Life-Cycle Assessment of Advanced Nutrient Removal Technologiesfor Wastewater TreatmentSheikh M. Rahman,† Matthew J. Eckelman,*,† Annalisa Onnis-Hayden,† and April Z. Gu*,†

†Department of Civil and Environmental Engineering, Northeastern University, 400 Snell Engineering Center, 360 Huntington Ave,Boston, Massachusetts 02115, United States

*S Supporting Information

ABSTRACT: Advanced nutrient removal processes, whileimproving the water quality of the receiving water body, canalso produce indirect environmental and health impactsassociated with increases in usage of energy, chemicals, andother material resources. The present study evaluated threelevels of treatment for nutrient removal (N and P) using 27representative treatment process configurations. Impacts wereassessed across multiple environmental and health impactsusing life-cycle assessment (LCA) following the Tool for theReduction and Assessment of Chemical and Other Environ-mental Impacts (TRACI) impact-assessment method. Resultsshow that advanced technologies that achieve high-levelnutrient removal significantly decreased local eutrophicationpotential, while chemicals and electricity use for these advanced treatments, particularly multistage enhanced tertiary processesand reverse osmosis, simultaneously increased eutrophication indirectly and contributed to other potential environmental andhealth impacts including human and ecotoxicity, global warming potential, ozone depletion, and acidification. Averageeutrophication potential can be reduced by about 70% when Level 2 (TN = 3 mg/L; TP = 0.1 mg/L) treatments are employedinstead of Level 1 (TN = 8 mg/L; TP = 1 mg/L), but the implementation of more advanced tertiary processes for Level 3 (TN =1 mg/L; TP = 0.01 mg/L) treatment may only lead to an additional 15% net reduction in life-cycle eutrophication potential.

■ INTRODUCTION

The U.S. Environmental Protection Agency (USEPA)initiatives to develop nutrient water-quality guidelines aremostly technology-driven and often encourage the adoption ofconventional, easily applicable, and economically feasibleprocesses. The total maximum daily load (TMDL) andwaste-load allocations in watersheds established under theClean Water Act (CWA) drive new technological challenges tokeep the nutrients in wastewater within acceptable limits. Atwo-tiered approach, technology-based and water-quality-basedapproaches, is followed to issue an effluent permit forwastewater discharge in the United States under the NationalPollutant Discharge Elimination System (NPDES). Thetechnology-based tier is based on the limits achievable, andthe water-quality-based tier limits nutrient discharges depend-ing on quality of the receiving water body. Althoughtechnology-based limits have specific guidelines, water-quality-based limits may vary widely depending on location, leading tovery strict nutrient limits for some sites.1 USEPA-recom-mended ecoregional criteria provide a starting point for statesto identify precise the nutrient levels needed to protect aquaticlife and maintain recreational or other uses on a site-specific orregional basis.2−4

To comply with increasingly stringent regulation on nutrientdischarges (N and P), WWTPs are facing the requirements to

increase the level of treatment by adopting advanced tertiarytreatment processes to further remove nutrients beyondconventional secondary processes with nutrient removal.5,6

Tertiary processes require the additional usage of energy,chemicals, and other material resources to meet effluent targets.Advanced wastewater treatment processes improve the waterquality of the receiving water body but can also lead todeleterious environmental and health impacts elsewhere in thetechnological life cycle that need to be evaluated. For example,during tertiary treatment, greenhouse gas (GHG) emissionsmay occur directly from the WWTP,7 as well as indirectly fromthe production and distribution of the electricity and chemicalsrequired.8 Indirect impacts are typically distributed regionally,or even globally, at different locations from locations other thanthe WWTP itself.The central goal of the present work is to evaluate the

balance between the local benefits achieved from the removal ofnutrients (e.g., reduced eutrophication) and potential indirectand distributed environmental damages due to additionalchemical, energy, and materials usage. In addition to

Received: October 29, 2015Revised: February 12, 2016Accepted: February 12, 2016Published: February 12, 2016

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eutrophication and GHG emissions, other life-cycle energy,environmental, and human health impacts will also beconsidered to help comprehensively assess the implicationsand trade-offs of newly proposed effluent limits across a rangeof treatment technologies and process configurations.Life-cycle assessment (LCA) is a multistage, multicriteria

modeling framework under international standards (ISO 14040and 14044) that has been widely used to characterize andquantify the potential environmental impacts of wastewater-treatment processes and plants.8−12 LCA inventories emissionsthat occur both directly at WWTPs themselves as well as theindirect observation of those that are associated with the supplychain of WWTPs and that may occur far from the actual plant(e.g., impact from the power plants that supply theelectricity).13 Emissions are then linked to midpoint (prob-lem-oriented) or end-point (damage-oriented) metrics forenvironmental quality and public health.14−17 LCA studies ofWWTPs have been conducted in various countries, particularlyin Europe and Australia, with relatively fewer studies based inthe United States. LCA studies have covered various scales andlevels of wastewater treatment processes, including nutrientremoval,8,10 emerging contaminants,18 selected tertiary pro-cesses,19,20 resource recovery,21 and water reuse technol-ogy.22−24 Some studies were conducted to reveal impacts ofthe entire treatment plants,14,15,22,25,26 and others weredesigned as decision support tools for treatment plantmanagement.12,27

