| Summary: | The separation of emulsified oils from water represents a significant challenge for water reclamation and clean-up, because such droplets remain suspended for long periods of time. Membrane technology offers a solution to such separations, but their major drawback is the requirement for frequent cleaning to remove foulants, which reduces membrane lifetime and incurs additional economic and environmental costs. Therefore, there’s motivation to design membranes with better robustness against fouling. There are generally two approaches to reducing the detrimental effects of membrane fouling. One way is to improve the effectiveness of foulant removal during membrane regeneration, so that a larger proportion of the original permeate flux can be recovered. The second approach is to improve the resistance against foulant deposition, so that the decrease in permeate flux is less severe. In this work, we look into both approaches to improve the anti-fouling properties of membranes used for the treatment of emulsified oily wastewater.
We first investigate the feasibility of using thermal treatment to improve the effectiveness of foulant removal. We selected a commercially available copolyimide called P84 to be a promising candidate due to its excellent thermal stability. The separation performance of fibrous membranes made of P84 was examined, and the effectiveness of different foulant removal methods was evaluated. Electrospun P84 membranes offer significant advantages over other materials for the separation of oil-in-water emulsions. In particular, the decline in permeate flux due to fouling is comparable to that observed for more oleophobic polymers like polyacrylonitrile (PAN), and less severe than that observed for several other commonly used polymers. More importantly, the P84 membranes also exhibit essentially 100% flux recovery through thermal treatment for at least three cycles, with no influence on the separation performance of the membranes. This work demonstrates the feasibility of using thermal treatment as an effective method to regenerate polymeric membranes with good thermal stability.
Next, we look into strategies to improve the resistance against foulant deposition. A simple approach to fabricate liquid-infused membranes (LIMs) with the potential to eliminate membrane fouling was presented. Fluorinated silane was attached to electrospun membranes made of a fluorinated polymer, and then the membranes were prewetted with lubricants made of perfluoropolyethers. The LIMs with fluorinated silane attached exhibits higher permeate flux, and better selectivity compared to the LIMs without the attachment, and better fouling resistance compared to the membranes without the infused liquid. A parametric study also elucidates that operating at the highest pressure at which the membrane can still maintain its permeation selectivity and choosing a less viscous infused liquid contribute to higher permeate flux. This work demonstrates the feasibility of using LIMs to permeate selectively a dispersed phase and their potential to eliminate foulant deposition.
Lastly, we further study the transport mechanism of the dispersed oil phase through LIMs to better understand the factors affecting the permeate flux and to later more effectively modified the LIMs to promote high flux. We used confocal laser scanning microscopy (CLSM) to construct three-dimensional images of the sequence of events responsible for coalescence of oil droplets and the formation of oil channels within the LIM. We show that the key to anti-fouling behavior is the higher affinity to the pore wall for infused liquid than permeating oil phase. Using image analysis, we find that the rate at which oil channels are opened within the membrane and the number of open channels ultimately govern the permeate flux through the LIMs. Oil concentration in the feed affects the capture of oil droplets and the subsequent coalescence of oil, which in turn affects the channel opening dynamics. The channel opening dynamics also depend on the viscosity of the infused liquid and the operating pressure. This work offers insight into the selective permeation of a dispersed liquid phase through a LIM and provides operating guidelines to promote higher flux.
Overall, this thesis presents two new approaches to ameliorate the detrimental effect of membrane fouling during the treatment of emulsified oily wastewater. The contributions from this thesis improves the competence of membrane separation technique for the treatment of emulsified oily wastewater.
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