FOR the first time in 150 years, salmon are returning to the Aire.

The fish have been seen in the river's lower reaches, at Leeds and down to Eggborough and Knottingley, although the Craven area has yet to enjoy a sighting. But Yorkshire Water is determined to change that.

During the next few years, the company will pump millions of pounds into a massive overhaul of its Bradford sewage treatment works at Esholt in a bid to make the River Aire cleaner and tempt salmon and other waterlife to return.

The company is to spend £55 million developing filtration and treatment processes at the massive site three miles north east of Shipley to improve the quality of the effluent pumped into the Aire.

"At Esholt it has been a long time since such far-reaching work was carried out," said Yorkshire Water project manager Rob Bainbridge. "There's been a European directive on making water more suitable for fish, so our works are to bring the water level to an even higher standard and meet those new regulations."

Esholt is Yorkshire Water's largest sewage treatment works; it handles waste from more than 300,000 homes and industrial waste equivalent to a population of another 442,000 people.

Mr Bainbridge said the project's end would bring a state-of-the-art site by 2009.

"It's about improving the river and to do that we need to improve the quality of the effluent. We are talking about providing processes that, technologically, would lead the market."

Mr Bainbridge's colleague, Judy Anderson, said the company was excited about other improvements, including a process to turn sludge - the waste matter left over from sewage treatment - into high-grade compost.

"We will be mixing our sludge with the city's green waste, such as grass clippings," she said. "We will then seed it and harvest the grass which can be used on brownfield sites and other reclaimed land and golf courses. It's a very green process."

The treatment works will also use gas produced from the waste treatment process to create electricity - enough, said Mr Bainbridge, to power a village of around 2,200 people.

Mr Bainbridge said improving water quality remained at the top of Yorkshire Water's agenda. "In a big way, it's about making the water good for the fish. We hope to see quite a few salmon in the Aire fairly soon."

But some anglers would argue that water quality was already good enough and that the weirs up and down the Aire were the real barrier to the return of salmon.

Conservationist Kevin Sunderland, of the Aire and Calder Rivers Group said that salmon returning to the Aire and heading upstream to spawn would find it very tricky to negotiate the river's many weirs.

"It's good news to see work which will improve water quality," he said. "They are spending £55 million at Esholt and similar at the Leeds site, so I'd like to ask for a little more to be spent on installing fish passes."

Mr Sunderland said the River Aire was a salmon river until pollution in the first half of the 19th century wiped out all life downstream of Shipley. Even before then, salmon had to run the gauntlet of dams and weirs which had been constructed for water power.

In the Bradford area there are only five weirs. Shipley, Bingley and Crossflatts have relatively small ones and the weir at Salts Mill is thought to be passable in half-flood conditions. The biggest obstacle is the weir at Hirst Mill near Saltaire.

"We'd need to see some work done providing fish passes to really see salmon make their way up to the Craven end of the Aire, " said Mr Sunderland.

The Environment Agency welcomed the improvement plans. The agency's Peter Baker said: "This is good news, we're pleased that the planned improvements at Esholt are going ahead. This investment will result in better water quality and will protect and improve the downstream fishery.

"The improvements at Esholt and at other large sewage treatment works in Yorkshire will allow the development of long-term sustainable fisheries in industrial rivers such as the River Aire."