Distribution, fate and risk assessment of antibiotics in sewage treatment plants in Hong Kong, South China

Abstract

Occurrence, removal, consumption and environmental risks of sixteen antibiotics were investigated in several sewage treatment plants (STPs) featuring different treatment levels in Hong Kong, China. Cefalexin, ofloxacin and erythromycin-H₂O were predominant with concentrations of 1020–5640, 142–7900 and 243–4740 ng/L in influent, respectively; their mass loads were comparable to levels reported in urban regions in China and were at the high end of the range reported for western countries. The target antibiotics behaved differently depending on the treatment level employed at the STPs and relatively higher removal efficiencies (> 70%) were observed for cefalexin, cefotaxime, amoxicillin, sulfamethoxazole and chloramphenicol during secondary treatment. ß-lactams were especially susceptible to removal via the activated sludge process while macrolides were recalcitrant (< 20%) in the dissolved phase. Two fluoroquinolones, ofloxacin (4%) and norfloxacin (52%), differed greatly in their removal efficiencies, probably because of disparities in their pK_a values which resulted in different sorption behaviour in sludge. Overall antibiotic consumption in Hong Kong was back-calculated based on influent mass flows and compared with available prescription and usage data. This model was verified by a good approximation of 82% and 141% to the predicted consumption of total ofloxacin, but a less accurate estimate was obtained for erythromycin usage. Risk assessment indicated that algae are susceptible to the environmental concentrations of amoxicillin as well as the mixture of the nine detected antibiotics in receiving surface waters.

Introduction

Pharmaceuticals are a class of emerging environmental contaminants that have been of increasing concern over the last decade. Taking into account their original functions, these compounds are believed to be intrinsically bioactive and bioavailable in the environment, resulting in unclear post-therapeutic effects on non-target organisms. Acute and chronic toxicities have been documented in model aquatic species (Fent et al., 2006), and mixture toxicity of pharmaceuticals with similar modes of action has also been observed in Vibrio fischeri (Backhaus et al., 2000) and the fathead minnow (Pimephales promelas) (Brian et al., 2007).

Sewage treatment plants (STP) are important point sources of pharmaceutical entry into the environment. Upon human administration, the parent compounds, metabolites or conjugates are partially excreted and later discharged at certain levels via STPs because of inadequate removal efficiencies in the plants (Golet et al., 2002, Metcalfe et al., 2004). Consequently, pharmaceuticals are widespread and well documented in various aquatic compartments including municipal sewage (Castiglioni et al., 2006), hospital sewage (Duong et al., 2008), groundwater (Hirsch et al., 1999) and surface water (Kolpin et al., 2002), usually at concentrations in the ng/L to a few ug/L range.

Amongst the broad span of therapeutic classes studied in the environment, antibiotics have received considerable attention due to their extensive human and veterinary utilization and, more importantly, their potential to promote antibiotic resistance (Pontes et al., 2009). To date, antibiotics have been studied mostly in urbanized cities in Europe (Castiglioni et al., 2006, Golet et al., 2002, Göbel et al., 2007, Hirsch et al., 1999, Lindberg et al., 2005, Lindberg et al., 2006, McArdell et al., 2003), North America (Karthikeyan and Meyer, 2006, Metcalfe et al., 2004, Miao et al., 2004) and Australia (Watkinson et al., 2007). Though some studies have previously examined antibiotic contamination in China (Gulkowska et al., 2007, Gulkowska et al., 2008, Peng et al., 2006, Yang et al., 2010), Japan and Vietnam (Managaki et al., 2007), information in the Asia-Pacific region is still lacking; this information is particularly important as many urban areas in this region are developing rapidly but have insufficient sewage treatment facilities.

In recent years, the Pearl River Delta (PRD) region in southern China has become one of the fastest growing economies and most densely populated areas in the world. Hong Kong, one metropolitan area in the PRD, is of special interest to investigate in depth; although it is a well-developed city, it is densely populated and in the process of improving its sewage treatment facilities to meet consumer demand. Daily, about 2.7 million m³ of sewage, of which almost 30% undergoes only preliminary screening (> 6 mm), is discharged (DSD, 2009). In fact, elevated antibiotic levels have been measured in local receiving water bodies compared to those reported in other countries (Gulkowska et al., 2007, Minh et al., 2009). On the other hand, overall annual antibiotic consumption has not officially been reported yet and a collective record system for all practitioners, public and private hospitals as well as over-the-counter antibiotics has not been established to date. A crude approximation of at least 14,219 kg/year, based on systemic prescription of antibiotics listed in the British National Formulary 5.1 was estimated by the Hong Kong Hospital Authority, the principal statutory body for managing hospital affairs, for public hospitals in 2007. However, this value is believed to be underestimated, as the private sector tends to have higher antibiotic prescription rates than public hospitals in Hong Kong (Dickinson and Chan, 2002). In addition to territory-scale surveillance of potential antibiotic sources by medical authorities, analysis of post-therapeutic residual concentrations in sewage after human administration can be an alternative tool to back-calculate antibiotic consumption by taking into account the specific pharmacokinetics of the targeted compounds. Good correlations between consumption rates and residual loads in influent have previously been demonstrated for different therapeutic classes (Heberer and Feldmann, 2005, Lindberg et al., 2005) as well as illicit drugs (Zuccato et al., 2008).

In this study, sixteen antibiotics, including β-lactams, sulfonamides, fluoroquinolones, macrolides, tetracyclines, amphenicols and trimethoprim were analysed in seven Hong Kong STPs employing different treatment methods: preliminary screening, chemically enhanced primary sedimentation and conventional activated sludge treatment. Investigations of β-lactams in STPs are particularly lacking (Andreozzi et al., 2004, Cha et al., 2006, Hirsch et al., 1999), probably due to their instability (Deshpande et al., 2004). Nevertheless, the β-lactams, cefalexin and amoxicillin, are the most commonly prescribed antibiotics in Hong Kong (Dickinson and Chan, 2002) and therefore their spatial occurrence and behavior is of particular interest. The major objectives of this work are therefore to provide a better understanding of antibiotic occurrence and mass inputs into the environment, as well as developing a back-calculation model for tracing overall antibiotic consumption in Hong Kong. Detailed characterization of removal efficiencies of the target antibiotics after different treatments and possible ecological risks due to effluent concentrations were also assessed.