Influence of roxithromycin as antibiotic residue on volatile fatty acids recovery in anaerobic fermentation of waste activated sludge
Graphical abstract
Introduction
Anaerobic fermentation is an effective technology for the stabilization and recycling of waste activated sludge (WAS) (Luo et al., 2016). WAS can be converted into value-added products via anaerobic fermentation, such as volatile fatty acids (VFAs), hydrogen and methane (Li et al., 2018). VFAs can be used as a carbon source for biotrophic microorganisms and as a platform block material for the production of biodegradable plastics (Rao et al., 2018). However, the yield of VFAs is usually too low to make the process competitive as a source of VFAs.
WAS not only contains an abundance of organic compounds, but also possesses potentially harmful pollutants such as antibiotics (Jang et al., 2018). In recent years, the fate and toxicity of antibiotics have raised concerns about its environmental and health impact. Antibiotics are usually detected in the influents of wastewater treatment plants (WWTPs) at concentrations ranging from 0.5 μg/L to 8.5 mg/L (Barbosa et al., 2016; Tran et al., 2018). It has been reported that antibiotics such as sulfamethazine and cephalexin can affect the structure of sludge flocs by stimulating the release of EPS (Hu et al., 2018; Lu et al., 2014). Our previous studies have shown that high levels of antibiotics inhibit biological nutrient removal by affecting reductase activity and the transformation of polyhydroxyalkanoates and glycogen (Chen et al., 2019). Since sludge adsorption is one of the main ways to remove antibiotics from wastewater (Ma et al., 2019), several studies have focused on the potential impact of antibiotics on anaerobic fermentation of WAS. These studies indicated that antibiotics can generally promote VFA production, but different antibiotics have been found to exert different effects on each process of fermentation. Earlier studies found that cefalexin can enhance the solubilization of WAS by promoting the release of extracellular polymeric substances (EPS) (Lu et al., 2014). Subsequently, ciprofloxacin was reported to promote VFA accumulation mainly by inhibiting the methanogenesis process (Mai et al., 2018). Recent studies have shown that clarithromycin mainly enhances solubilization and inhibits all other processes, and the mechanism of accelerated solubilization is mainly to promote the destruction of EPS (Huang et al., 2019). Meanwhile, it is reported that drugs similar to antibiotics, such as sulfadiazine, can also positively affect VFA production by enhancing hydrolysis and acidification during the anaerobic fermentation of WAS (Xie et al., 2019). Sulfamethazine can increase VFA production and inhibit methane production by promoting solubilization, acidification, and inhibition of methanogenesis (Hu et al., 2018). Although some attempts have been executed to discuss the effects of antibiotics and other drugs on anaerobic fermentation, no consensus has been reached on the mechanism.
Roxithromycin (ROX) is an emerging macrolide antibiotic derived from erythromycin. It is widely used in human health care to treat respiratory infections and skin and soft tissue infections caused by sensitive bacteria (Zhang et al., 2019a). In recent years, with the widespread use of ROX drugs, residual ROX is often detected in wastewater and sludge. ROX levels in the influents of WWTPs were reported to range from 3.7 μg/L to 0.1 mg/L (Yan et al., 2017; Zhang et al., 2019b). As a highly hydrophobic antibiotic, the removal efficiency of ROX in WWTPs was usually ranged from −190 % to 37 %, and its limited migration and high adsorption capacity of ROX in wastewater systems make it easy to adsorb on WAS (Gao et al., 2012; Le-Minh et al., 2010). With the extensive use of ROX, more ROX may remain in the sludge, which leads to its accumulation in WAS (Ni et al., 2019; Zhao et al., 2019). The concentration of ROX in WAS ranges from 69.6 μg/kg to 0.34 mg/kg (Li et al., 2013; Marx et al., 2015). ROX has been found to function as a bacterium by inhibiting protein synthesis once it enters the cell (Csongradi et al., 2017). Since protein is the main substrate of anaerobic fermentation, the presence of ROX may affect the fermentation process. Until now, previous studies have evaluated the toxicity of ROX in the aquatic environment (Alvarino et al., 2015; Zhang and Li, 2018). ROX was normally considered to have inhibition effect on bacterial ammonia oxidation in WWTPs (Ghosh et al., 2009). Therefore, ROX may have potential effects on anaerobic fermentation of WAS. However, whether and how ROX affects WAS anaerobic fermentation products (especially VFAs) remains scattered and inconclusive.
The purpose of this study was to investigate the effect of ROX on the recovery of VFAs from WAS anaerobic fermentation. First, the effect of different ROX doses on VFA production during anaerobic fermentation was evaluated. Then how ROX affects each process of anaerobic fermentation (i.e. solubilization, hydrolysis, acidification, and methanogenesis) was discussed in detail. Finally, the activities of key enzymes were analyzed to verify the role of ROX in each process. The findings obtained in this study are useful for engineers to pay more attention to the role of antibiotic residues in anaerobic fermentation processes and provide a basis for improving the design and optimization of sludge fermentation systems.
Section snippets
WAS and ROX
The WAS was taken from the secondary sedimentation tank of a municipal WWTP in Xiangtan City, China. Before fermentation, it was filtrated by a sieve (1.0 mm) and concentrated at 4 °C for 24 h. The main characteristics of the WAS were as follows: pH at 6.9 ± 0.1, total suspended solids (TSS) at 64603 ± 479 mg/L, volatile suspended solids (VSS) at 15700 ± 326 mg/L, total chemical oxygen demand (TCOD) at 18140 ± 1118 mg/L, soluble chemical oxygen demand (SCOD) at 216 ± 13 mg/L, total protein at
Effect of ROX on VFA production from anaerobic fermentation
The VFA production from anaerobic fermentation in the reactors was illustrated in Fig. 1. The total VFA accumulation (Fig. 1a) increased slightly with addition of ROX not exceeding 50 mg/L (p > 0.05). The maximum yield of total VFAs in the control was 295 mg COD/L on day 6, which is similar to the results of previous studies (Huang et al., 2019). The maximum VFA concentrations in the 10, 20, and 50 mg/kg TSS ROX reactors increased to 321.6, 410.1, and 386.5 mg COD/L, which were 1.09, 1.39, and
Conclusion
ROX can promote VFA recovery during WAS anaerobic fermentation. The presence of ROX (100 mg/kg TSS) increased the maximum accumulation of VFAs from 295 to 610 mg COD/L. In addition, ROX increased the accumulation of individual VFAs except n-valeric acid. Although ROX was partially biodegraded during WAS fermentation, its contribution to the increase in VFA was negligible. ROX can promote the solubilization and acidification of WAS, while inhibiting the hydrolysis and methanogenesis processes.
CRediT authorship contribution statement
Hongbo Chen: Conceptualization, Funding acquisition, Methodology, Writing - original draft. Xingning Zeng: Investigation, Data curation. Yaoyu Zhou: Resources, Visualization. Xiao Yang: Conceptualization, Writing - review & editing. Su Shiung Lam: Writing - review & editing. Dongbo Wang: Supervision, Project administration.
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 study was financially supported by the National Natural Science Foundation of China (NSFC) (51608464) and the Key Projects of Science and Technology Plan of Hunan Province (2018SK2027). The authors would also like to thank Universiti Malaysia Terengganu under Golden Goose Research Grant Scheme (GGRG) (Vot 55191) for supporting Dr Lam to perform this review project with Hunan Agricultural University.
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