INTRODUCTION

The demand for microfiltration (MF) and ultrafiltration (UF) in the water and wastewater treatment industry has increased greatly since the mid 1990s, due to the requirement for technology to comply with new disinfection standards for drinking water and more stringent regulations for wastewater discharge, respectively [1]. UF membranes, for example, have been reported to combine high pathogen removal ability with minimal energy consumption [2]. Additionally, application of MF/UF membranes in membrane bioreactors (MBR) has sharply increased in the recent years due to their ability to combine biological treatment and filtration in one compact process. A survey on MBR global market reports that this technology is expected to increase from $216.6 million in 2005 to $488 million in 2013 [3]. MF/UF has also been considered as a promising alternative for the pre-treatment to seawater desalination in the last ten years. MF/UF pre-treatment presents various advantages over the conventional pre-treatment techniques, such as environmental friendship in terms of limiting the sludge amount and use of chemicals, lower foot print and cost effectiveness.

Membrane fouling is the term used to describe the undesirable formation of deposits on membrane surfaces. Various membrane fouling types can be broadly classified as organic fouling, inorganic fouling, particulate deposition (colloidal) and biofouling [4]. Some of the important mechanisms of membrane fouling are, adsorption of feed components, clogging of pores, chemical interaction between solutes and membrane material, gel formation, and bacterial growth [5]. In general, fouling can be controlled by suitable pre-treatment of the feed and membrane cleaning strategies. Over longterm operation, membrane fouling is not completely reversible by simple physical cleaning (i.e. backwash and relaxation) therefore, chemical cleaners are used to remove remaining foulants from the membrane surface and restore the permeate flux. On the other hand, cleaning chemicals can potentially lead to removal or degradation of the membrane material, additives and alteration of membrane properties [6]. Generally, polymeric membranes are naturally hydrophobic and must be blended with additives and pore-formers to increase their hydrophilicity such as polyvinylpyrrolidone (PVP). However, the major concern associated with chemical cleaning remains the potential membrane degradation (or ageing) upon repeated fouling-cleaning cycles.

Although previous reports have reported possible chemical reactions between chemical agents and membrane, little is known on the nascent changes occurring at the molecular and structural level of membrane in progressive membrane ageing over prolonged period. Therefore, this study aimed to:

(1) adapt existing characterisation techniques capable of detecting nascent changes of membrane material to determine the loss of membrane integrity,

(2) carry out an arbitrary ranking of membrane stability against different chemical cleaning agents,

(3) gain a better understanding of the mechanisms occurring during membrane ageing,

(4) identify the most relevant membrane property capable to indicate these changes in membrane structural properties,

(5) assess the relative impact of fouling and cleaning on the filtration behaviour,

(6) assess current membrane cleaning protocols for polymeric microporous membranes during long-term progressive degradation of performance in parallel with membrane ageing and morphological changes and,

(7) characterise and possibly intervene the exact extent of the damage encountered by industrially-aged membranes during their lifetime.

MAIN OBJECTIVES AND OUTCOMES FROM INDIVIDUAL WORK PACKAGES

2.1 Critical assessment of the techniques characterising nascent changes in microporous membrane nature (work package 2 – see Appendix 1)

Various analytical tools are being used to observe and measure the mechanical, morphological and chemical effects of membrane ageing. The desirable characteristics of these methods generally include simplicity and lower cost. It is, however, important to use more sensitive and accurate methods jointly with novel and emerging technologies as it could allow the detection of nascent changes of the membrane. A thorough evaluation of the membrane characterisation techniques was executed by developing a ranking system to compare different criteria for the selection of the most suitable methods to assess membrane degradation or modification, as the result of chemical cleaning. The evaluation of the various analytical methods was reflected by the relative sensitivity of the tools in detecting the changes between new and aged flat sheet and hollow fibre polyvinylidene fluoride (PVDF) membranes. These membranes where statically aged in 2% NaOCl solution for the duration of one week. Clean water tests, contact angle, porometry, tensile test, Thermogravimetric analysis (TGA) and fourier transform infrared spectroscopy (FTIR) were completed on both flat sheet and hollow fibre membranes. Experimental reliability of the membrane characterisation techniques was also rigorously assessed for these membranes. The results from the sensitivity and reliably tests were used to rank all the tested characterisation techniques accordingly. Table 1 shows a summary of the recommended characterisation techniques for membrane ageing.

It was found that the various analytical techniques tested present different degrees of sensitivity and reliability towards the impact of the chemical agents. Therefore, the suitability of different membrane characterisation techniques as predictive tools for membrane ageing depends on membrane configuration (flat sheet/hollow fibre) and nature (material, average pore size). For example, clean water test was found to be more reliable for hollow fibre membranes, while contact angle was more suited for flat sheet membranes. Moreover, generally, different instruments operating range is limited, which makes them inappropriate for some samples. For instance, porometry analysis was only possible for membranes featuring pore size larger than 0.1 µm.

Finally, the relative contradiction observed between the testing methods indicated their limitations in measuring a single parameter. For example, many of the membrane characteristics (such as pore size, charge and roughness) can affect the contact angle value obtained from the goniometer. On the other hand, trends obtained from contact angle, membrane resistance and bubble point tests were successfully used to complement some proposed hypothesis regarding degradation mechanisms.

Findings in this section implied that in order to have a comprehensive record of the...