Balticsea 2020

Aluminium treatment in Björnöfjärden

The aim of the program Living Coast is to demonstrate that it is possible to restore vulnerable inlet systems to clear water, natural fish communities and a well anoxic seabed. Realising this vision of sound ecological status is a challenge, due to both current and historical nutrient loads, and success will require concrete and effective measures both onshore and offshore. To bind phosphorus in the bottom sediments an aluminum treatment was performed in the summer of 2012 and 2013. The work was carried out by Vattenresurs AB.

Why an aluminum treatment?
The ground water in Björnöfjärden bay system has unnaturally poor living conditions with anoxic bottoms, small access of light and extremely high nutrient concentrations. No organisms other than bacteria can survive in sediments and during large parts of the year there are not any fish in the water deeper than six meters.

The largest inflow of nutrients (phosphorus) to the water comes from the bay’s bottom sediments, where large amounts of nutrients have been stored in many decades. The natural supply of phosphorus-binding agents such as iron, aluminum and calcium, has not increased to the same extent as the phosphorus load in the last hundreds of years. Iron also loses their ability to bind phosphorus when the sediment becomes anoxic. Instead the iron-bound phosphorus is released and contributes to algal blooms in the overlying water, which in turn leads to more anoxic bottoms when they break down. A "vicious cycle" occurs where the same phosphorus contributes to frequent algal blooms and then anoxic bottoms.

In order to have better water quality in the bay by measures carried out on land, phosphorus emissions from previous years, “the old sins” which is released from the sediment, also needs to be corrected.

Fig. 1

How does it work?
The aluminum treatment was carried out by Vattenresurs AB, which uses a patented method where the aluminum solution is mixed into the bottom water and the bottom sediments with a kind of harrow that is pulled behind a barge (fig. 1). The solution is mixed down into three rounds so that it spreads evenly over the bottom surface. The added aluminum binds rapidly and permanently the phosphorus that would otherwise leak up to the water.

Fig. 2: All bottoms deeper than 6 meters are anoxic and have no life besides bacteria. These bottoms have been aluminum treated to reduce the leakage of nutrients to the water.

The aluminum treatment was carried out in two stages. Primarily bottom areas with a depth of 6-12 meters, from the north (Säbyviken) to south (Björnöfjärden), was treated during the summer of 2012. During the summer of 2013 the deeper bottoms and Torpe-Infjärden was treated (Figure 2).

What is aluminum treatment?
Aluminum treatment is the same substance that is used in drinking water treatment in our larger water treatment works. Aluminum treatment has been used to fix phosphorus in lakes for more than 40 years in Sweden, as well as the United States and other parts of Europe. No negative side-effects have been seen to date, as long as very acidic conditions do not prevail in the water. There is nothing to suggest that it would be different for the Baltic Sea. Unlike lakes, the Baltic Sea additionally has good buffering capacity, so there is no risk of having acidic water.

The aluminum treatment has made better water quality
As a result from the aluminum treatment the supply of phosphorus in the bay reduced by more than 90 %. This means that the treatment works well, despite the lack of oxygen sediments are able to bind phosphorus again. The phosphorus in the bay has also been much lower than in the reference bay Fjällviksviken, and now corresponds with the levels and visibility in the water that was in the middle of the last century.

As a consequence of the greatly reduced supply of phosphorus resulting from phosphorus treatment, the bay system is expected to have reduced growth of algae, clearer water and lower phosphorus levels in the water. This should lead to less oxygen consumption in the bottom water and recolonisation of benthic fauna and fish in the water.

The measures on land are at least as important to the good water quality to be maintained. Click here and learn more about our project and activities in the project Living coast.