Comparative Life Cycle Assessment of Advanced Wastewater

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Sustainability Engineering and Green Chemistry

Comparative Life Cycle Assessment of Advanced Wastewater Treatment Processes for Removal of Chemicals of Emerging Concern Sheikh Mokhlesur Rahman, Matthew J. Eckelman, Annalisa Onnis-Hayden, and April Z Gu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00036 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 3, 2018

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Environmental Science & Technology

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Comparative Life Cycle Assessment of Advanced

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Wastewater Treatment Processes for Removal of

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Chemicals of Emerging Concern

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Sheikh M. Rahman1, Matthew J. Eckelman1*, Annalisa Onnis-Hayden1 and April Z. Gu1† 1

Department of Civil and Environmental Engineering, Northeastern University, 400 Snell Engineering Center, 360 Huntington Ave, Boston, MA 02115, USA

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* Corresponding Author: [email protected], Tel: +1 617 373 4256; Fax: +1 617 373 4419

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†

Co-corresponding Author: [email protected], Tel: +1 607-255-2542; Fax: +1 607-255-9004

1 Environment ACS Paragon Plus


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ABSTRACT

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The potential health effects associated with contaminants of emerging concern (CECs) have

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motivated regulatory initiatives and deployment of energy- and chemical-intensive advanced

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treatment processes for their removal. This study evaluates life cycle environmental and health

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impacts associated with advanced CEC removal processes, encompassing both the benefits of

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improved effluent quality as well as emissions from upstream activities. A total of 64 treatment

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configurations were designed and modeled for treating typical U.S. medium-strength wastewater,

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covering three policy-relevant representative levels of carbon and nutrient removal, with and

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without additional tertiary CEC removal. The USEtox model was used to calculate

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characterization factors of several CECs with missing values. Stochastic uncertainty analysis

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considered variability in influent water quality and uncertainty in CEC toxicity and associated

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characterization factors. Results show that advanced tertiary treatment can simultaneously reduce

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nutrients and CECs in effluents to specified limits, but these direct water quality benefits were

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outweighed by even greater increases in indirect impacts for the toxicity-related metrics, even

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when considering order-of-magnitude uncertainties for CEC characterization factors. Future

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work should consider water quality aspects not currently captured in life cycle impact

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assessment, such as endocrine disruption, in order to evaluate the full policy implications of the

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CEC removal.

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KEYWORDS. Contaminants of Emerging Concern, Life Cycle Assessment, Wastewater

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Treatment, Advanced Tertiary CEC Removal, Tertiary Nutrient Removal.


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INTRODUCTION

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Since the mid-1990s, there has been increasing concern that a large number of unregulated

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yet widely used chemicals pose risks to our ecosystems and human health; such chemicals are

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referred to as “contaminants of emerging concern” (CECs). These CECs include both new

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emerging pollutants, such as nano-materials and antibiotic-resistant microbes, and existing

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chemicals with recently recognized health impacts, such as pharmaceuticals and personal-care

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products (PPCPs), endocrine-disrupting chemicals (EDCs) and other industrial and commercial

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compounds.1-3 CECs have been widely detected in the aquatic environment including drinking

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water systems, wastewaters, surface waters, and groundwaters worldwide.2,4-10 Though CECs are

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typically found in low concentrations (pico/nano/micro-gram/L levels), these can still be

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sufficient to cause harmful effects on human, animal, and plant organisms. Indeed, some CECs

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show no threshold no observed adverse effect level.1,11-13

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A number of initiatives and regulations have been introduced to address concerns about

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CECs; specifically, with the trend toward increasing water reuse and recycling, there are

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substantial efforts on monitoring and regulating CECs.14-16 Following the guidelines of the

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Unregulated Contaminant Monitoring Rule (UCMR), water utilities monitor and report

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occurrences of emerging contaminants that are selected by EPA for a 5-year cycle, largely based

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on the Contaminant Candidate List (CCL) process.17-18 Findings of both the UCMR and EPA’s

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Endocrine Disruptor Screening Program (EDSP) that tests for endocrine disruptor dose-

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response,19 along with relevant scientific advancements, may prompt the EPA to adopt new rules

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and standards regarding CECs.20 To comply with these potential future standards, additional

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treatment technologies would need to be implemented to reduce CEC concentrations in

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wastewater effluents.



