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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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1
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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 (
