Removal and coalescence of oil droplets in oily wastewater by nanofibrous membrane filtration

Abstract

The coalescence of oil droplets in membrane filtration is practically important, however it has not been studied extensively. The oil removal, filtering coalescence and settling coalescence of electrospun nanofibrous chlorinated polyvinyl chloride (CPVC) and cellulose diacetate (CDA) membranes were investigated and their mechanisms elucidated. Oil removal by the CPVC membrane (reaching 90.68%) was generally higher than in the case of CDA (reaching 84.22%). The ratios of D₅₀ of the filtered sample (D_50filtered) to D₅₀ of the raw sample (D_50raw) were all less than 1.0 owing to membrane interception. A higher oil concentration and a greater number of layers decreased the filtering ratio and increased the removal. Oil removal increased with operation time due to surface coalescence and formation of cake layer. The optimal flow rate of CPVC was 20 L h⁻¹ while that of CDA was 50 L h⁻¹, attributed to the difference in their filtering coalescence mechanisms: wetting coalescence for hydrophobic CPVC and collision coalescence for hydrophilic CDA in the pre-membrane, in-pore, and post-membrane stages. The values of D_50settled and the ratio of D_50settled-to-D_50filtered: higher oil concentration > lower one, CPVC > CDA, however, it does not depend on flow rates and membrane layers. This study provides a deeper insight into filtering/settling coalescence and suggests potential pathways to develop novel coupling processes.