Life cycle assessment (LCA) of urban water infrastructure: emerging approaches to balance objectives and inform comprehensive decision-making

Diana M. Byrne a, Hannah A. C. Lohman a, Sherri M. Cook b, Gregory M. Peters cd and Jeremy S. Guest *a
aDepartment of Civil & Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, USA. E-mail: jsguest@illinois.edu
bDepartment of Civil, Environmental, and Architectural Engineering, University of Colorado Boulder, Boulder, CO, USA
cDivision of Environmental Systems Analysis, Chalmers University of Technology, Gothenburg, Sweden
dSchool of Civil and Environmental Engineering, University of New South Wales, Sydney, Australia

Received 6th June 2017 , Accepted 22nd July 2017

First published on 26th July 2017


Life cycle assessment (LCA) has been widely used to quantify environmental impacts associated with urban water infrastructure, including wastewater, drinking water, stormwater, and integrated urban water systems. While LCA is applicable for the estimation of global environmental impacts, decision-makers must balance these impacts with local, often regulated, environmental and public health objectives. To characterize the state of the art for the use of LCA for urban water infrastructure, a thorough literature review was conducted of papers that applied LCA to wastewater (173 papers), drinking water (44 papers), stormwater (17 papers), and integrated urban water (22 papers) systems. Analyses revealed current preferences for functional unit basis (e.g., volume for wastewater), system boundaries (e.g., focus on operation), and impact assessment methodology (CML, ReCiPe, and Eco-Indicator). Based on these findings, LCA methodological recommendations for urban water infrastructure are made, and emerging opportunities to balance objectives and inform comprehensive decision-making are discussed. Critical opportunities include the integration of spatial considerations (e.g., spatialized characterization factors), water quantity (e.g., water quantity indicators), public health (e.g., integration with risk assessment), economic and social assessments (e.g., life cycle costing and social LCA), along with prioritization of continuous stakeholder engagement. Finally, research and development needs specific to the use of LCA for urban water infrastructure (e.g., development of new indicators coupled with case studies) are prioritized.


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Diana Byrne

Diana is a Ph.D. student in Environmental Engineering at the University of Illinois at Urbana-Champaign (UIUC). She earned a B.S. degree in Civil Engineering from Saint Louis University and a M.S. degree in Civil Engineering from the UIUC. Her research focuses on the development of quantitative sustainable design methods for evaluating environmental and human health impacts of water and sanitation infrastructure in both developed and developing communities.

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Hannah Lohman

Hannah is a M.S. student in Environmental Engineering at the University of Illinois at Urbana-Champaign, where she also received her B.S. in Civil Engineering. Her research is at the nexus of engineering, economics, and public health with an emphasis on navigating tradeoffs in the development and deployment of water, sanitation, and hygiene interventions in resource-limited settings.

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Sherri Cook

Dr. Cook is an Assistant Professor in the Environmental Engineering Program and in the Civil, Environmental, and Architectural Department at the University of Colorado Boulder. She received her M.S.E. and Ph.D. in Environmental Engineering from the University of Michigan and B.S. in Civil Engineering from Virginia Polytechnic Institute and State University. Dr. Cook's research interests focus on the design and development of sustainable water treatment and infrastructure systems, specifically investigating the operational limits of biological treatment systems; evaluating novel environmental biotechnology approaches to recover resources from waste and to improve civil infrastructure resilience; and developing sustainability assessments for technology development.

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Gregory Peters

Dr. Peters is a Professor in the Division of Environmental Systems Analysis at Chalmers University of Technology and is an Adjunct Academic in the School of Civil and Environmental Engineering at the University of New South Wales. He holds both a B.E. in Chemical Engineering and a Ph.D. on the biogeochemistry of selenium from the University of Sydney. Dr. Peters' research is aimed at expanding the application of sustainability assessment tools and improving their accuracy. Recent research projects relate to the clothing, water, agricultural and chemical industries, the regulatory sector, and the fate of nitrogen discharges in wastewater.

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Jeremy Guest

Dr. Guest is an Assistant Professor in the Department of Civil and Environmental Engineering at the University of Illinois at Urbana-Champaign. He holds B.S. and M.S. degrees in Civil Engineering from Bucknell University and Virginia Polytechnic Institute and State University, respectively, and a Ph.D. in Environmental Engineering from the University of Michigan. Dr. Guest's research interests include the advancement of sustainable technologies for engineered water systems, with an emphasis on the development of design tools to expedite innovation and align economic, environmental, and social trajectories for sustainable development.



Water impact

Although LCA is useful for characterizing global environmental impacts of urban water infrastructure, decision-makers must balance these impacts with the need to prioritize the protection of local environmental and public health. This review describes the state of the art, identifies emerging opportunities, and develops a path forward for LCA to better address the multiple sustainability demands of urban water systems.

