Surface modification of polypropylene microporous membrane to improve its antifouling characteristics in an SMBR: Air plasma treatment

Abstract

Polypropylene hollow fiber microporous membranes (PPHFMMs) were surface-modified by air plasma treatment. Morphological changes on the membrane surface were characterized by field emission scanning electron microscopy (FE-SEM). The change of surface wettability was monitored by contact angle measurements. The static water contact angle of the modified membrane reduced obviously with the increase of plasma treatment time. The relative pure water flux for the modified membranes increased with plasma treatment time up to 2 min, and then it decreased with further increase of plasma treatment time. Decreases in the tensile strength and the rate of tensile elongation at break of the modified membranes were also observed. The antifouling characteristics of the membranes in a submerged membrane-bioreactor (SMBR) for wastewater treatment were investigated. After continuous operation in the SMBR for about 110 h, flux recoveries after water and caustic cleaning are 11.66 and 34.99% higher for the 4 and 2 min air plasma treated membrane than those of the unmodified membrane. Result indicated that reversible fouling was only weakly dependent on membrane surface chemistry; in contrast, irreversible fouling exhibited a marked dependence on surface chemistry.

Introduction

Polypropylene microporous membranes exhibit high potentials for comprehensive application due to their high void volume, well-controlled porosity, high thermal and chemical stability, and low cost. However, the low energy surface and relatively high hydrophobicity probably lead to membrane fouling [1], [2]. In an attempt to improve the antifouling characteristics of these membranes, the modification of commonly used membranes such as polypropylene microporous membranes is very important. Different methods such as UV irradiation, plasma treatment, gamma irradiation, and chemical reaction have been employed to modify the membrane surface [3], [4], [5].

Among the various surface-modification techniques, plasma treatment is regarded as the most advantageous one [6], [7], the active species generated in plasma can activate the upper molecular layers on the surface, thus improving wettability, adhesion and biocompatibility without affecting the bulk of the polymer [8], [9], [10]. From a single material, by changing plasma gas or parameters, surfaces with various characters can be obtained. Many kinds of gases can be used as plasma gases, such as argon, helium, hydrogen, nitrogen, ammonia, nitrous oxide, oxygen, carbon dioxide, sulfur dioxide, water and tetrafluoromethane. Chemical nature of the plasma gases has strong influences on surface modification reactions. For example, air plasma is very effective in hydrophilic modification accompanied by extensive etching and by the implantation of both oxygen- and nitrogen-containing polar groups [11], [12]. Ampheric character may be obtained, which is connected to degrees of ionization of amino and carboxyl groups at different pH values [13].

The combination of membrane separation with the process of biological reactor is called membrane bioreactor (MBR) [14]. Studies on MBRs have received considerable attention due to the deterioration of the water environment all over the world [15], [16] and due to the advantages of MBRs compared with conventional activated sludge process. However, membrane fouling is still the major limitation to the large-scale application of an MBR process, which causes a decline in flux over time. Therefore many researches have been done for its enhancement [17], [18]. Physical rinsing and chemical cleaning have also to be applied frequently in the operation of an MBR, which increases the operation cost and shortens the life of the membrane [19].

The characteristics of membrane surface, such as surface wettability, surface charge and surface acidic/basic character play dominant roles in determining the antifouling characteristics. CO₂ [20] and NH₃ [21] plasma treated membranes have been conducted in our previous work, the surface characters were more acidic and basic due to the introduction of carboxyl and amino groups [22]. The antifouling characteristics in a submerged membrane-bioreactor (SMBR) increased to some extent.

As mentioned above, modification by air plasma treatment may introduce ampheric character onto the membrane surface, which may have some effects on the antifouling characteristics of the membrane. Based on the previous studied and such an expectation, the primary objective of this study is to investigate the effects of air plasma treatment on the membrane fouling during the filtration of activated sludge in a submerged aerobic MBR.