Elsevier

Water Research

Volume 201, 1 August 2021, 117348
Water Research

Ultrasound-enhanced coagulation for cyanobacterial removal: Effects of ultrasound frequency and energy density on coagulation performance, leakage of intracellular organic matters and toxicity

https://doi.org/10.1016/j.watres.2021.117348Get rights and content

Highlights

  • Low-frequency (LF) ultrasound causes more significant cell activity impairment and cell disruption.

  • High-frequency (HF) ultrasound causes mainly nonviolent and reversible damage to algal cells.

  • HF ultrasound is preferable to LF ultrasound when combining with coagulation on algal removal.

  • Avoiding severe destroy of algal cells is crucial for controlling secondary pollution of treated water.

  • Optimized intensities for 29.4, 470 and 780 kHz ultrasound are ~1.77, 1.77–3.42 and 1.77–3.42 J/mL.

Abstract

Ultrasound-enhanced coagulation is capable of effectively removing algal cells in algae-laden water. However, study differences in ultrasound settings, algal cell conditions and coagulant properties complicate the accurate evaluation of this technique for practical applications. No study has yet compared algae (and algal organic matters) removal among different frequencies of ultrasound in the ultrasound-coagulation process. In this study, the ultrasound at three typical frequencies, 29.4, 470 and 780 kHz, were applied for this purpose. The results showed that high-frequency ultrasound at 470 and 780 kHz had substantially greater improvement of coagulation than low-frequency ultrasound at 29.4 kHz (For example, the turbidity removal at 1 mg-Al/L of polymeric aluminum chloride increased by 204.2%, 571.9% and 563.2% under 29.4, 470 and 780 kHz ultrasound-coagulation, respectively, at 3.42 J/mL). Algal cells exhibited irreversible physical damage and the release of intracellular organic matters (such as odorous compounds) under low-frequency ultrasound with energy densities ≥ 3.42 J/mL, whereas high-frequency ultrasound was characterized by nonviolent impairment, including oxidative degradation and gas vacuole destruction (particularly reversible) resulting from ultrasound-induced radicals and cell resonance, respectively. Avoiding the severe destruction of algal cells is crucial for minimizing the toxicity and secondary pollution of the treated water. To achieve satisfactory removal, protected safety and better economy, the optimal energy density for each frequency was also determined. The findings from the analyses of the laboratory-cultured sample were confirmed via real eutrophic surface water. This study provides new insights and guidance for the ongoing study of harmful algal removal by ultrasound-enhanced coagulation.

Introduction

The eutrophication of surface waters can lead to many undesired issues, among which cyanobacterial blooms are of the most alarming ones and have attracted considerable attention due to their increasing frequency and intensity in recent decades (Rajasekhar et al., 2012). Coagulation has been widely employed in water treatment plants for algal-liquid separation. However, algal cells are organic particles that have negative zeta potential and exhibit considerable stability in water due to their hydrophilic effects, electrostatic repulsion, steric effects and small sizes (Edzwald, 1993; Henderson et al., 2008). A commonly used method to improve the efficiency of coagulation is to add oxidants prior to coagulation (i.e., preoxidation). These oxidants are characterized by strong oxidizability via which negatively charged algal organic matters (AOM) on the cell surface can be effectively degraded, leading to the destabilization and aggregation of algae cells because of the reduced electrostatic repulsion and steric effects among these cells (Qi et al., 2016; Xie et al., 2013). Nonetheless, the addition of oxidants may introduce additional problems involving other pollutants, such as intracellular organic matters (IOM, such as algal toxins and odorous compounds) and corresponding disinfection by-products (DBPs) (Fang et al., 2010; Xie et al., 2013).

