Water for the future. Do a reverse on reverse osmosis

Water is our most precious resource in Australia. Not natural gas, oil, coal or opals. Water. We must do everything to preserve it but sometimes, extraneous conditions make it hard.

Many will remember the millennium drought which saw harsh water restriction imposed. In response, our state governments built desalination plants to supplement our dwindling water supplies. The plants turned salt water to drinking water by a process called reverse osmosis to literally squeeze the salt out of the water.

What is osmosis?

Think about seasoning your juicy steak with salt for a few days. You might notice your steak is a lot bloodier when you go to cook it. Salt draws out the water from the steak. Osmosis is a natural occurrence where water will move to dilute areas of high salt concentrations.

Reverse osmosis

Desalination plants have high pressure pumps to force water through membranes. Water is forced through the membranes leaving the salt behind. Reverse osmosis may seem to be our solution for future water supply but it has problems.

  1. It is expensive to maintain the membranes and equipment
  2. Requires a lot of energy to drive reverse osmosis.

There are environmental issues with reverse osmosis. During treatment, a lot of chemicals are needed to clean and maintain the filters and membranes. These chemicals can be introduced into the water which goes to your home. The left-over salt water which has not been processed has an increased salt concentration which is then pumped out back into the ocean. This will cause problems for marine habitats.

Alternatives

In a cool little bit of technology, researchers from America have devised a more energy efficient method of desalination using electricity rather than pumps. They call it resin wafer electrodeionisation. It works by pumping water into ion exchange cells. Electricity is applied so it attracts the dissolved salts or ions to the positive and negative terminals: positive attracts negative and negative attracts positive. The walls of the cells stop the salts from going back into the cell.

Inside the cells, there are microbeads embedded with in a plastic which will replace the lost salts with hydrogen and hydroxide.

Diagram of how RW-EDI works. AEM and CEM stand for anion and cation exchange membranes. BP stands for bipolar membrane. The orange and brown balls are the embedded microbeads which supply hydrogen and hydroxide ions. Water is supplied into the cell pairs. When electricity is supplied, any salts are attracted to the respective electrode. Negative charged ions go the positive electrode and vice versa. The salts move through AEM and CEM into the concentrate streams between the cells and are stopped by the next set of membranes. <Image from the paper.>

Now, this is the part which I find cool. In controlled testing, resin wafer electrodeionisation was found to be 3 times more energy efficient than reverse osmosis. It could also match the amount of purified water it could generate. This means lower running costs in both maintenance and energy consumption.

These results are very promising for an alternative to reverse osmosis. It will be cheaper to run and maintain. It is a completely chemical free treatment method to produce pure water. There is no need for further treatment like chlorine or detergents.

We ought to get behind this technology. It has better energy efficiencies and productivity than reverse osmosis. It is a crucial option for the future water supply.

Without this technology, there is only one other option left if we run out of water.

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