There are several challenges in conducting and comparingthe LCA of WWTPs, as discussed recently in Corominas etal.,11 including internal assumptions, system boundaries, plantsize, operational conditions, and the technologies beinganalyzed. Although there are emerging standards andrecommendations for scoping LCAs of wastewater treatment,past studies have made a variety of assumptions concerningscope or system boundaries.11 Studies have generallyconsidered infrastructure materials and equipment for con-struction and maintenance, as well as the chemicals and energyrequired for operation.8,11 Transportation of chemicals andmaterials from the manufacturer to treatment plants has alsobeen included in several studies,15 as has sludge generation andmanagement,14,15 although their inclusion has not beenconsistent.LCA studies of nutrient removal are of particular relevance.

Several studies have evaluated the comparative impacts ofdifferent degrees of nutrient removal;5,8,10 however, theypredominantly evaluated secondary processes and focused onenergy use, eutrophication, and GHG emissions, while othercategories of environmental impact were not considered. Foleyet al. conducted a comprehensive life-cycle inventory study ofnine different treatment cases that were focused on achievingdifferent effluent N and P levels. Target effluent levels in thatstudy ranged from 50 to 3 mg of N per L for total nitrogen and12 to 1 mg of P per L for total phosphorus. The treatmentoptions consisted of biological processes for N and P removalwithout any tertiary processes for nutrient removal.8,9

The present study complements and extends previous workby conducting treatment process modeling and LCA for anarray of secondary and state-of-the-art tertiary processconfigurations that meet proposed limits for effluent nutrientlevels in the United States.6 These effluent levels range from 8to 1 mg of N per L for N and 1 to 0.01 mg of P per L for P,more stringent than those considered in previous LCA workand thus requiring more advanced treatment approaches. In

addition to biological nutrient removal, external carbonaddition is required for further N removal, and energy andchemical-intensive tertiary processes are used for higher Premoval. Here, we evaluate and compare the environmental andhealth impacts associated with different levels of wastewaternutrient removal technologies that have been specificallydesigned for three different levels of effluent limits, spanning27 treatment technologies and process scenarios for nutrientremoval. The objective of the present study is to provide acomprehensive view of potential trade-offs associated withadvanced nutrient removal processes across a broad range ofimpact categories, including human health effects, acidification,ecotoxicity, and ozone depletion that are less frequentlyreported. Variations associated with influent characteristics,operating energy, and chemical usage were evaluated viauncertainty analysis using a stochastic modeling approachbecause modeling the uncertainty associated with this variabilityis critical for proper interpretation.In a previous conference paper, we reported preliminary

results for LCA modeling of 15 treatment scenarios coveringthree levels of treatment, which demonstrated potentialincreases and trade-off among environmental impacts fromtertiary nutrient removal processes at different step-changes ineffluent limits.28 In the present work, we have extended thescope by incorporating additional treatment configurations,updating and modifying the treatment plant designs, updatingthe life-cycle impact assessment model employed, and includinga robust statistical analysis of uncertainty. Direct (local)environmental benefits resulting from decreasing nutrientconcentrations in effluent are compared against indirectenvironment impacts at regional and global scales that resultfrom the requisite material and energy use to construct andoperate tertiary treatment processes. Managing trade-offsbetween local benefits and regional or global impacts toenvironmental quality and human health have implications,both for modeling and for environmental policy.17,29

■ METHODS AND MATERIALSNutrient Removal Treatment Level Classification.

Nutrient removal processes for municipal wastewater havebeen designed and classified on the basis of their ability to meetthree levels based on the targeted effluent concentrations fortotal N and P (Table 1), which are representative of those

prescribed by NPDES permits.6 Level 1 treatment is achievablewith conventional nutrient removal technologies, such asbiological nitrogen removal along with chemical phosphorusremoval or biological nutrient removal without any externalcarbon addition. For the achievement of Level 2 effluent limits,a supplemental carbon source is generally required to enhance

