Abstract:
Dissolved air flotation (DAF) performance with two different naturally occurring cyanobacterial
morphologies was investigated with respect to the biomass removal efficiency,
the toxin release to water and the coagulant demand by different water background
natural organic matter (NOM). Coagulation (C)/Flocculation (F)/DAF bench-scale experiments
(2 min coagulation at 380 s 1 with polyaluminium chloride (0.5e4 mg/L Al2O3, the
dose depending on the water NOM content); 8 min flocculation at 70 s 1; 8 min DAF with
5 bar relative pressure and 8% pressurised recycle) were performed with single cells of
Microcystis aeruginosa and Planktothrix rubescens filaments spiked in synthetic waters with
different NOM contents (hydrophobic vs. hydrophilic NOM; moderate (2e3 mgC/L) vs.
moderate-high concentration (ca. 6 mgC/L)). For both morphologies, the results show no
apparent cyanobacterial damage (since the water quality did not degrade in dissolved
microcystins and the removal of intracellular microcystins matched the removal of chlorophyll
a) and high biomass removal efficiencies (93e99% for cells and 92e98% for filaments)
provided optimal coagulant dose for chlorophyll a removal was ensured. Charge
neutralisation by the polyaluminium chloride was the main coagulation mechanism of the
M. aeruginosa cells and most likely also of the P. rubescens filaments. The specific coagulant
demand was severely affected by NOM hydrophobicity, hydrophobic NOM (with a specific
UV254nm absorbance, SUVA, above 4 L/(m mgC)) requiring ca. the triple of hydrophilic NOM
(SUVA below 3 L/(m mgC)), i.e. 0.7 vs. 0.2e0.3 mg Al2O3/mg DOC.
