Abstract
Many reclamation treatments as well as integrated processes for the purification of olive mill wastewaters (OMW) have already been proposed and developed but not led to completely satisfactory results, principally due to complexity or cost-ineffectiveness. The olive oil industry in its current status, composed of little and dispersed factories, cannot stand such high costs. Moreover, these treatments are not able to abate the high concentration of dissolved inorganic matter present in these highly polluted effluents. In the present work, a review on the actual state of the art concerning the treatment and disposal of OMW by membranes is addressed, comprising microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), as well as membrane bioreactors (MBR) and non-conventional membrane processes such as vacuum distillation (VD), osmotic distillation (OD) and forward osmosis (FO). Membrane processes are becoming extensively used to replace many conventional processes in the purification of water and groundwater as well as in the reclamation of wastewater streams of very diverse sources, such as those generated by agro-industrial activities. Moreover, a brief insight into inhibition and control of fouling by properly-tailored pretreatment processes upstream the membrane operation and the use of the critical and threshold flux theories is provided.
Highlights
The olive oil sector has represented since several decades one of the most important industries in the Countries of the Mediterranean River Basin
Membrane processes are becoming extensively used to replace many conventional processes in the purification of water and groundwater as well as in the reclamation of wastewater streams of very diverse sources, such as those generated by agro-industrial activities [31,32,33,34,35,36,37]
The main handicaps in the treatment of olive mill wastewaters (OMW) by membrane bioreactors (MBRs) are basically, on one hand, the high residence times required, and on the other hand, the high load of recalcitrant organic compounds present in this type of effluent, that typically hinder the efficiency of biological processes; the high organic matter concentration in OMW multiplies the fouling propensity of MBR systems, principally because it represents a good culture medium for deleterious bio-fouling, which appears in the form of gel layers and pore blocking caused by microorganisms as well as the organic particles themselves
Summary
The significant boost of this industrial sector in the last years has brought an undesired side-effect: the amounts of olive mill wastewaters (OMW) by-produced have significantly increased, especially as a result of the change of the antique batch press method for the continuous centrifugation-based olive oil production processes currently used, which ensure much higher productivity (Table 1 and Table 2). On one hand, these continuous systems guarantee a higher yield in recovering olive oil from the olives, up to 21%, but on the other they lead to an increased production of wastewater streams due to water injection in the centrifugation processes [2].
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