Behavior of tetracycline and macrolide antibiotics in activated sludge process and their subsequent removal during sludge reduction by ozone

Highlights

• TCN, OTC, DOX and AZN were primarily transferred into the sludge via sorption.

• The cations of antibiotics had higher sorption affinity to sludge than other species.

• 86.4–93.6% of antibiotics in sludge can be removed at 102 mg O₃ g⁻¹ MLSS and pH 7.2.

• Alkaline pH favored the elimination of antibiotics during sludge ozonation.

Abstract

Ozonation is a promising means for the reduction of excess sludge in wastewater treatment plants. However, little information is available on the removal of antibiotics during sludge ozonation. Therefore, this study investigated first the behavior of four commonly-used hydrophobic antibiotics, including three tetracyclines (tetracycline, oxytetracycline, and doxycycline) and one macrolide (azithromycin) in activated sludge process and then their removal during sludge reduction by ozone. Results indicate that the studied antibiotics were primarily transferred into the solid phase of activated sludge via sorption, which was a reversible, spontaneous, and exothermic process governed by surface reactions. Sludge ozonation could effectively remove 86.4–93.6% of the antibiotics present in the sludge at an ozone dose of 102 mg per gram of mixed liquor suspended solids and pH 7.2. The removal of studied antibiotics mainly proceeded through desorption and subsequent oxidation. Increasing the initial pH from 5.0 to 9.5 obviously enhanced the antibiotic removal during sludge ozonation. This study demonstrated that the activated sludge process coupled with sludge ozonation can simultaneously reduce excess sludge and eliminate antibiotics.

Introduction

In recent years, antibiotics have been frequently detected in various environmental matrices such as water, soil, sediment, and biota (Gothwal and Shashidhar, 2015). The persistence of antibiotics in ecosystems could influence the evolution of microbial structure, thereby threatening the ecological health (Sharma et al., 2016). Moreover, the selection pressure originating from residual antibiotics on the environmental microbe could encourage the formation and spread of antibiotic-resistant bacteria and genes in a long term (Arias and Murray, 2009; Inyinbor et al., 2018). Antibiotic overuse and subsequent release has attracted the attention of environmental researchers and administrative departments in the world (Zhang et al., 2015). Many antibiotics are transported together with sewage into wastewater treatment plants (WWTPs), which were also an important point source of antibiotic contamination. The discharge of treated wastewater and excess sludge constitutes the release of antibiotics from the WWTPs to the natural environment. In recent years, a wide range of technologies (e.g., oxidation and advanced oxidation, adsorption, and membrane processes) have been applied to the elimination of antibiotics in the treated wastewater and other antibiotic-contaminated water (Benitez et al., 2011; Homem and Santos, 2011; Jiang et al., 2013). However, the elimination of antibiotics present in the sludge has not received sufficient attention.

As the annual production of excess sludge has been and will continue to increase in a foreseeable future, it is difficult to meet the more stringent standards and regulations for sludge disposal and land application relying only on the conventional sludge post-treatment processes (e.g., aerobic digestion, anaerobic digestion, and composting). Furthermore, the removal of many antibiotics (e.g., tetracyclines and macrolides) during sludge post-treatment processes was usually inefficient as a result of their resistance to biodegradation (Lindberg et al., 2006; Semblante et al., 2015), which will exacerbate the potential risk of excess sludge. Therefore, the great interest has been attracted on the in-situ sludge reduction based on various mechanisms (e.g., lysis-cryptic growth, uncoupling metabolism, and maintenance metabolism) (Guo et al., 2013). Among these processes, significant reduction of excess sludge could be achieved in the in-situ sludge ozone-reduction (Chu et al., 2009; Guo et al., 2013; Qiang et al., 2015). Considering the presence of electron-rich moieties (e.g., benzene ring and amino groups) in the molecular structures, many antibiotics could be efficiently degraded by ozone, as confirmed by previous studies performed in water phase (Homem and Santos, 2011; Jiang et al., 2013; Gomes et al., 2017). Thus, the in-situ sludge ozone-reduction process may also have the potential of eliminating many antibiotics present in the sludge. However, to date, little information is available on the elimination of antibiotics during sludge reduction by ozone (Carballa et al., 2007; Oncu and Balcioglu, 2013).

This study aims to elucidate the removal of four commonly-used and frequently-detected antibiotics, including three tetracyclines (doxycycline (DOX), oxytetracycline (OTC), and tetracycline (TCN)) and one macrolide (azithromycin (AZN)) during sludge ozonation. Since the antibiotic-sludge interactions could impact the degradation of antibiotics during ozonation, the behavior of antibiotics in the activated sludge process was first investigated by batch experiments. Thereafter, sludge ozonation experiments were conducted to investigate the removal of studied antibiotics at various ozone doses and initial pH values.