'Floc and Sink' Technique Removes Cyanobacteria and Microcystins from Tropical Reservoir Water

Abstract

Combining coagulants with ballast (natural soil or modified clay) to remove cyanobacteria from the water column is a promising tool to mitigate nuisance blooms. Nevertheless, the possible effects of this technique on different toxin-producing cyanobacteria species have not been thoroughly investigated. This laboratory study evaluated the potential effects of the "Floc and Sink" technique on releasing microcystins (MC) from the precipitated biomass. A combined treatment of polyaluminium chloride (PAC) with lanthanum modified bentonite (LMB) and/or local red soil (LRS) was applied to the bloom material (mainly Dolichospermum circinalis and Microcystis aeruginosa) of a tropical reservoir. Intra and extracellular MC and biomass removal were evaluated. PAC alone was not efficient to remove the biomass, while PAC + LMB + LRS was the most efficient and removed 4.3-7.5 times more biomass than other treatments. Intracellular MC concentrations ranged between 12 and 2.180 µg L^-1 independent from the biomass. PAC treatment increased extracellular MC concentrations from 3.5 to 6 times. However, when combined with ballast, extracellular MC was up to 4.2 times lower in the top of the test tubes. Nevertheless, PAC + LRS and PAC + LMB + LRS treatments showed extracellular MC concentration eight times higher than controls in the bottom. Our results showed that Floc and Sink appears to be more promising in removing cyanobacteria and extracellular MC from the water column than a sole coagulant (PAC).

Keywords: Dolichospermum; Microcystis; cyanobacteria mitigation; eutrophication control; geo-engineering; toxic bloom.

Figure 1

Chlorophyll-a concentrations (µg L⁻¹) in the top 5 mL (top light gray bars) and bottom 5 mL (lower dark gray bars), Photosystem II efficiency (Φ_PSII) (circles), and pH values (triangles) of 60 mL cyanobacteria suspensions incubated for 2 h in the presence of different concentrations (1, 2, 3, and 4 mg Al L⁻¹) of the coagulant PAC (polyaluminium chloride). The control is represented by 0 mg Al L⁻¹.

Figure 2

Chlorophyll-a concentrations (µg L⁻¹) in the top 15 mL ((A) top light gray bars) and bottom 15 mL ((B) lower dark gray bars), Photosystem II efficiency (Φ_PSII) in the top ((A) filled circles), and bottom ((B) open circles) and pH values ((A) triangles) of 1 L cyanobacteria suspensions from the Funil Reservoir incubated for 2 h in the absence (control) or presence of the coagulant (polyaluminium chloride, PAC 3 mg Al L⁻¹) and coagulant combined with ballast (lanthanum modified bentonite, LMB 0.2 mg L⁻¹, and local red soil, LRS 0.2 mg L⁻¹) separately or in binary mixtures (lanthanum modified bentonite, LMB 0.1 mg L⁻¹, and local red soil, LRS 0.1 mg L⁻¹). The dotted line indicates the initial chlorophyll-a concentration in the cylinders, error bars represent one standard deviation (n = 3), and similar letters indicate homogeneous groups according to the Holm–Sidak post-hoc test (p < 0.05).

Figure 3

Intracellular concentrations (µg L⁻¹) of MCs and their variants in the top of the cylinder (A) and bottom (B) in 1 L of cyanobacteria suspensions from the Funil Reservoir incubated for 2 h in the absence (control) or presence of the coagulant (polyaluminium chloride, PAC 3 mg Al L⁻¹) and coagulant combined with ballast (lanthanum modified bentonite, LMB 0.2 mg L⁻¹, and local red soil, LRS 0.2 mg L⁻¹) separately or in binary mixtures (lanthanum modified bentonite, LMB 0.1 mg L⁻¹, and local red soil, LRS 0.1 mg L⁻¹). The black diamonds represent the total intracellular MC in the treatments. Similar letters indicate homogeneous groups in the total MC according to the Holm–Sidak post-hoc test (p < 0.05).

Figure 4

Extracellular concentrations (µg L⁻¹) MCs and their variants in the top of the cylinder (A) and bottom (B) in 1 L of cyanobacteria suspensions from the Funil Reservoir incubated for 1 h in the absence (control) or presence of the coagulant (polyaluminium chloride, PAC 3 mg Al L⁻¹) and coagulant combined with ballast (lanthanum modified bentonite, LMB 0.2 mg L⁻¹, and local red soil, LRS 0.2 mg L⁻¹) separately or in binary mixtures (lanthanum modified bentonite, LMB 0.1 mg L⁻¹, and local red soil, LRS 0.1 mg L⁻¹). The black diamonds represent the total intracellular MC in the treatments. Similar letters indicate homogeneous groups in the total MC according to the Holm–Sidak post-hoc test (p < 0.05).