The impact of cell morphology and algal organic matter on algal floc properties.

Physical floc properties were systematically investigated by analysing the structure of algal and cyanobacterial flocs produced by five species (green algae (Chlorella vulgaris) and cyanobacteria (Microcystis aeruginosa (strain CS-564), Microcystis aeruginosa (strain CS-555/01), Dolichospermum circinale and Cylindrospermopsis raciborskii) using aluminium sulphate (alum) at different doses and pH values. The properties of spherical, compact flocs were determined using a laser diffraction instrument and a new in situ image analysis technique was validated to analyse the structure of more complex flocs. The incorporation of algal-derived organic matter (AOM) into the flocs was inferred by evaluating the dissolved organic matter concentration character before and after flocculation using liquid chromatography with organic carbon detection (LC-OCD). D. circinale, C. raciborskii, and M. aeruginosa (CS-564) produced large flocs (2-9 mm), while M. aeruginosa (CS-555) and C. vulgaris produced smaller flocs (<2 mm). While differences in physical floc properties were observed to result from changes in coagulation mechanism, the cell morphology and the AOM composition were the most influential factors. Examination of floc properties can give a rapid insight at the plant for trouble shooting, particularly through the use of the in situ techniques and provide a mechanism by which floc properties can be tailored to downstream processes.

Examination of the physical properties of Microcystis aeruginosa flocs produced on coagulation with metal salts.

Coagulation-flocculation (C-F) is a key barrier to cyanobacterial and algal cell infiltration in water treatment plants during seasonal blooms. However, the resultant cell floc properties, in terms of size, strength and density, which dominate under different coagulation conditions and govern cell removal, are not well understood. This paper investigated the floc properties produced during C-F of the cyanobacterium, Microcystis aeruginosa, under low and high doses of aluminium sulphate and ferric chloride coagulants and at different pH values, so as to promote charge neutralisation (CN) and sweep flocculation (SF) dominant conditions (or a combination of these). It was demonstrated that application of ferric chloride produced larger flocs that resulted in higher cell removal during jar testing. These flocs were also larger than those observed for natural organic matter (NOM) and kaolin, suggesting a role of algogenic organic matter (AOM) as an inherent bioflocculant. Under SF conditions, stronger flocs were produced; however, these had lower capacity for size recovery after exposure to high shear. Analysis of particle size distribution demonstrated that large scale fragmentation followed by erosion dominated for CN while erosion dominated under SF conditions. Overall, marked differences were observed dependent on the coagulation regime imposed that have implications for improving robustness of cell removal by downstream separation processes. While the cyanobacterium, M. aeruginosa, appeared to share general floc characteristics commonly observed for NOM and kaolin flocs, there were distinct differences in terms of size and strength, which may be attributed to AOM.