Benchmarking tertiary water treatments for the removal of micropollutants and pathogens based on operational and sustainability criteria

Journal of Water Process Engineering 46 (2022) 102587 Available online 3 February 2022 2214-7144/© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Benchmarking tertiary water treatments for the removal of micropollutants and pathogens based on operational and sustainability criteria Sabrina de Boer 1 , Jorge Gonz´ alez-Rodríguez * , 1 , Julio J. Conde 1 , Maria Teresa Moreira a CRETUS, Department of Chemical Engineering, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Galicia, Spain ARTICLE INFO Keywords: Review Tertiary treatment LCA Economic evaluation Emerging pollutants ABSTRACT In a context of increasing water scarcity, it is essential to ensure an integrated watershed management, savings in the consumption of water as a finite resource and improve the performance of wastewater treatment plants to guarantee the quality of treated effluents. Therefore, advanced technologies for tertiary wastewater treatment have been widely studied in recent decades. These treatments have been reviewed over the years mainly providing comparisons from a technical perspective. However, there is a lack of a holistic evaluation considering environmental and economic aspects together with the aforementioned technical aspects. In this review, treatment alternatives for micropollutant and pathogen abatement have been identified based on technologies implemented on a large scale (ozonation, ultraviolet treatment, adsorption on activated carbon or membrane filtration) as well as those treatments in the process of implementation, such as electrochemical, Fenton-based or photocatalytic techniques. Thus, a systematic bibliographic search was performed considering works applying pilot and full-scale equipment, leaving lab-scale results out of the analysis. The description of each process allowed the identification of the technical feasibility, operating costs and associated environmental impacts, providing a comparative assessment that will help decision-making in the development and application of the different technologies. The benchmarking results reveal that the selected treatment should be chosen based on the source and specific pollutants present in the wastewater, as there is no single solution for the treatment of micropollutants and pathogens. In addition, recommendations are presented for the publication of reliable process-related data to facilitate comparison between different technologies and treatment scenarios. 1. Introduction Population growth implies an increasing demand for natural resources such as water, energy and food [1]. In this context, one of the most serious problems to be faced is the increasing water scarcity [2]. This concept is defined as the imbalance between water demand and availability and is related to unbalanced consumption of water reserves, declining quality of drinking water due to contamination or saline intrusion of surface waters and aquifers and increased periods of drought [3]. The importance of access to safe drinking water and sanitation is highlighted and embedded in Goal 6 of the United Nations Sustainable Development Goals [4]. In order to address the problems related to water pollution, large investments have been made in wastewater treatment plants (WWTPs) [5]. Although WWTPs are capable of removing organic matter and nutrients (nitrogen and phosphorous), the occurrence of organic micropollutants (OMPs) such as personal care products, pesticides, endocrine disrupting chemicals or pharmaceuticals, in different environmental compartments requires changes in the design and operation of wastewater facilities to ensure their removal and the quality of the treated effluents [6,7]. As part of the implementation of the Water Framework Directive, the European Union defined a list of priority substances that could pose a risk to the environment and human health. The “Watch List” reported in 2015 included two pharmaceuticals, natural hormones, three macrolide antibiotics, pesticides, an ultraviolet filter and an antioxidant. Subsequently, three additional substances were added in 2018: an insecticide and two antibiotics. With the primary objective of OMPs removal, some tertiary treatment technologies, such as membrane filtration or adsorption on activated carbon, allow the retention of OMPs from the wastewater stream. However, the concentrated flow and the spent adsorbent have to be conveniently managed in a downstream process increasing the complexity of these techniques [8]. The possibility of * Corresponding author. E-mail address: [email protected] (J. Gonz´ alez-Rodríguez). 1 Sabrina de Boer, Jorge Gonz´ alez-Rodríguez and Julio J. Conde have contributed to the manuscript equally. Contents lists available at ScienceDirect Journal of Water Process Engineering journal homepage: www.elsevier.com/locate/jwpe https://doi.org/10.1016/j.jwpe.2022.102587 Received 3 October 2021; Received in revised form 30 November 2021; Accepted 16 January 2022 Journal of Water Process Engineering 46 (2022) 102587 2 implementing advanced oxidation processes (AOPs) based on nonspecific oxidation mechanisms [9] such as ozonation or UV radiation are options that have had variable success since their large-scale operation involves high operating costs due to their high energy requirements or lower kinetic rates. Beyond organic micropollutants, special attention should be also paid to contamination by heavy metals, which are generally classified as inorganic micropollutant as they are present in treated effluents in trace concentrations. Contamination due to heavy metals can occur naturally, caused by the entrainment of geological material into surface waters although most heavy metal pollution has been determined to be anthropogenic, deriving from the use of pesticides and fungicides (As) or pigments (As, Cd, Cr, Cu, Pb, Ni), as well as from the metallurgical (As, Cd, Hg), petrochemical (Cd, Pb) or pyrotechnical (As) sectors [10]. In this review, pathogen removal is also considered for benchmarking since tertiary treatments are widely used as methods for pathogen abatement [11]. The presence of pathogens is especially relevant in urban, hospital, livestock and agricultural