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Review

Life-Cycle Assessment of Wastewater Treatment: Enhancing Sustainability Through Process Optimization

by
Hajar Laouane
1,
Loubna El Joumri
2,
Amine Halhaly
2,
Yassine Arid
1,
Najoua Labjar
2 and
Souad El Hajjaji
1,*
1
Laboratory of Spectroscopy, Molecular Modeling, Materials, Nanomaterials, Water and Environment, (LS3MN2E-CERNE2D), Department of Chemistry, Faculty of Sciences, Mohammed V University in Rabat, Rabat P.O. Box 1014, Morocco
2
Laboratory of Spectroscopy, Molecular Modeling, Materials, Nanomaterials, Water and Environment, (LS3MN2E-CERNE2D), Ecole Nationale Supérieure d’Arts et Métiers, Mohammed V University in Rabat, Rabat P.O. Box 1014, Morocco
*
Author to whom correspondence should be addressed.
Sustainability 2026, 18(2), 605; https://doi.org/10.3390/su18020605
Submission received: 8 December 2025 / Revised: 29 December 2025 / Accepted: 5 January 2026 / Published: 7 January 2026
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Abstract

Rising quantities of a broad spectrum of contaminants due to high industrial and residential wastewater effluent loads have further raised the stakes with respect to environmental and health concerns. These demands, coupled with limitations in existing wastewater treatment solutions, have culminated in innovative supplementary solutions in the form of alternative wastewater treatments that, in general, encompass physical, chemical, or biological methods. By quantifying the resource consumption, pollution emissions, and ecological effects across the life-cycle in wastewater treatments, Life-Cycle Assessment (LCA) has proven valuable as a fundamental methodology for assessing and quantifying environment-related sustainability in wastewater treatments. Although valuable in its current applications, LCA is limited in its assessment of the relevant data related to the impacts of construction activities, novel contaminants emerging in wastewater treatment plants, and sludge disposal options. By considering pollutant type, wastewater treatment options, and important LCA methodological considerations, all encompassed within a structured framework including synthesis tables and comparative figures, our hope is that this study will prove valuable to rigorous decision-making processes based on related notions underpinning sustainability concerns in this domain.

1. Introduction

Water is central for life, economic growth, and maintaining the balance of nature. It is of particular importance in arid and semi-arid areas, where the lack of water has been exacerbated by reduced rainfall, increased demand, reduced water resources, and inadequate regulation [1]. The generation of wastewater has prominently increased due to intensively increasing rates of urbanization and industrialization, further worsening this problem. Wastewater poses a significant danger to public health and the environment if not adequately treated.
Pre-treatment, primary treatment, secondary treatment, tertiary treatment, and sludge treatment are treatment processes undertaken at Wastewater Treatment Plants (WWTPs) that have become essential to the modern wastewater treatment system [2,3].
Conventional methods of treatment are suspected to require large resource utilization, involving significant energy reserves and the application of chemicals, irrespective of their effectiveness. Furthermore, powerful greenhouse gases such as nitrous oxide (N2O) and methane (CH4) are released in the course of Biological Nutrient Removal (BNR) processes, thereby adding another factor that contributes to the urgent need for an inclusive assessment of treatment methods, specifically in the context of environmental sustainability.
LCA has become the principal framework within which to consider the environmental impact of wastewater treatment technologies. The recent sustainability literature also highlights the broad potential of LCA in identifying hotspots and trade-offs across complex systems and supporting mitigation pathways, reinforcing its relevance in wastewater decision-making [4].
Nevertheless, the use of LCA in the thorough evaluation of the effects of the micropollutants contained in wastewater and sludge is comparably limited. For instance, ref. [5] assessed the ecotoxicity and human toxicity potential of wastewater that contained almost a hundred priority and emerging contaminants. In a follow-up study, the same authors compared various wastewater reuse scenarios, using a variety of tertiary treatment techniques and considering eighty-four pollutants [6]. Similarly, refs. [7,8] applied LCA to advanced treatment technologies, including sand filtration, ozonation, and membrane bioreactors, to study the capacity of the respective technologies in removing organic micropollutants, heavy metals, and pathogens. However, many of these studies have overlooked or insufficiently addressed sludge management, and, in most cases, incineration is assumed to be the sole disposal option. This gap in the literature highlights the urgent need to extend the LCA framework.
Distinct from previous reviews that primarily describe wastewater treatment technologies or summarize LCA principles in isolation, this review provides an integrative and decision-oriented synthesis of wastewater treatment systems through an LCA lens. The systematic connections between pollutant classes, treatment families, and life-cycle environmental hotspots provide added value by enabling the interpretation of treatment performance beyond removal efficiency, standardizing the classification of pollutants in industrial, agro-industrial, and municipal settings. Furthermore, methodological decisions that have a significant impact on LCA results are carefully compared in this work. Synthesis tables and comparative figures that transform scattered LCA evidence into cohesive insights for wastewater treatment design, technology selection, and sustainability-oriented decision-making, particularly in water-scarce and agro-industrial regions, support these contributions.

2. Methodology

2.1. Review Approach

A literature review is required to identify critical gaps in earlier efforts that can be addressed in future research. To ensure that the results are not misleading and represent the highest quality of scientific evidence, literature scanning must be conducted correctly and systematically. For this study, a systematic review, screening various elements, was adopted. This choice is justified by the broad scope and heterogeneity of the topic, which encompasses diverse pollutant classes, multiple treatment families, and varied industrial contexts. While the variability in LCA modeling assumptions limits the feasibility of a strict meta-analysis, this approach enables critical comparison and the identification of methodological trade-offs across studies.

