Moderate pre-ozonation coupled with a post-peroxone process remove filamentous cyanobacteria and 2-MIB efficiently: From bench to pilot-scale study

https://doi.org/10.1016/j.jhazmat.2021.127530Get rights and content

Highlights

  • First proposal of moderate pre-ozonation enhanced removal of cyanobacteria and intracellular 2-MIB.

  • A dosage of 0.2 mg/L O3 can enhance coagulation of cyanobacteria with low cell lysis.

  • Pre-ozonation-coagulation-post-peroxone process remove algae and 2-MIB efficiently.

  • First verification with pilot-scale system to remove algae and 2-MIB.

Abstract

The increasing frequency and intensity of taste- and odour-producing cyanobacteria in water sources is a growing global issue. Odour events caused by 2-methylisoborneol (2-MIB) mainly arising from filamentous cyanobacteria have been a very common problem in water supply. Removal rates of filamentous cyanobacteria and 2-MIB by conventional water treatment, such as coagulation, and disinfection treatment processes is low. Hence, a moderate pre-ozonation of cyanobacteria (with little cell damage) was proposed in this study as an enhanced coagulation step to remove filamentous cyanobacteria and intracellular 2-MIB effectively, while avoiding the release of intracellular 2-MIB. A post-peroxone (O3/H2O2) process was applied after sand filtration to degrade the residual dissolved 2-MIB. Results show that moderate pre-ozonation (0.2 mg/L O3 oxidation for 20 min) can substantially enhance the coagulation efficiency for algae, with low cell lysis and high cell viability. Furthermore, 2.0 mg/L O3 combined with 2.0 mg/L H2O2 can degrade the residual dissolved 2-MIB nearly 100% after 20 min reaction. Based on the optimal dosages, a 0.6 m3/h pilot system, including pre-ozonation, coagulation and sedimentation, sand filtration, and post-peroxone processes, was continuously run for 14 days, and it was found that the proposed process can effectively and stably remove filamentous cyanobacteria and 2-MIB.

Introduction

The algae presented in lake and reservoir can cause a series of problems in drinking water treatment (Huisman et al., 2018, Tian et al., 2021). Algae are known to increase coagulant demand, shorten the filter runs, and cause microbial regrowth in distribution systems. Furthermore, their metabolites, such as the unpleasant tastes and odours, will deteriorate the quality of water (Zhang et al., 2016, Sun et al., 2018, Xu et al., 2019). 2-MIB has been regarded as representative of tastes and odours in drinking water and an earthy–musty flavour will be smelled when the concentration of 2-MIB is 4–10 ng/L (Cook et al., 2001). Hence, the odour compounds’ threshold concentration of 2-MIB in potable water is 10 ng/L in China (Xu et al., 2021).

It was reported that 2-MIB can be produced by more than 40 species of cyanobacteria (Izaguirre and Taylor, 2004, Jüttner and Watson, 2007), including the common bloom-forming genera, such as Pseudanabaena, Phormidium and Oscillatoria. According to our previous study, 2-MIB can exist both inside and outside of cyanobacterial cells and the proportion of the 2-MIB found intracellularly is about 60% for Pseudanabaena (Xu et al., 2019). Previous studies reported that the efficiency of extracellular 2-MIB removal by the conventional drinking water purification processes was low (less than 20%) (Srinivasan and Sorial, 2011, Zamyadi et al., 2015a), whilst removal of 2-MIB within intact cells as an intracellular compound was very effective (Li et al., 2019).

Numerous researchers have demonstrated that pre-oxidation can enhance the algal removal efficiency of subsequent coagulation and filtration processes in the water purification plant through decreasing the stability and viability of algal cells mainly (Chen et al., 2009, Xie et al., 2016, Jia et al., 2018); however, it is worth mentioning that algal cells are easy to lyse and a large amount of intracellular organic matter — including that causing tastes and odours — would be released if inapposite oxidants, such as ozone (O3), chloride dioxide, and permanganate, were used (Chen et al., 2009, Zamyadi et al., 2012, Xu et al., 2019). Hence, this should be seriously considered before applying oxidants to pre-oxidise algal cells. A moderate pre-oxidation should be undertaken to improve the removal efficiency of algal cells with little cell damage. However, the studies of moderate pre-oxidation on 2-MIB-producing cyanobacteria such as Pseudanabaena are few (Li et al., 2018). Although Li et al. (2018) investigated the impacts of KMnO4 on Pseudanabaena sp. and found that no significant variations in the Pseudanabaena cell until the exposure reached ct, the effect of moderate KMnO4 pre-oxidation on enhancing Pseudanabaena cell removal by coagulation was not studied. In addition, a variety of species of harmful cyanobacteria may be present in the natural cyanobacterial bloom water. Effects of the oxidant on different species of algae may be different. For example, Wert et al. (2013) studied the effect of O3 on Microcystis aeruginosa, Oscillatoria sp. and Lyngbya sp. and found that their lysis rates were different, and the unicellular Microcystis aeruginosa was more susceptible to oxidation than the filamentous Oscillatoria sp. and Lyngbya sp. However, to the best of our knowledge, the study of moderate oxidation of different cyanobacterial species contained in natural water was not found.

