Life-Cycle Assessment of Advanced Nutrient Removal Technologies

Feb 12, 2016 - Results show that advanced technologies that achieve high-level nutrient removal significantly decreased local eutrophication potential...
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Life Cycle Assessment of Advanced Nutrient Removal Technologies for Wastewater Treatment Sheikh M. Rahman, Matthew J. Eckelman, Annalisa Onnis-Hayden, and April Z Gu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b05070 • Publication Date (Web): 12 Feb 2016 Downloaded from http://pubs.acs.org on February 12, 2016

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Environmental Science & Technology

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Life Cycle Assessment of Advanced Nutrient

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Removal Technologies for Wastewater Treatment

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

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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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Department of Civil and Environmental Engineering, Northeastern University, 400 Snell

Co-corresponding Author: [email protected], Tel: +1 617 373 3631; Fax: +1 617 373 4419

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ABSTRACT

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Advanced nutrient removal processes, while improving the water quality of the receiving water

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body, can also produce indirect environmental and health impacts associated with increases in

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usage of energy, chemicals, and other material resources. The present study evaluated three

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levels of treatment for nutrient removal (N and P) using 27 representative treatment process

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configurations. Impacts were assessed across multiple environmental and health impacts using

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life cycle assessment (LCA) following the TRACI impact assessment method. Results show that

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advanced technologies that achieve high-level nutrient removal significantly decreased local

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eutrophication potential, while chemicals and electricity use for these advanced treatments,

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particularly multi-stage enhanced tertiary processes and reverse osmosis, simultaneously

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increased eutrophication indirectly and contributed to other potential environmental and health

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impacts including human and ecotoxicity, global warming potential, ozone depletion and

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acidification. Average eutrophication potential can be reduced by about 70% when Level 2

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(TN=3 mg/L; TP=0.1 mg/L) treatments are employed instead of Level 1 (TN=8 mg/L; TP=1

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mg/L) but the implementation of more advanced tertiary processes for Level 3 (TN=1 mg/L;

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TP=0.01 mg/L) treatment may only lead to an additional 15% net reduction in life cycle

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eutrophication potential.

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INTRODUCTION

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EPA’s initiatives to develop nutrient water quality guidelines are mostly technology driven that

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often encourages the adoption of conventional, easily applicable, and economically feasible

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processes. Total maximum daily load (TMDL) and waste-load allocations in watersheds

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established under the Clean Water Act (CWA) drive new technological challenges to keep the

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nutrients in wastewater within acceptable limits. A two-tiered approach- technology based and

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water quality based approaches is followed to issue effluent permit for wastewater discharge in

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the U.S. under the National Pollutant Discharge Elimination System (NPDES). The technology-

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based tier is based on the limits achievable while the water-quality based tier limits nutrient

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discharges depending on quality of the receiving water body. While technology-based limits

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have specific guidelines, water quality-based limits may vary widely depending on location,

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leading to very strict nutrient limits for some sites.1 EPA-recommended eco-regional criteria

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provide a starting point for states to identify precise nutrient levels needed to protect aquatic life

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and maintain recreational or other uses on a site-specific or regional basis.2-4

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In order to comply with increasingly stringent regulation on nutrient discharges (N and

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P), WWTPs are facing the requirements to increase the level of treatment by adopting advanced

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tertiary treatment processes to further remove nutrients beyond conventional secondary processes

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with nutrient removal.5-6 Tertiary processes require additional usage of energy, chemicals, and

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other material resources in order to meet effluent targets. Advanced wastewater treatment

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processes improve the water quality of the receiving water body but can also lead to deleterious

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environmental and health impacts elsewhere in the technological life cycle that need to be

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evaluated.

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When advanced tertiary processes are used to remove nutrients from wastewater to very

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low levels, direct greenhouse gas (GHG) emissions occur from the WWTP,7 as well as indirect

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emissions from the production and distribution of the additional electricity and chemicals

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required.8 Indirect impacts are typically distributed regionally or even globally, different

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locations from location other than the WWTP itself.

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The central goal of the present work is to evaluate the balance between the local benefits

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achieved from removal of nutrients (e.g., reduced eutrophication) and potential indirect and

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distributed environmental damages due to additional chemical, energy and materials usage. In

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addition to eutrophication and GHG emissions, other life cycle energy, environmental and

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human health impacts will also be considered to help comprehensively assess the implications

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and trade-offs of newly proposed effluent limits across a range of treatment technologies and

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process configurations.