2.2. Literature Retrieval and Screening

To ensure a complete dataset, the literature was retrieved utilizing top scientific databases, including ScienceDirect, Web of Science, and Scopus. First, a wide search for the years 2000–2025 using the terms “life-cycle assessment” and “wastewater treatment” produced about 1200 results. The time frame for the methodological establishment of LCA applications in the water sector is reflected in this initial pool.
The results were reduced to 310 documents by using particular keyword filters (such as “industrial,” “sludge management,” and “advanced treatment”) in order to focus on industrial applications and resource recovery. As seen in Figure 1, the screening procedure adhered to a defined protocol.
  • First screening: Titles and abstracts were examined to make sure they were pertinent to environmental impact indicators and technical treatment performance. As a result, the selection was narrowed down to 142 publications, most of which were critical reviews and peer-reviewed research studies written in both French and English.
  • Eligibility evaluation: To guarantee that LCA assumptions were transparent, full-text versions were reviewed. Excluded were studies that only examined degradation at the laboratory scale without system-level evaluation or that lacked clearly defined functional units and system boundaries.
  • Final inclusion: In the end, 85 studies were chosen for a thorough examination. Figure 2 shows the distribution of these publications by kind, including research articles, review articles, and international standards.

3. Sources and Importance of Wastewater Treatment

Wastewater contaminants are grouped into five main pollutant classes: bulk organics, nutrients, metals and metalloids, pathogens, pharmaceuticals, and personal care products (PPCPs) as representative emerging contaminants. Sources of wastewater (municipal, industrial, and agricultural) are treated as contextual categories rather than pollutant classes. This taxonomy is applied consistently throughout the following sections to improve clarity and comparability.

4. Bulk Organic Pollutants

Organic pollutants in wastewater arise from multiple sources and include a broad array of hazardous substances:

4.1. Organic Pollutants in Industrial Wastewater

Industrial waste is a major contributor to water contamination, particularly from sectors such as manufacturing, mining, and heavy industries. These processes result in large amounts of dangerous chemicals, most of which end up in nearby water bodies owing to unsuitable disposal of waste, accidental spillage, or inferior storage facilities [9].
Textile and chemical industries are regarded as organic wastewater pollutants generators, so the source is especially harmful. Dyes such as Methylene Blue and the various azo compounds are often employed and are notably resistant to biodegradation. When they are found in water, these compounds will potentially obstruct light penetration, leading to undesirable interference with the photosynthesis process, which can result in oxygen depletion, which ultimately results in ecological imbalance [10].

4.2. Organic and Emerging Pollutants in Domestic Wastewater

Domestic wastewater is commonly classified into two categories: (1) greywater, which comes from kitchens, bathrooms, and laundry, and (2) blackwater, which is generated from toilets and consists of human waste and disease-causing microorganisms. Owing to its lower pollutant load, greywater is generally easier to treat than blackwater [11].
The composition of domestic wastewater varies hourly, daily, and seasonally as a function of the water volume used and habits. A portion of the organic load present in wastewater is dissolved, and the remaining fraction constitutes solid particles, which ultimately generates sewage sludge. About 70% of organic matter in wastewater is secondarily contained in these solids, where they are available as a source of nutrition for bacteria [12].

4.3. Nutrients and Pesticide Residues from Agricultural Runoff

Another major contributor to water pollution is pesticides and agricultural runoff. Chemical agents, including fertilizers, pesticides, and herbicides, are common in today’s agriculture to increase crop production [13]. While these inputs are necessary for protecting food security, their improper handling and overuse can have negative implications for the environment and human health [14,15].

5. Impacts of Wastewater Contamination on Public Health and the Environment

Water pollution is the most concerning issue in developed and developing countries. Though considerable research has focused on the consequences of contaminated water on animal populations, little is known about the full public health impact of this contamination [16].
Human vulnerability to pollutants often arises from ingesting contaminated food or water, which may be engendered by exposure to polluted soil, plants, or animals. Among the most worrisome types of agents are Endocrine-Disrupting Chemicals (EDCs), which can disrupt hormonal systems and normal organismal growth [17]. The best known among the EDCs is Bisphenol A (BPA), a plastic additive, with exposure linked to fertility impairment, hormonal dysfunction, and neurological damage. It has also been associated with a heightened risk of breast cancer in women and developmental impairments in children [18]. In plants, BPA interferes with cell division and inhibits healthy growth [19].
In addition to BPA, the contaminants of Pharmaceutical and Personal-Care Products (PPCPs) are also becoming known as a significant threat to biodiversity, especially to aquatic organisms [16]. Assessing the direct consequences of such Emerging Contaminants (ECs) on human health is much simpler than measuring their ecological impact [20]. Urgent action is needed as freshwater resources grow more limited and water quality deteriorates.
These pollutant classes are relevant from the standpoint of life-cycle assessment because they influence treatment needs and related environmental trade-offs. Aeration demand and sludge production are often increased by high organic and nutrient loads, which have a significant impact on energy use and greenhouse gas emissions. When metals and persistent organic compounds are present, chemical or sophisticated treatment methods are frequently required, which increases material inputs and the production of hazardous sludge. Tertiary or polishing stages may be necessary to improve effluent quality due to pathogens and new contaminants such PPCPs, but often at the expense of increased chemical and electricity use. Therefore, the composition of pollutants directly influences the choice of technology and identifies the environmental hotspots that are captured by Life-Cycle Assessment (LCA). This provides the rationale for the treatment-focused analysis that is established in the subsequent sections.

6. Key Stages of Wastewater Treatment

Wastewater treatment has witnessed a significant evolution from a technological perspective, transitioning its focus from mere solids removal in the early 1900s to complex resource recovery facilities and systems currently [21,22]. Nonetheless, LCA studies have disclosed an environmental paradox: the enhancing of purification standards brings about a drastic increase in energy intensity and chemical dependency. For further clarification, historically, the success of a treatment process has been entirely based on effluent quality (e.g., low BOD, TSS removal). Nonetheless, from an LCA standpoint, this enhanced wastewater quality comes with a great burden and global impact, such as increased greenhouse gas emissions and colossal resource consumption. Consequently, treatment technologies ought to be classed and categorized based on their life-cycle profiles and not on their mechanical function. Biological processes usually employ insignificant chemicals but are characterized by important direct greenhouse gas (CH4, N2O) emissions, as well as requiring energy for aeration. Physical/chemical processes often involve lower direct emissions, but their impact on the environment is considerable due to coagulant and membrane employment. While hybrid systems ameliorate the recovery outputs, their environmental net threshold has to be carefully investigated and assessed through LCA to guarantee that the resources harnessed exceed the impacts of the additional infrastructure.
Today, wastewater treatment systems are considered to be one of the cornerstones of environmental protection strategies (Figure 3). By improving water quality, these systems help safeguard aquatic ecosystems, reduce health hazards, and create opportunities for the safe reuse of treated effluents in various sectors.