I. Introduction

Life cycle assessment (LCA) is often an effective tool to support decision-making, as its incorporation of multiple life cycle stages and impact categories can highlight cases of problem shifting.1 It allows for the consideration of trade-offs without requiring the weighting of results, and the ISO standards 14040/14044 have provided a framework for these studies to help ensure consistency and to prevent their misuse or misrepresentation.2,3 LCA results have often been used to target specific design and operational improvements, motivate environmental policies, and identify future research needs.4–6 Over the last two decades, LCA has been increasingly used as a tool to quantify environmental impacts associated with urban water infrastructure (wastewater, drinking water, stormwater, and integrated urban water systems). Despite the tool's robustness and wide application, LCA's global environmental focus may at times be in tension with urban water infrastructure's goals of protecting local public and environmental health. Because of this misalignment of goals, global environmental impacts (e.g., greenhouse gas emissions), which are spread over much larger spatial and temporal scales, can often be ignored by decision-makers who are focused on more pressing, local impacts.

Previous reviews concerned with the application of LCA to wastewater4 and integrated urban water systems1 have focused on summarizing the state of the art regarding LCA methodological choices (e.g., functional unit, impact assessment method). In addition, they have discussed LCA's limitations and recommended solutions, such as spatially differentiated characterization factors, and acknowledged the need to incorporate economic and social assessments when evaluating infrastructure alternatives.1,4 Ultimately, leveraging well-established methods from other disciplinary domains (e.g., public health, social science) will enable LCA to be a more effective tool for addressing the functional (e.g., manage water quantity), environmental (e.g., reduce impacts to local water bodies), social (e.g., protect public health), and economic (e.g., reduce life cycle costs) sustainability demands of urban water systems.7 To date, however, the true integration of such methods with LCA has been limited, especially in the context of urban water infrastructure.

The objectives of this frontier review are (1) to comprehensively describe the state of the art for LCA applied to urban water infrastructure (including wastewater, drinking water, stormwater, and integrated urban water systems); (2) to identify emerging opportunities to advance LCA (specifically relating to spatial considerations and water quantity) and integrate LCA into broader sustainability assessment frameworks (addressing public health, and economic and social assessments); and (3) to determine a path forward to more effectively use LCA in the research, planning, design, and operation of urban water infrastructure systems. These objectives were achieved by conducting a literature review of papers that applied LCA to urban water systems as well as papers discussing methods from other areas of research that were applicable to the multiple sustainability demands of urban water infrastructure.

II. State of the art

II.A Methodology for literature review of urban water LCAs

To better understand the use of LCA in research related to urban water infrastructure, articles focusing on drinking water, wastewater, stormwater, and integrated urban water were gathered through Scopus using title, abstract, and keyword search terms that accounted for the variety of names for each infrastructure type (Table S1 of the ESI). All searches were limited to research articles published from 1998 through May 2017 (including in press manuscripts). The searches yielded 1581 wastewater, 258 drinking water, and 259 stormwater research articles, with the implicit inclusion of integrated urban water (because these systems are comprised of two or more urban water infrastructure types). Each paper was then individually screened to identify the water system type and to determine if it met any of the following exclusion criteria: an LCA was not conducted, the focus was not an urban water system, or it was a review. A total of 173 wastewater, 44 drinking water, 17 stormwater, and 22 integrated urban water papers were retained for further analysis (full list in section S2 of the ESI).

II.B Application of LCA to urban water infrastructure

These 256 papers were analyzed to collect information regarding how decisions and assumptions were made in each requisite LCA phase: goal and scope definition, inventory, impact assessment, and interpretation.2,3 Search terms were used to determine the key decisions and assumptions included in each study (Table S3 of the ESI). The review of goal and scope definition phase focused on the functional unit and the life cycle stages (e.g., construction, operation, and end of life) included in the system boundary, while impact assessment and interpretation focused on the impact assessment method selected and inclusion of sensitivity and weighting.
Functional unit. In the majority of research articles, the basis of the functional unit was either volume, volume with a treatment objective, or person equivalent (defined as the total composition of mass and flow produced by one person each day). The basis, though, differed across infrastructure types (Fig. 1). For wastewater, volume (61.3%) was used most often, followed by a basis of volume with treatment objective (15.0%) and person equivalent (p.e.) (12.7%). For drinking water, volume (47.7%) and volume with a treatment objective (47.7%) were chosen most frequently. Similarly, for integrated urban water, volume (68.2%) and volume with a treatment objective (13.6%) were also chosen most often. Some studies of stormwater systems also used volume (41.2%) as the functional unit, but area (e.g., stormwater collection area, treatment area) was the most common choice (52.9%). These results align with similar reviews conducted on integrated urban water and wastewater studies, where volume was the most common functional unit.1,4
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Fig. 1 Literature review results for the basis of functional unit and impact assessment methodology by infrastructure type (wastewater, drinking water, stormwater, integrated urban water). Thickness of line corresponds to the number of papers. Volume was the most common basis for the functional unit overall, while volume with a treatment objective was equally common for drinking water and area was most common for stormwater. In general, CML was the most widely used impact assessment method with Eco-Indicator as most common for drinking water and ReCiPe (midpoint) as most common for stormwater. Full numeric results can be found in Table S4 of the ESI.