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Life Cycle Assessment (LCA) is a multi-stage, multi-criteria modeling framework under

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international standards (ISO 14040 and 14044) that has been widely used to characterize and

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quantify the potential environmental impacts of wastewater treatment processes and plants.8-12

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LCA inventories emissions that occur both directly at WWTPs themselves, as well as indirectly

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that are associated with the supply chain of WWTPs and that may occur far from the actual plant

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(e.g., impact from the power plants that supply the electricity).13 Emissions are then linked to

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midpoint (problem-oriented) or endpoint (damage-oriented) metrics for environmental quality

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and public health.14-17 LCA studies of WWTPs have been conducted in various countries,

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particularly in Europe and Australia, with relatively fewer studies based in the U.S. LCA studies

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have covered various scales and levels of wastewater treatment processes, including nutrient

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removal,8,10 emerging contaminants,18 selected tertiary processes,19-20 resource recovery,21 and

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water reuse technology.22-24 Some studies were conducted to reveal impacts of the entire

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treatment plants,14-15,22,25-26 while others were designed as decision support tools for treatment

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plant management.12,27

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There are several challenges in conducting and comparing LCA of WWTPs, as discussed

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recently in Corominas et al.11, including internal assumptions, system boundaries, plant size,

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operational conditions, and the technologies being analyzed. While there are emerging standards

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and recommendations for scoping LCAs of wastewater treatment, past studies have made a

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variety of assumptions concerning scope or system boundaries.11 Studies have generally

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considered infrastructure materials and equipment for construction and maintenance, as well as

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chemicals and energy required for operation.8,11 Transportation of chemicals and materials from

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the manufacturer to treatment plants has also been included in several studies,15 as has sludge

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generation and management,14-15 although their inclusion has not been consistent.

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LCA studies of nutrient removal are of particular relevance. Several studies have

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evaluated the comparative impacts of different degrees of nutrient removal;5,8,10 however, they

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predominantly evaluated secondary processes and focused on energy use, eutrophication, and

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GHG emissions, while other categories of environmental impact were not considered. Foley et

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al. conducted comprehensive life cycle inventory study of nine different treatment cases that

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were focused on achieving different effluent N and P levels. Target effluent levels in that study

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ranged from 50 mg N/L to 3 mg N/L for total nitrogen and 12 mg P/L to 1 mg P/L for total

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phosphorous. The treatment options consisted of biological processes for N and P removal

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without any tertiary processes for nutrient removal.8-9

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The present study complements and extends previous work by conducting treatment

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process modeling and LCA for an array of secondary and state-of-the-art tertiary process

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configurations that meet proposed limits for effluent nutrient levels in the United States.6 These

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effluent levels range from 8 mg N/L to 1 mg N/L for N and 1 mg P/L to 0.01 mg P/L for P, more

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stringent than those considered in previous LCA work, and thus requiring more advanced

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treatment approaches. In addition to biological nutrient removal, external carbon addition is

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required for further N removal, and energy and chemical-intensive tertiary processes are used for

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higher P removal.

Here we evaluate and compare the environmental and health impacts

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associated with different levels of wastewater nutrient removal technologies that have been

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specifically designed for three different levels of effluent limits, spanning 27 treatment

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technologies and process scenarios for nutrient removal. The objective of the present study is to

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provide a comprehensive view of potential trade-offs associated with advanced nutrient removal

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processes across a broad range of impact categories, including human health effects,

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acidification, ecotoxicity, and ozone depletion that are less frequently reported. Variations

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associated with influent characteristics, operating energy, and chemical usage were evaluated via

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uncertainty analysis using a stochastic modeling approach, since modeling the uncertainty

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associated with this variability is critical for proper interpretation.

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In a previous conference paper, we reported preliminary results for LCA modeling of 15

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treatment scenarios covering three levels of treatment, which demonstrated potential increases

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and trade-off among environmental impacts from tertiary nutrient removal processes at different

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step changes in effluent limits.28 In the present work we have extended the scope in the present

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study by incorporating additional treatment configurations, updating and modifying the treatment

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plant designs, updated the life cycle impact assessment model employed, and included a robust

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statistical analysis of uncertainty. Direct (local) environmental benefits resulting from decreasing

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nutrient concentrations in effluent are compared against indirect environment impacts at regional

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and global scales that result from the requisite material and energy use to construct and operate