7. Techniques Used for Wastewater Treatment

Recent advancements have focused on developing innovative strategies for wastewater management and meeting the need for clean water [23]. The choice of method depends on various factors, such as dye concentration, sewage composition, process type, the cost of the process, and the presence of other impurities in the wastewater [24]. Each approach possesses its own specific advantages but also comes with certain limitations. Techniques involving high installation and operational costs, prolonged processing times, low treatment efficiency, or the generation of hazardous byproducts are often unsuitable for large-scale industrial use [25]. Therefore, there is an inevitable need to explore alternative treatment systems capable of effectively and sustainably removing or degrading contaminants [26].
Treatment technologies cannot be assessed based only on removal efficiency from a life-cycle perspective. The environmental impact of wastewater treatment systems is frequently dominated by energy consumption, sludge production, and related emissions (Figure 4 and Figure 5). In light of this, Table 1 offers a process-level overview that connects the main pollutant categories with illustrative treatment methods. It is supplemented by indicated life-cycle implications derived from comparative life-cycle assessment studies [27,28,29,30]. Detailed cross-technology sustainability comparisons are further developed in Table 2, where treatment families are assessed using energy use, greenhouse gas emissions, and sludge-related trade-offs.

8. Sludge Treatment and Disposal

Sludge is the main sub-product of the wastewater treatment process and represents the principal environmental “hotspot” in the aforementioned process. Globally, sludge is the main driver of GHG emissions, comprising 30–50% of the total GHG emissions of a plant [31]. Biological stabilization and agricultural reuse are the main techniques promoted for sludge management and nutrient recovery (nitrogen, N; phosphorus, P). LCA studies highlight a countervailing effect of these techniques; in fact, they often result in high direct emissions of CH4 and N2O, concurrent with long-term soil ecotoxicity risks engendered by heavy metal accumulation [32,33]. To remediate this issue, thermal and chemical treatments (incineration, pyrolysis, gasification) considerably reduce sludge volume and effectively neutralize pollutants [34]; however, they shift the environmental burden to colossal fossil fuel consumption and air acidification [35,36,37]. Table 3 synthesizes the principal sludge treatment pathways applied in municipal and industrial wastewater systems and interprets them from a life-cycle assessment perspective by highlighting key system boundaries, dominant environmental hotspots, and trade-offs associated with energy use, emissions, and resource recovery.
From a Life-Cycle Assessment (LCA) perspective, the environmental performances of sludge treatment alternatives can differ greatly, and they also depend on the definition of system boundaries and modeling assumptions. It has been shown by comparative LCA studies that pathways (e.g., anaerobic digestion, composting, incineration, and landfill) cannot be evaluated solely by on-site performance. Instead, upstream processes such as energy usage and transportation, alongside downstream emissions and by-product utilization, play a significant role in defining environmental impacts. Systems incorporating anaerobic digestion often show improved environmental performance. This is achieved by creating biogas and syngas energy recovery through system expansion. This displaces conventional electricity or heat production. Therefore, allocation choices and the inclusion or exclusion of certain products play a decisive role in determining whether sludge management pathways act as environmental burdens or net contributors to resource recovery. This underscores the need for transparent boundary and allocation assumptions in LCA-based comparisons.

9. Sectoral Applications of Treated Wastewater Reuse (TWR): A Comparative LCA Perspective

Global water scarcity requires the diversification of Treated Wastewater Reuse (TWR) pathways, which determine agriculture (70% of withdrawals), industrial sector (20%), and groundwater recharge, representing the primary sectors of application [23,24,38]. Nonetheless, from an LCA standpoint, these pathways do not resemble one another, environmentally speaking, thus creating a distinct balance between “the added burden” of advanced purification processes and the “avoided impact” of freshwater extraction.
Critical analysis for agricultural sector: Its benefits consist mainly of nutrient recycling (N and P), which LCA determines to be a “credit” as it is an efficient substitute for synthetic fertilizer [38,39]. However, potential contact with edible crops is significant; the tertiary treatment raises the energy footprint, thus offsetting these benefits.
Critical analysis for industrial sector: The employment of treated wastewater in the industrial sector mainly concerns cooling systems and boilers [40], thus requiring high-quality water to curb scaling and corrosion issues. Despite being cost-effective, the LCA profile of industrial reuse is often controlled by the important chemical and energy intensities of membrane-based treatments (RO/UF), which are required to meet industrial standards.
Critical analysis for groundwater recharge: Managed Aquifer Recharge (MAR) and Soil Aquifer Treatment (SAT) offer a special LCA advantage; they harness natural soil processes for contaminant elimination, mitigating the need for energy-intensive chemical oxidation [25]. This pathway offers the most suitable compromise for long-term water storage, ensuring that energy pumping does not exceed the local water scarcity benefits. Table 4 illustrates the key LCA parameters for the three main reuse sectors, highlighting the trade-offs between resource recovery and environmental hotspots.

10. Life-Cycle Assessment (LCA) of Wastewater Treatment

Definition and Purpose of LCA

Life-Cycle Assessment (LCA) is a standardized framework used to quantify environmental impacts associated with wastewater treatment systems across their life-cycle, from construction and operation to sludge management and reuse pathways (Figure 6). In the context of WWTPs, LCA is particularly valuable for identifying energy- and emission-intensive hotspots, evaluating trade-offs between treatment efficiency and environmental burdens, and comparing alternative process configurations under consistent assumptions. Detailed methodological guidance is provided by ISO 14040/14044 [41,42]. The following sections focus on how these methodological choices influence the results reported in published wastewater LCA studies.

11. Key Steps in LCA

11.1. Goal and Scope Definition

Defining the objective and scope is the initial phase of LCA. In this section, the goals of this study are established, including the definition of the Functional Unit (FU), along with the description of system boundaries. Analyzing the life-cycle inventory and methodology for different impact categories, comparing technologies, assessing their environmental consequences, and carrying out environmental evaluations are a few examples of the goals [41,42].

