Sea threatens new treatment plant

Construction is at an unprecedented level in Gotland, and one consequence is that the wastewater treatment plant at Slite is no longer adequate. Plans are well advanced to extend the plant to four times its current capacity, but the extreme long-term high seawater levels during the spring led local authority principals to stop and rethink.

"There were acute problems getting the water from the plant to run out to sea, and there was a risk that seawater might flood into the treatment basins - which would have been absolutely devastating," says mechanical engineer Rikard Widén of Gotland Municipality.

Bearing in mind climate warnings of rising sea levels in the future, it was soon clear that the plans for the new treatment plant would have to be safeguarded. An inquiry into how high sea levels could rise was ordered from SMHI.

The results showed that there are likely to be occasions in the future when the surface level could rise by 1.4 metres compared to the current average. The calculations also showed that this spring´s highs of 65 centimetres above normal may occur once every other year on average.

"Now we know what we're dealing with. Situations we've thought of as very unusual to date could be common occurrences in the future, and we have to plan for that," Rikard Widén explains.

One of the problems at the treatment plant is that high seawater levels build great pressure at the opening of the discharge pipe some 100 metres out at sea, which prevents the water from the plant from flowing out.

And the higher the water levels, the greater the resistance faced by the outgoing water. Moreover, raised sea levels also increase the risk of seawater reaching the treatment basins and washing untreated water into the sea.

A wastewater treatment plant is generally the lowest-lying building in the community; the plant is at the end of the water chain and the pipes must therefore always point downwards. The project planning for a treatment plant is a meticulous affair, with careful consideration of planning permission in the event of changes. With tens of kilometres of pipe work, the angle of inclination is measured in parts per thousand, which equates to a precision of one millimetre for each metre.

One alternative for the Slite treatment plant, which will serve the whole of northern Gotland, would of course have been to build a brand new plant at a higher level.

"But that just wasn´t an option bearing in mind the costs. We have to re-use the existing plant and try to find technical solutions that will work even when sea levels are extremely high. We´re also talking about something that only happens perhaps every other year or so."

The future of the Slite wastewater treatment plant now entails the addition of a new pump house on the coast. In extreme high water situations, the pumps will be able to help pump water out into the sea. A backwater flap is also being built which can be shut to stop seawater from getting into the basins.

"We're basing our plans on future scenarios stretching 100 years into the future. Even if this new plant has a life span of 30 years, we must be able to count on re-using the basic parts of the plant for the entire foreseeable future," says Rikard Widén.

SMHI's calculations of present and future seawater levels encompass low, average and extreme levels, and an assessment on how often each situation may arise. The results are based on conclusions reached by the UN's Panel on Climate Change and future scenarios from SMHI's climate research. The sea level calculations also take account of land elevations.

The contacts at SMHI are Jan Andersson and Signild Nerheim.