An old aluminium smelter is being transformed into Surf Snowdonia, a £12m recreational facility set to open in August

The world’s longest man-made surfing wave is set to open in north Wales, marking the transformation of a heavily polluted aluminium works into a lake serving up six-foot barrels once a minute.

Surf Snowdonia will open on 1 August on the former site of the century-old Dolgarrog aluminium factory, between the green-wooded slopes of the Snowdonia national park and the river Conwy.

The £12m facility is 300m long and produces waves by pulling a snowplough-like wedge along under the water. The wedge travels 180m in 19 seconds under a central pier, before being sent back the other way and producing another pair of waves in the other direction.

“We had to convince people that, though it looked like a crazy idea, there were one or two sane people behind it,” said Steve Davies, the managing director of Surf Snowdonia and formerly a full colonel in the army.

Cleaning up the site was a major task, Davies told the Guardian: “A century of aluminium smelting had left its mark in terms of contamination.” Eighteen tanker-loads of heavy metals and hydrocarbons were removed from the site. About 25,000 cubic metre of materials were pulled from the old foundations, crushed and then re-used.

The wave-making technology was developed by Spain-based Wavegarden, a group of engineers who also happened to be surfers and who previously built a half-size prototype in Spain. The wedge will run behind a metal net to keep surfers safe without blocking the energy of the waves.

Facebook Twitter Pinterest An artist’s impression of how the Surf Snowdonia park will look like when it opens. Photograph: Surf Snowdonia

The wave will be 1.8m high at the centre but learner surfers will be able to tackle smaller waves at the edges and in the end zones. The water in the lake is rainwater which will have flowed through the hydroelectric plant on the hill above. The water will be cleaned using ultraviolet light rather than chlorine or other chemicals.

The aluminium plant provided sheet metal for aircraft in the second world war and was ringed by anti-aircraft guns and an air shelter. The latter has been adapted for wildlife and dubbed the “bat cave”.

Jo Dennison, a multiple British and Welsh surf champion, has been appointed head coach at Surf Snowdonia and has ridden the Wavegarden in Spain. “It is like a real wave – a point break,” she said. “The progression rate in training is also fantastic, as you get so many waves. It’s going to be a great training facility if you are working on your pop-up or some manoeuvres.”

She acknowledges that Surf Snowdonia is not going to replace ocean surfing, and the wave searching, local knowledge and unpredictability that is part of the sport’s charm for many surfers.

“I don’t think you can ever replace the sea,” she said. “But you can get fed up when you are doing more searching than surfing.” Surf Snowdonia will also guarantee waves in the warm summer months, when the ocean swell around the UK is at its weakest.

Davis, who has overseen the whole construction and overcome the setback of a small fire on the site, said: “My biggest concern is we become a wildfowl reserve, which would foul the water. But then we’d just turn the wave machine on and they’d scatter.”