Evaluation of stable isotope ratios (δ15N and δ18O) of nitrate in advanced sewage treatment processes: Isotopic signature in four process types

doi:10.1016/j.scitotenv.2020.144120

Science of The Total Environment

Volume 762, 25 March 2021, 144120

https://doi.org/10.1016/j.scitotenv.2020.144120 Get rights and content

Highlights

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Isotopic ratios of NO₃⁻ were determined in municipal wastewater treatment processes.
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Spatial change in stable isotope ratios was found within treatment steps.
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Isotope signatures rely on treatment types and steps.
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Treatment performance and characteristics may influence isotopic fluctuation.

Abstract

Stable isotope ratios of nitrate are a powerful tool to evaluate aquatic environment stress from treated and untreated sewage. However, there is generally a lack of knowledge on the change in stable isotope ratios within wastewater treatment plants. We investigated nitrogen and oxygen stable isotope ratios (δ¹⁵N and δ¹⁸O) of nitrate in four types of advanced treatment processes operated in parallel; (A) extended aeration activated sludge, (B) anaerobic-anoxic-aerobic (A²O), (C) recycled nitrification-denitrification, and (D) modified Bardenpho. The results exhibited spatial variation of δ¹⁵N and δ¹⁸O for nitrate within the treatment steps. The changes in δ¹⁵N and δ¹⁸O may result from the reactor conditions (aerobic, anoxic, and anaerobic) and the order of these processes. As decreasing nitrate concentration in the anoxic stages, the δ¹⁵N/δ¹⁸O ratio for nitrate increased at a rate of 1.3 to 1.6 coupling with the reduction in the nitrate concentration in the anoxic stages. The δ¹⁵N and δ¹⁸O signatures were attributed to process performance in regard to nitrogen removal. In particular, the modified Bardenpho process has higher nitrogen removal efficiency over other processes, producing effluent with lower nitrate concentration and higher stable isotopes (δ¹⁵N: 23.6 to 25.5‰, δ¹⁸O: 2.8 to 4.5‰). We concluded that the stable isotope signatures mirrored the treatment efficiency and effluent characteristics.

Graphical abstract

Introduction

To protect the aquatic environment from eutrophication, a survey on the contribution of pollution sources (i.e., atmospheric, fertilizers, soil, and sewage) is crucially important in water monitoring and assessment. Stable isotope techniques are useful to provide insights into the effects of anthropogenic disturbances on aquatic ecosystem functions (Soto et al., 2019; Kendall et al., 2010). Locations with elevated pollution levels usually have distinctive isotopic compositions that are suggestive of pollution sources (Gaston and Suthers, 2004; Risk and Erdmann, 2000). Stable isotope approaches using N and O of nitrate reveal nitrogen sources in water environment (Archana et al., 2018; Yi et al., 2017; Soto et al., 2019). In addition, discharge from sewage treatment plants significantly increase δ¹⁵N values in coral reefs (Lachs et al., 2019) and even in the whole food web, from phytoplankton to piscivorous fish (Hansson et al., 1997).

The treatment process in wastewater treatment plants (WWTPs) was selected from a variety of process types and it plays an important role in eliminating pollution loading from domestic pollution sources. The advanced sewage treatment (i.e., tertiary treatment) process, used for eliminating nutrient as well as organic substances, was selected to protect the aquatic environment from eutrophication (Metcalf and Eddy, 2013). Due to the multiple treatment process types, the effect of process types and performance on stable isotope signatures within sewage treatment plants are important to determine aquatic environment disturbance by treated sewage.

Despite this, there is generally a lack of focus on treatment types and process characteristics effects on isotopic fluctuation (Archana et al., 2016; Gaston and Suthers, 2004; Munksgaard et al., 2017; Sebilo et al., 2006). Moreover, little information is presently available about isotopic signatures within wastewater treatment system, only a few report on activated sludge (Kanazawa and Urushigawa, 2007) and constructed wetland (Chen et al., 2014; Reinhardt et al., 2006). Previous study had determined isotopic fluctuation during sewage treatment using model and pilot-scale reactors, showing trophic transfer throughout the microbial community (Onodera et al., 2015, Onodera et al., 2018); but the isotopic signatures in a large-scale WWTPs has not been studied. An interesting study shows a difference in stable isotope ratios in the different types of the treatment systems: Preliminary, Primary, CEPT (Chemically Enhanced Primary Treatment), Secondary, and Tertiary (Archana et al., 2016). However, to our best knowledge, isotopic signatures in different biological process types and biological treatment steps, including aerobic, anoxic, anaerobic conditions, has not been evaluated even though biological functions such as predation, organic degradation, nitrification and denitrification are key for isotopic fluctuation.

