One of the most pervasive problems facing the world is inadequate access to clean water, a problem only expected to grow worse with increased global population, urbanisation and climate change effects [1]. Membrane-based desalination serves as an important technological solution to this problem, but faces significant challenges due to fouling. World renowned membrane expert Prof. Menachem Elimelech at Yale University has stated [2] that “Novel non-invasive, in-situ and rigorous quantification of processes occurring during membrane filtration is the key to breakthroughs in our understanding of fouling phenomena.” Nuclear Magnetic Resonance (NMR) technology has been identified by Chen, Li and Fane [3] as the only non-invasive technique capable of measuring all relevant membrane regions (i.e. membrane, interface & bulk), and that its unique, but unrealised, sodium (23Na) species specification (and thus direct detection of salt concentration) is critical for direct in-situ measurement of undesirable concentration polarisation (the accumulation of salt on the membrane surface during operation). Here we look to realise this immense potential for a suite of NMR/MRI techniques, in which 23Na detection is a critical feature, to provide a fundamental understanding of the unexplored interplay between fouling and concentration polarisation in membranes. Such insight will subsequently be used to inform membrane module design. The following two aims will be pursued:
Membrane fouling is a key operating expense of membrane operations (1). Whilst Nuclear Magnetic resonance (NMR) and Nuclear Magnetic Resonance Imaging (MRI) has traditionally had applications in the medical fields; recent work has demonstrated that NMR allows non-invasive investigation of membrane plant operation and fouling that maybe occurring. NMR is particularly adept at monitoring flow disturbances within at spiral wound membrane brine spacer. Figure 1 a) shows the velocity distribution of a clean membrane showing an even flow field throughout most of the membrane construction. Figure 1 b) demonstrates the impact of biofouling in the brine spacer channel. It can be seen that flow through the membrane is heterogeneous, with preferential channelling through the membrane. High field MRI offers detailed insight into the operation of spiral wound membranes but has several practical implementation challenges in the field. Some of these challenges are cost, weight of components and the use of super conducting magnets with cryogenic cooling. Earth’s Field NMR (EFNMR) operates in the Earth’s magnetic field to and is relatively lightweight and simple to construct. EFNMR has been demonstrated to detect membrane biofouling (2-4) and sodium alginate fouling. Figure 2 provides results of NMR signal of a spiral wound membrane whilst being fouled with a sodium alginate solution. The blue bars are the signal at the feed end of the membrane. The increase in signal is caused by increasing stagnation in the brine spacer due to alginate fouling. A key problem with EFNMR is low signal to noise ratio. Development of a membrane industry specific NMR is currently ongoing with higher field strength delivering a higher signal to noise ratio whilst remaining portable. Extensive experience with portable NMR is found in oil field surveying with operating frequencies in the 10’s to 100’s of kHz range (compared with the EFNMR 2kHz operating frequency). These higher operating frequencies provide sufficient signal to allow practical measurement of oil and gas reservoir composition. Application of existing NMR technology as well as innovative magnet and coil designs such as the NMR CUFF (5) which allows the easy application and removal of NMR magnets to long circular objects gives a clear scope to develop a membrane specific NMR system. Examination of operating spiral wound membranes using NMR offers several possible applications such as early fouling detection, the ability to directly compare antiscalant or cleaning regimes and the impact on membrane hydrodynamics. NMR will may also be able to indicate critical flux points faster in spiral wound NF and RO membranes.