Table 1. Municipal Wastewater Nutrient Removal TreatmentLevelsa

effluent concentration(mg/L)

treatmentcategory level descriptor

totalnitrogen

totalphosphorus

level 1 conventional municipal nutrientremoval

8 1

level 2 enhanced nutrient removal 3 0.1level 3 best achievable performance 1 0.01

aAdopted from Clark et al.6

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denitrification and achieve the target effluent nitrogenconcentration. In addition, the enhanced tertiary chemicalphosphorus removal process is required to deliver the lowerLevel 2 Phosphorus concentration.6,30−32 Level 3 is known asthe best achievable performance level and targets extremely loweffluent nutrient levels to comply with ongoing or anticipatedregulations. A WERF-sponsored survey and independentstudies on advanced phosphorus removal technologies6,31,33

have indicated that, to attain these extremely low level ofnutrient levels, multiple tertiary processes for chemical Premoval are necessary, and the addition of more external carbonfor postdenitrification is required.3,26,28 WWTPs performanceat this level of treatment are also dependent on local weatherconditions, process implementation, and wastewater character-istics as well as skilled operation and maintenance.6

Treatment Plant Process Configuration Design Alter-natives. On the basis of the reviews of current and leading-edge treatment technologies for advanced nutrient removalfrom municipal wastewater,6 we designed a set of 27 treatmenttechnologies and process scenarios representing three differentlevel of treatments for this study (Figure 1 and Table S1). Forall of the treatment scenarios, an influent flow of 10 MGD andU.S. average representative influent properties were chosen asthe design influent parameters,34 as summarized in Table 2. Adesign life of 20 years was also chosen, which is typical forwastewater works in United States. For Level 1 treatment, atotal of seven treatment scenarios were considered that includechemical P removal process with biological nutrient removal(BNR) and combined chemical−biological P-removal pro-cesses. Chemical P removal processes were selected byconsidering different chemical addition strategies as an addition

Figure 1. Process diagram of the treatment scenario alternatives covering three levels of nutrient removal target. Process abbreviation: PC, primaryclarifier; MLE, modified Ludzack−Ettinger; Bardenpho, five-stage Bardenpho; UCT, University of Cape Town process; EC, external carbon;Filtration, traditional filtration; Filt. Cont. Back., filtration with continuous backwash; Sedimentation, sedimentation; Ball. Sed., ballastedsedimentation; Memb. Filt., membrane filtration; Rev. Osm., reverse osmosis. See more detail process description in Table S1.

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to the primary clarifier, to the secondary clarifier, or prior toboth primary and secondary clarifiers. A pair of the most widelyused BNR processes (5-stage Bardenpho and the University ofCape Town (UCT)) were selected as representative secondarytreatments with nutrient removal configurations. A total of 10treatment alternatives were considered each for Level 2 andLevel 3 treatments. To achieve Level 2 treatment, we adopted atertiary process following the BNR process. Tertiary processeswere chosen to cover each of the commonly applied processesincluding chemically enhanced sedimentation, ballasted sed-imentation, various filtration processes, and membranefiltration technologies. For Level 3 treatment, multistagetertiary processes were considered to meet the most stringenteffluent phosphorus and nitrogen limits.6,31,33

Process design parameters for all the primary, secondary, andtertiary processes were selected on the basis of MOP, relatedliterature,34,35 and manufacturer specifications. For secondarytreatment with nutrient removal processes, design simulationtools (e.g., BioWin or in-house spreadsheet models) wereapplied to assist the design process. The Total Solid RetentionTime (SRT) selected for each of the 27 plants was 10 days.Influent characteristics and typical kinetic parameter values ofmicrobial communities used in BioWin model were obtainedfrom the literature.34 External carbon addition for enhancednitrogen removal, chemical doses, and P removal rates fordifferent tertiary processes as well as other design parameterswere collected from the literature and are listed in Tables S2−S4.30−32,34,36 In addition, data regarding the sizes of thecommercial tertiary treatment processes were obtained fromtheir respective vendors’ Web sites, as noted in Table S2.Life-Cycle Assessment. The process units included in the

LCA model of nutrient removal processes were preliminary,primary, and secondary treatment with nutrient removal as wellas tertiary processes, where applicable. The system boundaryincluded influent pumping to effluent discharge and sludgepumping to sludge-treatment facilities, while the functional unitwas taken as 1 m3 of influent wastewater. Construction andoperation phases were included, while plant maintenance anddecommissioning after the 20 year design life were excluded.Downstream sludge management was also excluded for tworeasons. First, the impacts of sludge treatment and disposalfacilities on eutrophication (<5%), acidification (<20%), andglobal warming (10−20%) are reported to be small comparedto the other WWTP processes.25,37−39 Second, and moreimportantly, the range of environmental impacts from sludgetreatment and disposal reported in several of the earlier LCA

studies varied widely8,14,25,37−39 and are highly dependent onlocal factors that govern how the sludge is managed, includingextent dewatering, distance to final disposal, incineration versuslandfilling versus land application, and local climate, amongother factors. In contrast, WWTP operations are less dependenton location and local waste-management considerations. This isin line with Opher and Friedler, who reported methods toreduce the inventory data requirements for WWTP LCAstudies and who suggest that sludge treatment facilities beomitted from comparative analyses.40