11.1.1. Functional Unit (FU)

The Functional Unit (FU) serves as a critical normalizing factor in LCA. Its fundamental role is to establish a consistent basis for attributing all associated inputs and outputs, thereby ensuring the comparability of LCA results, particularly when evaluating alternative [41,42]. However, this volumetric measure alone often proves insufficient due to its inability to account for variations in influent quality or the efficiency of the WWTP. To address these limitations, some investigations have adopted the Inhabitant Equivalent (EH) as the FU, representing the biodegradable organic load defined by a five-day Biochemical Oxygen Demand (BOD 5) of 60 g of oxygen per day. Additionally, to capture the removal of both organic matter and nutrients, ref. [29] proposed expressing the FU in terms of kilograms of phosphate equivalents (kg PO43− eq) removed. Similarly, ref. [43] introduced the net environmental benefit approach, indicating a move towards more holistic performance indicators in the assessment of wastewater treatment systems.
In order to enhance the comparability of the studies, this review considers the volume-based functional unit—typically 1 m3 of wastewater treated to a defined discharge or reuse quality—as a common reference whenever possible as it best represents the core service provided by wastewater treatment systems and is widely used in the literature. In such cases, results are preferably discussed qualitatively rather than in terms of a direct numeric comparison, and deviations from the reference functional unit are explicitly acknowledged to avoid misleading interpretations and to highlight how indicator choice influences reported environmental performance.

11.1.2. System Boundaries and Scope Assumptions

As concerns the WWT life-cycle system boundaries, many studies have considered only the operational phase of the WWTP while neglecting the environmental impacts associated with the construction and demolition stages (Figure 7). Among those that considered the construction phase, a few studies identified its significant environmental contribution. In particular, for low-tech technologies, such as constructed wetlands and reed beds, the construction stage contributed as much as 80% of the overall impact in some categories [44,45,46]. Construction has also been reported as a critical stage for conventionally activated sludge systems and membrane bioreactors, with impacts reaching up to 43% and 31% of the total environmental burden, respectively [47]. Furthermore, ref. [48] noted that construction can account for as much as 20% of the total impacts for some categories.
The literature that has been reviewed shows that there is a lot of variation in the definitions of system boundaries that are used in wastewater treatment LCAs. Three main approaches can be distinguished: (i) process-level boundaries limited to unit operations; (ii) plant-wide boundaries, including influent pumping, aeration, sludge treatment, and auxiliary systems; and (iii) extended boundaries that incorporate infrastructure, construction materials, and downstream reuse or resource recovery pathways. Total environmental burdens are often underestimated in studies relying on narrow, process-only scopes. However, impact profiles can be significantly altered and comparative rankings between technologies can even be reversed by including the construction, sludge handling, and reuse stages. Consequently, conclusions based on LCAs for wastewater treatment systems are inherently conditional on boundary assumptions. This highlights the need for transparent and explicit scope definition when results are used to inform technology selection or sustainability-oriented decision-making.

11.1.3. Allocation and Treatment of Co-Products

Allocation procedures for wastewater treatment systems that include energy or nutrient recovery (e.g., biogas from anaerobic digestion or phosphorus recovery from sludge) vary considerably across studies. Some authors apply system expansion, crediting avoided electricity or fertilizer production, while others use energy- or mass-based allocation. The net impact results are particularly influenced by these methodological choices, especially in the context of climate change and cumulative energy demand. This highlights the importance of transparent reporting and sensitivity analysis when comparing recovery-oriented systems.

11.1.4. Inclusion of Direct Greenhouse Gas Emissions (N2O and CH4)

Across the reviewed literature, direct emissions of nitrous oxide (N2O) and methane (CH4) are identified as major contributors to the change in climate induced by wastewater treatment systems. In some cases, these emissions exceed those associated with energy consumption. However, their representation in LCA studies remains inconsistent, and reported results are highly sensitive to the choice of emission factors. These emissions are largely governed by operational conditions such as aeration strategies, dissolved oxygen levels, sludge retention time, the presence of anaerobic zones, and biogas capture efficiency, rather than treatment technology alone. As a result, relatively small changes in assumed operating parameters can substantially influence global warming potential estimates and even alter comparative rankings between treatment options. This highlights that LCA-based conclusions should be interpreted as conditional on operational assumptions and that explicit consideration of direct emissions and process control is essential when LCA is used to support wastewater treatment design, optimization, or policy decisions.

11.1.5. Sludge Management and Recovery Pathways in Wastewater LCA Studies

The treatment of sludge in LCA studies varies significantly. Across the literature reviewed, sludge handling typically involves stages such as thickening, dewatering, intermediate storage, transportation, and final disposal or reuse. However, transport distances and downstream processes are often oversimplified or omitted. The end-of-life options differ markedly between municipal and industrial contexts, commonly including land application, composting, incineration, and landfill, each of which is associated with distinct trade-offs in terms of climate change, eutrophication, and toxicity. In systems incorporating resource recovery, several studies apply environmental credits for recovered energy or nutrients, most often through system expansion approaches that account for avoided electricity or fertilizer production. However, these credits are highly sensitive to assumptions relating to recovery efficiency and background systems. Inconsistent treatment of these factors across studies contributes to divergent conclusions regarding the sustainability of sludge-based recovery pathways.

11.2. Life-Cycle Inventory (LCI)

The Life-Cycle Inventory (LCI) process aims to identify the inputs (such as resources) and outputs (such as effluents, waste, and emissions) associated with the entire life-cycle of the process studied. These inventories are characteristically expressed in physical units-such as kilograms (kg), cubic meters (m3), or kilowatt-hours (kWh). Foreground LCI data, representing primary data for the operation phase, is typically gathered through direct measurements, operational records, detailed design documents, sampling activities, and vendor-supplied information. In contrast, background information, like electricity production, concrete production, and chemical manufacturing, is generally sourced from established LCI databases such as Ecoinvent [28,49].

