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

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 D-50 of the filtered sample (D-50filtered) to D-50 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.

Automated summary

This study investigates the oil removal and coalescence mechanisms of electrospun nanofibrous membranes. The CPVC membrane shows higher oil removal rate compared to the CDA membrane. The oil concentration and the number of layers affect the filtering ratio and removal. The optimal flow rate varies for CPVC and CDA membranes due to their different coalescence mechanisms in different stages. The settled particle size and the ratio of settled particle size to filtered particle size depend on oil concentration and membrane type, but not on flow rate and membrane layers.