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CECs migrate to water bodies though various direct and indirect routes from point and non-

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point sources. Effluent from wastewater treatment plants (WWTPs) is considered to be one of

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the major sources, along with agricultural usages and storm runoff.21-24 Subsequently, scientific

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and engineering challenges exist in developing cost-effective remediation technologies to

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remove CECs from wastewater in compliance with proposed and future regulatory limits.25

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Traditional water and wastewater treatment processes are not designed to eliminate most CECs,

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especially those present at trace levels in drinking water and wastewater.1,26-28 Advanced

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treatment processes that are considered promising for CEC removal include advanced oxidation

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(such as UV-H2O2 oxidation and ozonation),29-30 adsorption,31-32 and advanced filtration

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processes.32-33 Reverse osmosis and UV-H2O2 oxidation processes are energy-intensive, while

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adsorption and ozonation processes require additional chemicals. Many of these advanced

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processes are in lab, pilot, or batch treatment scales; and even now the mechanisms of CEC

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removal in several of these processes are still not well understood.34-35 Although there have been

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reports exploring quantitative statistical and modeling approaches to estimate CEC removal

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efficiencies in conventional activated sludge36-37 and membrane bioreactor processes,36 for many

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CECs there is not yet consensus regarding removal efficiencies in advanced tertiary biological or

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chemical/physical processes.36-38

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The very low concentrations of many CECs in wastewater makes treatment processes both

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costly and technically challenging. Intensive usage of treatment chemicals and energy in

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advanced CEC removal processes have been associated with increased life cycle toxicity and

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other environmental impacts.31-33,39 In considering CEC effluent limits and the technologies

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required to meet these targets, it is appropriate to evaluate the balance between the

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environmental benefits achieved by CEC removal (e.g., reduced toxicity) and any unintended


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environmental costs due to additional chemical, energy and materials usage. It is also important

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to consider these trade-offs at a regional level if possible, rather than on a national or global

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basis, as local conditions can affect both the quantity of emissions and the health and

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environmental effects of these emissions.40-41 A quantitative, multi-endpoint consideration of

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these trade-offs is the central goal of the current study.

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Life Cycle Assessment (LCA) has been used extensively to characterize and quantify the net

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environmental impacts of wastewater treatment processes and plants and to compare treatment

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options.39,42-46 Because the scope of LCA includes both direct emissions from WWTPs as well as

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indirect emissions from producing and transporting all chemicals, energy, and infrastructure

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required for treatment, LCA can be applied to study both environmental benefits and costs

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associated with removing pollutants to meet more stringent regulations. Most studies to date

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have focused on nutrient removal, while very few studies have focused on CEC removal

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technologies. One study by Igos et al.39 for hospital wastewater considered treatment of ten

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pharmaceutical compounds using six different decentralized or centralized WWTP scenarios.

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The authors concluded that the direct environmental impacts from discharge PPCPs were

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negligible compared to the life cycle environmental impacts associated with the additional

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treatment processes required for their removal, implying that treatment leads to a net increase in

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impacts, rather than a decrease. Wenzel et al.47 found similar results for CEC treatment using

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ozonation and membrane bioreactors, but found that sand filtration lead to net reductions in

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overall impacts, due to its less intensive use of energy and treatment chemicals.

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Several other LCA studies have been conducted on removal of CECs such as PPCPs and

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EDCs, mostly considering isolated advanced wastewater treatment processes for CEC removal.

