Forward osmosis: the relationships between draw solution, membrane orientation and water flux

Neilly, Adam Geoffrey (2014) Forward osmosis: the relationships between draw solution, membrane orientation and water flux. Masters (Research) thesis, James Cook University.

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It is widely recognised that as freshwater resources become more stressed, desalination will become a reliable solution to water shortages being experienced in many countries, the by-product of which is waste concentrate. As a result, technologies for zero liquid discharge (ZLD) have been developed but they currently have very high capital and operating costs. It has been proposed that an integrated system using existing reverse osmosis desalination and salt recovery technology in conjunction with forward osmosis (FO) could be a solution for achieving ZLD that is economically viable, due to the relatively low capital and operating costs of FO. The FO process, however, is in the early stages of its development and more research is required in order to improve the process and to determine if the process is viable for enhancing water recovery from desalination.

In the existing body of FO research, a single proprietary membrane developed by Hydration Technology Innovations (HTI) consisting of a cellulose triacetate active layer embedded about a polyester screen mesh has been used for the vast majority of studies. In addition to this, the ammonium bicarbonate draw solution has been used extensively in the investigation of concentration polarisation phenomena that have been found to be responsible for causing lower-than-expected flux from the FO process, whilst only sodium chloride has been used in the development of flux prediction models. Subsequently, the lack of investigations utilising different FO membranes and different draw solutions represents a deficiency of knowledge in the field of FO.

This study aimed to improve the understanding of the impacts of concentration polarisation phenomena and its causes in order to help enable membrane manufacturers to develop membranes that reduce the impacts of these phenomena, ultimately increasing membrane flux and the economic viability of FO. This study used different draw solutions and membranes to those widely used in the literature and assessed the suitability of existing flux prediction models for these different draw solutions and membranes. The draw solutions consisted of ethanol, magnesium chloride, sodium thiosulphate and ammonium bicarbonate.

Despite the promising osmotic efficiency and low boiling point of ethanol, it was found that it was not a suitable draw solution because the membranes were not capable of rejecting small, polar and water-soluble organics such as ethanol. The experiments using the two draw solutions, magnesium chloride and sodium thiosulphate, both exhibited superior flux compared to those using the ammonium bicarbonate draw solution. However, neither of these solutions can be easily and economically removed and recycled, making them unsuitable as alternative draw solutions. Despite this, the experiments identified trends that other studies have found whilst using different membranes and draw solutions. These findings add further to the body of FO knowledge, particularly with regards to the behaviour of membrane flux with respect to concentration polarisation phenomena.

It was also found that the existing models used to predict flux from forward osmosis did not adequately account for the use of different chemical species, due to the assumption in the models that the membrane was achieving complete rejection of the draw solute, with an assumed reflection coefficient of one. Modification to the models by re-introducing the reflection coefficient was made. Different reflection coefficients were determined for each draw solution species for the same bulk osmotic pressure ranges, operation mode and membrane. This indicates that the different chemical species are rejected by the different membranes by varying amounts, resulting in different values for the reflection coefficient which consequently has a significant impact upon theoretical flux determination.

Item ID: 41372
Item Type: Thesis (Masters (Research))
Keywords: cell membranes; desalination; draw solution; environmental engineering; environmental technologies; flux modelling; flux; forward osmosis; membrane technologies; osmosis; reverse osmosis; saline water conversion; separation technologies; water flux
Date Deposited: 01 Dec 2015 05:08
FoR Codes: 09 ENGINEERING > 0907 Environmental Engineering > 090799 Environmental Engineering not elsewhere classified @ 33%
09 ENGINEERING > 0907 Environmental Engineering > 090703 Environmental Technologies @ 33%
09 ENGINEERING > 0904 Chemical Engineering > 090404 Membrane and Separation Technologies @ 34%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970109 Expanding Knowledge in Engineering @ 100%
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