The former train boss brought in to run Northumbrian Water a year ago has pledged to make it Britain's greenest utility by using the ancient technology of reed beds to clean up water, and sewage sludge to power its energy-hungry facilities.  Heidi Mottram, who ran Northern Rail for 24 years, said the only thing she had changed at Northumbrian since her arrival was to focus the company on becoming a beacon of renewable technologies.  Northumbrian is already leading the way in anaerobic digestion, the use of giant steel tanks to hasten the breakdown of organic matter and produce gas that is in turn used to create electricity.  The company uses sludge from its treatment works at its anaerobic plant at Bran Sands on Teesside and is constructing another plant at Howden on Tyneside. The two are expected to produce 20 per cent of Northumbrian's power needs.  Ms Mottram has also given the green light to the largest constructed reed bed in the world at Hanningfield near Chelmsford, where the company operates Essex & Suffolk Water. She says it will produce a natural filter that will be cheaper than mechanical treatment.  The greening of Northumbrian came as the company produced the sort of steady financial results that investors now expect from the sector. Pre-tax profits were 6 per cent better at ?181 million in the year to March 31. Revenues, helped by a rise in the regulated price cap, increased 5 per cent to ?738 million. The dividend is 7.9 per cent higher at 14.29p.  The recent reporting season proved controversial in the water sector. South West Water, owned by Pennon, cranked up its dividend after accepting a handout from the Government and Severn Trent Water admitted it is under investigation by two different regulators.