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CHAPTER 1
A guide book to studying the fate and effect of hazardous pollutants in biological treatment systems
Ferhan Çeçen
1.1 HAZARDOUS POLLUTANTS AND BIOLOGICAL REMOVAL
The present book deals specifically with the occurrence, effects, removal and monitoring of various types of hazardous pollutants during biological treatment. In that regard, the main focus is on the biological units of drinking water and wastewater treatment plants.
This introductory chapter provides an overview of all of the topics covered within the scope of the book. This chapter also presents the rationale behind the selection of topics and delineates in what aspects the book differs from others. While providing a brief look at the content of each chapter, also suggestions are made for the reader on how to best use the book.
As will be seen throughout this book, hazardous pollutants can be defined in many ways. Commonly, the term "hazardous pollutant" refers to a substance that is likely to threaten the health of humans and the environment upon its release to the environment. Hazardous pollutants may stem from both point- and non-point sources. However, point sources are the primary pathways by which hazardous compounds enter the water environment. Among point sources, the most important one is the discharges made from wastewater treatment plants (WWTPs) (Sathyamoorthy et al. 2013). To avoid the entrance of hazardous pollutants into the water environment, consisting of seas, oceans, lakes, and streams, wastewaters should be adequately treated in WWTPs. Another option to control the release of hazardous substances into the water environment is prevention of pollutant formation, an even more difficult task to accomplish. Since hazardous pollutants reach water bodies, many of them are inevitably found in waters that are extracted for drinking purposes. Thus, the presence of hazardous pollutants is also of concern in drinking water treatment.
1.1.1 Emerging need to control hazardous pollutants
At the beginning of water pollution control, the key issue in wastewater treatment was merely the removal of bulk organic matter and disinfection of treated effluent. Later, nutrient removal too became an important component of biological wastewater treatment. Beginning from the 1970s, hazardous pollutants began to receive attention both in the natural water environment as well as in water and wastewater treatment works. The underlying reason for this was the production of thousands of synthetic organic chemicals (SOCs). A large number of those pollutants were recognized, or at least believed, to have hazardous effects in animals and in humans, such as carcinogenicity and/or mutagenicity. In addition, hazardous inorganic pollutants too have begun to receive attention in recent decades.
Hazardous pollutants can be eliminated or transformed in many ways. However, among all processes the biological ones deserve special attention. Biological processes come into play both in the natural as well the engineered water environment, namely in drinking water and wastewater treatment systems. In both cases biotransformation or biodegradation of hazardous pollutants is possible. If full mineralization (ultimate biodegradation) does not take place, the stable products of transformation can even be more persistent and/or toxic than the parent compounds (Rucker & Kummerer, 2012). Therefore, today not only the hazardous pollutants in their original form but also their transformation products are studied in biological treatment systems. They are believed to pose risks for ecological systems and human health. While the effects of parent compounds are relatively better understood, there is still a lack of information on generation and effects of transformation products. Particularly, in biological treatment systems it is often difficult to predict and measure transformation products.
Among hazardous pollutants, many organic pollutants such as hormones, pharmaceuticals and personal care products (PPCPs), pesticides, UV filters, fragrances, antimicrobial agents, illicit drugs, brominated flame retardants, industrial products, household products, pesticides, disinfectants and antiseptics, have received special attention in the last few decades (Margot et al. 2015). A recent look at the conferences organized and papers written on the subject, indicates that a significant part of the research on hazardous pollutants centers around the removal of PPCPs from water and wastewater, due to their increased use in recent years. A substantial proportion of PPCPs originate from hospitals, while the rest are discharged domestically (Forrez et al. 2011). PPCPs, as well as many other compounds, cause concern because of their persistent and bioaccumulative properties, and health and ecological impacts. In order to determine such impacts, risks have to be assessed. On the other hand, risk assessment is a comprehensive task involving fields such as chemistry, process technology, toxicology and biology (van Leeuwen & Vermeire, 2007).
Nowadays, a large number of organic hazardous substances enter domestic wastewater treatment plants. These pollutants are often found at extremely low concentrations, namely at the level of a few µg/L or even ng/L. Therefore, they are referred to as micropollutants. If found at those levels a hazardous pollutant may not have an adverse effect on the biological processes going on in a bioreactor. However, if these pollutants are inadequately removed, their effects in the receiving water environment are quite important due to their toxicity, bioaccumulation, longrange transport properties etc. Therefore, in any case, they have to be removed by biological or other means. Also, inorganic hazardous pollutants may affect the treatment systems and then the water environment.
