Coagulant plus ballast technique provides a rapid mitigation of cyanobacterial nuisance

Cyanobacteria blooms are a risk to environmental health and public safety due to the potent toxins certain cyanobacteria can produce. These nuisance organisms can be removed from water bodies by biomass flocculation and sedimentation. Here, we studied the efficacy of combinations of a low dose coagulant (poly-aluminium chloride—PAC—or chitosan) with different ballast compounds (red soil, bauxite, gravel, aluminium modified zeolite and lanthanum modified bentonite) to remove cyanobacterial biomass from water collected in Funil Reservoir (Brazil). We tested the effect of different cyanobacterial biomass concentrations on removal efficiency. We also examined if zeta potential was altered by treatments. Addition of low doses of PAC and chitosan (1–8 mg Al L-1) to the cyanobacterial suspensions caused flock formation, but did not settle the cyanobacteria. When those low dose coagulants were combined with ballast, effective settling in a dose-dependent way up to 99.7% removal of the flocks could be achieved without any effect on the zeta potential and thus without potential membrane damage. Removal efficacy was influenced by the cyanobacterial biomass and at higher biomass more ballast was needed to achieve good removal. The combined coagulant-ballast technique provides a promising alternative to algaecides in lakes, ponds and reservoirs

The efficiency of combined coagulant and ballast to remove harmful cyanobacterial blooms in a tropical shallow system

We tested the hypothesis that a combination of coagulant and ballast could be efficient for removal of positively buoyant harmful cyanobacteria in shallow tropical waterbodies, and will not promote the release of cyanotoxins. This laboratory study examined the efficacy of coagulants [polyaluminium chloride (PAC) and chitosan (made of shrimp shells)] alone, and combined with ballast (lanthanum modified bentonite, red soil or gravel) to remove the natural populations of cyanobacteria collected from a shallow eutrophic urban reservoir with alternating blooms of Cylindrospermopsis and Microcystis. PAC combined with ballast was effective in settling blooms dominated by Microcystis or Cylindrospermopsis. Contrary to our expectation, chitosan combined with ballast was only effective in settling Cylindrospermopsis-dominated blooms at low pH, whereas at pH ≥ 8 no effective flocculation and settling could be evoked. Chitosan also had a detrimental effect on Cylindrospermopsis causing the release of saxitoxins. In contrast, no detrimental effect on Microcystis was observed and all coagulant-ballast treatments were effective in not only settling the Microcystis dominated bloom, but also lowering dissolved microcystin concentrations. Our data show that the best procedure for biomass reduction also depends on the dominant species

Chlorophyll-<i>a</i> concentration (micrograms per liter), Photosystem II efficiency (PSII), pH, zeta potential (mV) and percentage of cyanobacterial biomass removal in the top 5 mL and bottom 5 mL of 50 mL cyanobacteria suspensions incubated for one hour in a range of PAC (poly-alumnium chloride), chitosan and local red soil (LRS) series concentrations.

<p>Chlorophyll-<i>a</i> concentration (micrograms per liter), Photosystem II efficiency (PSII), pH, zeta potential (mV) and percentage of cyanobacterial biomass removal in the top 5 mL and bottom 5 mL of 50 mL cyanobacteria suspensions incubated for one hour in a range of PAC (poly-alumnium chloride), chitosan and local red soil (LRS) series concentrations.</p

Effect of the initial cyanobacterial biomass variation on the chlorophyll-<i>a</i> concentration on 5 mL top (painel A) and 5 mL bottom (painel B) using different local red soil concentration in presence of coagulant PAC (poly-alumnium chloride, 2 mg Al L^-1).

<p>Effect of the initial cyanobacterial biomass variation on the chlorophyll-<i>a</i> concentration on 5 mL top (painel A) and 5 mL bottom (painel B) using different local red soil concentration in presence of coagulant PAC (poly-alumnium chloride, 2 mg Al L^-1).</p

The exponential decay constants (const), r² and <i>p</i> values of the from exponential decay curves fitted to chl-<i>a</i> data over different ballast concentrations.

<p>Local red soil (LRS), bauxite (BAU), gravel (GRA), aluminum modified zeolite (AMZ), lanthanum modified bentonite (LMB) solely (Ballast) or with combination the flocculants PAC (poly-aluminium chloride, 2 mg Al L^-1; Ballast+PAC) or chitosan (2 mg L^-1; Ballast+CHI).</p

Percentage of cyanobacterial biomass (microgram of chlorophyll-<i>a</i> per liter) removal using different local red soil concentration in presence of coagulant PAC (poly-alumnium chloride, 2 mg Al L^-1).

<p>Percentage of cyanobacterial biomass (microgram of chlorophyll-<i>a</i> per liter) removal using different local red soil concentration in presence of coagulant PAC (poly-alumnium chloride, 2 mg Al L^-1).</p

Variation of the zeta potential values (mV) of concentrations factor for cyanobacteria in the bottom of test tubes compared to control in PAC (poly-alumnium chloride), chitosan and local red soil (LRS) series concentrations and different concentrations of the red soil and the flocculants PAC (2 mg Al L^-1;) or chitosan (2 mg L^-1).

<p>Variation of the zeta potential values (mV) of concentrations factor for cyanobacteria in the bottom of test tubes compared to control in PAC (poly-alumnium chloride), chitosan and local red soil (LRS) series concentrations and different concentrations of the red soil and the flocculants PAC (2 mg Al L^-1;) or chitosan (2 mg L^-1).</p

Chlorophyll-<i>a</i> concentrations (micrograms per liter) in the top 5 mL of 50 mL cyanobacteria suspensions incubated for one hour with different concentrations of the local red soil (LRS), bauxite (BAU), gravel (GRA), aluminum modified zeolite (AMZ), lanthanum modified bentonite (LMB) solely (A) or with combination the flocculants PAC (poly-aluminium chloride, 2 mg Al L^-1; B) or chitosan (2 mg L^-1; C).

<p>Statistically significantly exponential decay curves were fit to chl-<i>a</i> data over different ballast concentrations. Error bars indicate one standard deviation (<i>n</i> = 3).</p

Chlorophyll-<i>a</i> concentrations (micrograms per liter) in the top 5 mL (top light gray bars) and bottom 5 mL (lower dark gray bars) of 50 mL cyanobacteria suspensions incubated for one hour with different concentrations of the red soil and the flocculants PAC (poly-aluminium chloride, 2 mg Al L^-1; panel A and B) or chitosan (2 mg L^-1; panel C and D).

<p>Also included are the pH values (open triangles) of the suspensions and zeta potential in top (filled circles) and bottom (open circles).</p