Piyadasa, Chathuri (2017) Effects of commercially available pulsed electromagnetic field devices on bacterial viability and calcium carbonate precipitation. PhD thesis, Victoria University.

Abstract

Biofouling and scaling are two major problems in the operation of reverse osmosis (RO) membranes and other equipment. A variety of control measures are employed in practice, including the use of electromagnetic fields (EMF), which can avoid the use of chemical antifouling agents (e.g. halogen-based biocides) that may be toxic to humans or the environment. This is a fairly recent and controversial technology and, from the available documentation and literature, it is clear that the scientific basis for its purported effectiveness is not yet firmly established. In particular, the various conditions under which EMF technologies are likely to be effective for real world applications have not been established. This thesis reviews and collates the relevant literature on the problem of scaling and biofouling in RO membranes and heat exchanger systems (e.g. cooling towers), with a particular focus on the application of pulsed EMF technologies, including the broad documentation, relevant scientific studies, proposed mechanisms of action and further research directions. This study confirms that a lot more systematic scientific research is needed in order to validate the application and commercialization of EMF technologies as a pretreatment method to control fouling and scaling in various applications including RO membrane systems. Therefore, a number of carefully controlled laboratory experiments have been designed and carried out in order to test the inherent anti-bacterial and anti-scaling claims for two commercially available pulsed electromagnetic field (PEMF) devices, that were demonstrated in this study to operate at ~ 100 kHz but with different waveforms. For example, these commercially available devices are currently being marketed and employed to ostensibly manage biofouling. Since the reliable application and industry acceptance of such technologies requires thorough scientific validation – and this is currently lacking, we have initiated proof-of-principle research in an effort to investigate whether such commercially available PEMF devices can influence the viability (culturability) of planktonic bacteria in a pure aqueous environment. Thus, these two devices were first investigated via a static (i.e. non-flowing) treatment system. ‘Healthy’ Escherichia coli cells, as well as cultures that were physiologically compromised by silver nano-particles, were exposed to the PEMFs from both devices under controlled conditions. Although relatively minor, the observed effects were nevertheless statistically significant and consistent with the hypothesis that PEMF exposure under controlled conditions may result in a decrease in cellular viability and culturability. Notably, it has also been observed that under certain conditions bacterial growth is actually stimulated. These studies were then extended to flow conditions and to include another microorganism, P. fluorescens. Thus, the effect of the electromagnetic fields generated by the two commercial devices on the bacterial culturability of E. coli and P. fluorescens under flow conditions has been contrasted with previous static results. Specifically, for P. fluorescens, one of the two devices showed no significant inhibitory effect under static conditions but showed significant inhibition under several flow conditions (low and high) and for different exposure times. For the other device, static conditions are actually stimulatory to growth. However, under low flow conditions, the effect is inhibitory and, under high flow conditions, is either inhibitory or stimulatory depending on exposure time. Also, the marketing and implementation of commercially available pulsed-electromagnetic field (PEMF) devices to, ostensibly, control scaling in processes such as reverse osmosis (RO) and cooling-tower installations, is based on the notion that such devices enhance the coagulation of inorganic particles such as calcium carbonate. In order to provide a scientific basis for these claims, the precipitation characteristics of calcium carbonate under the influence of the PEMFs from the two devices has also been investigated under controlled conditions. Thus, the rate and profile of calcium carbonate precipitation in the presence and absence of PEMF exposure of parent calcium nitrate and sodium carbonate aqueous solutions were tracked, in parallel, by UV absorption at 350 nm and by turbidity measurements. The morphology of the corresponding crystalline precipitates was also assessed using SEM. From these studies, is apparent that exposure of the parent solutions to the PEMF from one of these devices, but not the other, can influence both the profile of calcium carbonate precipitation and the morphology of the resulting microcrystals, consistent with enhanced particle coagulation. It is evident from these studies that PEMF induced anti-bacterial and anti-scaling effects depend on a wide range of variables such as waveform, extent of flow, type of bacteria and PEMF exposure duration. The effect of other parameters such as frequency, pH, temperature, other dissolved species etc. also need to be considered, but were not addressed in these studies. Our investigations suggest that the uncertainties, if not confusion, in this area are a result of the high level of complexity, due to the aforementioned wide range of possible variables. This can only be addressed by systematically conducting controlled experiments, along the lines of those reported here, in order to properly isolate the effects of all such variables. In particular, it is imperative to define the conditions under which such devices might be commercially viable.

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