Life-Cycle Inventory. A life-cycle inventory (LCI), orcomprehensive list of resource inputs and emissions acrossthe life cycle, was compiled for each of the treatment scenariodesigns. Operating energy and chemical requirements forprimary and secondary processes were calculated directly fromthe plant designs using BioWin. The reactor sizes were basedon design model simulations to meet the targeted effluentquality. Materials required for plant construction were limitedto concrete and steel requirements. The inventory data alsoincludes direct emissions of the three greenhouse gases CO2,CH4, and N2O from secondary with nutrient removalprocesses, following EPA methodology,41 where CO2 andCH4 were modeled based on the stoichiometry of degradationof organic compounds in biological processes, and N2O wasmodeled from the nitrification−denitrification stoichiometry.Fugitive emissions from WWTPs are dependent on variousdynamic factors including influent characteristics (BOD andnutrient concentrations) and operating conditions (temper-ature, pH, dissolved oxygen, SRTs, and reactor types).42−44

However, the current study assumes annual average influentproperties and static operating conditions. In addition,following IPCC guidelines, biogenic CO2 emissions fromWWTPs are excluded.45 Table S3 lists the calculated life-cycleinventories of different treatment scenarios. Each LCIcomponent was matched to the most closely related unitprocesses in the US-EI 2.2 LCI database (Earthshift,Huntington, VT), which is the ecoinvent database modifiedfor the U.S. energy system and assembled in the LCA softwarepackage SimaPro 8.1 (PRe Consultants, Amersfoort, Nether-lands). Table S5 lists the US-EI 2.2 unit processes that havebeen entered in SimaPro.

Life-Cycle Impact Assessment. Life-cycle impacts of theWWTPs were assessed using the Tool for the Reduction andAssessment of Chemical and Other Environmental Impacts(TRACI 2.1) impact assessment method developed by theUSEPA.17 Such assessment methods link emissions to impactsthrough multimedia fate transport exposure effect models, oftenstitching together multiple models to calculate characterizationfactors (impact per unit of emission) for each impact categoryacross hundreds or thousands of potentially emitted substances.Details of the TRACI methodology can be found in theliterature.17,29

A list of selected impact categories from the TRACI model ispresented in Table S6. The impact categories of smogformation, respiratory effects, and fossil-fuel depletion wereomitted from the analysis to focus on impacts related to waterquality and global environmental concerns. Eutrophication inthe TRACI 2.1 model considers the effect of substancesemitted to water and air, and so includes atmosphericdeposition. Emissions of N, P, and COD−BOD are scaled toequivalent nitrogen in the affected water body (kg N eq).46 Theglobal impact categories are ozone depletion potential,expressed in terms of CFC-11 equivalents, and global warming

Table 2. Influent Wastewater Properties Used in LCAAnalysisa

element description influent

design life, yrs. 20flow, MGD 10total carbonaceous BOD, mg BOD/L 200volatile suspended solids, mg COD/L 160total suspended solids, mg TSS/L 200total Kjeldahl nitrogen, mg N/L 35total P, mg P/L 8nitrate N, mg N/L 0pH 7.2alkalinity, mg/L as CaCO3 250temperature, °C 20

aBased on Tchobanoglous et al.34

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potential, expressed as CO2 equivalents. Acidification potentialis expressed in terms of H+ equivalents, thus altering the pH ofwater bodies. Both ecotoxicity and human health toxicity in theTRACI model are estimated using the UNEP−SETACconsensus toxicity model USEtox47 and are expressed incomparative toxic units (CTU). Human toxicity (CTUh) ischaracterized by human health cancer and noncancer potentialand is estimated by morbidity increase in the total humanpopulation per unit mass of an emitted chemical. Eco-toxicity(CTUe) represents an estimate of the potentially affectedfraction of species (PAF) integrated over time and volume perunit mass of a chemical emitted.TRACI impact assessment absolute results can be

normalized by United States annual totals for each impactcategory, allowing for comparison across indicator results usinga common benchmark. Normalization factors are presented,along with sample normalized impact results, in Table S9.Uncertainty Analysis. We performed a series of

uncertainty analyses that considered the variability in theinfluent wastewater characteristics (carbon to phosphorus ratio,C/P), as well as chemical and electricity usages variationsreported for different tertiary treatment processes. For WWTPwhose influent C/P or C/N ratios are unfavorable for effectivebiological nutrient removal, an external carbon source such asmethanol is often supplemented. For uncertainty analysis, wesimulated the scenarios with C/P ratio ranged from the 15 to25 mg/mg. Variation ranges for ferric chloride, alum, andelectricity were based on literature values, with the mean equalto that used in BioWin process modeling. All ranges andparameter values are listed in Table S7. These direct inputs

were combined with existing log-normally distributed un-certainty ranges for all indirect inputs and emissions (back-ground processes) established for the US-EI unit processes. A10 000 run Monte Carlo simulation was conducted in SimaProto investigate the effects of these parameter distributions on theoverall LCA results, providing sets of summary statistics on thedistributions of the LCA results. We also discuss modeluncertainty for the derivation of characterization factors in theTRACI model and implications for interpreting the results.

