The studies of ultrasound enhancement on ultrafiltration for Radix astragalus extracts and cleaning of fouled membrane

Abstract

This study investigated the effects of operational parameters on both ultrafiltration (UF) and ultrasound-assisted UF (USUF) processes of a natural product, Radix astragalus (RA) extracts. Dead-end flat sheet (DEFS) and cross-flow hollow fiber (CFHF) membranes were used in the processes. Permeate flux and fouling resistances were investigated in the UF of RA extracts, in both DEFS and CFHF modules. Transmembrane pressure (TMP) was an important factor which could significantly influence the flux and resistances either in the DEFS or CFHF mode. 10 k or 30 k Da molecular weight cut-off (MWCO) of membrane was suitable for clarifying the RA aqueous extracts. By analyzing the fouling resistances, it was found that concentration polarization and reversible fouling were two main resistances which could significantly affect the performance of UF process. The quality of RA extracts has been improved after UF because of lower soluble solid and higher total polysaccharides. Polysaccharides and proteins were demonstrated as the main substances of membrane foulants in UF of RA extracts. US technique was introduced to both DEFS and CFHF UF processes of RA extracts. Effects of ultrasonic parameters, including frequency, power and irradiation mode, on permeate flux and fouling resistances during both UF processes were investigated. Ultrasonic irradiation made strong impacts on the UF processes, especially at low frequency and high output power. In DEFS, 12-15% enhancement in flux was observed at the US of 28 or 45 kHz frequency. Upon ultrasonic irradiation, the reversible resistance, including concentration polarization and cake layer, was sufficiently reduced as revealed by the quantitative resistance analysis using the resistance-in-series model. In CFHF, the flux enhancement could be up to 42% at the US of 45 kHz and 120 W, but it was 29% only at the US of 100 kHz and 600 W. Concentration polarization and cake layer resistances could be effectively decreased by US, which led to exciting flux performance. Though satisfactory enhancements were obtained, hollow fiber UF membrane was more susceptible to the US irradiation of high power and low frequency than the flat sheet membrane. It is necessary to carefully control the US power when it is applied to the UF process. Response surface methodology (RSM) with a central composite rotatable design (CCRD) was employed to optimize the process of USUF for RA mixtures with hollow fiber membrane. The effects and mutual interactions of various parameters, namely ultrasonic power, ultrasonic irradiation mode, TMP and temperature, on flux reduction (Y₁) and process duration (Y₂) were investigated, simultaneously. The results showed that TMP was the most significant parameter, followed by the temperature, ultrasonic power and irradiation mode. The optimum conditions were found to be ultrasonic power of 120 W, continued ultrasonic irradiation mode, TMP of 0.64 bar and temperature of 20 ℃. The two predicted response values were 55.3% and 53 minutes for Y₁ and Y₂, respectively, which were in good agreement with the results obtained from the confirmation experiments, about 57.0-60.3% and 53-58 minutes. The kinetics of various resistances, including adsorption, pore blocking and cake layer resistances, were quantified as functions of time or time and TMP in UF and USUF processes. Mechanisms for the effects of US irradiation on membrane fouling were demonstrated by comparing with the dynamics of different resistances in these two processes. Semi-empirical models were developed for predicting flux decline in UF and USUF of RA extracts. According to the analysis on the fouling resistances and the phenomena of US irradiated hollow fiber membranes, acoustic cavitation, bubble collapse and micro-jet were believed to be the main mechanisms leading to membrane fouling reduction and permeate flux improvement.