Beating the Blues by Floc & Lock

‘Floc & Lock’ is a treatment to mitigate blooms of blue-green algae (the blues) by imposing phosphorus limitation. First the bloom is sunk to the bottom using a flocculent (Floc), providing immediate relieve of the nuisance, then the sediment, including the precipitated bloom, is capped with a solid phase phosphorus binder (Lock), ensuring a long term effect. The thesis focusses on Lake Rauwbraken (Tilburg, Netherlands) where blooms of toxic blue-green algae led to prolonged swimming bans in 2004 and 2007. The lake was treated in April 2008 and remained free of cyanobacterial nuisances for more than 10 years. In 2011 the long term effect was put to the test by blue-green algae that emerged from the lake sediment. But, as a result of the phosphorus limitation, this starting bloom died off in an early stage and ended in a minor non-toxic scum. The thesis explains the lake system analysis, reports on the direct and longer-term treatment effects and includes the fate of lanthanum which was the active ingredient in the solid phase phosphorus binder.