There are multiple functional units that can be acceptable, but the common focus on per unit volume could introduce biases in wastewater and drinking water LCAs, especially when researchers and industry seek to compare findings across regions or countries. For example, domestic water consumption varies regionally and globally as a result of differences in climate, infrastructure capabilities, and societal expectations.8 Globally, annual per capita domestic water withdrawal ranges from 1.7 m3 (Somalia) to 292 m3 (Armenia),9 with significant variability even within developing countries where individuals in poorer communities are more likely to consume less water than those in wealthy cities.8 Depending on the functional unit selected, results can be impacted drastically.1 In the context of wastewater LCAs, however, more concentrated waste streams would probably incur greater impacts per m3 treated, but reduced impacts per capita due to increases in efficiency of treatment processes. The contaminant concentrations in municipal wastewater vary by 2–3 fold for mixed waste streams10 based on local water consumption and by up to two orders of magnitude when comparing mixed to source separated streams (e.g., urine).11 In the case of stormwater LCAs, selection of area as the functional unit was most common due to treatment system design based on collection areas in a region, but it should be noted that water quantity determinants such as precipitation, evaporation, and infiltration will influence the total volume of water entering the system.

Consideration of the LCA objective and the differences in water quality and use is critical to reduce the potential for unintended biases in the results. The functional unit in the context of urban water systems should include a water quantity metric and a management objective (Fig. 2). Water quantity may be defined by the volume of water (e.g., m3 of drinking water produced), the volume of precipitation over a catchment area (e.g., precipitation for a one acre catchment), or the volume generated by one person (e.g., wastewater generated by one p.e.). The management objective considers the water utility treatment and management goals beyond total quantity. This criterion includes water quality objectives – either as contaminant treatment requirements (e.g., log inactivation requirements in disinfection) or effluent/product water standards (e.g., effluent ammonia less than 5 mg-N L−1) – as well as flow management (e.g., required infiltration). In the case of drinking water and wastewater LCAs, selection of volume with a management objective of water quality may be appropriate when comparing two or more systems in one region because the study is more likely to be independent of per capita water consumption biases. However, if systems are compared across regions with differing per capita water usage, selection of person equivalent (instead of volume) may be more appropriate to enable comparisons between studies. This also enables studies to include a wider range of alternatives (e.g., dry toilets; low-pressure or vacuum sewers) which may have environmental benefits that surpass traditional technology. Stormwater studies most frequently selected area as a functional unit; however, including an infiltration or management goal could help mitigate differences in results stemming from varying rain intensity and duration across studies. It should be noted that alternative functional units (e.g., the grey images in Fig. 2) could also be appropriate if they are better suited to the goal of the specific LCA and the water management strategy such as the use of treated wastewater for groundwater recharge. Once careful consideration has been given to determine an appropriate functional unit, the next step is to determine which life cycle stages to include to assure that both the functional unit and system boundary allow for generalizable results and comparison between studies.


