Bioassay-based identification and removal of target and suspect toxicants in municipal wastewater: Impacts of chemical properties and transformation
Graphical Abstract
Introduction
Massive use of pharmaceuticals, personal care products, and cleaning agents in daily life has resulted in tons of chemicals entering aquatic network and ending up in municipal wastewater treatment plants (WWTP) (Han et al., 2020, Han et al., 2020). Municipal wastewater is a complex cocktail that may cause joint toxicity to aquatic organisms (Affek et al., 2018, Cheng et al., 2018, Laquaz et al., 2018). Ignoring joint toxicity would lead to high uncertainty in risk assessment (Kortenkamp, 2021). To effectively mitigate environmental hazards of WWTP effluents, a series of regulations and policies for wastewater discharge control have been enacted, e.g., the Clean Water Act in the United States that defined 65 categories of 129 priority pollutants for control (de Baat et al., 2019). Whole-effluent toxicity (WET) is required in national pollutant discharge elimination system (NPDES) permit program by the U.S. Environmental Protection Agency (https://www.epa.gov/npdes/permit-limits-whole-effluent-toxicity-wet). In many countries, however, regular monitoring of sewage treatment efficiency is still mainly depended on conventional chemical indicators, such as total nitrogen, total phosphorus, chemical oxygen demand (COD), and few priority contaminants (Guo et al., 2021). Biological indicators showing aggregate toxic effects are lack (Pérez-Alvarez et al., 2018, Williams et al., 2019). Consequently, effluent has often been found to contain various contaminants and exhibit profound bioactivity, presenting potential risk to the ecosystem in receiving waterways even when wastewater has been treated using a series of processes (Iloms et al., 2020, Ma et al., 2019, Muhammad et al., 2021, Qiu et al., 2021). Prioritizing pollutants based on bioactivity would be a useful strategy for improving WWTP efficiency, however, it is rather challenging to identify key toxicants among tons of substances in municipal wastewater (Guo et al., 2021, Xu et al., 2021).
Toxicity identification evaluation (TIE) has been developed to identify causative toxicants in mixtures since the 1990s (USEPA, 1991), by integrating chemical and bioactivity analyses. With the advance of WET, TIE has become a promising technique in wastewater toxicity identification, with a special advantage in determining toxicity contributions from metals and ammonia (Adamsson et al., 1998, Ra et al., 2016). TIE is also capable of identifying individual organic contaminants, yet it mainly focuses on target analytes, and thus may ignore toxicity contribution from nontarget contaminants (Daflon et al., 2017, de Melo et al., 2013). Nontarget analysis is especially needed to be incorporated in toxicity identification in this ever-changing world with increasing emerging contaminants (Bay et al., 2018, Lee et al., 2017, Priac et al., 2017).
Municipal wastewater contains a mixture of organic contaminants related to residential use with high uncertainty in chemical types, which makes it difficult to identify responsible toxicants (Koppe et al., 2020). In addition, pollutants experience degradation and transformation products (TPs) are formed during sewage treatment processes, which makes toxicity identification more difficult (Han et al., 2020, Han et al., 2020, Menger et al., 2021, Zind et al., 2021). Therefore, it is critical to perform TIE to recognize principal toxicants in municipal wastewater with an effort on improving toxicity identification of organic contaminants using new analytical techniques, e.g., nontarget analysis. Another challenge in wastewater toxicity identification is the selection of appropriate test organisms in toxicity assessment. Large variations in inter-species sensitivity during wastewater bioassays have been observed. For example, Zhou et al. (2015) used four species (bacteria, algae, crustacean, fish) to evaluate toxicity of the influent and effluent of coking wastewater and found that the crustacean Daphnia magna exhibited the highest sensitivity. Multiple species in toxicity characterization are recommended to track toxicity variations during wastewater toxicity assessments (Shi et al., 2017).
The present study aimed to (1) identify chemical toxicants in the influent of a typical municipal WWTP using the TIE approach; (2) assess removal efficiencies of bioactivity and key toxicants at individual WWTP treatment stages; and (3) evaluate the transformation processes of key toxicants during the whole WWTP treatment process. Two test organisms, i.e., D. magna and zebrafish (Danio rerio) embryos were selected as test organisms in the present TIE due to their wide usage and representativeness in assessing aquatic toxicity to invertebrates and vertebrates, respectively.
Section snippets
Sample collection
A typical WWTP with classical A2/O treatment process and a capacity of 49,000 tons/d of domestic wastewater in Guangzhou, China was selected as a representative WWTP facility in the present study. In addition to the influent and effluent samples, wastewater samples at intermediate treatment stages were also collected, including the anaerobic zone, anoxic zone, oxic zone, secondary clarifiers, and ultraviolet disinfection zone of the WWTP in March, 2021. Water samples (8 L of each) were
Toxicity screening
Acute toxicity of wastewater at each treatment stage to D. magna and zebrafish embryos is shown in Fig. 2. The influent caused 100% death of D. magna within 24 h, indicating high acute mortality. The wastewater at anaerobic and anoxic stages caused significant (p < 0.05) mortality to D. magna, while wastewater at and after aerobic stage exhibited no lethality, indicating the toxicity to D. magna pronouncedly diminished as the wastewater treatment progressed. The influent caused 37 ± 12%
Conclusions
Municipal WWTP influent exhibited extremely high toxicity to D. magna and zebrafish embryo (100% mortality), and the toxicity was pronouncedly reduced after a series of treatment processes, nevertheless considerable lethal and sublethal effects on zebrafish embryos were still observed for the effluent. TIE suggested that organic contaminants, metals, and ammonia jointly caused the toxicity, showing the complexity in toxicity identification of municipal wastewater. Based on quantitative results
CRediT authorship contribution statement
Yuan Liu: Conceptualization, Methodology, Writing − original draft. Faxu Li: Methodology, Writing − review & editing. Huizhen Li: Methodology, Resources, Writing − review & editing. Yujun Tong: Methodology, Writing − review & editing. Weizong Li: Methodology, Writing − review & editing. Jingjing Xiong: Methodology, Writing − review & editing. Jing You: Supervision, Resources, Writing − review & editing.
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.
Acknowledgments
This work was financially supported by the Guangdong Provincial Department of Science and Technology (2019B151502020), the National Natural Science Foundation of China (U1901220 and 41977343), Guangzhou Municipal Science and Technology Bureau (202102010098 and 202103000046), and Innovative Research Team of Department of Education of Guangdong Province (2020KCXTD005). We thank Dr. W. Tyler Mehler for help on manuscript edition.
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