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Most previous quantitative work has reported that the local benefits of CEC removal are small



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compared to the impacts from the treatment processes, which are often distributed over the life

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cycle at a regional or global level.48-51 In the few studies that found relatively large benefits from

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advanced treatment, results were dominated by metals whose removal contributed the most to

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life cycle benefits, while many organic CECs were omitted due to missing characterization

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factors.49,51 Due to data gaps and uncertainties in CEC toxicity, some previous studies adopted a

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qualitative approach to describe possible human and ecotoxicity impact of CECs,52 or focused

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solely on the impacts from the treatment processes per unit mass of PPCP removal, without

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considering the benefits.53 With the rising demand for water conservation and water reuse, there

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is an ongoing need for analysis that compares the life cycle benefits of co-removal of nutrients

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and CECs across different process configurations as advanced tertiary technologies develop and

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achieve wider implementation.

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Here we report life cycle environmental and health impact results for CEC removal

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technologies utilizing a variety of advanced processes designed for both nutrient and CECs

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removal in different plant configurations. This new analysis builds on the LCA work noted

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above while expanding to cover a wide range of target substances and treatment processes,

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including conservative ecotoxicity estimates for all CECs, and addressing uncertainty through a

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multi-tiered analysis. Secondary biological and tertiary nutrient removal processes were selected

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to achieve three different representative levels of nutrient removal for municipal wastewater,

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coupled with processes for CEC removal, in order to achieve targets that have been proposed in

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recent policy discussions.54-55 Net life cycle environmental and health impacts of treatment are

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evaluated with a focus on human toxicity and ecotoxicity, as the primary motivations for CEC

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removal, but also including impact categories of eutrophication, acidification, and global

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warming.


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This work builds on our previous assessment of tertiary treatment processes for nutrient

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removal,45 which combined wastewater process simulation results with LCA, revealing large

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increases in global warming and ozone depletion associated with more stringent effluent

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standards, and even an increase in life cycle eutrophication for RO tertiary treatment processes,

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for which indirect impacts overwhelmed the direct water quality benefits of reduced nutrient

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concentrations in effluent. Our previous work focused on reduced nutrient loading, but not the

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effects of micropollutants or any additional treatment that might be required for their removal. In

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this study, we focus on life cycle impacts and benefits associated with CEC removal, extending

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preliminary work56 while updating models to include new life cycle inventory data and

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characterization factors and uncertainty analysis, leading to improved representativeness and

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interpretability of the LCA results.

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METHODS

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Selection of CECs and Their Concentrations in Wastewater

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Among the CECs identified in the literature,2,5,7-8,10,32 35 CECs are selected for the current

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study that are present in at least 50% of reported plants surveyed or monitored, with a preference

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for CECs with available information on degradation, transformation, and removal during the

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treatment processes. This set includes 19 pharmaceuticals and personal care products (PPCPs), 2

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pesticides, 4 natural and/or synthetic hormones, and 11 industrial and commercial chemicals

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(ICCs). Concentrations of CECs in secondary effluent vary widely, ranging between 10 ng/L and

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3250 ng/L. The medians of the reported concentrations are used in the present study as influent

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levels feeding to the tertiary CEC removal processes and listed in Table 1 along with the

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minimum and maximum of the reported values. The CEC concentrations reported here are



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representative of the typical occurrence data in the US wastewater, and do not apply to any

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specific site. Spatial and temporal variations in CEC concentrations across the country can occur

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from different CEC production rate and consumption practices, water consumption, capacity of

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WWTPs, persistence and metabolism of CECs, as well as climatic conditions such as

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temperature and rainfall.4 For example, recorded high concentrations of specific CECs in densely

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populated areas reflect the influence of factors such as population density, food and drug

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consumption habits, and land use patterns.4,57

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Treatment Plant Design Alternatives

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When implemented, CEC treatment processes typically follow advanced schemes for nutrient

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removal.22 Hence, combining both nutrient removal and CEC removal processes may be

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necessary to understand the comprehensive environmental costs and benefits of more stringent

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limits for treatment plants. With that in focus, treatment configurations designed for the current

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study include secondary biological processes targeting three different effluent nutrient limits

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(Level 1: TN = 8 mg/L, TP = 1 mg/L; Level 2: TN = 3 mg/L, TP = 0.1 mg/L; Level 3: TN = 1

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mg/L, TP = 0.01 mg/L,55 details in Table S1), and both with and without additional CEC

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removal processes.