■ RESULTS AND DISCUSSION

Life-Cycle Eutrophication Potential. LCA considers life-cycle eutrophication potential attributed to both direct effluentnutrients and indirect emissions from the production anddelivery of energy and chemical inputs involved in the varioustreatment processes to achieve different nutrient-removaltargets. As shown in Figure 2a, as more rigorous treatmenttechnologies are applied to achieve higher nutrient-removalefficiency, the effluent nitrogen and phosphorus concentrationswere greatly reduced as expected. Effluent total nitrogenconcentrations were all within the limit of each targetedtreatment level (8, 3, and 1 mg/L in Levels 1, 2, and 3,respectively). For Level 1 plants, nitrogen removal was achievedthrough biological processes. For the achievement of effluentnitrogen level beyond Level 1, external carbon sources such asmethanol are added to remove additional nitrogen in thoseplants. Effluent total phosphorus concentrations in all of theLevel 1 treatment plants were less than the target value of 1mg/L. For Level 2 and Level 3 treatments, employment ofadvanced tertiary phosphorus removal technologies led the

Figure 2. Comparison of effluent phosphorus and nitrogen concentration and respective eutrophication potential for various treatment processesdesigned to achieve three different levels of effluent nutrients limits. See the process abbreviation of plant ID and description in Table S1. The errorbars represent a 95% confidence interval of the variability due to uncertainty in nutrient removal rate in tertiary processes and chemical andelectricity usage. Note: NRP, nitrogen removal process; Chem, chemical P removal (the numbers refers to different feeding strategies); BNR,biological nutrient removal (1: 5-stage Bardenpho; 2: UCT process); EC, external carbon; MF, membrane filtration; Fil1, filtration; Fil2, filtrationwith continuous backwash; Sed1, sedimentation; Sed2, ballasted sedimentation; MF, membrane filtration; RO, reverse osmosis.

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plants to achieve target effluent P concentrations of 0.1 and0.01 mg/L, respectively.Figure 2b depicts life-cycle eutrophication potentials,

encompassing both direct and indirect eutrophication effects.Both chemical consumption and electricity requirements arehigher for treatment processes to achieve Level 2 and Level 3nutrients limits than those required to deliver Level 1 effluentquality. However, indirect eutrophication potentials also varywidely within each treatment level. For example, of the twoBNR processes configurations analyzed, UCT plants (Plant IDswith BNR2) have higher contributions to life-cycle eutrophi-cation due to the higher pumping and recycling of activatedsludge than do plants with a Bardenpho process (Plant ID withBNR1; see Table S1 for treatment scenario ID and plantconfigurations). Processes that involve multistage tertiarytreatment and process that require additional electricity andchemicals use resulted in higher-level indirect eutrophicationpotential. In particular, indirect eutrophication for configu-rations that include reverse osmosis (RO) are almost five timesgreater than that of Level 1 due to the high electricity use forRO and deposition of NOx emitted from power plants.

Operation- and construction-phase chemicals and materialsrequirements per m3 of influent wastewater and electricityrequired for each process are reported in Table S4. Concreteand steel requirements are amortized over the 20 year lifetimethroughput of each treatment plant and thus varied relativelylittle among options.Overall, the results indicated that the average eutrophication

potential can be reduced by about 70% when Level 2treatments are employed instead of Level 1. However,implementation of the more advanced tertiary processes inLevel 3 treatment may only lead to about 15% furtherreduction in life cycle eutrophication potential, with a markedincrease in indirect eutrophication impacts. Moreover, Level 1plants can reduce by almost two-thirds the eutrophicationresulting from the BOD-only removal treatment scenario(results are shown in Table S8). Therefore, the extent ofeutrophication potential reduction is not proportional orcomparable to the effluent nutrient reductions provided byincreasing treatment levels. In other words, the benefits ofeutrophication reduction achieved by the advanced nutrientremoval were diminished or reversed by increases in indirect

Figure 3. Global warming, ozone depletion, and acidification potential for different treatment levels. See the process abbreviation of plant ID anddescription in Table S1. The error bars represent a 95% confidence interval of the variability due to uncertainty in nutrient removal rate in tertiaryprocesses and chemical and electricity usage. Note: NRP, nitrogen removal process; Chem, chemical P removal (the numbers refers to differentfeeding strategies); BNR, biological nutrient removal (1: 5-stage Bardenpho; 2: UCT process); EC, external carbon; MF, membrane filtration; Fil1,filtration; Fil2, filtration with continuous backwash; Sed1, sedimentation; Sed2, ballasted sedimentation; MF, membrane filtration; RO, reverseosmosis.