wastewater, since the release of microorganisms into water bodies contributes to the spread of pathogens and antibiotic resistances. Several review articles have recently been published on the benefits and drawbacks of advanced tertiary treatments for wastewater polishing, mainly focusing on the technological aspects of treatments. For instance, experts from NEREUS COST Action analyzed the best available technologies for water reuse for crop irrigation considering ozonation, activated carbon adsorption, chemical disinfectants, UV radiation, advanced oxidation processes and membrane filtration [12]. The conclusion of the expert group is that a single advanced treatment method is not sufficient to minimize the release of chemicals of emerging concern and antibiotic-resistant microorganisms. Luo et al. analyzed the removal efficiency of the selected micropollutants in 14 countries and regions, analyzing different tertiary systems such as coagulation–flocculation, activated carbon adsorption, advanced oxidation processes, nanofiltration, reverse osmosis, and membrane bioreactors [13]. Rizzo et al. analyzed consolidated versus new tertiary treatment methods, concluding that the lack of comparative research between the two categories complicates the evaluation of the most suitable and cost-effective solution for the treatment of emerging contaminants [14]. Bui et al. performed a multicriteria assessment of advanced treatment technologies for micropollutants removal, including very brief references to environmental considerations and only including some of the available tertiary treatments, i.e., adsorption, ozonation, UV/H 2 O 2 , membrane processes and membrane bioreactors [15]. The analyzed reviews are mainly focused on the technical aspects, lacking the economic and environmental perspectives. The search for new technological alternatives must meet the following objectives: technological feasibility in the construction and operation of the equipment, operational efficiency and reliability and reduction of environmental impacts and costs. Based on the score in each of the aforementioned sections, decision making will be better supported by evidence and contrastable data [16]. In accordance with European directives, environmental and socioeconomic factors, including consideration of human health, must be considered in the assessment of advanced technologies. In this sense, the environmental impacts associated with tertiary treatments can be elucidated using the internationally standardized Life Cycle Assessment (LCA) methodology. Considering the environmental approach, Pesqueira et al. conducted a literature review solely on the application of LCA in tertiary wastewater treatment, however, the scope of the review focuses on the removal of priority substances and pollutants of emerging concern mentioned in European legislation, including a total of 18 papers [17]. In this study, the focus was further extended to a total of 40 papers dealing with LCA in tertiary treatments. In consequence, the main objective of this keyword-based literature review is to perform a holistic analysis of the main technological developments in tertiary treatments from a sustainability perspective, including not only efficiency variables, but also environmental impacts and cost estimation. The key aspects of the technologies were identified and evaluated for pilot and full-scale studies considering technical and sustainability approaches, concluding that the combination of multiple treatment processes is essential to meet the effluent requirements. Moreover, the use of LCA methodology as a powerful tool for decisionmaking can highlight specific hotspots of the technologies, complementing the information provided by economical and technological evaluation. Accordingly, the main challenge is to emphasize the key data of each technology considering a joint technical, environmental, and economic approach, providing useful information about the main drawbacks of present studies and desirable targets for future research. 2. Bibliographic search methodology The literature search was performed using the search tool provided by the SCOPUS database in March 2021. The selection of manuscripts addressing the efficiency of tertiary wastewater treatments for the removal of micropollutants under technological, economic and environmental criteria was conducted. Considering the scope of the review, the search was limited to the technologies applied to wastewater treatment, including the keywords “wastewater treatment” or “waste water treatment” in the search string. In addition, since the goal of tertiary treatments is the removal of micropollutants, heavy metals and pathogens, these words and their relevant abbreviations were considered in the formulation of search parameters by including their respective keywords. For this purpose, the search procedure used in the literature review is summarized in Fig. 1, along with the specific keywords and Boolean operators. Tertiary treatments were classified into seven distinct groups to facilitate the search process, i.e., ozonation, ultraviolet, catalyst-based, pressure-driven, activated carbon adsorption, electrochemical and irradiation treatments. Moreover, the results were filtered and reduced considering their publication after 2010, written in English and in a final step of publication. The obtained results applying each step are presented in the Table S1 in the Electronic Support Material (ESM) 1. The analysis of environmental indicators estimated by the LCA methodology have been also analyzed performing a complementary search, using specific keywords as “life cycle” or “LCA”. The results of the bibliographic search are compiled in ESM 2 and ESM 3 in the Supplementary Information. ESM 2 includes the bibliographic information of all studies analyzed after the second refining step along with their basic bibliographic data, while ESM 3 presents a standardized table with the technoeconomic data extracted from the selected studies, focusing in micropollutant removal and operational conditions. 