ReviewA review on the occurrence of micropollutants in the aquatic environment and their fate and removal during wastewater treatment
Introduction
Over the last few decades, the occurrence of micropollutants in the aquatic environment has become a worldwide issue of increasing environmental concern. Micropollutants, also termed as emerging contaminants, consist of a vast and expanding array of anthropogenic as well as natural substances. These include pharmaceuticals, personal care products, steroid hormones, industrial chemicals, pesticides and many other emerging compounds. Micropollutants are commonly present in waters at trace concentrations, ranging from a few ng/L to several μg/L. The ‘low concentration’ and diversity of micropollutants not only complicate the associated detection and analysis procedures but also create challenges for water and wastewater treatment processes.
Current wastewater treatment plants (WWTPs) are not specifically designed to eliminate micropollutants. Thus, many of these micropollutants are able to pass through wastewater treatment processes by virtue of their persistency or/and the continuous introduction. In addition, precautions and monitoring actions for micropollutants have not been well established in most WWTPs (Bolong et al., 2009). Consequently, many of these compounds may end up in the aquatic environment, becoming threats to wildlife and spelling trouble for drinking water industry. The occurrence of micropollutants in the aquatic environment have been frequently associated with a number of negative effects, including short-term and long-term toxicity, endocrine disrupting effects and antibiotic resistance of microorganisms (Fent et al., 2006, Pruden et al., 2006). To date, discharge guidelines and standards do not exist for most micropollutants. Some countries or regions have adopted regulations for a small number of micropollutants. For example, environmental quality standards for a minority of micropollutants (e.g. nonylphenol, bisphenol A, DEHP and diuron) have been stipulated in Directive 2008/105/EC (European Parliament and The Council, 2008). Nonylphenol and nonylphenol ethoxylates have also been recognized as toxic substances by the Canadian government (Canadian Environmental Protection Act, 1999). Other micropollutants, such as pharmaceutical and personal care products (PPCPs) and steroid hormones, are not included in the list of regulated substances yet. To set regulatory limits for micropollutants, further research on biological responses to these compounds (both acute and chronic effects) is of particular importance. Furthermore, scientific community and regulatory agencies should gain insight into not only the impact of individual micropollutants, but also their synergistic, additive, and antagonistic effects.
Several review papers have been published with regard to the occurrence of micropollutants in different water bodies such as wastewater (Deblonde et al., 2011) and groundwater (Lapworth et al., 2012), as well as treatment methods for micropollutant removal (Bolong et al., 2009). In addition, Verlicchi et al. (2012) reviewed the pharmaceutical removal efficiency in conventional activated sludge systems and in MBR fed by municipal wastewater, while Liu et al. (2009) focused on the physical, chemical and biological removal of endocrine disrupting compounds (EDCs). However, no attempt has been made to provide a comprehensive summary of the occurrence of miscellaneous micropollutants in aquatic systems as well as the removal of micropollutants in conventional and advanced treatment processes. In this review, we systematically summarized the recent occurrence of various micropollutants in the aquatic environment and delineated the behavior and removal of micropollutants during conventional as well as advanced wastewater treatment processes.
Section snippets
Occurrence of micropollutants in the aquatic environment
Sources of micropollutants in the environment are diverse and many of these originate from mass-produced materials and commodities. Table 1 summarizes the sources of the major categories of micropollutants in the aquatic environment.
The recent occurrence (2008 to date) of the micropollutants in the aquatic environment has been reviewed in terms of their aqueous concentrations in different types of waters, including wastewater, surface water, groundwater and drinking water. Of all aqueous media,
The removal and fate of micropollutants in WWTPs
Municipal WWTPs are designed to control a wide range of substances, such as particulates, carbonaceous substances, nutrients and pathogens. While these substances can be efficiently and consistently eliminated, the removal of micropollutants is often insufficient. Hence, the evaluation of the fate and removal of micropollutants during wastewater treatment is imperative for the optimization of treatment processes, in order to prevent the release of these potentially harmful micropollutants.
Overview of treatment alternatives for micropollutant removal
No specific treatment is now available to assure the complete removal of various micropollutants due to their diverse properties. Reliable processes that are able to eliminate both bulk substances as well as micropollutants are yet to be developed. An overview of the current treatment options is present in the following sections to reveal the performance of each technique for micropollutant removal and to identify the need for improvement.
Assessment of micropollutant removal from municipal wastewater and recommendations for future research
Micropollutants have been frequently detected in wastewater as well as important drinking water sources, such as rivers, lakes and groundwater. The evaluation of micropollutant removal from municipal sewage should cover a series of issues from sources to end uses, including selection of micropollutants with high occurrence and ecotoxicological relevance, determination of possible sources, investigation on their occurrence and fate in WWTPs and receiving waters, and estimation of their
Conclusion
Enormous research effort has been directed toward the assessment of occurrence of micropollutants in the aquatic environment. In particular cases, the occurrence levels of some micropollutants in surface waters were much higher than their PNECs, which revealed an environmental concern. WWTP effluent has been considered as the primary source of many micropollutants in aquatic systems. Given their diverse properties (e.g., hydrophobicity and biodegradability) and low concentrations,
Conflict of interest
On behalf of all the co-authors, I declare that: (1) we have no financial and personal relationships with other people or organizations that can inappropriately influence our work; and (2) there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “A review on the occurrence of micropollutants and their fate and removal during
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