ULTRAVIOLET light has long been used in water-treatment plants to help with disinfection. As long as the water is clear enough not to absorb the rays, exposing it to a discharge lamp will destroy the DNA of viruses, bacteria and protozoa that could otherwise cause illness when swallowed. The ultraviolet that exists in sunshine can also be used this way. A rough and ready method for sanitising water, for example, is to pour some into a clear plastic bottle, aerate it by shaking and then place it in the sun for six hours. Now a group of German companies including KACO, a solar-power firm, have gone one better. They have developed an industrial-scale solar-driven purification system that removes not only bugs but also chemical pollutants.

The first of their devices has been built as demonstration project at the German Aerospace Centre in Lampoldshausen. This organisation, which was among the group of developers, wanted to clean the water that it uses to cool its experimental engines. This cooling water inevitably gets contaminated both with rocket fuel and with substances produced by that fuel's combustion.

The trick the researchers use to boost the cleansing action of ultraviolet is to add a light-activated catalyst to the effluent. They have come up with two sorts, one for heavily polluted water and one for water that is only lightly polluted.

For water containing serious industrial contaminants, like that at Lampoldshausen, they use iron sulphate as the catalyst and add a small amount of hydrogen peroxide and sulphuric acid to assist the process. Thus treated, the water is pumped through arrays of transparent glass tubes in a 49 metre-long solar reactor. In a complex series of reactions, one of which is promoted by ultraviolet light, the iron reacts with both the hydrogen peroxide and some of the water itself to produce powerful oxidising agents known as hydroxyl radicals. These destroy the polluting molecules. Once the water has been cleaned, the acid is neutralised, causing the iron to become insoluble. It precipitates out as iron oxide and can then be recovered and recycled, leaving water pure enough to discharge into a river.

For less heavily polluted water, similar equipment is used, except that the glass tubes are coated with titanium dioxide. This substance is a semiconductor, meaning that when electrons (which are negatively charged) are knocked free from its crystal structure, the result is a positively charged “hole” that can move around inside the crystal. This hole is also a powerful oxidiser. One of the ways a hole can be created is for the electron to be displaced by ultraviolet light. A titanium-dioxide system, which is still in development, could purify water to the point where it was suitable not merely for discharge, but for drinking.

The iron-sulphate system at Lampoldshausen, meanwhile, is capable of treating 4,500 litres of waste water in an hour or two. It would be even more efficient in sunnier climes, and these are expected to become the main market for the process, which KACO has started to produce under the brand name RayWOx.

The German Aerospace Centre reckons RayWOx will be good at removing certain pollutants that are particularly hard to get rid of at the moment, notably antibiotics, X-ray contrast media and hormones. KACO, meanwhile, says it will be able to supply the system complete with photovoltaic cells to power both the control electronics and the pumps, making it not just cheap to operate, but also ideologically sound—a green, mean, cleaning machine, powered entirely by the sun.