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The designs of the treatment alternatives are based on 10 MGD (37,854 m3/d) influent flow

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with typical US domestic wastewater characteristics (TN = 35 mg/L and TP = 8 mg/L, details in

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Table S2 and variations considered in the uncertainty analysis) and typical design life of 20

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years, following previous studies.43,58-60 As 20-50 years are common lifespans of wastewater

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treatment plants, the selection of 20 years as the design life represents a conservative assumption.

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In addition to two conventional biological nutrient removal (BNR) processes (5 stage Bardenpho


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and University of Cape Town (UCT)), external carbon addition is considered for additional

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denitrification for both Level 2 and Level 3 treatment scenarios.61-62 Four commonly used

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tertiary processes—including ballasted sedimentation, traditional filtration, filtration with

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continuous backwash, and membrane filtration technologies—are selected for higher-level

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phosphorus removal. Several studies have demonstrated that application of tertiary processes in

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multiple stages with external carbon addition may achieve target TN and TP concentrations,63-66

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assuming reliable and stable operation of the treatment plants.55 Furthermore, four advanced

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tertiary processes are evaluated as CEC removal processes and they are ozonation, UV-H2O2

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oxidation, reverse osmosis, and activated carbon adsorption processes.

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In total, there are eight scenarios for Level 1 treatment (including six with CEC removal), 30

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scenarios for Level 2 treatment (including 24 with CEC removal), and 26 scenarios for Level 3

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treatment (including 20 with CEC removal). A breakdown of treatment configurations is

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provided in Figure 1 with details in Table S3. In-house spreadsheet models for preliminary

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treatment coupled with BioWin simulations (EnviroSim, Hamilton, ON) are used to finalize the

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reactor design including sizes, solids retention time (SRT), chemical and O2 requirements of the

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BNR processes for nutrient removal. The materials, energy, and chemical requirements of the

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BNR reactors are kept static for the subsequent uncertainty analyses. The total SRT for all the

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treatment scenarios is set to 10 days, which is in line with the usual operating condition of a

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BNR process.58,67

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Process configuration and design parameters such as chemical dose, energy use, and P

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removal rate of the tertiary nutrient removal processes are determined based on Manual of

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Practice (MoP) and previous literature (Table S4 and S5).45,54,61,68-69 For the adsorption process,

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granular activated carbon (GAC) is assumed with regeneration from spent media (see Tables S4



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and S6). Rejected brine from the RO process, which also includes the CECs removed during

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treatment, is assumed to be directly disposed to the sea without any further treatment (see Tables

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S4 and S7).

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Different CECs categories exhibit various levels of susceptibility to and removal efficiency

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from the various wastewater treatment processes. CEC removal efficiency in the treatment

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process depends on several factors including operating chemical dose, contact time,

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biotransformation, mixture effects, and variations in flow and temperature.4,70-71 Water quality

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parameters such as dissolved organic carbon (DOC) also affect the CEC removal efficiencies,

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chemical doses, and energy use in the CEC treatment processes.72 However, due to the lack of

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mechanistic understanding and quantitative models for predicting the DOC level associated with

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different levels of nutrient removal, we did not consider the influence of DOC in our analysis,

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instead used ranges for removal efficiency to account for uncertainty in influent wastewater

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quality generally. The CEC removal efficiencies for each advanced tertiary treatment processes

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are selected as the median of the reported values collected from literature, which represent

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typical operating conditions, rather than the full range of variability that might be seen in actual

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treatment plants. CEC removal efficiencies and chemical and energy usage by ozonation, UV-

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H2O2 oxidation, RO membrane filtration, and activated carbon adsorption processes are gathered

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from available literature of pilot or lab-scale studies and are summarized in Table 1 and Table

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S4.8,10,32,39,73-74 Despite a thorough review and a preference for well-studied compounds, removal

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rates of several CECs are still missing. For these CECs and treatment processes, rates are

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assumed as the median of the removal rates available for those CECs with available data, as

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noted in Table 1.


