Fouling control by threshold flux measurements in the treatment of different olive mill wastewater streams by membranes-in-series process

• Optimum treatment sequence: pH–T flocculation + UV/TiO2 photocatalysis + UF + NF.
• UF and NF membranes exhibit threshold fluxes with 54.5% less fouling on the latter.
• Fouling control of membranes-in-series process for olive mill wastewater treatment.
• Required membrane area, overdimension and constant fouling parameter reduced.
• Steady-state permeate flux and membrane longevity enhanced by the adopted sequence.

This paper deals with the measurement of the critical and threshold flux for modelization, prediction and control of the fouling issues of batch membranes-in-series processes, in detail ultrafiltration (UF) followed by nanofiltration (NF) and reverse osmosis (RO), for the reclamation of olive mill wastewater (OMW-2). Results suggest the existence of threshold flux values for both UF and NF membranes, with minimum constant fouling attained on the latter, that is 54.5% lower. UF + NF in series after pH–T flocculation process followed by photocatalysis with ferromagnetic-core titanium dioxide under ultraviolet irradiation (UV/TiO2) as pretreatment to inhibit fouling issues guarantees COD values in the permeate of 1.3 g L− 1. This value complies with irrigation water quality standards. Moreover, the adoption of this treatment sequence helps reducing the required membrane area, equal to 104.6 m2 and 81.4 m2 for the UF and NF membranes, respectively, leading to a limited need of overdesign of the membrane plant. In addition, especially the use of the applied UV/TiO2 photocatalysis process not only enhances the productivity but also ensures minimization of the constant fouling build-up on both membranes. This latter effect sensibly increases the longevity of the membranes, reducing the capital and operating costs of the treatment.