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tertiary treatment processes. Managing trade-offs between local benefits and regional or global

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impacts to environmental quality and human health have implications both for modeling and

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environmental policy.17,29

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METHODOLOGY

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Nutrient Removal Treatment Level Classification

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Nutrient removal processes for municipal wastewater have been designed and classified based on

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their ability to meet three Levels based on the targeted effluent concentrations for total N and P

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(Table 1), which are representative of those prescribed by NPDES permits.6 Level 1 treatment is

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achievable with conventional nutrient removal technologies, such as biological nitrogen removal

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along with chemical phosphorus removal or biological nutrient removal without any external

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carbon addition. In order to achieve Level 2 effluent limits, a supplemental carbon source is

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generally required to enhance denitrification and achieve the target effluent nitrogen

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concentration. In addition, enhanced tertiary chemical phosphorus removal process is required in

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order to deliver the lower Level 2 Phosphorus concentration.6,30-32 Level 3 is known as the best

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achievable performance level and targets extremely low effluent nutrient levels to comply with

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ongoing or anticipated regulations. A WERF–sponsored survey and independent studies on

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advanced phosphorus removal technologies6,31,33 have indicated that, in order to attain these

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extremely low level of nutrient levels, multiple tertiary processes for chemical P removal are

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necessary and addition of more external carbon for post-denitrification is required.3,26,28 WWTPs

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performance at this level of treatment are also dependent on local weather conditions, process

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implementation, wastewater characteristics, as well as skilled operation and maintenance.6

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Treatment Plant Process Configurations Design Alternatives

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Based on the reviews of current and leading-edge treatment technologies for advanced

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nutrient removal from municipal wastewater,6 a set of 27 treatment technologies and process

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scenarios representing three different level of treatments were designed for this study (Figure 1

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and Table S1). For all the treatment scenarios, an influent flow of 10 MGD and U.S. average

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representative influent properties were chosen as the design influent parameters,34 as summarized

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in Table 2. A design life of 20 years was also chosen, which is typical for wastewater works in

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U.S. For Level 1 treatment, a total of seven treatment scenarios were considered that include

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chemical P removal process with Biological Nutrient Removal (BNR) and combined chemical-

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biological P-removal processes. Chemical P removal processes were selected considering

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different chemical addition strategies: as addition to the primary clarifier, to the secondary

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clarifier, or prior to both primary and secondary clarifiers. Two of the most widely used BNR

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processes―5-stage Bardenpho and The University of Cape Town (UCT)―were selected as

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representative secondary treatment with nutrient removal configurations. Ten treatment

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alternatives were considered each for Level 2 and Level 3 treatments. In order to achieve Level 2

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treatment, a tertiary process is adopted following the BNR process. Tertiary processes were

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chosen to cover each of the commonly applied processes including chemically enhanced

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sedimentation, ballasted sedimentation, various filtration processes, and membrane filtration

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technologies. For Level 3 treatment, multi-stage tertiary processes were considered in order to

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meet the most stringent effluent phosphorus and nitrogen limits.6,31,33

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Process design parameters for all the primary, secondary and tertiary processes were

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selected based on MOP, related literature,34-35 and manufacturer specifications. For secondary

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treatment with nutrient removal processes, design simulation tools (e.g., BioWin or in-house

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spreadsheet models) were applied to assist the design process. The Total Solid Retention Time

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(SRT) selected for each of the 27 plants was 10 days. Influent characteristics and typical kinetic

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parameter values of microbial communities used in BioWin model were obtained from the

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literature.34 External carbon addition for enhanced nitrogen removal, chemical doses and P

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removal rates for different tertiary processes, and other design parameters were collected from

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the literature and are listed in the Tables S2–S4 in the Supporting Information.30-32,34,36 In

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addition, data regarding the sizes of the commercial tertiary treatment processes were obtained

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from their respective vendors’ websites, as noted in Table S2.

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Life Cycle Assessment

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The process units included in the LCA model of nutrient removal processes were

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preliminary, primary, secondary treatment with nutrient removal, and tertiary processes, where

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applicable. The system boundary included the influent pumping to effluent discharge and sludge

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pumping to sludge treatment facilities, while the functional unit was taken as 1 m3 of influent

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wastewater. Construction and operation phases were included, while plant maintenance and

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decommissioning after the 20-year design life were excluded. Downstream sludge management

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was also excluded, for two reasons. First, the impacts of sludge treatment and disposal facilities

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on eutrophication (