In this study, we show how stable isotopes of nitrate change during the treatment process in large-scale WWTPs. We focus on the comparison of isotopic fluctuation in four types of advanced treatment process: (A) extended activated sludge process, (B) anaerobic-anoxic-aerobic (A²O) process, (C) recycled nitrification-denitrification process, and (D) modified Bardenpho process. In addition, the relationship between treatment characteristics and isotopic signature (δ¹⁵N and δ¹⁸O) of nitrate was also determined. These treatment processes were operated in parallel in the WWTP, and fed with same influent wastewater, so that the experiment site was suitable for the accurate comparison between the different processes.

Section snippets

Advanced sewage treatment processes

A schematic diagram of four types of advanced treatment processes operated in a WWTP is shown in Fig. 1. The processes were extended activated sludge process (process A), anaerobic-anoxic-aerobic (A²O) process (process B), recycled nitrification-denitrification process (process C), and modified Bardenpho process (process D) (Metcalf and Eddy, 2013). The rapid filtration process and disinfection process were applied before final effluent discharge. Carbon source (e.g. methanol) is not used for

Water quality in treatment processes

Basic water quality taken by on-site measurement is summarized in Table 1. The influent wastewater was divided into four types of treatment process in WWTP. Thus, each process type was fed with the same influent wastewater. Temperature of influent wastewater changed during a sampling period of one year; 23.8 °C on October, 18.6 °C on March, 23.7 °C on June, and 26.6 °C on August. The temperature increased slightly from influent to final discharging in the range of 0.8 to 1.4 °C. The pH was

Relationship between final effluent and stable isotope ratios (δ¹⁵N and δ¹⁸O)

Isotope signatures (δ¹⁵N and δ¹⁸O) for nitrate in effluent from biological treatment differed between each type of process (Fig. 3). The difference in δ¹⁵N and δ¹⁸O of NO₃⁻ can be considered as a result of the effect of treatment characteristics, particularly removal efficiency. The large shift was found especially while nitrate concentration was very low. After that, the isotopic shift was cancelled out because nitrate concentration was higher and the isotope ratio became more robust. To focus

Conclusion

Stable isotope ratios (δ¹⁵N and δ¹⁸O) of nitrate were investigated in four types of advanced treatment processes operated in parallel in large-scale WWTPs. Spatial variation of δ¹⁵N and δ¹⁸O for nitrate within the treatment steps were found. The δ¹⁵N/δ¹⁸O ratio for nitrate increased at a rate of 1.3 to 1.6 coupling with the reduction in the nitrate concentration in the anoxic stages. The δ¹⁵N and δ¹⁸O signatures were attributed to process performance in regard to nitrogen removal. We concluded

CRediT authorship contribution statement

Takashi Onodera: Conceptualization, Validation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing, Visualization. Kazuhiro Komatsu: Investigation, Resources, Writing – review & editing. Ayato Kohzu: Methodology, Validation, Formal analysis, Resources, Writing – review & editing. Gen Kanaya: Methodology, Validation, Resources, Writing – review & editing. Motoyuki Mizuochi: Investigation, Project administration. Kazuaki Syutsubo: Supervision, 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.

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Evaluation of stable isotope ratios (δ15N and δ18O) of nitrate in advanced sewage treatment processes: Isotopic signature in four process types

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Advanced sewage treatment processes

Water quality in treatment processes

Relationship between final effluent and stable isotope ratios (δ15N and δ18O)

Conclusion

CRediT authorship contribution statement

Declaration of competing interest

Mar. Pollut. Bull.

Water Res.

Water Res.

J. Exp. Mar. Biol. Ecol.

Chem. Geol.

Water Res.

Mar. Pollut. Bull.

Geochim. Cosmochim. Acta

Estuar. Coast. Shelf Sci.

Water Res.

Water Res.

Sci. Total Environ.

Mar. Pollut. Bull.

Science of the Total Environment, The

Mar. Environ. Res.

Evaluation of stable isotope ratios (δ¹⁵N and δ¹⁸O) of nitrate in advanced sewage treatment processes: Isotopic signature in four process types

Relationship between final effluent and stable isotope ratios (δ¹⁵N and δ¹⁸O)