Reducing the Discharge of Micropollutants in the ... - ACS Publications

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Reducing the Discharge of Micropollutants in the Aquatic Environment: The Benefits of Upgrading Wastewater Treatment Plants Rik I. L. Eggen,*,†,‡ Juliane Hollender,†,‡ Adriano Joss,† Michael Schar̈ er,§ and Christian Stamm† †

Eawag, Swiss Federal Institute of Aquatic Science and Technology, CH-8600, Dübendorf, Switzerland Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, CH-8092, Zürich, Switzerland § FOEN, Federal Office for the Environment, Water Division, CH-3003, Bern, Switzerland water bodies at some stage in their lifecycle. For this transfer of chemicals to water bodies, several flow paths need to be considered (Figure 1). Diffuse sources of contaminants include transport from agricultural land and runoff from urban areas where surface waters may be polluted by biocides and other chemicals leaching from building facades, automobile emissions, tire wear and dry and wet atmospheric deposition.3,4 In developed countries with existing sewer systems, wastewater from households and industry is a major source of chemicals entering the aquatic environment despite the treatment taking place in the wastewater treatment plants.5,6 Additionally, untreated wastewater can also be discharged via combined sewer overflow or leaking sewers. In case of a well-maintained Micropollutants (MPs) as individual compounds or in complex urban water management system, the discharged volumes mixtures are relevant for water quality and may trigger amount to about 10% of the total municipal sewage.7 Limited unwanted ecological effects. MPs originate from different MP removal occurs in the sewer and about half of the MPs are point and diffuse sources and enter water bodies via different removed or transformed in state-of-the-art WWTPs,8 and if flow paths. Effluents from conventional wastewater treatment they are not easily photochemically degraded in the surface plants (WWTPs), in which various MPs are not or not waters, these MPs tend to persist in the water body.9 completely removed, is one major source. To improve the As a consequence of these sources and pathways for water quality and avoid potential negative ecological effects by chemicals, numerous compounds can be found, mostly at micropollutants, various measures to reduce the discharge trace concentrations in the μg/L to ng/L range, in freshwater should be taken. In this feature we discuss one of these ecosystems particularly in densely populated regions.10,11 The measures; the benefits of upgrading WWTPs toward reduced chemical pollution of aquatic ecosystems by our global society MP loads and toxicities from wastewater effluents, using the is effectively a long-term experiment. We currently have only recently decided Swiss strategy as an example. Based on (i) fullscant knowledge of the impacts on complex ecosystems by this scale case studies using ozonation or powder activated carbon large diversity of MPs12,13 at low exposure concentrations14 that treatment, showing substantial reduction of MP discharges and form mixtures15 of ever changing compositions.16 One concomitant reduced toxicities, (ii) social and political exception, where ecological impact is rather well established, acceptance, (iii) technical feasibility and sufficient costis the case of endocrine disrupting compounds (EDCs). EDCs effectiveness, the Swiss authorities recently decided to implecan mimic or interfere with natural hormone activities and their ment additional wastewater treatment steps as mitigation toxic potential has been demonstrated with fish exposed in situ, strategy to improve water quality. Since MPs are of growing caught from rivers or studied in whole-lake experiments.17−19 global concern, the concepts and considerations behind the In all cases, the fish showed physiological and anatomical Swiss strategy are explained in this feature, which could be of aberrations that could be attributed to EDCs. Some of these use for other countries as well. It should be realized that aberrations had negative impacts on reproductive success; upgrading WWTPs is not the only solution to reduce the fertilization and survival rates of eggs from fish at polluted sites discharge of MPs entering the environment, but is part of a was significantly reduced.20,21 For EDCs, mixture toxicity has broader, multipronged mitigation strategy. been clearly demonstrated and must be taken into considMICROPOLLUTANTS ARE OF GROWING CONCERN eration in assessing exposure effects.22,23 The contamination of freshwater ecosystems with synthetic Besides estrogenicity, a wide range of existing bioassays, targeting a variety of biological end points in vitro or in situ, chemicals is increasing worldwide.1 As the global population have shown that wastewater contains MPs that may impair increases and economies in many regions grow considerably, aquatic ecosystems through many modes of toxic action.24 production of chemicals is also predicted to increase. Currently, already more than 100 000 compounds in commerce are registered in Europe,2 many of which may get transported into Published: June 10, 2014 ‡

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© 2014 American Chemical Society

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dx.doi.org/10.1021/es500907n | Environ. Sci. Technol. 2014, 48, 7683−7689

Environmental Science & Technology

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Figure 1. Schematic presentation of diffuse and point source entry paths of micropollutants in the environment (adapted and with courtesy from FOEN).

