Elsevier

Minerals Engineering

Volume 72, 1 March 2015, Pages 108-114
Minerals Engineering

Removal of Phormidium sp. by positively charged bubble flotation

https://doi.org/10.1016/j.mineng.2014.12.008Get rights and content

Highlights

  • Phormidium sp. removal was attempted by flocculation and charged bubble flotation.

  • Positively charged bubble flotation enhanced Phormidium sp. removal.

  • We demonstrated the importance of positively charged bubble in the chain cell breakup.

  • Optimal removal was obtained with bubble at a 30% injection rate, a zeta potential >16 mV and bubble formed at 6 bars.

Abstract

The objectives of this study were to investigate the behavior of Phormidium sp. during flocculation and negatively or positively charged bubble flotation in order to optimize algal removal processes and identify mechanisms underlying the efficiency of flotation with positively charged bubbles. The nuisance of Phormidium sp. significantly decreases water quality in natural watershed and clogs filter bed in water treatment plant. Although dissolved air flotation has been recently adopted for algae removal, the best method has not been fully investigated. According to theories on dissolved air flotation, the operational conditions affect removal of the process and in this study, the optimum bubble generations was also investigated for better algal removal. Bubbles were generated at two levels of saturated pressure and measured at different bubble concentrations (10%, 20% and 30%), in the absence and presence of coagulants. Bubbles forming at 6 bars and 3 bars were observed at zeta potentials of −30 mV to + 27 mV. The chain-like algae were cultured in the laboratory for 20 days. At the stationary phase, Phormidium sp. sizes ranged from 2 μm to 10 μm in diameter and about 100–200 μm in length. Over a pH range of 4.0–7.0 (increments of 0.5), the negative zeta potentials were −4 mV to −12 mV. Algal removal by flocculation was determined by jar tests and by the batch dissolved air flotation (BDAF) method with bubble generation and flotation. We obtained optimal Phormidium sp. removal with positively charged bubble flotation at a 30% bubble rate at >16 mV and a bubble formed at 6 bars, with removal of up to 85% and 93% of cells and chlorophyll a, respectively. We also demonstrated the efficacy of using positively charged bubbles to remove Phormidium sp. cells and the importance of positively charged bubbles in the rarely reported interaction between bubbles and chain-like algae.

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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