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eutrophication potential brought by the increases in energy andchemical use.Life-Cycle Greenhouse Gas Emissions, Ozone Deple-

tion, and Acidification. Advanced treatment schemesimplementation for enhanced nutrient removal can also leadto other negative environmental impacts including GHGemissions, ozone depletion, and acidification arising from directWWTP emissions and indirect, upstream emissions. Figure 3apresents the global-warming potential of different plantalternatives. Direct emissions of CH4 and N2O from thebiological reactors were found to be the predominant source ofGHG emissions. Additional fugitive emissions from nutrientremoval increase the global warming results by more than threetimes compared to the BOD-only removal plant (results shownin Figure S8). Treatment processes that are highly electricityintensive, such as RO, are associated with much higher levels ofindirect GHG emissions from power plants. Due to thegenerally higher levels of chemical use in the sedimentationprocesses compared to those in filtration processes, tertiarysedimentation processes produce higher global warming

potential than the filtration processes. Between the two typesof filtration processes, filtration with continuous backwashgenerated slightly more global warming potential than thetraditional filtration.Nearly 90% of ozone-depletion potential (ODP) results from

chemical use in various treatment processes, especially from therelease of chlorinated substances. Application of morechemical-intensive treatment processes such as tertiarysedimentation and filtration processes, where coagulants andpolymers are required at high stoichiometric ratios for achievinglow nutrients levels,31 resulted in elevated ozone-depletionpotentials (Figure 3b). It is important to note that these resultsdo not include the ozone depletion effects of fugitive N2Oemissions, although these have been shown in other work todominate life-cycle ODP.9,48 The TRACI LCIA method doesnot currently classify N2O as an ozone-depleting substance,even though the EPA has published a recommended ODPvalue.49 Lane and Lant discussed the implication of includingN2O in ODP calculations and recommend a value of 0.017 kgCFC-11 eq per kg of N2O emitted.48 If this value is adopted in

Figure 4. Ecotoxicity and human health toxicity (cancer and noncancer) associated with different treatment levels. Toxicities were measured incomparative toxicity unit (CTU). See the process abbreviation of plant ID and description in Table S1. The error bars represent a 95% confidenceinterval of the variability due to uncertainty in nutrient removal rate in tertiary processes and chemical and electricity usage. Note: NRP, nitrogen-removal process; Chem, chemical P removal (the numbers refers to different feeding strategies); BNR, biological nutrient removal (1: 5-stageBardenpho; 2: UCT process); EC, external carbon; MF, membrane filtration; Fil1, filtration; Fil2, filtration with continuous backwash; Sed1,sedimentation; Sed2, ballasted sedimentation; MF, membrane filtration; RO, reverse osmosis.

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the present work, N2O emissions from bioreactors would beresponsible for more than 99% of ODP for all of the plantoptions.Acidification potential also increased as more stringent

effluent limits were met by adopting additional tertiaryprocesses. Figure 3c reveals that acidifying substances releasedduring chemical production (e.g., H2SO4 during alumproduction) or from fossil-fuel combustion (e.g., SO2) are themain drivers of acidification impacts. Figure 3c also reveals thatthe electricity-intensive RO process also has high acidificationpotential, again due to SO2 emissions from fossil power plants.Life Cycle Ecotoxicity and Human Health Impacts.

Each treatment-plant option has life-cycle ecotoxicity andhuman-health impacts that must be weighed against thebenefits of reduced eutrophication. These toxicity impacts arenot due to emissions from the treatment processes themselvesbut rather to upstream emissions from the extraction andindustrial production of chemicals, materials, and energy.Figure 4 presents both human toxicity and ecotoxicitypotentials for each treatment option, expressed in comparativetoxicity units (CTU). Due to differences in exposure pathways,modes of action, and types of effect, human health toxicity isassessed in separate cancer and noncancer impact categories.17

For cancer-related impacts, compared to Level 1 treatment, theimplementation of Level 3 treatment technologies couldpotentially increase cancer-related human-health impacts by60−440%, with alternatives that utilize RO at the high end ofthat range due to releases of carcinogenic emissions from powerplants. The ranges stated here reflect results across treatmentprocess configurations at the same treatment level; uncertaintyanalysis results with each configuration are described in thefollowing section. Similarly, noncarcinogenic human healthimpacts and ecotoxicity also increase with treatment levels.Even the Level 1 plants showed at least twice the ecotoxicitythan that which resulted from the baseline BOD-only treatmentalternative (Table S8). For both human and ecological toxicity,the production of treatment chemicals is the highestcontributor to impacts, while emissions from producing theelectricity required for pumping and wastewater recyclingwithin the treatment plants are the second-largest contributors.As for other impact categories, among the tertiary P removalprocesses, plants that incorporate sedimentation processes yieldhigher ecotoxicity and carcinogenetic impact than the filtrationprocesses due to higher chemical requirements. Treatmentalternatives that utilize microfiltration have a comparable levelof life cycle toxicity to other filtration processes; however, forLevel 3 treatments, they led to higher carcinogenic impactsbecause microfiltration is followed by a sedimentation process.The choice of BNR process also impacted toxicity potentials, ashigher electricity requirements for internal recycling for theUCT process than for Bardenpho resulted in additional power-plant emissions.Uncertainty Analysis in LCA Assessment of Nutrient