controls, to best management practices and end-of-pipe solutions. Source control may include the promotion of green chemistry that should result in the reduction of emissions of persistant chemicals. However, success can only be expected on the long-term. For selected, critical substances, targeted restrictions may be the most appropriate measure to prevent MPs from entering surface waters as was successful since 2005 for nonylphenol.30 Information campaigns on optimal usage, storage and disposal of chemicals may lead to behavioral changes. However, compound-specific individual regulations alone, or even coupled with changes in consumer behavior, are unlikely to be sufficient to reduce the loading of the many thousands of chemicals that are used in different ways and can enter the water cycle over a variety of pathways. In addition, for human pharmaceuticals restrictions of effective drugs are probably not appropriate due to ethical reasons. Thus, source control must be complemented with other measures, and wastewater treatment has a dominant role to play. Wastewater streams generated by hospitals or industry that contain elevated MP concentrations can be pretreated before being discharged to sewers, or measures (such as separate urine collection in hospitals) can be adopted to reduce the input of MPs into wastewater.31 The catalogue of measures increases if one also includes reducing discharge of pollutants from diffuse sources such as agriculture.32

The diversity of compounds and the related effects suggests that different trophic levels and the interactions among them may be affected by MPs. As an example at a higher level of biological organization, assessment of traits using the SPEAR (SPEcies At Risk) index indicates that pesticides, as a class of MPs, alter the taxonomic composition of macroinvertebrate communities in streams25 and reduce the survival of sensitive species.13 Release of MPs can also result in indirect effects, resulting in complex and unforeseen interactions between MPs, native populations and basic ecosystem functions.26 In addition to the impacts of the continuous discharge of MPs on ecosystems, there may also be consequences for the quality of raw water for human consumption. In many locations in Europe, groundwater is replenished by bank filtration.27 This is especially problematic in densely populated urban areas with tight water cycles, such as the well-known example of the city Berlin.28 Increased public awareness of these issues is often linked with a decreased acceptance of the presence of MPs in freshwater, particularly when human exposure is possible. Potential human health effects are likely to be a stronger driver for (political) action than the ecotoxicological effects of MPs, even though the latter are more consequential. Pathways for human exposure include swimming or other recreational use of surface waters and drinking water consumption, which, in the case of riverbank filtration, often involves no further treatment after groundwater extraction. Recently, this awareness resulted in a political consensus among the countries of the River Rhine basin. At the 15th conference of the Rhine ministers in 2013, an agreement was made toward a significant reduction in the discharge of persistent MPs derived from different sources.29

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LIMITED REMOVAL OF MICROPOLLUTANTS IN CONVENTIONAL WASTEWATER TREATMENT There are substantial differences in the technologies used for wastewater treatment and in the level of treatment achieved in different countries and even within a single country. Nonetheless, wastewater treatment, wherever it is performed, has a common set of objectives (shown schematically in Figure 2): (i) to improve hygienic conditions of the receiving waters by functioning as a barrier for fecal bacteria and pathogens, (ii) to improve the water quality of receiving waters (i.e., removing degradable organic compounds to minimize oxygen depletion in receiving waters), and

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REDUCING MP CONCENTRATIONS IN THE AQUATIC ENVIRONMENT REQUIRES A MULTIPRONGED AND MULTIBARRIER STRATEGY Given the thousands of MPs as well as the diversity of their use and pathways, effectively reducing the discharge of MPs to the aquatic environment requires a combination of complementary measures. These can be very diverse and range from source 7684



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Figure 2. Schematic diagrams of current and future demands and outcomes on wastewater treatment showing the current situation (left panel) in which the loading of degradable organics, pathogens, nutrients and some MPs are reduced and the anticipated future situation (right panel) with increased amount of wastewater and additional treatment with the main aim to improve the MPs removal but with the positive side effect of improved removal of pathogens and color.

on selected municipal WWTP to reduce MP load and toxicity in wastewater. The aim of these measures is to improve the protection of drinking water resources and the water quality in aquatic ecosystems. The measures will be implemented mainly in densely populated regions where wastewater from WWTP discharges constitutes an important impact on water quality. In these regions an efficient wastewater infrastructure is already in place and therefore only needs to be optimized regarding MP elimination. This national strategy for MP elimination in wastewater treatment··· Has a Sound Scientific and Technical Basis. Much of the scientific basis for the strategy on wastewater treatment was gained through the Swiss National Research Program (NRP) 50, “Endocrine Disruptors: Relevance to Humans, Animals and Ecosystems” (www.nrp50.ch), which was conducted between 2002 and 2007. NRP50-funded projects identified and confirmed that effluents from WWTPs are important entry paths for MPs in general and EDCs in particular into aquatic ecosystems. From 2006 to 2010, the multiproject “Strategy MicroPoll” aimed at quantifying loads and toxicities of MPs from WWTP effluents, to prioritize both specific MPs and catchments, and to assess options for WWTPs to improve removal of MPs.34 The results35 are supported by studies of the international community in this field.36,37 Is Based on Broad Societal and Political Acceptance. Reports in the Swiss media on the occurrence of pharmaceuticals in streams and drinking water raised public awareness and

