive oil factories. Therefore, the new twophase olive mill effluents (TPOME) are made up of the mixture of effluents (1) and (2), the total volume of TPOME generated being around 0.25 l/kg of olives processed. In addition, the solid residue (two-phase olive pomace, TPOP) has a high organic matter concentration giving an elevated polluting load and it cannot be easily handled by traditional technology which deals with the conventional three-phase olive cake. So, this paper aims to report the main features and characteristics of TPOME, and of TPOP, as compared to the classical olive mill wastewater (OMW) and olive cake derived from the three-phase manufacturing process. The advantages and disadvantages of the two-phase decanting process will be summarized. Among the treatments reported for TPOME, aerobic processes in completely mixed and activated sludge reactor showed high COD removal efficiencies. Kinetic constants of the aerobic processes were also compared at different operational conditions. The report also includes the following findings: assays of anaerobic digestion of wastewaters from the washing of olives, of olive oil and the two together using fluidised-beds and hybrid reactors; the kinetics, performance, stability, purification efficiencies and methane yield coefficients; other purifying methods for TPOME treatment including oxidation (with different chemical oxidants) and physico-chemical treatments (using different coagulants and flocculants); the anaerobic digestibility of TPOP using different influent substrate concentrations; kinetic studies of anaerobic digestion of TPOP and mass balances to predict the behaviour of the reactor and simplified kinetic models for studying the hydrolysis, acidogenic and methanogenic steps of one and two-stage anaerobic digestion of TPOP. Finally, other methods for treatment and use of TPOP were mentioned, including composting, production of alcohols, mannitol, and other added - value compounds such as monosaccharides, oligosaccharides, arabinose and glucose. Furfural and activated carbons were also produced from TPOP and included in the report.