Recent change in nutrient discharge from municipal wastewater in China's coastal cities and implication for nutrient balance in the nearshore waters

https://doi.org/10.1016/j.ecss.2020.106856Get rights and content

Highlights

  • Rapid improvement in wastewater treatment in China’s coastal cities has reduced nutrient discharges to nearshore waters.

  • Higher TP and NH3-N removals than TN in wastewater treatment increased TN/TP and TN/NH3-N ratios in effluent.

  • Continuous increase in TN/TP and TN/NH3-N ratio in total municipal wastewater discharge was observed from 2006 to 2015.

  • Municipal wastewater discharges might influence spatial variations of nutrient ratios in nearshore waters.

Abstract

Municipal wastewater discharge into the adjacent seas is the most common practice of final wastewater disposal in coastal cities worldwide. To mitigate serious water pollution, treatment of municipal wastewater has advanced rapidly in China's coastal cities since 2005. However, because of differential nutrient removal efficiencies in wastewater treatment, ratios between different nutrients in wastewater can change significantly after being treated, which imposes a bottom-up effect for the growth of phytoplankton in receiving waters. In this study, we had characterized temporal changes of TN, TP and NH3–N discharges and their ratios in municipal wastewater in China's coastal cities during 2006–2015. Results showed that in 2015, a total of 296 Gg of TN, 37 Gg of TP, and 256 Gg of NH3–N were discharged into nearshore waters by municipal wastewater. Much higher TP and NH3–N removal efficiencies (82% and 92%, respectively) than TN (63%) in wastewater treatment were observed, which had increased TN/TP and TN/NH3–N mass ratios by about twice and five times, respectively, in the effluents relative to the influents. As the percentage of municipal wastewater being treated increased, significant increases in TN/TP and TN/NH3–N ratios in the total municipal wastewater discharge into nearshore waters had occurred. Simple correlation analysis showed that differences in nutrient ratios in municipal wastewater discharge in coastal cities might be related with the spatial variations of nutrient ratios in nearshore waters. Our results suggest that a long-term strategy for municipal wastewater management in coastal cities should not merely focus on the reduction of nutrient discharge, but also consider more balanced nutrient regimes to maintain aquatic biodiversity in nearshore waters.

Introduction

Intensified human activities have largely increased the nitrogen (N) and phosphorus (P) discharges into the aquatic ecosystems in recent decades (Peñuelas et al., 2012; Liu et al., 2016; Tong et al., 2017a). As a sink for nutrient discharge from the terrestrial ecosystems, rapid changes in human activities have resulted in significant modifications to nutrient concentrations and stoichiometry in estuaries and nearshore waters (Davidson et al., 2014; Tong et al., 2015; Burson et al., 2016; Lu et al., 2018). This alteration can further lead to frequent occurrences of dead zones and harmful algal blooms (HABs) (Diaz and Rosenberg, 2008; Conley et al., 2009; Burson et al., 2016; Breitburg et al., 2018). Since the 1960s, dead zones have spread exponentially in nearshore waters. They have occurred in more than 400 coastal ecosystems worldwide, with a total area of over 245,000 km2 (Diaz and Rosenberg, 2008). At the same time, the vast majority of nearshore waters are also experiencing significant increases in eutrophication and subsequent occurrence of HABs (Davidson et al., 2014; Lu et al., 2018; Xiao et al., 2019). For instance, in China, a total of 1392 HABs events have been recorded in the nearshore waters between 2000 and 2017, with a total area of 2.52 × 105 km2 (Tong et al., 2017b; Zeng et al., 2019).

Two major anthropogenic nutrient sources for nearshore waters are the riverine nutrient input and the wastewater discharge in coastal cities (Paerl et al., 2014; Bryhn et al., 2017; Caron et al., 2017; Lu et al., 2018; Lemley et al., 2019). Due to the large runoff volume, riverine nutrient input has been reported to be an important factor in influencing nutrient dynamics in nearshore waters (Strokal et al., 2017; Wu et al., 2019). However, this influence is usually restricted to the estuaries or nearshore waters which are adjacent to the large rivers (Tong et al., 2015; Best, 2019; Grill et al., 2019). For instance, the two largest Yangtze and Pearl Rivers in China had contributed about 92% and 96% of all the riverine N and P discharges into nearshore waters (Tong et al., 2015, 2017b), while the nutrient contributions by other rivers could almost be neglected (Wu et al., 2019). With the growing population and urbanization process in coastal cities, nutrient discharge by municipal wastewater is becoming a widespread problem particularly in poorly flushed nearshore waters (Martí et al., 2009; Davidson et al., 2014; Caron et al., 2017; Otim et al., 2018). Municipal wastewater discharge into nearshore waters is the most common practice of final wastewater disposal in the coastal cities (Otim et al., 2018). Marine wastewater treatment is usually applied after land-based physical and biological treatment, which can utilize the natural processes to assimilate, disperse and dilute sewages (Xu et al., 2011; Otim et al., 2018). However, with large increases in volumes of discharge, their negative impacts on the nutrient concentrations and stoichiometry are occurring in many coastal ecosystems worldwide, such as the North Sea in Europe (Burson et al., 2016; Caron et al., 2017), the Kavala Gulf in Greece (Sylaios et al., 2005), the Hudson-Raritan Estuary in the USA (O'Shea and Brosnan, 2000), and nearshore waters near Hong Kong in China (Xu et al., 2008).