Despite the effects of hazardous pollutants on receiving waters, there is still no consensus about which hazardous substances and discharges need to be regulated and reduced. This arises partly due to the still existing uncertainties about the fate and effects of hazardous pollutants, both in water as well as in wastewater treatment systems, and thereafter in the natural water environment. In some places, bans have been introduced for the use of some specific organics such as alkylphenol ethoxylates with the aim to protect flora and fauna in receiving waters. Yet, regulations have not been developed for the discharge of micropollutants from WWTPs. In few cases limits have been set for specific organics such as benzene which serves as a surrogate parameter for other organic micropollutants. It is believed that upcoming regulations will be increasingly stricter regarding the release of such pollutants (Clouzot et al. 2012).
Similarly, in drinking water treatment systems, hazardous pollutants are encountered at very reduced concentrations, at even lower levels than in wastewater treatment. The hazardous pollutants found in the raw water flowing into a drinking water plant can stem from point- or non-point sources. In addition, beginning from the 1970s it was also recognized that the common practices applied in the treatment of drinking water, such as chlorination and ozonation, could lead to generation of a number of hazardous organic and inorganic pollutants. While wastewater discharges indirectly affect human health, the presence of hazardous pollutants in a finished drinking water may have direct effects.
1.1.2 Challenges in removal of hazardous pollutants
Different treatment options are available for the removal of hazardous pollutants. In engineered systems biological treatment is hardly ever applied alone, it often finds use in combination with physicochemical treatment.
Compared to biological processes, physicochemical processes often transfer the pollutant from one phase to another. Biological processes are relatively cheaper. In addition, they lead to complete destruction of hazardous pollutants whenever they can be effectively applied. Thus, the complete disappearance of many hazardous pollutants would largely depend on the effectiveness of biological treatment units. However, in contrast to physicochemical processes that are more straightforward to apply and whose outcomes are more predictable, in biological treatment there are still many unresolved issues regarding the effects, removal and fate of hazardous pollutants.
Determining the appropriate regime for the removal of hazardous pollutants from drinking water and wastewater continues to be a challenge. Although in both cases hazardous pollutants often exist as micropollutants, their removal and effects are quite problematic. In fact, most of the problems encountered in biological removal of such pollutants arise due to their extremely low concentrations and not the opposite.
With the increased use of hazardous substances and their discharge into WWTPs, there is a great need today for optimum removal in WWTPs. Yet, biological units of conventional treatment plants were initially designed to remove organic carbon; then they were expanded to include nutrient removal. But, they did not particularly aim to remove hazardous pollutants. Consequently, hazardous pollutants are mostly poorly removed in existing WWTPs and a significant number of them are released into the water environment with the effluent. In addition, most of them are simply transferred from liquid to solid phase by sorption onto sludge.
Many laboratory-scale studies are conducted to assess the fate and effect of hazardous pollutants in biological treatment. However, in most research studies hazardous pollutants are examined as individual compounds. In reality, in full-scale treatment, hazardous pollutants exist in a matrix composed of hazardous as well as non-hazardous organic and inorganic substances. Therefore, laboratory-scale experimenting on single pollutants may not yield adequate results if extrapolated to full scale. Also in the natural water environment, hazardous pollutants do not exist alone. Due to their high numbers, their combined effects are usually measured rather than their concentrations (Clouzot et al. 2012).
1.2 PURPOSE OF THE BOOK
The basic features of this book that distinguish it from others may be summarized as follows:
(a) Targeting engineered systems only: Many studies are available on the fate and transport of hazardous pollutants once released from point- and non-point sources into the water environment. However, this book narrows down the subject and addresses mainly the biological processes taking place in the engineered water environment, and everything that centers around biological treatment. The natural water environment is only mentioned when needed, for example, when origins, occurrence and effects of hazardous pollutants are discussed.
