Water is often cited as one of the great emerging investment opportunities. But there are not that many ways to tap into it. That partly explains the buzz around HaloSource, whose shares will start trading on AIM next week. The company makes low-cost water purifying kits which it hopes will become the standard for white goods in the developing world. Although the technology is derived from work by a British scientist, the company is now based in Seattle. But it decided to float on AIM as it says London investors are more interested in technology stories focused on the developing world. Today it will announce it has placed £50 million of shares, which may encourage others to follow its lead.