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Fig. 2 Recommendations for basis of functional unit (by infrastructure type) to include both a water quantity metric (volume, area, or person equivalent) and a management objective (flow or quality). The functional unit selection for integrated urban water systems is case-specific. The specific function of the infrastructure will vary even within water system type (e.g., stormwater infrastructure could be designed for conveyance or infiltration), and therefore no universal functional unit exists. Alternative functional units (grey images) may be appropriate if they are better suited to the goal of the specific LCA and the water management strategy.
Life cycle stages. The life cycle stages included in published LCAs varied based on the goal and scope of the LCA. Almost all studies included operation (98.4%), but fewer studies quantified the impacts of construction (55.5%) or end of life (25.4%), either because a comparative LCA was conducted and similar processes (among alternatives) were appropriately excluded or because these phases were assumed to be negligible. Several wastewater and drinking water studies that included construction within the system boundary found that impacts from the construction phase were negligible compared to those of the operation phase.12–16 In particular, for treatment processes with resource-intensive operation (e.g., high energy input), the environmental impacts may be much greater for facility operation than for plant construction, maintenance, and final disposal.15 There have been exceptions to this generalization, for example the work of Lundie et al. (2004) showed that in a decentralized system with local integrated water management, construction impacts were 40–50% of the conventional LCA indicator scores.17 Therefore the construction phase should be included in studies where decisions related to construction materials are important (e.g., materials that cause major impacts or that are derived from resource-limited substances), and for infrastructure having minimal operation and maintenance requirements (e.g., bio-infiltration rain garden for stormwater management).18 While many papers did not include end of life, the majority (52.9%) of stormwater papers included this stage. The decision to include construction and/or end of life stages should take into consideration the resource intensity of each life cycle stage, as well as the goal of the LCA and type of water infrastructure.
Impact assessment. Although endpoint and midpoint indicators endeavor to characterize human and environmental health impacts, translation of emissions to impacts may not be well aligned with a local focus of urban water systems. A common endpoint methodology utilized was Eco-Indicator, while midpoint methodologies included CML, TRACI, EDIP, and Eco-Points. ReCiPe and Impact 2002+ were also used and can fall into either category based on how it was applied. Wastewater LCA papers commonly used CML (27.5%) and midpoint ReCiPe (11.3%) as the chosen impact assessment methodology, while drinking water systems utilized Eco-Indicator (18.5%) and midpoint ReCiPe (12.3%). Stormwater methodologies included midpoint ReCiPe (31.6%), TRACI (15.8%), and Impact 2002+ (10.5%), while integrated urban water commonly used CML (19.4%) and midpoint ReCiPe (16.1%) (Fig. 1). CML was also found to be most frequently used as reported by reviews of wastewater and integrated urban water.1,4 The dominant impact assessment methodology selected varied by type of infrastructure, but selection of midpoint versus endpoint can influence the results due to the key differences in how environmental relevance of indicators is taken into account. Midpoint models provide a greater level of certainty, whereas endpoint models are generally considered to be more understandable to decision-makers because the information is consolidated into a single score.19 The selection of methodology can influence results and should be based on the LCA's goal, which may include determining a single system's environmental impacts or comparing two or more system options.19–22 In the case of an LCA conducted for a single system, a single score is not needed and midpoint methodologies may be more accurate and transparent. In contrast, for comparative LCAs, endpoint methodologies that consolidate environmental impacts into a single score may facilitate environmentally informed decision-making. As one example, endpoint methodologies – by including a wide range of effects – have the potential to help an analyst discover issues which had not been imagined at the start of an LCA study. Although endpoint categories begin to quantify human health impacts, LCA still does not have fully established ways to address these effects. Selection of methodology is ultimately based on the research objective; however, selecting a method that has the potential to be supplemented with additional assessments such as health could better shift research objectives to match the primary goals of urban water infrastructure.
Interpretation. Life cycle interpretation, the fourth LCA phase, allows decision-makers to analyze the results and make design recommendations to ensure consistency with the goal and scope of the study. Sensitivity analysis, a key component in LCA interpretation, is used to determine how uncertainty in input parameters influences the LCA results, but less than half of papers included this type of analysis. Of the papers considered, wastewater, drinking water, stormwater, and integrated urban water system LCAs included a sensitivity analysis with a frequency of 38.7%, 43.2%, 41.2%, and 40.9%, respectively. User-defined weighting criteria of different impact categories allows for the aggregation of these results to present them in a readily understandable format for decision-makers. Across papers using midpoint indicators, 16.8% included weighting criteria, and considering each water system type, wastewater, drinking water, and integrated urban water system papers used weighting criteria with a frequency of 14.8%, 34.8%, and 14.3%, respectively. No stormwater papers using midpoint methodologies included weighting criteria. Although weighting criteria are not required according to ISO standards, they can be useful for improving the accessibility of LCA results. However, improving the accessibility for interpretation requires embedded assumptions during the selection of weights. This may introduce accuracy and uncertainty issues, that if not properly addressed, may result in misleading messages.23 To help address this, a sensitivity analysis on the weighting criteria should always be conducted to determine how the uncertainty in weights influences the resulting outcome. The complete results of the literature review provide a more detailed data breakdown of the study (Table S4 of the ESI).
Integration into decision-making. Studies of urban water infrastructure should match the primary objectives of urban water systems, which are to manage water quantity and water quality in a way that protects local and regional human and environmental health. To develop this effort, spatial considerations, water quantity, public health, and economic and social factors should be considered to better inform decision-making. These factors also make it more feasible to make generalizations across studies because they holistically consider the impacts associated with urban water infrastructure. Urban water system decision-making, often controlled by governmental or private organizations, requires the integration of economic and environmental considerations to show how performance meets the public's expectations.17 LCA has the potential to be a key component for decision-making in the water industry, and if it can be supplemented with emerging approaches that include spatial considerations, water quantity, public health, and economic and social assessments, studies could be used to better protect public and environmental health and achieve more sustainable urban water infrastructure.

III. Emerging opportunities

Although meeting expectations of stakeholders often requires considering human health and economic and social implications, very few researchers have begun to supplement urban water infrastructure LCAs with these assessments (Fig. 3). To address this, motivation and opportunities to incorporate spatial considerations, water quantity, public health, and economic and social assessments – either by advancing the current environmental LCA framework or integrating LCA within broader sustainability assessment frameworks (Fig. 4) – are discussed in the following sub-sections, followed by a final section providing a path forward for the application of LCA to urban water infrastructure.
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Fig. 3 Literature review results for the intersection of LCA with economic, social, and health assessments. Area of box is proportional to the number of papers (enumerated on each line) indicating inclusion of the assessments identified by the color and the associated dots on the right. Integration of LCA with economic assessments was the most common (62 papers total), followed by health (11 papers total) and social (5 papers total).

