The benefits of hybridising electrodialysis with reverse osmosis

In a recently published paper in the Journal of Membrane Science, Prof. Syed Zubair, Prof. Lienhard and I reported on how the cost of achieving high purity water with electrodialysis, an electrically driven technology, can be reduced through hybridisation with reverse osmosis, a pressure driven technology.

In reverse osmosis, multiple stages are required to recover a significant portion of the feed water. However, the membranes block salt very well and the product water is very pure.
In reverse osmosis, multiple stages are required to recover a significant portion of the feed water. However, the membranes block salt very well and the product water is very pure.

In reverse osmosis, water is pressurised and forced through a membrane that is capable of blocking upwards of 99.5% of salts. In electrodialysis, an electrical current is passed through a stack of membranes and pulls salt ions from a feed stream (known as the diluate) into a more concentrated stream (known as the concentrate). Reverse osmosis can provide very high purity water, but the feed water must pass through multiple stages before a large fraction is recovered. Meanwhile, in electrodialysis, because salt, rather than water is removed, almost all of the water can be recovered as a final product, but high product purity requires multiple stages to ensure all of the salt has been removed. The idea of hybridisation is to leverage the ability of reverse osmosis to achieve very high purity and the ability of electrodialysis to recovery a large portion of the feed water as product.

In electrodialysis salt, rather than water, is removed from the feed water. This means that the vast majority of the feed water is recovered as product water. However, to achieve excellent purity multiple stages are required.
In electrodialysis salt, rather than water, is removed from the feed water. This means that the vast majority of the feed water is recovered as product water. However, to achieve excellent purity, multiple stages are required.

We analysed different ways of combining reverse osmosis and electrodialysis systems and compared the cost of water from these systems to the cost of water from standalone electrodialysis systems. We found that the hybrid systems allowed for the greatest cost reductions when high product purity was required. This is because the number of stages of electrodialysis required rises rapidly as higher purity is needed. The inclusion of reverse osmosis can reduce this requirement.

Ultimately, this methodology can be used by engineers to guide their choice between hybrid and standalone systems. The operation of hybrid systems is still unproven but this study suggests there is good potential for these systems to reduce water costs when high purity is required.

This blog is based upon “The benefits of hybridising electrodialysis with reverse osmosis” by Ronan K. McGovern, Syed M. Zubair and John H. Lienhard V, published in the Journal of Membrane Science.

The author’s version of the manuscript may be downloaded here.

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