Removal of tetracycline in nitrification membrane bioreactors with different ammonia loading rates: Performance, metabolic pathway, and key contributors☆
Graphical abstract
Introduction
Tetracyclines (TCs) are one of the most widely used broad-spectrum antibiotics in the livestock and aquaculture industries (Huang et al., 2020). A recent study showed that the average concentrations of TCs in soil and surface water in China were significantly higher than those of quinolones, macrolides, and sulfonamides (Lyu et al., 2020). A large amount of wastewater rich in NH4+-N and TC is generated in the pharmaceutical industry, and the traditional treatment methods have limited removal effect on TC (Zhang et al., 2006). In addition, animal farm wastewater was also characterized by high NH4+-N and TC concentrations (Zhou et al., 2013), which poses a significant risk of antibiotics resistance gene transmission (Chen et al., 2022). Therefore, it is of great importance to develop an efficient, stable, and economical process to control both NH4+-N and antibiotic pollution in response to the urgent need for ammonium-rich antibiotic wastewater treatment.
Biological treatment processes are expected to be an eco-friendly and cost-effective technique for handling antibiotic wastewater (Fischer & Majewsky, 2014), and it has been demonstrated that biodegradation and adsorption are two of the most important removal mechanisms of antibiotics during biological wastewater treatment processes (Yu et al., 2018). An extended sludge retention time (SRT) and a high sludge concentration is generally thought to be beneficial for the removal of both NH4+-N and antibiotics (Kumwimba & Meng, 2019). Membrane bioreactors (MBRs) can achieve complete sludge retention for attaining high sludge concentrations and long SRTs by effective biomass-effluent separation with membrane modules, providing sufficient time for the growth of slow-growing microorganisms like ammonia oxidizing bacteria (AOB) (Shen et al., 2014) and contributing to the development of specialized microbial species capable of decomposing compounds (such as antibiotics) of lower biodegradability (Shao et al., 2019; Shi et al., 2021). The combination of these factors suggests that MBRs may be more advantageous in terms of high NH4+-N antibiotic wastewater treatment.
In addition, a large number of studies have been reported that AOB were capable of improving the degradation of a wide range of toxic or refractory organic pollutants (Joss et al., 2006; Park et al., 2017). For example, Park et al. (2017) performed batch tests using activated sludge from a MBR and demonstrated AOB make a huge contribution to the removal of a variety of micropollutants. Much evidence confirmed the contributions of AOB to the biodegradation of TC, and suggested that ammonia monooxygenase (AMO), a key enzyme of AOB catalyzing the first step of ammonia oxidation to nitrite, played a pivotal role in the co-metabolism of TC (Shi et al., 2011; Wang et al., 2021; Yang et al., 2023). In our previous studies, we found that the abundance of AOB and the ammonia oxidation activity of sludge in MBR increased with the elevation of NH4+-N loading rates (ALRs) within a certain range (Wang et al., 2016; Xu et al., 2022). Thus, the highly activated nitrifying sludge in MBR with high ALRs is expected to play a more significant role in enhancing TC removal. Although MBRs have been widely adopted to treat antibiotic pharmaceutical wastewater (Hou et al., 2016; Xiao et al., 2019), and been confirmed to be able to efficiently remove NH4+-N and TC simultaneously (Sheng et al., 2018), the influent NH4+-N concentration in most studies was generally lower than that of 300 mg/L (Sheng et al., 2018), information about TC removal in MBRs with high ALRs is still very limited. Meanwhile, the presence of high TC concentrations in wastewater may affect the nitrification process. Katipoglu-Yazan et al. (2015) found that 50 mg/L of TC caused the nitrifying bacteria to be phased out, eventually leading to complete inhibition of the nitrification capacity of the sludge. Thus, the effects of high concentrations of TC on the performance of MBR with high ALRs should also be studied.
In this study, three parallel lab-scale nitrification MBRs with different ALRs were operated (named AN50, AN500, and AN1000) with the increasing TC dosage (1 mg/L-50 mg/L). The TC removal efficiencies, removal routes and metabolic pathway in MBRs were determined. The influences of TC, in turn, on the nitrification performances of MBRs were also evaluated. This study demonstrated the potential of MBR in the treatment of wastewater with high concentrations of TC and ammonia nitrogen.
Section snippets
Experiment setup and operating conditions
Three sets of lab-scale MBRs were operated, all with an effective volume of 2 L and a hydraulic retention time of 14 h. The membrane modules were made of PVDF hollow fiber membrane with a pore size of 0.05 μm and a membrane flux of 7.5 L/(m2·h). The influent and effluent of the MBRs were realized by peristaltic pumps. Pressure sensors were installed between the membrane modules and effluent pumps to record the variation in transmembrane pressure. Aeration was performed with blower pumps
Tetracycline removal
The other operating conditions of the three MBRs with different NH4+-N loads were maintained constant, and the TC concentration in the influent was gradually increased in stages, in the order of 1, 5, 20, and 50 mg/L. The TC concentration in the effluent during operation is recorded in Fig. 1(A), and the removal rate statistics are shown in Fig. 1(B). At an influent TC concentration of 1 mg/L, the removal rate of AN50 was close to 90% in the first week and then decreased to about 80%, which may
Conclusions
The removal efficiency of TC by MBRs with different ALRs and the interaction between TC and nitrification sludge were investigated. The results confirmed that three MBRs with different ALRs (AN50, AN500, and AN1000) could achieve efficient simultaneous removal of ammonia and TC when the influent TC concentration increased in a gradient of 1, 5, 20, 50 mg/L. AN500 and AN1000 exhibited higher removal rates than AN50 at low concentrations (≤5 mg/L). In contrast, when raised to 20 and even 50 mg/L,
Credit authors statement
Huaihao Xu: Methodology, Data curation, analysis, preparing original draft; Yuepeng Deng: Methodology, analysis; Mingji Li: Analysis; Kaoming Zhang: Analysis, Data curation; Jie Zou: Data curation; Yunhua Yang: Reviewing and editing; Peng Shi: Reviewing and editing; Yiping Feng: Reviewing and editing; Chun Hu: Resources, reviewing and editing; Zhu Wang: Resources, analysis, Methodology, reviewing and 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.
Acknowledgements
We gratefully acknowledge the generous support provided by NSFC (51608134 and 52070047), Guangzhou city science and technology project (202102010460 and 2023A03J0082), Guangdong natural science foundation (2021A1515011898 and 2023A1515012324), Featured Innovation Project of Guangdong Education Department (2019KTSCX135), State Key Laboratory of Pollution Control and Resource Reuse Foundation (PCRRF19010) and Innovation and Entrepreneurship Training Program for College Students (202211078141).
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This paper has been recommended for acceptance by Yaoyu Zhou.