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Considering sludge management, life cycle impacts of the sludge treatment and disposal are

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often small compared to the liquid stream treatment, and can vary widely depending on the

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selected treatment and disposal units.45,75-77 Therefore, this analysis considers the single

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configuration of sludge treatment and solids handling processes that include gravity thickener,

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anaerobic digestion, and disposal to a sanitary landfill, including fugitive emissions (Text S1,

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Table S13, Table S14).

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Life Cycle Assessment

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LCA modeling is based on a functional unit of 1 m3 of influent wastewater. The system

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boundary considers both liquid and solids streams of the treatment facilities including influent

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distribution through different treatment processes, treatment processes, sludge management, and

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final effluent discharge, following previous work.45,78 Life cycle inventory data were assembled

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for plant operation (chemicals, energy) from process simulations and for plant construction

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(steel, concrete) from sizing of the various units. Direct emissions of CO2, CH4 and N2O from

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secondary biological processes are estimated following an EPA method that models CO2 and

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CH4 based on the stoichiometry of degradation of organic compounds in biological processes

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and N2O from the nitrification-denitrification stoichiometry.79 Fugitive emissions are estimated

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based on the annual average influent properties and static operating conditions, though they may

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be dependent on various dynamic factors including influent characteristics (BOD and nutrient

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concentrations), and operating conditions (temperature, pH, dissolved oxygen, SRTs, reactor

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types).80-82 The estimated direct CO2 emission from the treatment processes are considered to be

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biogenic CO2, following IPCC guidelines for WWTPs,83 and has no impact on the global



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warming potential estimation. For tertiary nutrient and CEC removal processes, operating

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energy, and chemical requirement are obtained from literature.32,39,68-69,84

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All material and energy inputs are summarized in Tables S4-S5. Inventory data are matched

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with unit processes (Table S8) from the US-EI LCI database (Earthshift, Huntington, VT), which

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includes modified unit process data from the ecoinvent LCI database adjusted for U.S. energy

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inputs and compiled in the SimaPro 8.1 software package (PRé Consultants, Amersfoort,

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Netherlands).

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Life cycle impacts of all emissions were assessed using the Tool for the Reduction and

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Assessment of Chemical and Other Environmental impacts (TRACI 2.1) impact assessment

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method, developed by the USEPA.40-41 The impact categories of eutrophication, acidification,

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global warming, ozone depletion, ecotoxicity, and human health—carcinogenic and

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non-carcinogenic—were selected for the current study, as these represent the water quality,

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global emissions, and human health concerns most frequently included in wastewater treatment

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LCAs. Table S9 presents the selected impact categories with a description of their estimation

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approach and reference substances. Ecotoxicity and human health toxicity in TRACI 2.1 model

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are estimated using the UNEP-SETAC consensus toxicity model USEtox.85 For the CECs

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disposed with the effluent, the freshwater emission compartment of the USEtox model is

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assumed for calculation of the fate factor. USEtox considers the exposure routes of direct

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inhalation and ingestion of drinking water, as well as ingestion of foods where pollutants may

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have bioaccumulated. (Potential CEC removal during drinking water treatment is not considered

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in the model.) Of the 35 CECs considered, some had existing characterization factors (CFs) for

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ecotoxicity and human health endpoints pre-run in USEtox or derived by Alfonsín et al. for

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PPCPs.85-87 CECs with missing CFs were evaluated in the USEtox model using input values for


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physical/chemical properties and toxicity results from EPA’s EPISuite™ model.88 Table S10

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presents the USEtox input parameter values for 10 CECs that were evaluated in this manner,

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while Table S11 presents all USEtox CFs by data source. Only three CECs (o-hydroxy

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atorvastatin, p-hydroxy atorvastatin, and TCEP) were unable to be evaluated for all three

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endpoints (ecotoxicity, human health cancer, and human health non-cancer toxicity). Where no

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toxicity data were available, CFs are set to the median of the CFs available for other CECs. The

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median values are 5.7×103 CTUe for the ecotoxicity, 2.5×10-8 CTUh for the cancer related

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human health toxicity, and 2.6×10-6 CTUh for non-carcinogen human health toxicity.