Removal Technologies. In addition to variations in influentcharacteristics (i.e., influent C/N and C/P ratios) that mayaffect process design, there are also uncertainties associatedwith chemical and energy use due to varying operating andambient conditions. The process design parameters andassociated chemical, pumping energy for secondary treatmentand nutrient-removal processes design are well-establishedaccording to standard design guidance.34 However, for theadvanced nutrient-removal tertiary processes, especially those

emerging technologies that exist only at pilot scale with limitedfull-scale data, these parameters carry greater uncertainty.Uncertainty analysis using the Monte Carlo simulation for

varying influent C/N and C/P ratios, as well as with a varyingrange of chemical and energy usage, were performed, and a95% confidence interval of the analyses are shown as error barsin Figures 2−4. The results reveal that the uncertaintyassociated with variations in chemical and energy input hadmarginal impacts on Level 1 treatment but presentedobservable impacts on higher-level treatments. The influenceof the tertiary process input variation on eutrophicationpotential was relatively low compared to the other impacts.This is due to the fact that life-cycle eutrophication is lessaffected by energy and chemical usage than by the effluentnutrient concentration, which varied only slightly from thevariations in chemical dose and energy use. Configurationsincluding the RO process exhibited the highest uncertaintiesamong all the tertiary nutrient removal processes due tovariations in electricity requirements.The highest uncertainty among the tertiary processes is

associated with ecotoxicity and human-health toxicity impactcategories, as is typical in LCA. There are several reasons forthis. First, impacts for these categories are driven by indirectemissions from electricity and chemicals production, which aregreater for tertiary processes. Second, chemical and electricityuse information is collected from limited available literature. Awide range of electricity requirements for RO processes wasreported in the literature, which led to the wide variation intoxicity results of the scenarios with RO processes. Third,uncertainty in emissions from background unit processes has amore pronounced effect when toxicity impacts are charac-terized, whereas uncertainty in global warming potential issmall, with well-established characterization factors, such as thefact that the resulting confidence intervals are relatively narrow(Figure 3a).Another important source of uncertainty stems from the use

of the TRACI LCIA model itself, which is the basis for all of thecharacterization factors used in the present analysis. AlthoughTRACI was developed to be specific to North America and isconstructed from USEPA models and data sources, there is stilluncertainty in applying national average factors to a specificlocation, particularly for eutrophication. Specifically, TRACImodels both N and P emissions as eutrophying, even though Nis most commonly limiting for coastal areas and P forfreshwater systems. Other LCIA models, such as ReCiPe,50

have separate impact categories for marine and freshwatereutrophication. There are several reasons for TRACI’scombined approach, discussed in detail in Norris,46 the mostimportant of which is that TRACI considers eutrophication notjust in the receiving water body but also in water bodiesdownstream. To this end, TRACI models the fate and transportof nutrients in water bodies over the entire continent.Transport and deposition of eutrophying air emissions arealso included. Here, we are considering site-generic WWTPsand so use continental average characterization factors. For site-specific results, new characterization factors would have to bederived for eutrophication (as well as for acidification andhuman- and ecotoxicity).

Comparison with Previous Studies. Although there arefew LCA studies focusing on tertiary P removal processes,several studies presented different impact categories forsecondary with nutrient removal processes. Most of theimpacts observed in the current study are comparable with

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the findings of other studies for plants with similar processesand scopes (as discussed in the Materials and Methods section,sludge management and disposal is excluded from this study).Coats et al. finds eutrophication potential of biological andbiological plus chemical P removal plants in the range of0.012−0.016 kg N eq/m3 treated of wastewater.10 In our study,Level 1 plants that include similar processes find eutrophicationpotential in the range of 0.016−0.018 kg N eq/m3 (Figure 2),which is comparable with the Coats et al. study. Globalwarming is more widely reported than other impact categories.Here, we report global-warming results of 0.6−0.9 kg CO2 eq/m3, compared to 0.4−0.5 kg CO2 eq/m

