Coagulation and precipitation of cyanobacterial blooms

Abstract

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 coagulanta "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 H 2 O 2 ) 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. recyclable sediment phosphorus (P) pool (Moss, 2010). The most notorious symptom of eutrophication is a massive proliferation of cyanobacteria (Paerl et al., 2011). Blooms of cyanobacteria are viewed problematic as they may produce nasty odours, cause fish kills due to hypoxia/anoxia, and may impair ecosystem services, such as drinking water production, irrigation, recreation, aquaculture and fisheries (Paerl and Paul, 2012). Eutrophication is expected to further increase in the upcoming decades because of human population growth, intensified

Removal of cyanobacteria from a water supply reservoir by sedimentation using flocculants and suspended solids as ballast: Case of Legedadi Reservoir (Ethiopia)

“…Previous studies used a ‘floc and sink’ technique in which ballast material was introduced before the addition of an effective dose of coagulant [ 13 , 14 , 19 , 20 , 24 , 26 , 51 ].…”

“…Thus, there is a need to look for fast, easy, cheap, and safe technologies as curative/effect-oriented methods. To this end geochemical engineering approaches, such as “floc and lock” and “floc and sink”, have been developed [ 13 , 14 ]. In the floc and sink technique, flocculants/coagulants such as chitosan, poly aluminium chloride (PAC), iron chloride, and Moringa oleifera seed-extract (MOS) with ballast materials (clays, natural soils, lanthanum-modified bentonite, and Phoslock) have been used to effectively coagulate cyanobacteria as intact cells and to settle to the sediment of different lakes/reservoirs [ 14 ].…”

“…In Legedadi Reservoir, the high turbidity caused by particulate materials entering the reservoir through rivers was considered a unique feature. In previous studies, ballast materials such as clay and soil were added with a coagulant to lake/reservoir water [ 14 , 20 ]. The high turbidity of the Legedadi Reservoir, however, was expected to bring sufficient autochthonous ballast such that, adding additional ballast is not needed.…”

‘Floc and Sink’ Technique Removes Cyanobacteria and Microcystins from Tropical Reservoir Water

“…The coagulation and precipitation of cyanobacteria biomass and phosphate (P) is a promising tool to manage eutrophication and its nuisance [19]. This technique combines a ballast and a coagulant (Floc and Sink) that moves intact cells and P out of the water column, bound to ballast, toward the sediment, and is proven safe [19][20][21][22][23].…”

Kinetics of Microcystin-LR Removal in a Real Lake Water by UV/H2O2 Treatment and Analysis of Specific Energy Consumption

“…There are two possible approaches to address the problem of the presence of MC-LR in waters: (i) remove the cyanobacteria that produce them or (ii) directly eliminate the free toxin [ 1 ]. The conventional treatments present in a drinking water treatment plant (DWTP; e.g., coagulation, flocculation, sedimentation and filtration) allow one to implement the first of the two approaches [ 22 , 23 , 24 , 25 ]. However, these treatments are not able to remove the MC-LR already secreted by cyanobacteria and present in the water in dissolved form [ 26 ].…”