Removal of Phormidium sp. by positively charged bubble flotation
Introduction
Filamentous algae at the base of the water column in fresh watersheds form a periphyton community under high light conditions, consisting of series of cells joined end-to-end, giving them a thread-like appearance (Bellinger and Sigee, 2010). In lakes or ponds, these normally develop into massive surface populations known as “pond moss” or “pond scum,” especially in summer (Bellinger and Sigee, 2010, Canter-Lund and Lund, 1996, Sze, 1998, Paerl et al., 2001). Among the chain-like filamentous algae, Phormidium sp. is the one of the most common species in floating mats on the water surface (Canter-Lund and Lund, 1996, Sze, 1998). This algal nuisance reduces water quality and causes the filter bed to become clogged in conventional water treatment plants (Graham and et al., 1998, Hargenshainer and Watson, 1996, Bauer and et al., 1998). Effective removal of filamentous algae can be achieved by flocculation, negatively charged bubble flotation, and positively charged bubble flotation, of which the latter remains a challenge. Dissolved air flotation (DAF) has been widely used to remove algae due to its improved floatability and applicability in natural systems (Gao et al., 2010, Henderson et al., 2010a, Henderson et al., 2010b, Teixeira et al., 2010, Henderson et al., 2008, Wang et al., 2008). These studies demonstrated that bubble characteristics affect the collision mechanism of particles, and bubbles in turn affect removal efficiencies (Han et al., 2006, Han et al., 2004, Han et al., 2001, Han and Dockko, 1998). Bubble generation under saturated pressure with covalent metal ions can change the zeta potential from negative to positive, thus enhancing removal efficiency (Han et al., 2004). However, for spherical or spheroid algae, maintaining negative to low positive zeta potential is also key to successful algal removal (Henderson et al., 2010a, Henderson et al., 2008, Taki and et al., 2008). Similar sized bubbles increase collisions of the bubble–particle attachment, thus improving removal efficiency (Han et al., 2001). Additionally, the similar size distribution of bubble and particle would result the better removal on flotation process; however, there have been few studies on the collision mechanism of bubbles and filamentous cell aggregates and optimal tools for removal of filamentous algae. We aimed to (1) evaluate Phormidium sp. removal efficiencies by flocculation and flotation (including negative and positive bubbles) and (2) investigate the mechanism by which chains of Phormidium sp. are broken by positively charged bubbles. We optimized bubble formation by determining the ideal concentration (amount of bubbles injected), zeta potential (by adding dosed coagulants), and bubble size (by changing saturating pressures) for cells removal of Phormidium sp. and chlorophyll a reduction.
Section snippets
Phormidium sp. culture and observation properties
Phormidium sp., a representative chain-like algae, was purchased from Korea Marine Microalgae Culture Center (KMMCC #1218, sampled from Upo, South Korea swamp surface water) and cultured in the laboratory. Cells were grown in a 2-L de-ionized water flask at 28 °C in Jaworski’s medium (JM) with a 24-h light conditions (300 lux) and 0% salinity. The culture was shaken at 150 rpm for 20 days. This culture regime was intended to mimic the optimal growth conditions of Phormidium sp. in nature.
We
Characteristics of Phormidium sp.
The morphology of Phormidium sp. under cultivated conditions is provided in Table 2 with an image. Phormidium sp. is a chain-like algae with cells formed in segments, each of which is spheroid. Most of the segments are surrounded by a continuous cuticle to form the filament. These chain-like cells range from 2 to 10 μm in diameter and about 100–200 μm in length (Table 2).
Removal efficiencies
Fig. 1 shows the separation efficiencies of the flocculation and flotation methods, with Phormidium sp. removal expressed in
Effects of the zeta potential of the bubble
Bubble zeta potential changed from negative to positive with the addition of Al3+ (Fig. 5a). Without coagulant, the zeta potential was −30 mV; with 1 mg/L and 5 mg/L Al3+, zeta potentials were +16 mV and +27 mV, respectively. At 0.5 mg/L Al3+, the zeta potential was neutral (see in Fig. 5). The generation of positively charged bubbles was also reported in a previous study (Han et al., 2006). For example, the bubble zeta potential was positive in the presence of 10−2 M Al3+ at pH 2–8.2 with a maximum
Conclusions
In this study, Phormidium sp. removal was attempted by flocculation and by charged bubble flotation. We found that positively charged bubbles provided the most efficient algal removal. We surmise that collision of positive bubbles and filamentous cells occurs via breakup of the chain-like cells and attachment of the detached shorter algae to the bubbles, which then float to the surface. In addition, the Phormidium sp. chains become smaller and spherical-like shaped. Thus, bubble-generating
Acknowledgements
This research was supported by Korea Ministry of Environment as Eco-Innovation Project (413-111-008). This research was also supported by Integrated Research Institute of Construction and Environmental Engineering at Seoul National University.
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