11.3. Life-Cycle Impact Assessment (LCIA)

Before calculating environmental impacts as part of Life-Cycle Assessment (LCA), it is essential to proceed with an appropriate impact assessment method to determine the categories of interest, whether at the mid-point or the end-point. Several methodologies are available for this purpose, including Eco-Indicator99 [50], ReCiPe [51], EDIP 2003, IMPACT 2002+, and CML 2001 [52]. Of these methods, CML 2001 is the most commonly used by researchers because of its broad set of impact categories, its high relevance for wastewater treatment assessments at the intermediate level, and its ability to produce accurate results [53]. In the LCIA phase, LCI data relating to emissions to air, water, and soil are multiplied by the corresponding Characterization Factors (CFs) to convert flows into quantified environmental impacts [54].
Characterization factors are provided to practitioners either by published literature or by LCA software [55]. These CFs are developed on the basis of cause-and-effect chains, i.e., from pollutant emissions to their effects on the environment or human health systems [56]. The CF is generally calculated by multiplying three factors: the Fate Factor (FF), which describes the residence time of pollutants in receiving compartments; the eXposure Factor (XF), which refers to the concentration of pollutants absorbed by the target compartment (e.g., humans); and the Effect Factor (EF), which takes into account the impact of exposure routes such as ingestion or inhalation. The combination of the fate factor and the exposure factor gives the Absorption Factor (IF) of a substance [55].

11.4. Interpretation of Results

Interpretation is the final phase of the LCA methodology. This step is essential for collecting and evaluating data from the LCIA results since it assesses the degree of certainty of the results. Interpretation begins by examining the accuracy of the results and determining how effectively they meet the study’s original goals. According to [41], the interpretation phase in LCA involves three main components: (a) identifying significant issues based on the findings from LCI and LCIA; (b) evaluating the study through completeness, sensitivity, and consistency checks; and (c) formulating conclusions, recognizing limitations, and providing recommendations.

12. Main Environmental Indicators Used in LCA

12.1. Footprints as Environmental Indicators

Footprint indicators are commonly used in environmental assessments to describe how human activities place pressure on natural resources, particularly in terms of carbon emissions, water use, energy demand, and impacts on biological systems. LCA provides a coherent framework for quantifying these footprints across the entire life-cycle of a system or service, allowing key environmental hotspots and trade-offs to be identified. Applied to wastewater treatment, footprint-based LCA enables a more integrated evaluation of resource consumption and emissions from WWTPs and supports more informed sustainability-oriented decisions. In recent years, an increasing number of studies have combined carbon, water, energy, and ecological footprints to better capture interactions and trade-offs between environmental dimensions, offering a more realistic picture of the overall sustainability performance of wastewater treatment systems [57,58].

12.2. Carbon Footprint

Carbon Footprint (CF) is one of the most well-known environmental indicators used to evaluate the contribution of human activities to climate change (Figure 8). It measures volumetrically all the direct and indirect Greenhouse Gas (GHG) emissions that are released by a product, service, or process life-cycle in CO2 equivalents [59]. Although the CF generally refers only to carbon dioxide, it can also account for other carbon-containing gases such as CH4 and N2O, according to the scope and data availability [60].
In the context of wastewater treatment systems, the CF assessment plays a key role in assessing energy intensiveness and GHG emissions, contributing to the development of more sustainable treatment alternatives. Studies have shown that both direct (such as CH4 and N2O produced from integrated biological processes) and indirect (mainly originating from electricity consumption) emissions are comparably significant to the overall CF of WWTPs [30]. However, when anaerobic systems like Anaerobic–Anoxic–Oxic (AAO), Sequencing Batch Reactors (SBRs), and oxidation ditches were compared to aerobic systems, results indicated that aerobic systems are more GHG-intensive than anaerobic ones, thus allowing for the recommendation of anaerobic processes where feasible.
Other experimental studies have concentrated on fugitive GHG emissions, with an emphasis on nitrification–denitrification reactors. Ref. [61] measured the GHG emissions from full-scale bioreactors and found that methane was the predominant direct emission.

12.3. Water Footprint

The Water Footprint (WF) is a comprehensive, multi-dimensional indicator that provides an assessment of the total volume of freshwater used and/or degraded within the entire process or product life-cycles either directly or indirectly as per human activities. It is closely related to the term “virtual water,” which is the hidden flow of water embedded in goods and services [62].
In 2011, the Water Footprint Network (WFN) proposed a standard methodology for WF assessment. This standard covered four main steps: (1) goal and scope definition; (2) quantification of water footprint; (3) sustainability assessment of the water footprint; and (4) response identification [63].
As a multidimensional metric, the WF is composed of three sub-indicators:
  • The Blue Water Footprint (BWF) measures the consumption of surface and groundwater.
  • The Green Water Footprint (GrWF) accounts for the use of rainwater stored in soil.
  • The Gray Water Footprint (GWF) estimates the volume of freshwater required to dilute pollutants to acceptable environmental standards [63,64].
The data quantified are usually the volumes of water that are consumed (via evaporation or incorporation into products) or polluted per unit of time or per functional unit [65].

12.4. Energy Balance

The Energy Footprint (EF) is an important parameter used to evaluate energy resource requirements in connection with a product or process [66]. The ReCiPe midpoint method can be used to calculate the EF in wastewater treatment, and it will also convert energy input into one impact score [67]. The EF is usually presented in kilograms of oil equivalent to per cubic meter of treated water (kg oil eq/m3).
Expanding on this approach, ref. [68] introduced the concept of the energy footprint of a supply chain, where the eventual energy usage of carriers is weighted by their respective land use indices. This provides a more complete perspective on the energy impacts throughout the life of a product or service.
The EF concept can also be broken down into sub-categories, depending on the energy source. These include the fossil energy footprint [69], the nuclear energy footprint [70], and the renewable energy footprint [71]. This classification permits a more detailed account of energy dependence and sustainability, especially when comparing traditional and alternative treatment technologies.