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Fig. 4 Opportunities for integration of spatial considerations, water quantity, public health, and economic and social assessments within each LCA phase (inventory, impact assessment, interpretation). Some opportunities are for advancements within LCA (e.g., apply spatially differentiated characterization factors) while others are for integration of LCA within broader sustainability assessment frameworks (e.g., those which incorporate quantitative risk assessment). Across all four categories, these opportunities should be coupled with continuous stakeholder engagement.

III.A Spatial considerations

Motivation. Accuracy of LCA results can be affected by spatial variations within data that are used at both the inventory (i.e., quantity of an emission) and impact assessment (i.e., characterization factors) levels. At the inventory level, direct emissions (e.g., wastewater treatment plant effluent, drinking water at point of consumption) within the life cycle inventory of emissions can play a significant role in impact categories, such as eutrophication and toxicity,1 which are concentration-dependent.22 Moreover, these emissions have nonlinear environmental and human health responses, and they have thresholds, after which a significant change in the effects of contaminant exposure can occur. In addition to the concentration of the emitted contaminant, locality-specific environmental factors will also influence an emission's dose–response relationship. For example, in terms of eutrophication potential, water bodies can be limited by nitrogen, phosphorus, or both with the limited nutrient varying both spatially and temporally.24 A local water body's conditions regarding nutrient limitations will further affect an emission's dose–response curve and threshold value. The inherent nonlinearity of dose–response relationships for an emission, along with additional potential variations caused by an emission location's environmental factors, suggests that spatial considerations can improve accuracy of LCA results for these systems.

In addition to the mismatch between potential impacts from the water industry (e.g., local aquatic impacts from nutrient discharge) and LCA (which assumes large dilutions of contaminants and neglects threshold effects), trade-offs between spatial scales can also occur. For example, improving effluent quality through increased treatment could require additional materials or energy, simply shifting the problem from the local to the global spatial scale25 or from one locality to another. Simultaneous consideration of local and global impacts can help stakeholders balance impacts that directly affect their location's human and environmental health with impacts spread over much larger spatial and temporal scales (e.g., greenhouse gas emissions) during decision-making.

Opportunities. For urban water systems, opportunities to incorporate spatial considerations exist at three different phases of LCA: (1) inventory, by accounting for direct emissions; (2) impact assessment, by using spatially differentiated characterization factors; and (3) interpretation, by separately considering local impacts (Fig. 4). At the inventory level, LCA can account for direct emissions to the local environment to enable disaggregation of impact categories such as eutrophication and toxicity.1 This can allow for local water quality emissions to be directly compared within the global context of LCA while also considering direct aquatic emissions relative to local thresholds.

To more accurately estimate local aquatic and public health impacts of direct emissions, characterization factors can be modified to consider a finer spatial resolution; for example, by modifying fate factors for phosphorus emissions to freshwater26 or deriving soil sensitivity factors.27 While finer spatial resolution within LCIA methods is becoming available in terms of methodological development,28–33 commercial LCA software is currently not capable of matching regionalized impact assessment methods to regionalized emissions6 which could limit the potential for wider adoption of spatial considerations within LCA. As an example, the original version of TRACI, a common U.S. impact assessment method, distinguished between the impacts of nitrogen and phosphorus emissions on the state level;34 however, state level differentiation was removed in TRACI 2.0 due to a lack of user implementation and characterization factors were instead summarized at the country-level.35 Additionally, characterization factors could be calculated to account for the nonlinearity of dose–response curves with mechanistic thresholds; however, a lack of data on background concentrations makes linear dose–response relationships more practical.36 As they become more common and accessible through existing LCA methodologies and software, spatially modified characterization factors can strengthen LCA's applicability to local water infrastructure.

Finally, if spatial considerations are not incorporated into the inventory or impact assessment phases of LCA, decision-makers can still account for these considerations during the interpretation step. Local water quality impacts can be considered separately (i.e., through other analyses such as risk assessment) but concurrently with the global impacts quantified using LCA; this can help ensure that LCA results are relevant for decision-makers who must also consider more urgent local needs.

III.B Water quantity

Motivation. Reliance on indicators that solely report water withdrawal or consumption can mask sustainability implications of urban water infrastructure and management decisions,37 including water availability, water transfer, infrastructure design, and water quality. First, freshwater availability varies both temporally and spatially, therefore impacts of water withdrawals are not uniform, and there are already 4 billion people experiencing severe water scarcity around the globe.38 However, despite some attempts to include landscape-scale hydrological changes in the assessment of water impacts,39 LCA is not equipped to address water scarcity or consider the effects of changes in natural hydrology.7 Well-established life cycle impact assessment methodologies (e.g., TRACI, CML) were not designed to address water availability but rather have focused solely on quantifying the impacts of emissions (i.e., water quality).

