Hundreds of Norwegian hydropower plants threaten fish and bottom-dwelling animals by exposing them to water that is oversaturated with air. The danger increases with wilder, wetter weather and more flooding. The solution may be to use sound in a new way.

Professor Ole Gunnar Dahlhaug and postdoctoral fellow Wolf Ludwig Kuhn at the Norwegian University of Science and Technology (NTNU) have three goals: To understand the problem of gas supersaturation at hydropower plants, to solve it, and to help correct mistakes made during Norway's more than 100 years of hydropower production.

"Hydropower is known as clean and green, but it turns out that it is neither completely clean nor completely sustainable. We have to try to solve this problem," says Ole Gunnar Dahlhaug.

The problem is that many hydropower plants can threaten fish and bottom-dwelling animals that live in rivers, reservoirs or fjords downstream. When the water discharged from the hydropower plant contains too much dissolved air, fish can get gas bladder disease, which is related to decompression sickness in humans.

Gas bladder disease results when harmful, deadly air bubbles form inside the body. That's what happened to the small trout that was found in the river Otra below one of Norway's largest power stations a few years ago. It had all the signs of gas bladder disease: protruding eyes, bleeding, infection and wear and tear on the fins.

Supersaturation can occur when air is sucked in through the intake tunnels on the upper side of hydropower plants. The pressure becomes so high that the air dissolves in the water on the journey down to the turbines. When discharged from the lower side of the power plant, the water looks like skimmed milk due to huge amount of gas bubbles. Gas supersaturation can also occur during floods and high water flow.

In some areas, life along long stretches of rivers downstream of power plants has almost been eliminated, such as in Otra, which is regulated by Brokke Power Plant in Agder. Calculations made by the research institute NORCE LFI (Laboratory for Freshwater Ecology and Inland Fisheries) show that many of Norway's 1,800 power plants can be affected by the supersaturation problem. Almost 30 per cent are in the high-risk class. The greatest risk of harm comes from the largest facilities that discharge to rivers. There are roughly 200 of these in Norway.

"We know that climate change means more flooding, so we believe that the problem will only get worse. That's why we should take it more seriously," Dahlhaug said.

Eliminating this risk has been the goal of the DeGas project led by Dahlhaug for the past five years. Here, mechanical engineers from NTNU's Department of Energy and Process Engineering have joined forces with biologists from the Norwegian Institute for Nature Research (NINA) and SINTEF researchers.

Kuhn and Dahlhaug have now applied for a patent for a new technology, along with Bjørn Winther Solemslie from NINA and Ulrich Puig from Norce.

With the help of an acoustic transducer, which is a cylindrical device that can shoot sound waves through water, the researchers have managed to remove dissolved air in river water.

The device uses power ultrasound. The pressure waves from the device cause the water to evaporate locally, and the air molecules move into the zones where the water has evaporated. There they form bubbles. The bubbles grow larger as they rise to the surface and eventually dissolve.

So far, the tests show that the method works. The researchers started in a specially built, 20-metre-long and 36-centimetre-deep trench in the hydropower laboratory at NTNU.

In the first experiments, the water flow was 4 liters per second. Now they have increased the speed to 600 liters per second by testing the equipment at a small power plant outside Trondheim. The largest power plants can have a flow rate of as much as one million litres of water per second. "The next step is 2000 litres, and gradually we will increase to 100,000 litres per second," said Kuhn.

The researchers believe the method is effective, and relatively cheap to install and operate. At least two things must be in place before this can become a commercial spin-off from NTNU: More testing and a market that will use the technology.

"The market will not be there until the authorities introduce requirements and the facts are put on the table, so that we know what is actually happening around Norwegian hydropower plants," says Dahlhaug.

It has been almost 60 years since researchers first uncovered problems at large run-of-river power plants with dams in the USA and Canada. Authorities address the problem, but not in Norway. The height differences here are greater here, which makes Norwegian power plants more vulnerable. Nevertheless, decompression sick animals have been allowed to continue to float under the radar of owners and regulatory authorities.

The higher the air saturation, the higher the mortality rate. Insects, larvae and other small animals float up and die when the water is at 110 per cent saturation. This is also the tolerance limit for fish, but it also depends on how long the fish are exposed.

North American and Chinese power plants have measured maximum values of 150-160 per cent air saturation. A study from NORCE LFI shows that there can be an oversaturation of close to 230 per cent at some Norwegian power plants.

"We consequently have a bigger problem here, because we should not be above 100 per cent," says Ole Gunnar Dahlhaug.

At present, there are no requirements for power companies to monitor or limit gas supersaturation. Research has shown that the problem may affect many more power plants than previously thought. Several thousand new dams are being planned in the world. The NTNU researchers believe that more people should know about the problems.

"A report from a new power plant in Brazil shows that they killed 9 tons of fish during a test weekend. They did not know that they had such challenges. This shows that we must be aware and adapt the power plants in the planning phase," Kuhn said.

The NTNU researchers join the ranks of researchers who believe that politicians and the Norwegian Water Resources and Energy Directorate (NVE) should demand that the power plant owners initiate monitoring. This will illuminate which power plants have problems.

"A single year is enough to get the facts on the table," says Kuhn, while Dahlhaug adds that the measure will cost vanishingly little for the power companies.

With Norway's increasingly wilder and wetter weather, are there increasing numbers of dead fish and bottom-dwelling animals?"

Dahlhaug says no one is currently studying this issue or trying to tally up possible mortality.

"There is no research or counts, but NORCE, among others, assumes that there are probably more than we realize, in paart because fish or animals that float to the surface due to too much gas will quickly be picked up and eaten by other animals. And there are probably very few of us who will be able to see the small animals that are left on the bottom, or insects that float up and past us because they have gas bubbles in them," he said.