Finding superconductors in powerful electronic devices and scanners may not be too much of a surprise, but a surprising example of an application of superconductivity - a totally quantum phenomenon - is a million miles away from the delicacy of Josephson junction powered SQUIDs (Superconducting Quantum Interference Devices). It is in sewage treatment. We live in a paradoxical world that is awash with water – it almost defines our planet – and yet at the same time where there is a shortage of clean drinking water. It shouldn’t be that way. The world contains around 200,000,000,000 litres of water for every living person.
If you think of that in terms of consumption, assuming a typical 5 litres a day, the water out there should last over 100 million years. And that would be if it were all used up, where we know in practice that most of the water we consume is released back into the environment in short order. Of course that 5 litres only represents our direct consumption. A typical Western water user will be responsible for up to 10,000 litres a day. In part this is due to washing, watering the garden and flushing the toilet, but also because of the indirect use in the production of the goods we buy and the foods we eat. Just one hamburger takes around 3,000 litres, while a 1 kg jar of coffee requires a massive 20,000 litres. (Though once again, most of this water will be recycled – it doesn’t remain in the product.)
The problem, of course, comes not from poor availability of water per se, but the lack of clean drinking water in the right place for those who need it. Arguably this makes any water shortage more of an energy problem than anything else –that’s the energy required to clean up the water, whether it is desalination or removing dirt and sewage, and to get the water to where it is needed. And superconductivity can play its part in overcoming this. Most existing waste water treatment – whether cleaning up sewage or cleaning water from a river to use in an industrial plant – is expensive to build and has to be on a large scale to be cost effective. There are many circumstances where a smaller, distributed system would work better and, surprisingly, superconductors offer a solution to cleaning water that is both more cost effective and compact than a conventional treatment plant. What’s more it works more quickly too.
The process makes use of a powerful superconducting magnet to separate off the suspended material in the water. This is obviously fine for magnetic metals, but it seems an unlikely solution for the rest – the typical gunk that we associate with sewage and polluted water. But by adding a substance known as a ferromagnetic adsorbent to the water this mess become accessible to magnetic fields. The suspended particles stick the adsorbent material, which is then dragged out of the water by the magnets leaving clean water behind. The only way to get a sufficiently strong magnetic field is to use superconductors. Quantum physics to the rescue.