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Additionally, the toxicity characterization factors of most of the brine ions are not available in

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the USEtox model, and were collected from the Zhou et al. study.89 The seawater emission

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compartment of the USEtox model is selected to characterize the toxicity of the CECs disposed

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along with the brine (Table S11).

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Uncertainty Analysis

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Uncertainty analysis has been conducted to analyze the effects of variation in influent

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wastewater quality, chemical and energy use in tertiary processes, CEC concentrations in

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wastewater, and removal rates of CECs in advanced tertiary processes, using two sets of Monte

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Carlo simulations. First, we have considered variability in the influent carbon to phosphorus

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(C/P) ratio (since influent C/P ratio can have noteworthy effect on nutrient removal)90 together

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with variations in chemical and energy use in the tertiary processes. Since a low C/P ratio

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reduces P removal efficiency,90 influent C/P ratios ranging from 15 mg/mg to 25 mg/mg are

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considered by adding COD, where acetate is used as the external carbon source. Ranges for

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chemical and energy inputs of the tertiary processes, reported in the literature, are used to create



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uniform probability distributions (Table S12). Direct inputs in the BNR processes are kept static.

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A 10,000 run Monte Carlo simulation is conducted in SimaPro for both these direct inputs as

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well as existing log-normal distributions of all indirect inputs and emissions (background

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processes in the LCI).

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Second, we have investigated the possible changes in the benefits associated with reduction

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of direct CEC emissions by varying the CECs concentrations, removal efficiencies, and toxicity

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CFs. Though nutrient removal process designs are well established, advanced tertiary processes

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for both nutrient and CEC removal still exist only at the pilot scale or limited full-scale, and so

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have less robust data. A relatively wide range of CECs concentrations in wastewater and their

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removal efficiencies in the CEC removal processes are reported in the literature. CEC

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concentrations and removal efficiencies are considered to vary uniformly for the Monte Carlo

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simulations, within the ranges listed in Table 1. Toxicity CFs are among the most uncertain

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among impact categories. Rosenbaum et al. provided guidance for CFs estimated by the USEtox

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method, with uncertainty of 1-2 orders of magnitude variation for the ecotoxicity and 2-3 orders

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of magnitude for the human health impacts.85 Accordingly, we apply a standard variation of 0.01

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to 100 times of the base values of the characterization factors for the ecotoxicity and 0.001 to

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1000 times of the base values for the human health toxicity to run the Monte-Carlo simulation

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with 100,000 iterations, which is conducted in MATLAB 2015b (Mathworks, Natick, MA).

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RESULTS AND DISCUSSION

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Life Cycle Assessment

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Life cycle impacts for ecotoxicity and human toxicity (carcinogenic and non-carcinogenic)

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are presented in Figure 2 for all treatment scenarios. As expected, reduction of CECs in the

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effluent due to advanced tertiary treatment leads to reduced direct environmental and health

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impacts. However, these reductions can hardly be seen as life cycle toxicity is completely

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dominated for all three impact categories by indirect emissions from upstream production of

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chemicals, energy, and (to a lesser extent) construction materials. This means that although

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advanced tertiary processes reduce the CECs concentration in the effluent locally, their

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implementation may in fact lead to net increases in life cycle toxicity overall.

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Figure 3 isolates just the CEC removal processes in order to visualize the benefits of

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reducing CECs in WWTP effluent versus the additional toxicity impacts from upstream

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emissions. Contributions of CEC removal to life cycle human toxicity are essentially negligible

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(

Comparative Life Cycle Assessment of Advanced Wastewater

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