13. Integrated Comparison of Environmental Indicators Across Wastewater Treatment Systems

Across the reviewed literature, carbon, energy, and water footprints exhibit strong interdependencies, which limit their interpretation in isolation. Conventional activated sludge and biological nutrient removal systems generally show moderate water footprints but high energy demand and carbon intensity, largely driven by aeration and indirect electricity-related emissions (Figure 9). In contrast, anaerobic-based systems reduce net energy use and carbon footprints through biogas recovery; however, they may require post-treatment, which increases water and material inputs. Advanced and hybrid technologies consistently achieve superior effluent quality but display the highest combined energy and carbon footprints, particularly when applied at full scale. Reuse-oriented scenarios frequently reduce overall water footprints and may offset energy-related impacts when freshwater substitution is credited, although these benefits depend strongly on local water scarcity and background electricity mixes. These findings demonstrate that environmental indicators respond differently to technology choice, operational scale, and reuse objectives, reinforcing the need for integrated indicator-based assessment rather than single-metric evaluation when comparing wastewater treatment options. While this indicator-based comparison highlights cross-footprint trade-offs associated with technology choice and reuse objectives, LCA studies further reveal that overall environmental performance is strongly shaped by system boundaries, sludge management strategies, and methodological assumptions, as synthesized in the following section.

14. Synthesis of LCA Results for Wastewater Treatment Systems

Throughout the reviewed literature, LCA results consistently identify energy consumption, direct greenhouse gas emissions, and sludge management as the dominant contributors to the environmental footprint of wastewater treatment plants. Conventionally activated sludge systems generally have a higher electricity demand and climate impact than anaerobic-based configurations, primarily due to the requirement for aeration. Advanced treatment technologies improve effluent quality and micropollutant removal but are typically associated with increased energy use and material inputs, which can offset environmental gains. The choice of sludge treatment pathway has a strong influence on the overall results: studies that include energy recovery from anaerobic digestion often report net reductions in climate impacts when avoided electricity is credited. However, incineration and landfilling tend to increase global warming and toxicity indicators unless recovery is explicitly modeled. Overall, comparative conclusions across technologies are highly sensitive to functional unit choice, system boundaries, and allocation assumptions, which reinforces the need for harmonized and transparent LCA frameworks when using these results to support wastewater treatment decision-making.

15. Environmental IMPACT Assessment Methods

In the LCIA phase, multiple methodological structures are used to convert LCI data, such as emissions and resource consumption, to significant environmental impact categories. These impact categories are links between the various inventory result types and the potential environmental consequences associated with them.
Typical environmental impact categories involve climate change, human toxicity, ecotoxicity, acidification, eutrophication, and resource depletion. Indicators for these categories are usually classified as midpoint or endpoint indicators. Midpoint indicators characterize impacts at an intermediate stage along the cause–effect chain (e.g., kg CO2-equivalents per unit of gas for climate change), whereas endpoint indicators describe more concrete effects, such as a rise in sea level or global average temperature. While endpoint indicators might provide a more easily comprehensible perspective on environmental damage, they tend to be more uncertain than midpoint indicators [72].
LCIA methods can be broadly categorized into two groups based on their focus:
  • Resource-based methods concentrate on the depletion of natural resources (inputs), often using indicators such as Cumulative Energy Demand (CED).
  • Emission-based methods evaluate the environmental burden of emissions (outputs) through detailed impact models.
Among the widely adopted emission-oriented LCIA methodologies are the following:
  • CML: This method emphasizes midpoint impact categories and includes both core and optional categories.
  • IMPACT 2002+: Building upon CML 2002 and Eco-indicator 99, this method links 14 midpoint categories to four endpoint (damage) categories.
  • ReCiPe: A successor to CML 2002 and Eco-indicator 99, ReCiPe harmonizes midpoint and endpoint modeling into an integrated framework, facilitating both detailed and aggregated environmental assessments [72].
Although the occurrence of Emerging Contaminants (ECs) is extensively documented, their quantitative integration into LCIA remains constrained by the lack of robust Characterization Factors (CFs) for the majority of the micropollutants [55,73]. Which represents a methodological bottleneck originating in the complexity of modeling their environmental fate and discriminating between aqueous and sludge phases during treatment [74,75]. In order to remediate this “toxicity gap,” recent works have emphasized the need to merge LCA with Environmental Risk Assessment (ERA) to incorporate local toxicity thresholds (e.g., PNEC), as well to employ advanced multicompartment fate models, allowing for a more accurate characterization of the ecotoxicity effects in the receiving environments [76].
External normalization is a key characteristic of methods such as CML 2001, IMPACT 2002+, and ReCiPe 2016. This stage contextualizes impact scores using regional or local reference values, allowing for a better interpretation of the magnitude and relevance of the results [77].

16. Challenges, Limitations, and Future Perspectives

Current Limitations of LCA in Wastewater Treatment

The use of LCA has made a substantial contribution to existing knowledge by uncovering environmental impacts that had previously been overlooked. Despite these advantages, LCA has some intrinsic limitations, leading to incomplete data delivery.
One of the biggest limitations is the lack of consideration of economic aspects, leaving out essential cost-related insights. In addition, hygienic factors, such as bacterial or viral presence, are not adequately considered in the LCA; neither are risk-related aspects, such as accidents or uncertainties. Aspects such as land use and visual impacts are also often neglected [78].
Furthermore, LCA cannot directly determine the real environmental impacts of a system. The ISO 14042 standard [79] states that while LCIA offers a structured approach, it does not predict specific impacts, evaluate safety risks, or determine if environmental thresholds are exceeded. The actual environmental effects of emissions are influenced by time, place, and mode of emission. For example, a point emission from a single source will have a very different environmental footprint from that of continuous emissions from multiple, diffuse sources. This underlines the need to complement LCA with other assessment tools to achieve a holistic approach to environmental management [78].
Another important issue for the use of LCA in wastewater treatment is to provide good quality and reliable data. The inventory phase, often combining experimental or full-scale data with existing databases, plays a crucial role. The degree of precision of the inventory is also set on the basis of the study’s objectives, determining where data collection efforts should focus. As for the remainder of the model-based approaches, it is essential to design this phase carefully to capture key aspects that can significantly influence the LCA results. For instance, non-biogenic gaseous emissions generated from the secondary treatment are rarely factored into LCA studies.

