Identification of fouling mechanisms in cross-flow microfiltration of olive-mills wastewater

Microfiltration (MF) is recognized as a promising green process that can efficiently treat different types of wastewater. However, the efficiency, economic viability and technological feasibility of MF can be severely affected by fouling. The occurrence of specific fouling mechanisms depends on the specific membrane materials and related physico-chemical properties, characteristics of solutes present in the feeding solution, and operating conditions. The present study aims at evaluating the extent of membrane fouling and its impact on flux decline during olive mills wastewater (OMWW) microfiltration in a semi-pilot plant. The possibility to restore the initial membrane performance was evaluated in terms of cleaning efficiency (CE). OMWW MF was carried out using both polymeric (polyvinylidene fluoride, PVDF) and ceramic membranes (zirconia and α-alumina) at transmembrane pressures (TMP) up to 1.7 bar. The highest permeate flux was obtained using α-alumina membrane (35.4 ± 1.6 L m−2 h−1 at 1.2 bar TMP), the zirconia membrane, despite a slight decrease in flux compared to the alumina membrane, allowed the better restoration of the initial water permeability after cleaning procedures (CE =74 %). PVDF membrane was found to be the least suitable for treatment of OMWW due to low permeate flux (5.3 L m−2 h−1 at 1.2 bar TMP) and the inability to restore initial water permeability (CE = 8 %). Each peculiar fouling mechanism was estimated by fitting the experimental filtration data to the linearized equations of the Hermia model, single and multistage, corresponding to pore blocking (complete, intermediate and standard) and cake formation. Results indicated that for α-alumina and zirconia membranes the dominant fouling mechanism was cake layer formation. For the PVDF membrane Hermia multistage model revealed that the initial flux decline is appropriately described by standard pore blocking whereas after the first 10 min of operation the fouling mechanism switched to cake layer formation.