Fate of antibiotic resistance genes in mesophilic and thermophilic anaerobic digestion of chemically enhanced primary treatment (CEPT) sludge
Graphical abstract
Introduction
In most full-scale WWTPs, a combination of physical, chemical and biological processes has been employed to reduce organic and inorganic pollutants such as solids, nitrogen and phosphorus (Li et al., 2017, Wan et al., 2016, Wang et al., 2009). In general, physical processes (e.g., sedimentation, flotation and filtration) have been used for primary treatment prior to biological processes. Among the various primary treatment options, CEPT using hydrolyzed coagulants such as alum and ferric chloride (FeCl3) is widely used in WWTPs (De Feo et al., 2008, Wang et al., 2009). Compared to conventional sedimentation, enhanced removal of pollutants can be achieved via CEPT (Li et al., 2017). Additionally, CEPT has various advantages such as: (1) low energy consumption, (2) easy operation and maintenance and (3) less expensive (De Feo et al., 2008, Jiang and Graham, 1998, Jordπo and Volschan, 2004). However, addition of coagulants in CEPT inevitably leads to an increase in mass of primary sludge (i.e., CEPT sludge) due to enhanced removal of solids (Wan et al., 2016). Thus, it is essential to further process CEPT sludge.
AD is considered one of the practical processes to reduce quantities of sludge prior to the final stabilization processes (e.g., landfill, incineration and other applications) (Ju et al., 2016, Li et al., 2011). Considering the characteristics of CEPT sludge (e.g., organic rich and easily biodegradable), AD may represent a feasible option for treatment of CEPT sludge with high energy recovery. Furthermore, the presence of metal ions derived from coagulants, particularly the ferric ion, has been shown to enhance reactor performance of AD (Ju et al., 2016). Hao et al., 2017, Park and Novak, 2013 reported that both enhancement of VS removal (i.e., VSR) and methane production could be achieved by the addition of iron during AD.
Previous studies have revealed that the WWTP is one of the largest reservoirs of ARGs encoding tetracyclines, sulfonamides, quinolones, β-lactam antibiotics, macrolides and multidrug resistance and plays a significant role in the proliferation of potential risks to the environment (LaPara et al., 2011, Lee et al., 2017, Ma et al., 2011). Therefore, the fate of ARGs in AD has recently received considerable attention with the removal of organic matter being a priority for AD (Diehl and LaPara, 2010, Ma et al., 2011, Sun et al., 2016). Currently, most of the literature reporting the fate of ARGs in AD has focused mainly on the correlation between environmental variables (e.g., temperature, HRT and pre-treatment) and ARGs (Ma et al., 2011, Sun et al., 2016, Tong et al., 2016, Zhang et al., 2015). Additionally, exchange of ARGs between different bacterial cells via mobile genetic elements such as integrons also plays an important role as a proxy for HGT in AD (Diehl and LaPara, 2010, Tian et al., 2016).
Recently, Chen and Zhang, 2013, Li et al., 2017 have suggested that a coagulation process can effectively remove not only organic matter but also ARGs. Although extensive efforts have been dedicated to understanding microbial community shift and reactor performance during AD of CEPT sludge (Ju et al., 2016), no research to date has studied regarding the fate of ARGs during AD of CEPT sludge. Furthermore, the effect of temperature on reactor performance and fate of ARGs during AD of CEPT sludge has not been clarified until now. In our study, AD of CEPT sludge was investigated to understand the fate of ARGs under two different temperatures (i.e., mesophilic (35 °C) and thermophilic (55 °C)). Although alum based coagulants are widely used in WWTPs, recent studies have shown that residual aluminum is harmful to humans (Wang et al., 2009). Also, residual alum can lead to decrease in specific methanogenic activity in AD (Cabirol et al., 2003). Thus, in this study, FeCl3 was used as the coagulant for the CEPT process.
Section snippets
Preparation of CEPT sludge
To mimic the CEPT process, a series of jar tests were carried out using a digital programmable jar tester (SF6 model, Misung Scientific, Korea). All jar tests were conducted in triplicate. The wastewater used in the jar tests was collected from a municipal WWTP (treats 600,000 m3/d of wastewater) in Gwangju, Korea. The coagulant under investigation was FeCl3 with dosages varying from 1.53 to 100 mg/L. For each jar test, 0.5 L of wastewater was rapidly mixed with coagulant at 150 rpm for 1 min,
Reactor performance
According to previous studies (Elsayed et al., 2016, Hao et al., 2017), VSR during AD is considered one of the key parameters to evaluate reactor performance. Table 1 summarizes the VSR efficiencies during AD of non-CEPT and CEPT sludge under two different temperatures. Under mesophilic conditions, AD achieved VSR (%) 37.84 ± 2.42 for non-CEPT and 43.54 ± 0.70 for CEPT sludge. This result is consistent with previous research (Park and Novak, 2013), which reported a positive relationship between the
Conclusions
Under mesophilic conditions, but not thermophilic conditions, CEPT sludge showed a positive effect on methane production. Higher removal of ARGs and intI1 can be achieved under thermophilic conditions than mesophilic conditions. However, there were no significant differences in ARGs profiles between non-CEPT and CEPT sludge. To our best knowledge, no study to date has reported either reactor performance or the fates of ARGs and intI1 in AD of CEPT sludge under mesophilic and thermophilic
Acknowledgement
This research was supported by Public Technology Development Program based on Environmental Policy funded by the Ministry of Environment, South Korea.
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