3 for Level 1 equivalentplants from Falk et al. and 0.4−0.6 kg CO2 eq/m

3 for Foley etal. (Figure 3).5,9,51 Disparity in these results can be attributed todifferences in scope, process simulation tools, and emissionfactors. The scope of Falk et al. study included sludge treatmentand disposal and administrative activities that are not includedin the current study and that contribute to higher energy use.Electricity requirements for Level 1-equivalent plants werereported as 0.6 kWh/m3 by Falk et al.5,51 and 0.28−0.35 kWh/m3 in the current study (Table S3). However, the life-cyclecarbon intensity of the electricity factor used here (760 g CO2eq/kWh) is 25% higher than the combustion-only factor usedin Falk et al. due to the inclusion of the fuel cycle and otherindirect emissions. Life-cycle GHG emission factors forchemicals production are also taken from updated sourcesand are 15−130% higher than those from Falk et al. Finally, thefugitive emission estimation factors used in their study resultedin lower N2O emissions than the EPA model used here, whichalso contributed to the difference in GHG emissions results.Implications. The goal of this study is to understand the

benefits and trade-offs of advanced wastewater treatmenttechnologies with consideration of both local water-qualitybenefits and systems-level impacts. The LCA analysis amongdifferent nutrient-removal technologies demonstrate thatpushing for a higher level of treatment can further improvethe eutrophication potential locally but have other negativeimpacts or co-costs to the environment and human health thatare not often considered in nutrient management and policy-making. The results demonstrate that, even with considerationsof the uncertainties, the implementation of more advancednutrient-removal processes may lead to more intensifiednegative environmental and health impacts as unintendedconsequences for the benefits gained from nutrient removal andeutrophication reduction. Watershed-based permitting throughTMDLs have expanded the scale of water-quality management,although LCA could help to inform regional policymaking atyet larger scales to ensure overall net benefits to water quality.Beyond just affecting local water quality, wastewater treatmentin the United States is a significant contributor to overall energyuse and air emissions.52,53 While developing regulations onnutrient removal, regulatory agencies may consider multipleenvironmental goals for wastewater treatment to balancebetween intended benefits and unintended co-benefits andco-costs. A major focus of wastewater engineering currently isreducing net energy use and improving resource recovery. Asshown here, LCA applied to novel wastewater technologies andprocess configurations can provide early estimates of thesetrade-offs.As mentioned in the Materials and Methods section, the

current study does not include the effects from the sludgetreatment and disposal units, which depend on the quantity andcomposition of sludge from each treatment-process config-

uration, as well as on site-specific management practices. Werecommend combining the results shown here with sludgegeneration information and site- and practice-specific LCAresults for sludge management (for example, as reported inAlanya et al. and Yoshida et al.)54,55

Another opportunity for ongoing research is to improve theresolution of eutrophication models used in life-cycle impactassessment to the point where emissions can be both spatiallyand temporally specified for use in watershed-level analysis.This is currently achievable for direct effluent discharges but ismuch more challenging for indirect emissions (for example, airemissions from power generation, some portion of which maybe deposited in the watershed in question). Investigating thegeographic variability of the results of WWTPs can be done bymodifying the geographic locations of the input variables inLCA analyses. This would allow for regional assessment of thelife cycle implications of meeting stricter nutrient limits andmay also inform a more regional strategy to wastewatertreatment in the United States.Advanced tertiary treatment for N and P reduction will also

have co-benefits that could be quantified using LCA, forexample, in enabling water reuse or the reduction of chemicalsof emerging concern (CECs, which have received much lessattention in the LCA literature than macronutrients and yet areincreasingly targeted in wastewater engineering and policy.Previous studies include Munoz et al., who applied LCA toanalyze the water quality benefits achieved from CEC removalin Spanish urban WWTPs,18 and Igos et al., who comparedCEC removal in decentralized and conventional WWTPsagainst the environmental costs of treatment,56 finding that thebenefits of reducing pharmaceuticals are negligible compared tothe costs of additional treatment. Consideration of the multiplelife-cycle co-benefits of tertiary treatment may provideadditional useful information in comparing treatment optionsto meet future water-quality regulations.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.est.5b05070.

Tables listing characteristics of treatment plant alter-natives, design criteria and inventory basis of thetreatment processes, life-cycle inventories, chemicaldoses and P removal rate in tertiary processes, a list ofmatched unit processes, a list of LCA impact categories,ranges of parameters for uncertainty analysis, LCA resultsfor baseline BOD-only treatment scenario, and normal-ized scores for the Level 1 plant- BNR1. (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*Tel: +1 617 373 4256; fax: +1 617 373 4419; e-mail: m.eckelman@neu.edu.*Tel: +1 617 373 3631; fax: +1 617 373 4419; e-mail: april@coe.neu.edu.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis study was funded by Water Environment ResearchFoundation (WERF) and Northeastern University Interdisci-plinary research grant. The authors thank the anonymous plant

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staff and technicians for their assistance in sampling and processinformation collection. Special thanks are also given to theWERF Nutrient Challenges Program Team, especially Dr. AmitPramanik (WERF), Dr. J. B. Neething (HDR Inc.), Dr. DavidStensel (University of Washington), David Clark (HDR Inc.),Dr. Ryujiro Tsuchihashi (AECOM), and Dr. Julian Sandino(CH2M Hill) for their advice and support.

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