(b) Focus on hazardous pollutants in biological treatment: Another aim of the book is to provide a comprehensive document that entirely focuses on biological water and wastewater treatment alone. Although many books have been written on hazardous pollutants, such a unified document on biological treatment alone did not exist to date. To fill this gap, this book delineates the fundamental characteristics of hazardous pollutants and concentrates on their behavior and effects in biological treatment systems. To achieve this objective, it is divided into successive chapters that follow each other in a logical order, but also can be read alone.
(c) Focus on fundamental principles and processes rather than specific groups: While most books written on hazardous pollutants examine a specific hazardous pollutant, a hazardous pollutant group or a specific biological process alone, the aim of this book is different. In the present book, all subjects related to hazardous pollutants are put into a context where the main focus is on principles that would apply to all kinds of hazardous pollutants. In that respect, pollutants are also not differentiated as macro- or micropollutants, whenever appropriate.
(d) Focus on experimental methods and application: The book aims to cover all aspects of hazardous pollutants as far as biological treatment is of interest. First, it contains basic information on the characteristics, occurrence and origin of hazardous pollutants. In addition, it addresses the fate and effects of hazardous pollutants in biological wastewater and drinking water treatment units (surface and groundwater), both at laboratory- and pilot/full-scale. As also reflected in the title, one of the major goals of this book is to serve as a guide in experimental research. Therefore, details are provided about analysis of various types of hazardous pollutants, experimental methodologies and evaluation of experimental data. Computational methods are also mentioned since for experimenters they provide valuable information on biodegradability and inhibitory properties of hazardous pollutants before conduction of experiments. The book also tries to delineate the link between the biological processes and the microbial ecology of systems containing hazardous pollutants. In that respect, it includes a discussion on the use of molecular microbiology and genetic tools.
1.3 CONTENT OF THE BOOK
Figure 1.1 illustrates the major fields on which the book focuses. Complementary to this figure, the following discussion clarifies the content of each chapter. To provide a quick idea of the content of each chapter, Table 1.1 lists some of the most common concepts in each chapter.
Chapter 1: A guide book to studying the fate and effect of hazardous pollutants in biological treatment systems
The present chapter can be read as a stand-alone chapter that provides an insight into the content and rationale of this book. At the same time, this chapter provides an opportunity for a reader to choose among a number of topics.
Chapter 2: Hazardous pollutants in the water environment
This chapter provides a review on the properties of hazardous pollutants in the water environment. It first addresses the underlying historical development that led to the definition of "hazardous pollutants". The chapter differentiates then between toxicity and exposure hazards of these pollutants. The physicochemical properties of chemicals, such as partitioning between phases and degradability that affect their fate, transport, and exposure and toxicity hazards are also discussed. The chapter also focuses on the tools used in estimation of relevant physicochemical properties including molecular, partitioning and degradability properties. In that respect, a large overview is provided on the computational tools EPI (Estimation Program Interface) Suite, Online Chemical Modeling Environment (OCHEM), and the Organisation for Economic Co-operation and Development (OECD) Toolbox, all based on the Quantitative Structure-Activity Relationships (QSAR) technique.
In the chapter, hazardous pollutants in the water environment are classified according to their origins and occurrence. Also discussed are the pathways to reach WWTPs and the role of WWTPs in the fate and transport of hazardous pollutants in the environment. The pollutant groups discussed are persistent organic pollutants (POPs), including many pesticides and specific groups such as polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDDs/PCDFs), and also new POPs such as some insecticides, and brominated flame retardants like hexabromocyclododecane (HBCD) and polybrominated diphenyl ethers (PBDEs). Also new POPs used in the manufacturing of industrial products are mentioned. Great emphasis is put on the behavior and fate of PPCPs in WWTPs and in the water environment.
The chapter also discusses the regulatory frameworks that deal with hazardous pollutants. The main focus is on the Toxic Substances Control Act of U.S. EPA (Environmental Protection Agency), the Toxic Substances Management Policy of Environment Canada, and the Regulation for Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) of the European Union (EU).
Since there are many hazardous pollutants in the water environment nowadays, there is a need to examine them in special groups. Thus, the chapter discusses the groupings of hazardous pollutants as "priority pollutants", "emerging pollutants", "micropollutants", "macropollutants", and "xenobiotic organic compounds (XOCs)". Also mentioned are the conditions and profiles for chemicals to become planetary boundary threats.
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