Mitigation of fouling on hollow fiber membrane using ultrasonic transducer

Xu Li, Purdue University

Abstract

Membrane filtration process offers an innovative and precise separation technique for wastewater treatment. However, the membranes are easily to be fouled on the surface by retentate when the filtration system operates for a long period. One of the critical issues in the development of effective processes is the decline in system performance due to fouling, which limits the economic efficiency of the processing operation. In this research, ultrasound was employed to mitigate and clean a sludge fouled hollow fiber membrane. Specifically, the mechanisms of ultrasound cleaning, temperature effects, ultrasound power effects, ultrasound frequency effects and fouling cake layer effects on ultrasound cleaning were evaluated. In initial studies, cavitation as the primary ultrasound cleaning mechanism was examined by Particle Dynamics Analysis (PDA) laser system. Results reveal that the increase of ultrasound frequency result in smaller cavitation bubble size, higher bubble travel speed and lower bubble density. They are considered to be important in detaching fouling from membrane surface. Subsequently, a hollow fiber membrane system was set up with pumping DI water as feed solution in order to obtain better understanding of membrane characteristics. Results indicate that the increase of transmembrane pressure and feed solution temperatures have significant impact on improving permeate flux. Conversely, the rise of ambient temperature around membrane shows minor effects on permeate flux even though the presence of ultrasonic transducer release an amount of heat into water in which a membrane cartridge was submerged. The primary experiments effectively lay the foundation for later researches. In further studies, ultrafiltration (UF) membrane were fouled by sludge solution in hollow fiber module and subsequently subjected to continuous ultrasound in a water tank at different frequencies, power intensities and duration of treatment. Results indicate that increased power intensity and lower frequency are able to effectively enhance permeate flux. Lower frequency contributes to remove both tightly and loosely bound solids from fouled membrane. However, permeate turbidity test and SEM images suggest that higher ultrasound power and lower frequency have the potential to cause membrane damages. Hence, a gentle method is required for wastewater treatment in an ultrasound-membrane system. Finally, continuous ultrasound was utilized to clean sludge fouled membrane in an offline membrane filtration system. Namely, ultrasound was instead of traditional methods, such as chemical cleaning and back flushing, to clean fouled membrane. Results indicate a better cleaning performance that ultrasound can perfectly restore the original membrane flux at different frequencies. The results of this research elucidate the cavitation bubbles, temperature, ultrasound frequency and ultrasound power influencing ultrasonic cleaning of sludge fouled membrane. A new laboratory-scale hollow fiber membrane filtration system based on ultrasound was developed for entire experiments, suggesting that utilization of this technology will lead to improved membrane performance.

Degree

M.S.E.

Advisors

Nnanna, Purdue University.

Subject Area

Industrial engineering|Mechanical engineering|Environmental engineering

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