In addition to external challenges such as water scarcity, urban water infrastructure, particularly stormwater systems, can be specifically designed for managing water quantity (i.e., from impervious areas). While this has been conventionally done using traditional infrastructure (e.g., pipe networks, underground storage tanks), green infrastructure (e.g., rain gardens, green roofs, permeable pavement) is becoming increasingly popular as it provides greater opportunities for infiltration and a more natural solution for water storage. Green infrastructure is focused on managing both water quantity and water quality40 and typically encourages infiltration unlike traditional infrastructure. Neglecting to consider these distinctions in approaches to water management limits the usefulness of LCA for decision-making surrounding infrastructure designed for water quantity management (e.g., comparisons of green and traditional infrastructure).

Aside from being a design focus, water quantity can affect and be affected by water quality, the latter of which is captured by LCA (to a limited extent depending on whether spatial considerations are addressed). Degrading water quality can lead to decreased water availability by polluting previously drinkable water sources,41 while changes to water flows can cause combined sewer overflows or impact the capacity of water bodies to dilute contaminants. Finally, the uncertainty associated with water quantity data and models can impact water quality predictions.42 To accurately calculate direct emissions to water, which can then be included during impact assessment, water quantity needs to be considered within the LCA.

Opportunities. As with spatial considerations, opportunities to incorporate water quantity into LCA of urban water infrastructure exist within three LCA phases: (1) inventory, by using water quantity models to better predict water quality and water transfer; (2) impact assessment, by using impact categories that specifically address water availability; and (3) interpretation, by separately considering water quantity metrics (Fig. 4). First, dynamic water quantity modeling can be used to improve the prediction of water quality.43 Various hydraulic and hydrologic models, either in the form of equations (e.g., rational method) or software packages (e.g., SWMM, HEC-HMS), are freely available and can either directly model emissions to water or output discharge and infiltration quantities that can then be connected to emissions through concentrations. Examples of incorporating water quantity within the inventory step include using average annual rainfall and runoff coefficients to predict mass of pollutants from a green roof44 or using rainfall data to consider various precipitation scenarios for green and gray stormwater infrastructure comparisons.45 In addition to water quality, spatial considerations for water availability can be considered at the inventory level by differentiating between surface water and groundwater,41 as well as more specific classifications such as green water flows.46

At the impact assessment level, impact categories that specifically address water quantity have been developed. As part of the UNEP/SETAC (United Nations Environment Programme; Society of Environmental Toxicology and Chemistry) Life Cycle Initiative, the Water Use in LCA (WULCA) working group has recommended using the AWARE (Available WAter REmaining) method to calculate a water scarcity footprint.47,48 This water quantity indicator can be calculated directly from water consumption, which is accounted for within the life cycle inventory. Furthermore, users can distinguish between agricultural, non-agricultural, or unknown land use and apply monthly characterization factors at both the country and sub-watershed scales.47,48 This level of detail allows for spatial and temporal variability within water quantity and builds on previously proposed indicators such as demand to availability ratios.49

Finally, decision-makers could consider water quantity using metrics such as infiltration rates, water availability, or runoff volumes separately but concurrently with LCA results, combining the two into a multi-criteria decision-making framework. Incorporating water quantity at the inventory, impact assessment, or interpretation levels will help to mitigate the current limitations of only considering water quality when water quantity also influences urban water infrastructure's design and environmental impacts.

III.C Public health

Motivation. Urban water systems are designed to protect public health by providing access to safe drinking water and reducing the impacts of wastewater and stormwater on natural water bodies. These water systems can contain microbial and chemical contaminants, and simultaneous consideration of both is becoming more necessary in relation to what is referred to as the human exposome.50 Public health considerations, specifically regarding emerging chemical and microbial contaminants, can enable LCA to be used as a sustainability decision-making tool that is in line with the original objectives of urban water infrastructure.

Although LCA is designed to address fugitive chemical emissions, emerging contaminants such as pharmaceuticals, pesticides, and disinfection by-products are not yet comprehensively included in impact assessment methodologies. For instance, disinfection by-products (DBPs) generated by the interaction between free chlorine and organic matter are of growing concern due to their carcinogenic effects.51 While UV disinfection as opposed to chlorination can avoid the production of DBPs, its implementation requires additional electricity and its lack of chlorine residual can lead to contamination within the distribution system.52 Navigating the health trade-offs between these two disinfection options would be advantageous for the water industry. Pharmaceuticals also pose potential risks in wastewater, either through urine and feces53 or unused medications.54 However, the potential environmental and health impacts of pharmaceuticals are difficult to evaluate as research regarding the impacts of their active ingredients is ongoing.54–57

In addition to emerging contaminants, LCA's focus on chemical pollutants fails to capture the potential impacts of microbial contaminants, both well-known (e.g., Cryptosporidium parvum) and emerging (e.g., brain-eating amoeba) in urban water systems, which lead to multiple waterborne disease outbreaks every year.58 Despite our increasing ability to develop an inventory of microbial communities in urban water systems using improved DNA sequencing methods,59 microbes, both as contaminants and as probiotics,60 are not included within current impact assessment methodologies. This is due to both a historical lack of characterization data as well as LCA's traditional focus on regional and global impacts (section III.A). Accounting for microbial impacts could increase the usefulness of LCA for industry and regulatory decision-makers, who must balance the protection of public health with the global environmental and health consequences that this protection may generate.