17. Future Perspectives

LCA provides a well-established tool for monitoring energy and material flows, the resources consumed, the generation of solid waste, and emissions across entire systems. By providing this detailed “system map,” LCA establishes a foundation for holistic environmental management [78]. One of its key strengths is that it extends environmental analysis beyond isolated issues, employing quantitative methods that enable a broad and objective assessment of impacts, thus promoting informed decision-making.
Waste and water treatment systems are among several product categories that can benefit from the LCA framework’s flexibility. For example, ref. [27] highlighted that while their research generated valuable new inventory data on Wastewater Treatment Plants (WWTPs), the absence of integrated LCIA models limited the depth of the comparison and interpretation of environmental consequences.
Beyond identifying environmental impacts, LCA plays an important role in assessing trade-offs when integrating new or upgraded technologies, balancing both environmental and economic considerations [80]. Ref. [81] found that meeting stricter effluent standards, such as those outlined by the EU Urban Waste Water Directive (10–15 mg N/L and 1–2 mg P/L), improved effluent quality but simultaneously led to increased energy consumption, chemical usage, and sludge production. Similarly, in nutrient recovery efforts, struvite precipitation improved phosphorus recovery and effluent quality but introduced significant costs, with chemical additions for pH control accounting for up to 97% of total struvite production expenses [82]. These examples underscore how LCA can illuminate critical trade-offs and inform more balanced decision-making in WWTP management.
Despite these advantages, many LCA studies exclude important life-cycle stages from their boundaries, such as construction, wastewater collection, transportation, and demolition. The question of whether or not to include the construction phase has been the subject of debate among researchers. While some argue that the negative environmental impacts of construction are minimal, recent findings suggest notable effects: ref. [83] reported that construction accounted for 51–71% of mineral resource scarcity impacts, 18–27% of fossil resource scarcity, 21–28% of water consumption, and 8–13% of the total direct Global Warming Potential (GWP) impact. This suggests that choices of system boundaries need to be made carefully to ensure complete and accurate assessments of treatment systems.
Sensitivity and uncertainty analyses are other important aspects in the application of LCA in WWT. Sensitivity analysis helps determine how changes in input parameters affect the LCA outcome, and uncertainty analysis accounts for errors associated with measurement inaccuracies, methodological choices, and mathematical assumptions. Studies such as [84] have employed both analytical and numerical methods to explore uncertainties, and the broader application of these approaches is essential to strengthen the validity and reliability of LCA outcomes.
Finally, prospects for future research include integrating LCA with other system-level approaches, including economic and social assessment and valuation. While economic assessments are increasingly being incorporated into LCA studies, the inclusion of social assessments remains rare. Notably, ref. [85] observed that none of the 20 reviewed studies incorporated social LCA, reflecting a major research gap. Integrating economic and social dimensions more fully into LCA has the potential to enhance its practical effectiveness and contribute to the development of more sustainable wastewater treatment solutions.

18. Conclusions

Industrial wastewater remains one of the most significant contributors to environmental degradation and public health challenges, especially in agro-industrial areas. The above review studied the sources and characteristics of the major pollutants and elaborated on the suitability and limitations of established treatment procedures. Despite technological advances, many existing solutions face barriers related to cost, energy use, and sustainability, especially in developing regions.
To solve these challenges, comprehensive strategies will be required, which will combine advancements in treatment technologies with rigorous environmental impact assessments, such as LCA, to support decision-making and minimize unintended environmental burdens. Furthermore, context-specific solutions tailored to local industrial practices, economic conditions, and regulatory environments will be critical in achieving effective wastewater management.
Overall, the path forward will require the joint efforts of industry, researchers, and public policy-makers based on integrated strategies that are not only technologically and economically viable but also societally and environmentally responsible. Through enabling innovations, facilitating knowledge transfer, and reinforcing institutional capacity, we can move toward a future where industrial activity supports both economic development and the long-term health of ecosystems and communities.

Funding

This investigation was made possible through generous doctoral research funding from Morocco’s National Center for Scientific and Technical Research (CNRST) as part of their prestigious PASS fellowship initiative. The research team wishes to formally recognize CNRST’s crucial financial backing, which enabled the execution of this scientific study.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study.

Conflicts of Interest

The research team confirms the absence of any financial or interpersonal affiliations that might create bias or compromise the objectivity of this study’s methodology, findings, or conclusions.

