Removal of filamentous algae by positively charged bubble flotation: a comparative study with spherical/ spherical-like shapes

Abstract

Abstract Removal of filamentous algae by positively charged bubble flotation: a comparative study with spherical/ spherical-like shapes

Bui Thi Thuy Dept. of Civil and Environmental Engineering College of Engineering Seoul National University

The objective of this study was to compare removals of filamentous algae with spherical/ spherical-like algae during charged bubble flotation process, optimize operation conditions for better removal of filamentous algae and observe the change of algal morphology to identify mechanisms underlying the efficiency of flotation with positively charged bubbles. Three selected shapes in this research are spherical (Microcystis sp.), oval (Chlamydomonas angulosa) and filamentous algae (Phormidium sp.), cultured in the laboratory under certain cultivated conditions. Physical properties of three differing algae were understood as morphology, size distribution and zeta potential in various pH. Under pH conditions of 4.5~7, all algae species showed negative charges

-10 mV for filamentous shape, ~ -20 mV for oval shape and ~ -40 mV for spherical shape. The size of these shapes ordered increasingly as spherical < oval < filamentous algae. Removal of differing algal shapes were conducted by the batch dissolved air flotation (BDAF) method with bubble generation from saturated pressure of 6 bars, in the presence of aluminium at 30% of bubble rate. Filamentous algae presented the greatest results (i.e. 86% for cell removal and ~93% for chlorophyll a reduction) when comparing to spherical/ spherical-like algae (i.e. ~75% for cell removal and ~80% for chlorophyll a reduction). For better filamentous algal removal, the optimum operational condition was also investigated. Bubbles were generated at two levels of saturated pressure (3 bars, 6 bars) and measured at different bubble concentrations (10%, 20%, and 30%), in the absence and presence of coagulants. Bubbles averaging 30 µm and 45 µm in size were observed at zeta potentials of -30 mV~+27 mV. We obtained optimal filamentous algal removal with positively charged bubble flotation at a 30% bubble rate at >16 mV and a bubble size of 30 µm, with removal of up to 85% and 93% of cells and chlorophyll a, respectively. Also, by positively charged bubble flotation, only filamentous cell changed its size arrangement and promoted spherically or ovally being to adopt with the stress environment. We also demonstrated the efficacy of using positively charged bubbles to remove filamentous cells and the importance of positively charged bubbles in the rarely reported interaction between bubbles and chain-like algae.

Keywords: Bubble generation conditions, chain-like algae, filamentous algae removal, flotation, positively charged bubble, zeta potential

Student number: 2012-23968