Opportunities. Public health can be incorporated with LCA by using quantitative risk assessment or developing new health-focused impact categories (Fig. 4). Quantitative risk assessment (QRA), which follows the framework of hazard identification, exposure assessment, dose response, risk characterization, and risk management, can complement LCA due to its inclusion of site-specific parameters and ability to capture microbial contaminants. Kobayashi et al. have proposed four methods for the integration of LCA and QRA: (1) considering LCA and QRA separately in multi-criteria decision-making framework, (2) using LCA as a screening tool to provide more focus for QRA, (3) expanding the perspective of QRA to include additional impact categories from LCA, and (4) utilizing the additional information of QRA (e.g., dose response models, exposure assessment) to perform a more site-specific LCA.61 Finally, outside of these four QRA integration approaches, endpoint LCA methodologies can enable LCA and QRA results to be calculated in terms of a common unit of disability-adjusted life years (DALYs), allowing for direct comparisons of trade-offs between environmental impacts (globally focused, using LCA) and human health risks (locally focused, using QRA);25,62 however, consistency of assumptions regarding the temporal and spatial variability of DALY estimations will be necessary.63

Outside of quantitative risk assessment integration, human health impacts can be accounted for during the impact assessment phase of LCA. Use of endpoint rather than midpoint methodologies can provide more concrete health outcomes (e.g., damage in terms of DALYs), although this introduces additional uncertainty, with different LCIA methods providing different results.13,64 Finally, additional indicators for health, such as relative health indices which take into account human exposure and disease severity,65 may be necessary particularly as water technology changes to incorporate new techniques such as water reuse.

III.D Economic and social assessments

Motivation. Aside from improving models and methodologies within LCA, decision-making regarding urban water systems requires that environmental sustainability be considered along with economic and social dimensions, in accordance with the tripartite sustainability model.66,67 Economic considerations are necessary for the applicability of LCA in the private sector where cost often governs decision-making.68 Similarly, as society raises expectations of social responsibility, LCA's ability to support decision-making diminishes for those seeking to holistically assess sustainability.69 One relevant example stems from frequent challenges to utilities regarding odor issues (in collection systems, at treatment facilities, and potentially at sludge reuse sites). Odor is difficult to parametrize in an LCA health impact framework, but is a critical amenity issue in the water industry that affects land resources and social impacts.70 Recognizing the significance of other sustainability metrics has prompted researchers to establish methods for integrating economic and (though less developed) social assessments within the existing environmental LCA framework.
Opportunities. Economic and social assessments can be incorporated with LCA by using existing life cycle frameworks (life cycle costing, techno-economic analysis, social life cycle assessment; Fig. 4). While life cycle costing (LCC) is the most commonly accepted economic assessment method for infrastructure,71 other strategies have been used including replacement cost methodology72 and cost–benefit analysis.73 Although similar to LCC in many aspects, techno-economic analysis (TEA) focuses on a product's sale prices and potential revenue. Therefore, as water infrastructure continues to shift toward resource recovery (e.g., water, energy, nutrients),74 TEA will become more advantageous. Both LCC and TEA, which involve creating a cost inventory throughout a system's life cycle integrate well with the existing LCA framework, in which a life cycle emissions inventory is generated.

Recent attention to odor assessment in LCA has resulted in the development of two new approaches. One is a spin-off of the improved consensus around the modelling of chemical contaminants, embodied in the steady-state USEtox model. Marchand et al. (2013) suggested adjusting the environmental descriptors in USEtox 1.0 to enhance its relevance to the environmental assessment of a particular local area.75 Although the intent was to connect with human health as an area of protection, this is confounded by the absence of clear link between odor perception and health effects,76 so instead the ultimate odor burden was expressed in terms of 11 different midpoint indicators, one for each different type of smell (sweet, rancid, fecal etc.).75 Peters et al. (2014) criticized this approach on operability grounds and suggested a dynamic but otherwise simpler midpoint indicator model based on diffusion and reaction of odorants, with economic resources as the ultimate area of protection.77

Including social assessments in LCA is less developed and applied less frequently than economic assessments;6 however, frameworks with potential social indicators and damage categories have been proposed.69,78 In particular, UNEP/SETAC has developed guidelines for social life cycle assessment (SLCA) along with methodological sheets providing example SLCA indicators.79,80 Social indicators relevant to urban water systems include health and safety (e.g., number of consumer complaints, presence of management measures to assess consumer health and safety), public commitment to sustainability issues (e.g., existence of (legal) obligation on public sustainability reporting), and local employment (e.g., percentage of workforce hired locally).80,81 In the context of the environmental LCA framework, SLCA includes developing an inventory and assessing the impacts of a company's conduct toward its stakeholders,69 where stakeholders can include the worker, consumer, local community, society, and value chain actors.79 SLCA is often discussed in parallel with environmental LCA and LCC in a comprehensive sustainability assessment, called life cycle sustainability assessment (LCSA).5