It is very important to efficiently remove 2-MIB in drinking water treatment processes for water-related researchers and practitioners. Although powdered activated carbon (PAC) is the most commonly used to adsorb and eliminate 2-MIB, it is not very efficient and stable (Nerenberg et al., 2000, Andreadakis et al., 2010). Furthermore, the typical oxidants, such as permanganate and chlorine, are ineffective for 2-MIB removal. Hence advanced water treatment processes such as ozonation and ozone-based advanced oxidation process (e.g., O3/H2O2) are required for effective removal of 2-MIB.

It was reported that 2-MIB is not reactive with O3 essentially (kO₃,2-MIB = 0.35 M−1 s−1 (Peter and von Gunten, 2007)), albeit that ozonation can degrade 2-MIB when high doses of O3 are added. This is mainly because O3 can react with natural organic matter in water and produce hydroxyl radicals, which can degrade 2-MIB efficiently (von Sonntag and von Gunten, 2012). Although a high dose of O3 (i.e., >2 mg/L) can enhance 2-MIB abatement, a great deal of bromate may be generated when the contained bromide in water is at a moderate level (i.e., 50 mg/L) (Meunier et al., 2006, Mizuno et al., 2011). To overcome this dilemma, H2O2 can be added during ozonation (i.e., O3/H2O2, the so-called peroxone process) (Koch et al., 1992, Park et al., 2006, Mizuno et al., 2011).

Previous studies of 2-MIB and algal removal through enhanced coagulation involving pre-oxidation were mainly carried out in the laboratory (Cook et al., 2001, Jung et al., 2004, Mizuno et al., 2011, Zoschke et al., 2012), and their removal for practical application in the drinking water purification plants had not been studied extensively. Hence, moderate pre-ozonation enhanced coagulation for algal cell removal with low or no cell damage and post-peroxone degradation of 2-MIB in the filtered water were studied in the present work. A continuous-flow pilot-scale study was undertaken to investigate removal of algae and 2-MIB through a system including pre-ozonation, coagulation, sedimentation, filtration, and post-oxidation combining O3 and H2O2. The influences of pre-ozonation, coagulation, and filtration on different species of cyanobacteria obtained from a natural drinking water source were also explored, along with the relevant mechanisms.

Section snippets

Source water characterisation

Source water for this study was selected as Chengbei Reservoir, which is an important drinking water source in Weifang city, Shandong province, China. Its average depth is 7.3 m. Algal blooms have occurred in the summer and autumn every year in this reservoir recently and the taste & odour problem caused by 2-MIB mainly seriously affects quality of the raw water. The odour-producing cyanobacteria is mainly Pseudanabaena limnetica during the summer and autumn. The main characteristics of the

Enhanced efficiency of algal coagulation with pre-ozonation

The effects of O3 doses on algal cell density and DOC and turbidity of water are shown in Fig. 2. The cell density of total algae in raw water was 1.62 × 107 cells/L, and cell density slightly decreased to 1.50 × 107 cells/L (p < 0.01) and 1.47 × 107 cells/L (p < 0.01) upon oxidation by 0.15 and 0.2 mg/L O3 for 20 min, respectively. However, the density of total algae reduced markedly to 1.16 × 107 cells/L (p < 0.01) and 1.09 × 107 cells/L (p < 0.01) with 0.3 and 0.5 mg/L O3, respectively, when

Conclusions

Moderate pre-ozonation (0.2 mg/L O3 treatment for 20 min) caused low damage to the filamentous cyanobacteria (initial density of 1.62 ×107 cells/L and mainly composed of Pseudanabaena limnetica, Cylindrospermopsis raciborskii, and Leptolyngbya foveolarum) in this study. It can also enhance the coagulation efficiency for algal cells from 43.3% to 80.2%, with low lysis of algal cells, while a great amount of cell-bound 2-MIB was removed. A post-peroxone process with 2.0 mg/L O3 and 2.0 mg/L H2O2

CRediT authorship contribution statement

Hangzhou Xu: Conceptualization, Investigation, Funding acquisition, Writing – original draft. Jing Zhang: Methodology, Data curation, Visualization. Wenjuan Wang: Investigation, Resources, Methodology. Yizhen Li: Investigation, Resources, Formal analysis. Haiyan Pei: Conceptualization, Supervision, Writing – review & editing, Project administration, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Natural Science Foundation of China (51878386, 52000121), and the Natural Science Foundation of Shandong Province (ZR2020QE284). We thank the staff of the drinking water treatment plant in Weifang, Shandong province, for their help. We also acknowledge Dr. David I. Verrelli for revising the English in the manuscript.

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