Shifts of nutrient concentration and stoichiometry can affect the growth of plankton and community structures (Gilbert, 2017; Van Meerssche and Pinckney, 2019), which further affects the overall functioning of the entire food web (Grosse et al., 2017). According to the Liebig's law of the minimum, nutrient in the least supply relative to the requirements for organisms is considered to be the limiting nutrient (Cloern, 1999). Redfield demonstrated that the chemical composition of phytoplankton tends towards an atomic N/P ratio of 16 in nearshore waters (Redfield, 1960; Davidson et al., 2012). However, the understanding about the typical nutrient limitations is constantly being revised due to the alterations of nutrient discharges from the human activities. Nearshore waters were traditionally considered to be N-limited (Blomqvist et al., 2004; Howarth and Marino, 2006). However, recent studies have pointed out that this nutrient regime is changing due to unbalanced reduction of nutrient in wastewater discharge (Xu et al., 2011; Burson et al., 2016). Compared with N, anthropogenic P discharge can be more easily controlled by mitigation measures (Burson et al., 2016; Tong et al., 2018). Change in nutrient discharges can cause a resultant increase of N/P ratios in coastal waters, thereby shifting from being N-limited to being P-limited (Xu et al., 2011; Burson et al., 2016). The elevated N/P ratio could favour some typical algal species with stronger competitive abilities for using P such as toxin-producing dinoflagellates and nanoflagellates, and suppress the colonial Phaeocystis blooms (Cao and Wang, 2012). Change in nutrient regime can finally affect the normal functioning of the entire marine food webs (Sterner and Elser, 2002; Peñuelas et al., 2012; Van de Waal et al., 2014).

China has experienced a large-scale and rapid improvement in wastewater treatment in coastal cities since 2005. Despite the far-ranging and potential consequences of nutrient imbalance for coastal ecosystems, previous studies have not closely documented the temporal changes of nutrient ratios in municipal wastewater discharge with shifting implementation of wastewater treatment plants (WWTPs). In this study, we had compiled an influent and effluent nutrient dataset from 398 WWTPs distributed in China's 52 coastal cities. Together with information on the volumes of municipal wastewater discharges, temporal changes of nutrient discharges and their ratios in the municipal wastewater were characterized during 2005–2016. These results can provide valuable information for understanding the potential consequences about large-scale practices of wastewater treatment in coastal cities on coastal ecosystems from the perspective of nutrient stoichiometry.

Section snippets

Data source

In this study, the volumes of treated and untreated municipal wastewater in each year during 2006–2015 in China's coastal cities were obtained from the Ministry of Housing and Urban-Rural Development, China. The information about TN, TP and NH3–N concentrations in the influents and effluents of 398 WWTPs distributed in China's 52 coastal cities was obtained from the previous national-level investigation performed by our group (Qi et al., 2020; Tong et al., 2020). According to the adjacent seas

Results

With growing population and urbanization process in the coastal cities in China (MEE, China, 2015; Liu and Su, 2017), the volume of municipal wastewater discharges into nearshore waters had increased significantly from 2006 to 2015 (Fig. 2A). On the national scale, the volume of municipal wastewater discharge into the nearshore waters was ~8 × 109 m3 in 2006, and this volume had increased by over 30% to ~11 × 109 m3 in 2015. Due to improvement in wastewater collection and treatment, the volume

Discussion

Nutrient discharge through municipal wastewater in coastal cities is becoming a widespread problem in estuaries and nearshore waters worldwide (Xu et al., 2011; Davidson et al., 2014; Paerl et al., 2018). Changes in nutrient discharges can induce the consequent shifts of nutrient concentrations and stoichiometry in the receiving waters, which could further impose a bottom-up effect for the growth of phytoplankton (Sterner and Elser, 2002; Xu et al., 2011; Burson et al., 2016; Tong et al., 2020

Conclusions

Our results suggest that due to differential removal efficiencies for nutrients, ratios between different nutrients in municipal wastewater discharge in the coastal cities are shifting consistently. This change might be an underestimated factor in influencing nutrient dynamics in nearshore waters. For majority of the nearshore water sampling sites, DIN/PO43− and DIN/NH4+-N mass ratios have far exceeded the reference ratios that are necessary to sustain a healthy coastal ecosystem. Therefore, we

CRediT authorship contribution statement

Xiaoyan Zhang: Conceptualization, Data curation, Formal analysis. Miao Qi: Investigation, Data curation, Formal analysis. Long Chen: Conceptualization, Data curation. Tianyu Wu: Investigation, Formal analysis. Wei Zhang: Formal analysis, Writing - original draft. Xuejun Wang: Conceptualization, Writing - original draft. Yindong Tong: Conceptualization, Formal analysis, Writing - original draft, 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.

Acknowledgement

This study is funded by the National Key Research and Development Program, China (No.2018YFA0903000) and the National Natural Science Foundation of China (No. 41977324, No. 41630748 and No. 41671492).

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