Most challenges surrounding SLCA involve acquiring appropriate data, as social LCA is very site-specific69 with unequal effects on different societal groups.78 The demand for site-specific data becomes even more challenging when involved organizations are undetermined;82 yet, this point of flexibility is when LCA has the greatest potential to inform decision-making.83 In addition to data limitations, further challenges arise when trying to force social LCA into the existing environmental LCA framework. For example, social indicators are often qualitative84 and are related to a company's conduct rather than the product that company is producing, creating further complications for normalization to the functional unit.85 Due to these many methodological challenges and the difficulty of SLCA application,86 future advancement of SLCA must balance method development with implementation through case studies.87 These case studies can employ tools such as the social hotspot database,88 which can ease the burden of SLCA data collection. Furthermore, using the social hotspot database in conjunction with established input–output LCA models can connect social risks to monetary flows, allowing for identification of social impacts without detailed process-modeling.89 Ultimately, improvements such as these can make SLCA more simple to conduct, more closely aligned to environmental LCA, and ultimately more relevant to stakeholders.

IV. Path forward

Stakeholder participation

Underlying all emerging opportunities is the need for stakeholder participation. Outside of the traditional LCA framework, stakeholders can be integrated with engineering planning and design in a continuous feedback loop,74 to enhance locality-specific decision-making. At the highest participation level, stakeholders can be actively involved in the planning process, for example, by using system dynamic modeling to develop water management alternatives and test the system's response.90 Examples of integration of stakeholder participation with environmental LCA include allowing for community input via rankings of social acceptability91 and using household surveys to evaluate public preferences regarding access to water.92 Within SLCA, stakeholder participation can be used to determine which social indicators should be evaluated.93 During interpretation, environmental, economic, and social impacts can be considered concurrently in a multi-criteria decision-making framework or using multi-objective optimization.94 While this may require weighting of different sustainability dimensions, this weighting can be effective if done transparently,95 ultimately leading to a shift away from technology-centered sustainability projects focused on short-term economic optimization.67

Specific research needs

To continue developing the emerging opportunities discussed above, there are specific research needs that must be addressed. Within public health, emerging contaminants first need to be identified so they can ultimately be included; therefore, checking for new compounds, for example in wastewater sludge,96,97 and identifying the impacts of active pharmaceutical ingredients98 are key to the development of life cycle inventories and indicators. Within risk assessment, challenges related to QRA – and consequently its integration with LCA – need to be overcome,99 such as relating aggregated risks to a functional unit100 (including antibiotic resistant bacteria101) and addressing the uncertainty and variability among exposure parameters and initial concentrations, which can inhibit fair comparisons.102 New indicators that incorporate fate and transport for odor, account for ecosystem services,103 and establish relationships between state and pressure variables could enhance the assessment of social impacts. While not specifically discussed here, there is a need for continued research into incorporation of biodiversity into LCA – which often uses potentially disappeared fraction (PDF) of species as an indicator104,105 – and its relationship to other emerging needs such as water quantity (e.g., influence of flow fluctuations on aquatic species loss106). Finally, it is important to note that the identified emerging needs and methods are interconnected (e.g., water quantity assessments could benefit from improved spatial considerations107); therefore, they should be considered collectively.

Universal research needs

Case studies will be required to test these emerging approaches, find their limitations, determine their cost-effectiveness, and ultimately standardize assumptions to be able to make comparisons. As these emerging methods often involve other areas of research outside of environmental science and engineering (e.g., public health, economics), collaboration among various fields of research will be necessary and may present new challenges. For example, risk assessment is often associated with a tolerable effect threshold while LCA is typically less focused on absolute values of impacts but rather how alternatives compare. It will be essential for these communities to communicate these challenges to ensure that all aspects of the analysis are valid and that results will be relevant to stakeholders from different backgrounds. Urban water infrastructure is constantly evolving (e.g., developing new technologies,108 shifting toward resource recovery109,110) and adapting to challenges (e.g., climate change111,112); therefore, the need for these emerging methods will be case-specific and depend on how much complexity is required for the specific problem. For example, in some cases, coupling process models with LCA will be advantageous and allow for sustainability trade-offs to be linked to specific design decisions. Overall, considering these emerging methods within the goals of the specific urban water infrastructure being studied can ensure that LCA is supporting these objectives, making water infrastructure an opportunity for urban environmental solutions rather than just problems.113,114 This way, LCA will be an invaluable and relevant tool for evaluating urban water infrastructure.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The authors would like to acknowledge financial support from the Department of Civil and Environmental Engineering (UIUC) for partial funding for the first and second authors, as well as the National Science Foundation Graduate Research Fellowship Program for partial funding for the first author.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ew00175d

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