How to deal with plankton blooms

Posted on September 23, 2013

High nutrient levels in water bodies often lead to dense surface blooms of algae and cyanobacteria, which can reduce water quality and threaten human health.   A variety of engineering, physical, chemical and biological methods have been proposed for the control of surface blooms,  but none have proved to be completely satisfactory.  An integrated approach, where complementary techniques are used in combination, seems to be the solution.  Findings from laboratory and field experiments suggest that blooms can be dispersed effectively through the joint application of hydrogen peroxide, polymeric ferric sulphate (PFS) and clay.  Hydrogen peroxide acts as an algicide, breaking up large algal colonies and shutting down photosynthesis within two hours.  Without further treatment the inactivated algae remain on the surface and prevent light penetrating the water.  For this reason, they’re then treated with PFS (a flocculent) and clay, which acts as ballast and causes the floc to sink to the bottom.  In tests of the method in a two metre-deep enclosure in LakeChaohu, Eastern China, one of the most polluted and nutrient-rich lakes in the world, most of the treated algae were deposited on the bottom.  At the same time there were large and rapid improvements in water quality, in the form of reduced turbidity and lower nitrogen and phosphorus concentrations.  A spin-off benefit of this technique is that oxygen levels in the bottom sediment are increased as a result of the breakdown of the hydrogen peroxide in the floc.  Optimum application rates for the three active agents were: hydrogen peroxide 60 mg/l,  PFS 20 mg/l and clay 2 g/l.

Reference:    Wang, Z., Li, D., Qin, H. & Li, Y.   2012.   An integrated method for removal of harmful cyanobacterial blooms in eutrophic lakes.   Environmental Pollution 160, 34-41.