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Figure 1. Workflow of literature identification, screening, and final inclusion.
Figure 1. Workflow of literature identification, screening, and final inclusion.
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Figure 2. Distribution of included documents by publication type.
Figure 2. Distribution of included documents by publication type.
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Figure 3. General flowchart of wastewater treatment process.
Figure 3. General flowchart of wastewater treatment process.
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Figure 4. Trade-off between treatment performance and energy demand across wastewater treatment technologies.
Figure 4. Trade-off between treatment performance and energy demand across wastewater treatment technologies.
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Figure 5. Conceptual framework linking wastewater characteristics, treatment selection, and life-cycle sustainability trade-offs.
Figure 5. Conceptual framework linking wastewater characteristics, treatment selection, and life-cycle sustainability trade-offs.
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Figure 6. The four stages of life-cycle assessment.
Figure 6. The four stages of life-cycle assessment.
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Figure 7. System boundary levels in wastewater LCA.
Figure 7. System boundary levels in wastewater LCA.
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Figure 8. Relative contribution of major sources to climate change impacts in wastewater treatment systems based on life-cycle assessment studies.
Figure 8. Relative contribution of major sources to climate change impacts in wastewater treatment systems based on life-cycle assessment studies.
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Figure 9. Comparative ranges of carbon, energy, and water footprints reported for major wastewater treatment technologies in life-cycle assessment studies.
Figure 9. Comparative ranges of carbon, energy, and water footprints reported for major wastewater treatment technologies in life-cycle assessment studies.
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Table 1. Pollutant–treatment linkages with indicative life-cycle implications.
Table 1. Pollutant–treatment linkages with indicative life-cycle implications.
Pollutant CategoryRepresentative Treatment TechnologiesEnergy Demand (kWh·m−3)Sludge GenerationClimate Impact Relevance (GWP)Key LCA-Related Trade-Offs
Bulk organic matter (COD, BOD)Conventional Activated Sludge (CAS), anaerobic reactors (UASB)0.3–0.8 (CAS); 0.1–0.3 (UASB)High (CAS); Moderate (UASB)Moderate–High (aeration, CH4/N2O)Energy-intensive aeration vs. biogas recovery potential
Nutrients (N, P)Biological Nutrient Removal (BNR), chemical precipitation0.4–1.0HighHigh (N2O emissions)Trade-off between nutrient removal efficiency and GHG emissions
Heavy metalsChemical precipitation, electrocoagulation, membrane filtration0.2–1.5High (metal-rich sludge)ModerateHazardous sludge disposal and chemical consumption
Synthetic dyes and recalcitrant organicsAdvanced Oxidation Processes (AOPs), adsorption0.8–2.5Low–ModerateHigh (electricity-driven)High energy demand and potential toxic by-products
PPCPs and micropollutantsOzonation, AOPs, Membrane Bioreactors (MBR)0.9–2.0ModerateModerate–HighImproved effluent quality vs. increased energy and material use
PathogensDisinfection, ultrafiltration, tertiary treatment0.1–0.6LowLow–ModerateAdditional energy demand for polishing steps
Table 2. Performance and environmental trade-offs of wastewater treatment families.
Table 2. Performance and environmental trade-offs of wastewater treatment families.
PollutantWastewater ContextTreatment TechniquesEfficiencyTypical Removal EfficiencyLCA Key Hotspots and Environmental ProfileIndicative Benchmark
PAHs/BPAMunicipal/IndustrialAOPs, ozonationHighPPCPs: 60–95%Substantial energy profile. Major impact on Global Warming Potential (GWP) due to electricity use and ozone generation.~1.5–3.5 kg CO2 eq/m3
Synthetic dyesIndustrialAdsorption (activated carbon)ModerateDyes: 60–90%Adsorbent material generation. Environmental burden shifted to activated carbon production (embodied energy) and regeneration processes.Extensive fossil resource depletion
Heavy metalsIndustrialElectrocoagulationHighMetals: 80–99%Electricity and electrode consumption, leading to mineral resource depletion and greenhouse gas emissions; metal-rich sludge generation.~0.5–2.0 kWh/m3
PharmaceuticalsMunicipal/IndustrialMembrane filtration (MBR)HighCOD: >95%; PPCPs: 60–95%High operational energy demand and membrane fouling; aeration for membrane cleaning is the dominant hotspot. Ecotoxicity decreased, but GWP increased.~0.8–2.5 kWh/m3
Pathogenic microorganisms (E. coli, etc.)Municipal/IndustrialUltrafiltration, ozonation, UV irradiationHighPathogens: >99%High energy demand, especially for ozonation and UV irradiation, increasing GWP. Ultrafiltration hotspots due to membrane manufacturing (embodied energy) and frequent backwashing/cleaning chemicals.~0.1–0.5 kWh/m3 (disinfection stage)
Table 3. Most applied techniques for sewage sludge treatment.
Table 3. Most applied techniques for sewage sludge treatment.
Treatment MethodOperating ConditionsPrimary LCA Hotspot and Environmental BurdenIndicative Carbon Footprint (Kg CO2 eq/t DM)Resource Recovery
Composting55–65 °CDirect emissions: high CH4 and N2O during degradation150–300Nutrients (N, P)
Agricultural ReuseAmbientToxicity: heavy metal and pathogen leaching in soil/underground water and transfer to food chainLow (transport only)High (especially in terms of enhancing soil fertility)
Anaerobic Digestionk35–55 °CPotential methane leakage: during storage−50 to +100 (Net)Energy (biogas)
Incineration>850 °CAir emissions: high toxic and pollutant gases (NOX, SOX, CO2, CO…); high consumption of fossil fuels for startup400–800Thermal energy
Pyrolysis400–800 °CInput energy for heating; toxic compounds and residues in biochar (if not fully immobilized)100–400Bio-oil (energy, chemical synthesis); biochar
Gasification700–1000 °C, gasification agent (steam, O2 and/or CO2)Requires high energy input (mainly fossil fuels); generation of tar, a subproduct characterized by its high-toxicity and its fouling effect on the reactors’ material100–300Very high and clean energy (mainly H2)
LandfillingAmbientExcessive use of land sources and important generation of CH4 due to anaerobic decay200–500None (negative)
Hydrothermal processes (HTC, HTL)180–370 °C, high pressure requiredNecessitate high energy input; effluent toxicity200–500High-density fuels
Table 4. LCA parameters and environmental trade-offs across wastewater reuse sectors.
Table 4. LCA parameters and environmental trade-offs across wastewater reuse sectors.
SectorPrimary Reuse PathwayKey LCA Benefit (Avoided Impact)LCA Hotspot (Negative Impact)Treatment Intensity (LCA Cost)
AgricultureIrrigation (edible/non-edible)Avoided synthetic fertilizer production (N, P)Soil ecotoxicity from heavy metals/micropollutantsLow to medium (secondary + disinfection)
IndustryCooling towers, boilers, scrubbingFreshwater preservation in industrial clustersHigh chemical footprint from scaling inhibitorsMedium to high (advanced membrane/AOP)
EnvironmentManaged Aquifer Recharge (MAR)Salinity regulation; long-term aquifer securityEnergy consumption for high-pressure injectionMedium (leverages natural SAT processes)
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Laouane, H.; El Joumri, L.; Halhaly, A.; Arid, Y.; Labjar, N.; El Hajjaji, S. Life-Cycle Assessment of Wastewater Treatment: Enhancing Sustainability Through Process Optimization. Sustainability 2026, 18, 605. https://doi.org/10.3390/su18020605

AMA Style

Laouane H, El Joumri L, Halhaly A, Arid Y, Labjar N, El Hajjaji S. Life-Cycle Assessment of Wastewater Treatment: Enhancing Sustainability Through Process Optimization. Sustainability. 2026; 18(2):605. https://doi.org/10.3390/su18020605

Chicago/Turabian Style

Laouane, Hajar, Loubna El Joumri, Amine Halhaly, Yassine Arid, Najoua Labjar, and Souad El Hajjaji. 2026. "Life-Cycle Assessment of Wastewater Treatment: Enhancing Sustainability Through Process Optimization" Sustainability 18, no. 2: 605. https://doi.org/10.3390/su18020605

APA Style

Laouane, H., El Joumri, L., Halhaly, A., Arid, Y., Labjar, N., & El Hajjaji, S. (2026). Life-Cycle Assessment of Wastewater Treatment: Enhancing Sustainability Through Process Optimization. Sustainability, 18(2), 605. https://doi.org/10.3390/su18020605

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