(iii) to remove the nutrients nitrogen and phosphorus that are responsible for the eutrophication of aquatic ecosystems By achieving these objectives, most existing water quality standards are met and therefore MPs are often not seen as a problem. Conventional wastewater treatment is protective of recreational and bathing water, it tackles major threats to aquatic biodiversity and to ecosystem function by preventing for example oxygen depletion, and, furthermore, reduces the requirements for drinking water treatment when water supply intakes are downstream of WWTPs. Nevertheless, the increasing use of chemicals along with growing populations and increasing urbanization pose new challenges to wastewater treatment that are not met by conventional WWTPs. In conventional WWTPs, approximately half of the MP load is eliminated either by sorption to sludge or by degradation.8 However, many hydrophilic compounds do not sorb to the sludge and are either persistent over the retention time in the plant or are transformed into mostly unknown products. Thus, MPs or their transformation products are continuously discharged to the receiving waters,33 resulting in the chronic exposure of aquatic organisms.

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THE SWISS NATIONAL STRATEGY FOR UPGRADING SELECTED WWTPS With the adaptation of the water protection act in March 2014, the Swiss government decided to implement technical measures 7685



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Is Adaptable in Time. The upgrade at 100 WWTPs will be realized over the next 20 years. This allows time to optimize MP removal technologies or to select other technologies as they become available. In addition, implementation over this time period allows the optimization of investments by combining the upgrades with standard maintenance and renewal needs of WWTPs. Is Financially Feasible. The cost of wastewater treatment depends on the condition of the WWTP and the technology that is installed. In Switzerland, the average cost for wastewater treatment including nutrient removal is around 0.7 CHF/m3 wastewater. With the planned upgrade, these treatment costs are expected to increase by 10−20% for WWTPs serving >80 000 persons and by 20−50% for WWTPs serving between 8000 and 80 000 persons, depending on the specific conditions and the selected technology. WWTPs serving 80 000 persons for which larger load reductions can be achieved, (b) WWTPs serving >8000 persons that contribute >10% of the dry-weather stream flow and are thus characterized by low dilution in the receiving water, and (c) WWTPs serving >24 000 persons discharging into sensitive waters or important drinking water reservoirs. Based on these criteria, about 100 out of the 700 WWTPs are identified resulting in roughly 50% of the municipal wastewater in Switzerland being additionally treated to remove MPs. The total load released via the Rhine to Germany and The Netherlands will be accordingly reduced. This upgrading procedure will be complemented by closing down small WWTPs that are not upgraded, and diverting the wastewater to larger plants.

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TAKING ACTION ON MICROPOLLUTANTS−A LUXURY FOR RICH COUNTRIES ONLY? Over the last century, plans for significant changes in wastewater management were often initially met with skepticism, primarily on the basis of anticipated costs and because gains for human and environmental health were unproven. Examples include the introduction of (i) sanitary sewers, (ii) biological wastewater treatment and (iii) nutrient elimination. In hindsight, all of these improvements are seen to be both effective and beneficial. Today, these developments, which were constructed and implemented over decades, are largely taken for granted. All over the world, densely populated regions are facing water reuse issues of different kind. These include drinking water contamination, loss of consumer confidence, loss of good water resources for irrigation and recreational value in urban and peri-urban zones as well as ecological value of the aquatic environment. These problems can only be solved over a sustainable and efficient infrastructure integrated in an appropriate strategy. Its buildup is usually a long-term process that needs strategic planning and financing. MP elimination only causes a slight cost increase, if integrated in a long-term strategy. Nevertheless, MP elimination at WWTPs is only a part of a broader strategy which should comprise also source control and further mitigation measures for diffuse pollution, which is not flowing through WWTP like urban surface runoff, runoff from intensive agriculture or roads. Further direct treatment at specific point sources might result in significant efficiency gain, for example, specific industries, hospitals, or nursing homes. Prevailing trends in global development, specifically the increases in population, urbanization, economic welfare, and use of chemicals, results in increased pressures on water resources: More water will be needed for drinking water, for industrial processes, and to produce food on irrigated agricultural land for a growing population.44,45 It is likely that raw water for drinking water production and/or for irrigation will contain an increasing fraction of discharged wastewater. 7686



