Surface modification of polypropylene microporous membrane to improve its antifouling property in MBR: CO₂ plasma treatment

Abstract

To improve the antifouling property of polypropylene hollow fiber microporous membranes (PPHFMMs) in a membrane-bioreactor (MBR) for wastewater treatment, the PPHFMMs were subjected to surface modification by CO₂ plasma treatment. Structural and morphological changes on the membrane surface were characterized by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Water contact angle, which reflects the hydrophilicity of the membrane surface, was measured by the sessile drop method. Results of XPS clearly indicated that the plasma treatment introduced oxygen containing polar groups on the membrane surface. The static water contact angle of the modified membrane reduced obviously at first and then kept almost constant with the increase of CO₂ plasma treatment time. To assess the relation between the plasma treatment and the membrane fouling in a MBR, filtration for activated sludge was carried out using synthetic wastewater. The PPHFMMs after CO₂ plasma treatment showed better flux recovery after cleaning than that of the unmodified membrane. Fouling index (FI) for the CO₂ plasma treated PPHFMMs was lower than that of the unmodified PPHFMM.

Introduction

Increasingly the shortage of water is a serious problem all over the world because of the population growth and the expansion of industry activities. Therefore, there is a growing impetus for wastewater recycle and reuse. Interests in the membrane-bioreactor (MBR) technology for wastewater treatment have continually increased [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. The advantages offered by MBR over conventional treatment technologies are well known and the main of them are given below [11]: (1) complete removal of solid; (2) effluent disinfections; (3) separation of hydraulic retention time (HRT) and sludge retention time (SRT); (4) higher loading rate capability and longer SRT; (5) lower/zero sludge production; (6) rapid start-up; (7) more compact size; (8) lower energy consumption. Negative aspects, however, include membrane fouling and concentration polarization (which are to some extent exacerbated by membrane fouling) [11], [12], [13], [14], [15]. When membrane fouling occurs, a thick gel layer (which can be both biological or abiotic in composition) is formed onto the membrane surface and into the membrane pores, which causes the permeate flux to decline quickly. There have been many investigations concerning the mechanisms of membrane fouling [16], [17], [18], [19], [20] and the processes to restrict fouling and to enhance flux [21], [22]. A series of methods, such as pretreatment of feed solution [23], using air sparging [24], unsteady flow [22], limiting the flux and flushing by big bubbles [21], adding activated carbon [25], suitable design of membrane modules [8], back washing and cleaning [26], have been described in the literatures.

In general, membrane fouling occurs more seriously on hydrophobic membranes than hydrophilic ones because of hydrophobic interaction between solutes, microbial cells, and membrane materials. Membrane fouling can also be attributed to the adsorption of organic species, the precipitation of less soluble inorganic species, and the adhesion of microbial cells at the membrane surface. It is widely accepted that membrane fouling is less troublesome for the hydrophilic membranes than for the hydrophobic ones [12]. As a result, much attention has been made to reduce membrane fouling by modifying hydrophobic materials to relative hydrophilic [27], [28], [29], [30]. In our previous work [31], [32], [33], [34], it was found that grafting hydrophilic polymers on the membrane surface can enhance the resistant property of protein adsorption for polypropylene microporous membranes. This result indicates that the static antifouling property of polypropylene microporous membrane can be improved by surface modification. However, to our knowledge, few results [35], [36] have been reported to describe the effect of surface modification on the dynamic antifouling property of a polymeric membrane in MBR for wastewater treatment. Therefore, the primary objective of this study is to investigate the effects of CO₂-plasma treatment on the membrane fouling during the filtration of activated sludge in a submerged aerobic MBR.