Boundary flux optimization of a nanofiltration membrane module used for the treatment of olive mill wastewater from a two-phase extraction process

• NF of secondary-treated olive mill (OMW) and olives washing (OWW) wastewater streams.
• Secondary treatment comprised pH-temperature flocculation + UV photocatalysis + UF.
• Optimization of NF operation by the merge of the critical and threshold flux theory.
• Reduced NF long-term fouling (28.6–33.3%) after the proposed secondary treatment.
• Increased feed recovery rates (85–90%) and boundary fluxes (12.3–19.6 L h−1 m−2).

The boundary flux theory was used to modelize the performance of a nanofiltration (NF) membrane in the treatment of the effluents exiting a two-phase olive oil extraction process, in particular olive mill (OMW) and olives washing (OWW) wastewater streams. The results obtained from the pressure-cycling experiments exhibit a boundary flux pattern with very low long-term fouling for all analyzed feedstocks. This observation is confirmed by the low values of the sub-boundary fouling parameter (α) obtained by the fitting of the experimental flux data measured during batch-run operation to the boundary flux model. Reduced long-term fouling (28.6–33.3%) occurred during NF operation of the effluents after the proposed secondary treatment, which comprised pH-temperature flocculation followed by ultraviolet (UV) photocatalysis with ferromagnetic-core nanocatalyst and an ultrafiltration separation step (UF). Otherwise, a decrease of long-term fouling in the range of 57.1–60% was observed by using the secondary-treated 1:1 (v/v) mixture of OMW and OWW as feedstock. Moreover, processing both feedstocks to the complete secondary treatment led to increased feed recovery rates (85–90%) and boundary flux values (12.3–19.6 L h−1 m−2). Finally, the standard limits to reuse the purified effluents for irrigation were successfully achieved.