Coagulation and precipitation of cyanobacterial blooms

Eutrophication is the prime water quality issue in inland waters. Eutrophication and its key symptom, harmful cyanobacterial blooms, is expected to further increase in the future, which highlights the importance of managing the issue. The reduction of external nutrient load is crucial but might not bring fast relief to eutrophic waters due to ongoing diffuse pollution and legacy nutrients in the sediment. In this context, in-lake measures are needed to speed-up recovery. In this review, we discuss different in-lake measures based on coagulation and precipitation of cyanobacteria and/or phosphate for different lake categories (e.g., shallow or deep, mainly external or internal nutrient load, occurrence of perennial or summer blooms). In deep lakes with an external nutrient load higher than the internal load, a “Floc and Sink” method could be used in which a coagulant (e.g. aluminium salts, Al-salts; chitosan) combined with a ballast (e.g. soil, clay) removes a cyanobacterial bloom out of the water column. In case the deep lake suffers from high internal load, a phosphate (P)-fixative (e.g. lanthanum modified bentonite or Al-salts) can be used to “Lock” the legacy P, possibly combined with a coagulant – a “Floc and Lock” technique. The latter approach will target both the particulate P in a bloom and the internal P load. A shallow lake that suffers from summer blooms and in which the internal load is higher than the external load, a “Lock” strategy of winter application of a P-fixative is proposed to prevent bloom development. In shallow lakes with perennial blooms, an agent to damage the cells (such as H₂O₂) is required together with a coagulant and a ballast to avoid recolonization of the water column due to resuspension – a “Kill, Floc and Sink/Lock” method. The selection of the most promising in-lake measures and materials should be based on a proper system diagnosis and tests prior to a full-scale intervention. These methods can be effective, but evidently reduction of external nutrient loads, both from point- and non-pointed sources, is an absolute necessity to restore aquatic ecosystems in a holistic sense.