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(iii). Understanding the Impact of Micropollutants, In the Context of Confounding Factors, On the Structure and/or Function of Natural Aquatic Ecosystems. Although MPs are known to be ubiquitous in inland waters,11 the effects of MPs on complex aquatic ecosystems is poorly understood.12,52,53 Several factors make it difficult to predict such impacts. There are potentially thousands of MPs to consider, they occur in low concentrations and in changing mixtures,15,16 hence it is difficult to relate the occurrence of MPs to ecological effects in real systems. The molecular and physiological mechanisms by which MPs may affect organisms directly or indirectly are diverse, and furthermore, MPs are generally not the only stressors present in the aquatic environment. Organisms in natural ecosystems experience multiple stressors such as changes in UV or light intensities, temperature, pH or also biological stressors such as predators or pathogens.12 This makes it difficult to disentangle the effects of multiple stressors or to elucidate the potential interactions with other factors that vary in natural ecosystems. Confounding factors may exacerbate or dampen the toxic effects.26,53 Hence, it is a major challenge to establish causal links between MPs and effects on the structure and/or function of natural aquatic ecosystems. The upgrading of wastewater treatment plants offers realworld experiments. By studying the consequences of removing MPs from wastewater on the complex ecosystems in receiving waters, we can gain insight into how MPs affect their structure and function.54 An improved understanding of these effects could provide the basis for optimizing treatment processes so that they target the most relevant MPs. (iv). Promoting Societal Discussion on Reducing MP Concentrations in the Aquatic Environment. It is undeniable that chemicals play an important role in society worldwide.2 Chemical production can be anticipated to further increase to support human health and personal care, animal husbandry, food production, manufacturing, construction, and many other industrial sectors. The expanding use of chemicals has had and will have positive effects on human welfare but, at the same time, poses risks to environmental and human health. The need for all chemicals in current use as well as options to reduce use or to substitute more toxic with less toxic chemicals should be assessed. A societal understanding of the price involved in the production and use of chemicals and a common agreement on the willingness to accept risks are needed. This requires a thorough evaluation of trade-offs between benefits brought by chemicals and costs associated with their production and use in various sectors.

In the past, adequate water quality downstream from WWTPs was often achieved partly through dilution. In the future, this is not likely to be a reliable strategy in densely populated urban areas. Developing and emerging countries face even more severe challenges. Rivers flowing out of Beijing, for example, contain about 95% wastewater and 90% of the flow is subsequently extracted for use in agricultural irrigation.11 Water reuse (ranging from indirect, nonpotable reuse to direct potable reuse) is a growing trend in regions subject to water scarcity.46 The presence of MPs in wastewater could present a serious constraint on water reuse, or at least, impose greater demands for treatment of reclaimed water before reuse. A further consequence of urbanization, particularly when it is accompanied by improved economic welfare, is the demand for urban recreational areas. Water bodies like rivers and lakes can contribute significantly to the quality of urban life. Thus, improving the water quality in urban lakes and rivers is an essential element in developing the urban environment.

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CHALLENGES AND OPPORTUNITIES RELATED TO UPGRADING WWTPS The decision to pursue an MP removal strategy in urban water management by upgrading WWTPs brings with it a series of challenges and opportunities for science, engineering and society. Although the current approach, exemplified in this feature, is a good starting point, further development is needed. Here we provide a few examples, although we are aware that there are many other relevant and exciting issues. (i). Developing and Implementing Sound Evaluation and Monitoring Tools. For evaluating and optimizing treatment processes, it would be instructive to monitor the effectiveness of the treatment technology by assessing the chemical and toxicological quality of the effluent along treatment trains. This is particularly needed to understand the effects of differences in process layout and wastewater composition on MP removal (i.e., different industrial inputs or differences among process alternatives) and to design appropriate control and operation strategies. Comprehensive methods require the assessment of possible byproducts and transformation products, as well as their (eco)toxicological relevance. These products are formed during the oxidative treatment process, but not in processes relying on sorption to activated carbon. New approaches like using online (bio)sensors could open new possibilities for automated monitoring procedures. (ii). Understanding and Predicting the Biological and Oxidative Degradability of Chemicals. From a scientific as well as a practical perspective, it would be advantageous to be able to predict if and how a given MP is degraded in different treatment processes. The development of such predictive models, which are currently based on quantitative structure property relationships in case of chemical-physical processes47,48 and a rule-based system based on biological reactions described in databases and literature49 may be facilitated by using new experimental models. These include meta-genomics to understand the microbial processes responsible for degradation50 or ozone chemistry to predict possible oxidative reactions.51 Better understanding of compound persistence is expected to be an important fundament for a sound source control strategy, that is, to identify when persistent compounds ought to be either substituted or collected at its point of use for special treatment.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

We thank Janet Hering, Crista McArdell, Hansruedi Siegrist and three anonymous reviewers for helpful comments on the manuscript. We acknowledge Peter Penicke for supplying Figure 2. 7687



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