Given the above, environmentally friendly and efficient techniques are urgently needed. Ultrasound is one such technique, as only sound energy is involved during treatment. Ultrasonic effects in water include physical/mechanical effects in multiple forms, such as intense shock waves, high shear forces, liquid microjets and resonances, and chemical effects with strong oxidizing substances, such as hydroxyl radicals (Muthupandian, 2011; Rajasekhar et al., 2012). Discussions regarding the effectiveness of ultrasound have dominated research on harmful algae control/removal in recent years (Rajasekhar et al., 2012; Wu et al., 2011). This technique has been widely accepted as a potential alternative for the effective removal of cyanobacteria containing gas vacuoles (Fan et al., 2017a, 2017b), especially Microcystis aeruginosa (M. aeruginosa). Another important trait of ultrasound is its promising performance in collaboration with other techniques for algal removal, such as coagulation (Huang et al., 2020b; Li et al., 2019b) and microbubbles (Broekman et al., 2010). Previous studies have demonstrated a significantly enhanced coagulation removal of algal cells through either low-frequency or high-frequency ultrasonic pretreatment (Heng et al., 2009; Shi et al., 2008; Zhang et al., 2009a), however, the lack of a systematic comparison between low-frequency and high-frequency ultrasound on the coagulation removal of algae is not conducive to the global evaluation of the application of ultrasound in practical engineering. A recent study (Peng et al., 2020b) preliminarily tested the performance of ultrasonic pretreatment at frequencies of 20 (0.038 W/mL), 740 (0.113 W/mL) and 1120 kHz (0.108 W/mL) on the coagulation removal of algal cells and showed that high-frequency ultrasound was preferable to low-frequency ultrasound. However, the intensity settings among these frequencies were inconsistent, and only one polymeric aluminum chloride (PAC) dosage was used in that study (10 mg/L, the exact dosage of Al was not reported). Our previous work (Huang et al., 2020b) investigated the specific mechanisms for the enhanced coagulation removal of cyanobacterial cells by low-frequency ultrasound and found that with the addition of 29.4 kHz ultrasonic pretreatment at intensities of 0.6, 1.11 and 2.22 J/mL, 4 mg-Al/L of polymeric aluminum chloride (PAC) achieved increased removal from 44.1% to 59.7%, 67.0% and 74.9% without the undesired leakage of AOM. In that work, different mechanisms were definitely correlated with certain treatment intensities, including the cell activity reduction (≥ 0.6 J/mL), cell stability impairment (≥ 1.11 J/mL) and cell aggregation (≥ 1.11 J/mL). Li et al. (2019b) investigated the effects of high-frequency ultrasound on enhancing the coagulation removal of cyanobacterial cells and the corresponding mechanisms and showed that the coagulation removal was increased by approximately 7.0%, 10.2%, 8.2% and 6.3% (PAC = 10 mg/L), respectively, upon ultrasonic pretreatment at frequencies of 120, 430, 740 and 1120 kHz and an energy density of 5.4 J/mL. They recommended a mild high-frequency ultrasonic pretreatment (e.g., 740 kHz and 6 J/mL) to avoid the overmuch accumulation of proteins and the excessive production of radicals for a better algae removal effect.

Although the effects of low-frequency and high-frequency ultrasound on the coagulation removal of algal cells and the mechanisms involved have been investigated in different studies, the conclusions cannot be compared directly due to the multifarious ultrasound settings, such as frequency range and intensity; the diversity of algal cell conditions, including source, initial concentration and growth phase; and the dissimilar coagulant properties affected by reagent purity, preparation method and basicity. There is still a lack of studies that compare between low-frequency and high-frequency ultrasound combined with coagulation with respect to algae (and AOM) removal. In addition, no direct proof is available regarding the safety of the ultrasound-coagulation process. Therefore, this study aimed to reveal the differences in the effects and mechanisms of ultrasound on the coagulation removal of algae (and AOM) among a low frequency (29.4 kHz) and two high frequencies (470 and 780 kHz) of ultrasound. Moreover, the toxicity of the treated water and the leakage of intracellular odorous compounds (2-methyl-isoborneol, 2-MIB) were determined to identify the relationships between safety and ultrasound settings for reference. Of note, both the laboratory-cultured algal sample and the field sample were employed, which provides new scientific and engineering insights and guidance for the ongoing study of ultrasound techniques to enhance the coagulation removal of harmful algae.

Section snippets

Materials

Low-frequency and high-frequency ultrasonic systems were obtained from the Institute of Acoustics, Chinese Academy of Sciences (Figure S1 of the supporting information [SI]). The low-frequency transducer employed (29.4 kHz) is a horn-type device and has a titanium alloy CT4 head with a diameter of approximately 2 cm. The two high-frequency transducers are plate-type devices, and the diameter of the transmitting surface is approximately 9 and 7 cm for the 470 and 780 kHz transducers,

Methods

Calorimetry was used to determine the acoustic power (W) entering the experimental solution (Raman and Abbas, 2008). The energy density (J/mL), which is calculated by multiplying the acoustic power per unit volume of medium (W/mL) by treatment duration (s) (Rajasekhar et al., 2012), was used to indicate the intensity of ultrasonic pretreatment. More detailed information can be found in our previous publications (Huang et al., 2020a, 2020b). In this study, the ultrasound energy density was set

Influences of ultrasound settings on the coagulation removal of algal cells and aom

Fig. 1 (top) displays the influence of ultrasonic pretreatment at different frequency ranges on the coagulation removal of M. aeruginosa cells (as indicated by turbidity). Ultrasound effectively enhanced the turbidity removal at low PAC dosages (≤ 4 mg-Al/L in this study). For example, the turbidity removal ratio at 2 mg-Al/L of PAC increased from 56.2% without ultrasound to 94.1%, 95.0% and 95.5% with the assistance of 29.4, 470 and 780 kHz ultrasound, respectively, at an energy density of

Conclusions

In this study, the performance of low-frequency and high-frequency ultrasound in enhancing the coagulation removal of algal cells and the underlying mechanisms were systematically evaluated, and the intensity of ultrasonic pretreatment at different frequency ranges was optimized with respect to removal efficiency, safety and economy. Based on the results, the following key conclusions can be drawn:

Coagulation removal of algal cells at a low coagulant dosage can be substantially enhanced by

Declaration of Competing Interest

We declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in the manuscript entitled, “Ultrasound-enhanced coagulation for cyanobacterial removal: effects of ultrasound frequency and energy density on coagulation performance, leakage of intracellular organic matters and toxicity”.

Acknowledgment

This study was financially supported by the Institute of Water Environment Rehabilitation, Tongji University.

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