ReviewA review of emerging organic contaminants (EOCs), antibiotic resistant bacteria (ARB), and antibiotic resistance genes (ARGs) in the environment: Increasing removal with wetlands and reducing environmental impacts
Graphical abstract
Introduction
In recent years there has been growing concern about the release of organic compounds of anthropogenic origin, known as emerging organic contaminants (EOCs), to the environment. These EOCs include a diverse group of thousands of chemical compounds, such as pharmaceuticals and personal care products (PPCPs), pesticides, hormones, surfactants, flame retardants, plasticizers and industrial additives, among others. Metabolites and intermediate degradation products of parent compounds are also included (Farré et al., 2008). The ubiquity of EOCs in the environment poses a threat to many non-target living organisms since they are designed to remain biologically active for long periods. The presence of antibiotics is of special concern due to the development of antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs). These substances are extensively used in both human and veterinary medicine against microbial infections and are excreted from the body of the treated organisms, together with their metabolties, within a few days of consumption. It has been widely demonstrated that conventional sewage treatment plants (STPs) are inefficient in the removal of many PPCPs, including antibiotics, ARBs and ARGs, thus contaminanting the receiving ecosystems with a complex mixture of bioactive agents and bacteria (Cacace et al., 2019, Corno et al., 2019, Manaia et al., 2018).
Once in the environment, antibiotics can lead to the continuous selection for ARB that contain ARGs (Choo, 1994, Costanzo et al., 2005, White et al., 2006: Ávila and García, 2015, Sui et al., 2015, Shiffett and Schubauer-Berigan, 2019). Although ARB are a threat to public health, ARGs are the underlying mechanism of an increasing antibiotic tolerant microbial consortia. In recent years, medical professionals and scientists globally have become concerned over the prevalence of ARGs and ARB that have appeared as the result of over prescription/production of antibiotics. Overuse of antibiotics can range from doctors prescribing them ineffectively to patients for viral infections (Gonzales et al., 1997), patients using other people’s antibiotics or old antibiotics and the use of antibiotics as growth promotors and feed additives in livestock and poultry (Kim and Aga, 2007). In 2019, antibiotic-resistant bacteria and fungus caused more than 2.8 million infections and 35,000 deaths in the United States alone (CDC, 2019). Currently, there are approximately 260 different antibiotics in about 20 different families or classes (Everage et al., 2014). A focus of this paper is to review how constructed and natural weltands can enhance removal of antibiotics.
In this paper, we review the occurrence and impact of EOCs, especially PPCPs, ARB and ARGs in the environment and address the potential of wetlands to remove these compounds from wastewaters. For PPCPs, we focus on antibiotics which are the main cause of ARB and ARGs. There are already several reviews on the occurrence of PPCPs in the environment and removal by constructed wetlands (García et al., 2010; Zhang et al., 2014), but few specifically focused on ARB and ARGs (Liu et al., 2019). The main objective of this paper is to discuss the prevalence of PPCPs, ARB, and ARGs in aquatic ecosystems and the mechanisms of removal using constructed and natural wetlands.
Section snippets
Sources of PPCPs, antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs)
After intake, antibiotics rarely become fully metabolized in the body and thus are partially excreted in their original form (Zhang et al., 2009). Together with their metabolites, they are excreted from the body through urine and feces within a few days of consumption. In rural areas, direct excretion from medicated cattle in animal husbandry facilities is the main entrance route into the environment, together with manure application as fertilization amendments (considering also biosolids from
Antibiotic resistance acquisition
It is important to understand that antibiotic resistance can be present in all bacteria, not solely pathogenic bacteria (Hawkey, 1998). Bacteria often have two distinct types of resistance to antibiotics, intrinsic and acquired resistance. Intrinsic resistance is a naturally occurring trait in the organism, while acquired resistance is the evolution of sensitive bacteria to resistant bacteria (Hawkey, 1998). Organisms most often develop resistance to antibiotics because of spontaneous mutations
PPCPs, ARB and ARGs in the environment and their impacts
The continued use of antibiotics is likely to increase the frequency of antibiotic resistance in the environment (Gillings and Stokes, 2012). For instance, soil samples in the Netherlands were shown to contain up to 15 times more genes-encoding resistance in 2008 when compared to soil samples from 1970 (Knapp et al., 2010). Furthermore, antibiotics can survive for extended periods of time in the environment and free DNA carrying ARGs can last up to two years in the soil.
Urban wetlands may
Efficiency of conventional wastewater treatment on PPCPs, ARB and ARGs removal
Secondary municipal wastewater treatment using conventional activated sludge (CAS) does not substantially remove PPCPs and removal rates are highly compound specific (Conkle et al., 2008; Ávila and García, 2015, García et al., 2010; Zhang et al., 2014). Removal mechanisms during CAS treatment include microbial degradation and sorption to particulate matter (Conkle et al, 2010). However, sludge can retain significant concentrations of PPCPs that can be released back into the environment after
Removal of EOCs, ARB and ARGs in constructed wetlands
Constructed wetlands (CWs) for wastewater treatment are a state of the art technology with thousands of full-scale applications at a global scale. CWs are being increasingly used for decentralized wastewater treatment due to their simple design and because operation and maintenance costs are typically much lower than conventional systems. Wetlands do not require any chemical addition and their sludge production is negligible (Álvarez et al., 2017, Paing et al., 2015). Efficiency of CWs for the
PPCPs in small-scale domestic (septic) treatment systems
In many areas with a low-density population, such as suburban and semi-rural situations, sewage treatment for a significant proportion of the population is provided by small-scale (septic tanks) and package plant treatment systems. For example, over 85% of the population in some areas of the United States use septic systems (Godfrey et al., 2007) mainly because conventional centralized systems for such populations is very expensive (Matamoros et al., 2009). Coastal Louisiana is an example of an
Reduction of PPCPs, ARB and ARGs in natural wetlands
Natural wetlands can also reduce PPCPs, as well as ARB and ARGs, in treated sewage effluents. Direct discharge to natural wetlands after treatment in a centralized STP can often result in significant removal of these compounds in a relatively small area. Hayward et al. (2018) reported that ARGs generally decreased in tundra wetland ecosystems receiving domestic wastewater, and that removal rates were inversely correlated with HRT. In this study, short HRTs (2 days) produced the highest ARGs
Conclusions
The presence of antibiotics selectively favors ARB and ARGs, so their reduction in wetlands needs to be an area of focus. Research on disinfection methods and the behavior of EOCs, ARB, and ARGs should continue to inform adaptive management of treatment systems to reduce the impact on the receiving environment, ensuring safety for both humans and other organisms. Overall, a well-managed treatment system coupling municipal wastewater treatment systems to wetlands can result in significant
CRediT authorship contribution statement
Joan Garcia: Writing - original draft. María Jesús Garcia-Galan: Writing - original draft. John W. Day: Writing - original draft. Raj Boopathy: Conceptualization, Writing - review & editing. John R. White: Writing - original draft. Scott Wallace: Writing - original draft. Rachael G. Hunter: Writing - original draft.
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