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Abstract
The contamination of waterways with algae and cyanobacteria is a challenging problem for water treatment plants in many parts of the world. Dissolved air flotation (DAF) is a popular method for the clarification of algae-laden water. Pre-treatment by coagulation-flocculation (C-F) is necessary for effective flotation; however, as raw water characteristics can vary rapidly during an algae bloom, coagulation can be difficult to optimise. Hence, eliminating the dependence of DAF on C-F is desirable.
An alternative to C-F prior to DAF is the chemical modification of microbubble surfaces generated during flotation. This adaptation has been effective in floating various laboratory cultured algae and cyanobacteria cells; however, research has been limited to laboratory studies using commercially available polymers. The purpose of this work was to synthesise purpose-designed cationic polymers to closely adhere to a microbubble surface and test the use of these modified microbubbles in DAF at both at bench- and pilot-scale.
A collection of hydrophobically-associating cationic polymers was synthesised. Selected polymers were then used to modify microbubble surfaces in bench-scale flotation experiments to treat two strains of Microcystis aeruginosa without pretreatment with C-F. Results obtained using the synthesised polymers were compared with a commercially available flocculant aid. Greater than 95% cell removal was obtained for one strain of M. aeruginosa with all polymers tested. Polymer performance was differentiated via charge residuals, where more negative zeta potentials indicated lower polymer residual, suggesting stronger bubble adherence. While the removal efficiency achieved for the second strain was only 37%, this was improved to >95% by adding extracellular organic matter extracted from the original strain. Subsequently, polyDADMAC and the best-performing synthesised polymer were trialled using a purpose built DAF pilot plant where they were used to treat algal blooms in a waste stabilisation pond. It was found that the use of polyDADMAC-modified bubbles resulted in contaminant removal, similar to that of conventional DAF. However, 46 % lower removal efficiencies were obtained for the synthesised polymer, contrasting with bench-scale results of 98% turbidity removal. For the first time, modified-bubble flotation has been demonstrated in field experiments, proving to be an alternative for conventional DAF.