3. Bibliometric analysis The bibliometric analysis includes all the results after the manual refinement obtained from Scopus using the methodology explained above. These papers have been taken into account for the technological and economic analysis performed during this review, and the keywords were extracted and analyzed according to their occurrence and relationships, as shown in Fig. 2. To clarify the data and homogenize the results, the substitution of keywords by synonyms or abbreviations was conducted considering the formation of clusters involving keywords with high similarity (ESM 4). In view of the results, the keywords can be classified into four groups considering the main topic addressed: (i) generic keywords: this group formed by wastewater treatment, tertiary treatments, wastewater reuse and domestic wastewater, terms that represent the target of the findings and the definition of the field of study; (ii) target compound: the most repeated keywords were pharmaceuticals and personal care products (PPCPs), organic micropollutants (OMPs), compounds of emerging S. de Boer et al. Journal of Water Process Engineering 46 (2022) 102587 3 Fig. 1. Search methodology for the different tertiary treatments. Fig. 2. Map and network of keywords (elaborated with VosViewer®). S. de Boer et al. Journal of Water Process Engineering 46 (2022) 102587 4 concern (CECs) and antibiotics (ABs) and represent the main target compounds evaluated; (iii) technologies: this group encompasses the different technologies used for tertiary wastewater treatment such as filtration, ozonation, UV or AOPs; and (iv) treatment effectiveness: this group includes the methods carried out for the evaluation of the technologies, such as removal, removal efficiency, by-products identification or toxicity. Fig. 2 allows tertiary treatments to be classified between large-scale applied technologies and more innovative processes, and these trends are represented in the diagram. Focusing on the wastewater treatment circle and its relationships, this keyword usually appears together with adsorption techniques as granulated activated carbon or powdered activated carbon (GAC-PAC), ozonation, filtration and UV, indicating that the technologies most applied as advanced treatments for the removal of PPCPs and OMPs are those mentioned above. On the other hand, there is an evident relationship between tertiary treatment and titania, photocatalysis and solar treatment, as well as a growing interest in pathogen removal. This keyword appears related to others such as disinfection or presence in water of antibiotic resistant bacteria (ARBs) and antibiotic resistance genes (ARGs), showing the current concerns regarding thetransfer of antibiotic resistance to pathogens in the environment. Another issue to highlight is the close relationship between electrochemical methods and heavy metals (HMs), showing the preferences for the use of this type of processes for the treatment of wastewater containing heavy metals. Adsorption techniques such as GAC-PAC and filtration-based treatments to remove contaminants were usually studied together, as can be seen by the proximity of points and width of the relation line. In general, low presence of cross-sectional keywords as “economic assessment” was observed considering the significance criteria (only words with an occurrence of more than 3 were considered for the study). As a result, although some articles incorporate the economic evaluation of the technology, the analysis from the environmental point of view is missing. 4. Technological aspects of tertiary treatments 4.1. Ozone-based treatments Ozonation (O₃₃) is a heterogeneous process applied for the oxidative Fig. 3. A) Schematic representation of an ozone treatment unit. B) Worldwide distribution of the analyzed studies (circle area is proportional to the treated flow rate and color represent the continent). C) Overview of the quality requirements of the influent and typical operational parameters and consumables ranges of the analyzed studies. D) Removal efficiencies for the most investigated compounds (n >2). The acronym list is available in Supplementary Information (ESM 3). S. de Boer et al. Journal of Water Process Engineering 46 (2022) 102587 5 treatment of drinking water and more recently for wastewater. Conversely to fully water miscible oxidants (e.g., H₂O₂), mass transfer optimization from the gaseous to the aqueous phase must be considered. Typically, bubble column reactors are used to contact the ozone with the water stream (Fig. 3A). Ozonation has been studied mainly in continuous processes, whereas batch operation is the setup of choice at pilot scale to optimize reaction conditions [18–20]. Most of the reviewed studies considered the application of ozonation to effluents from conventional secondary treatment plants. However, since ozonation uses non-specific oxidation pathways to transform target compounds, the versatility of this process allows its wide application, such as for the removal of pollutants from surface groundwaters [21–23] as well as other types of wastewaters, e.g., industrial effluents [24,25] or reject streams from ultrafiltration and reverse osmosis units [26]. 70% of the reviewed studies were located in Europe, whereas none of them was in Africa, as depicted in Fig. 3C. The typical temperature of ozonation-treated effluents ranges from 10 to 30 ◦C depending on climatic conditions and WWTP location, high temperatures can lead to volatilization of compounds facilitated by gas bubbling. In general, a pH above 8 favors ozone decomposition mediated by hydroxyl anions, but this condition is not met in most wastewaters, the pH values in the selected studies vary in the range of 6.0 to 10.5, respectively. To a much greater extent than hydroxyl anions, dissolved organic matter (e.g., phenols and amines) induces the formation of hydroxyl radicals through different pathways [27,28]. It should be noted that the efficiency of the process can be hampered by high loadings of radical scavenging species such as carbonates, halogen ions or nitrogen oxides [29,30]. Consequently, the abundance of these species implies not only a reduction of the treatment efficiency but also the formation of toxic oxidation products when halogen ions are present in the reaction matrix (bromate formation). In a typical configuration, relatively short removal times to achieve micropollutant removal vary from 5 to 30 min, but can extend to longer periods, as is the case for nonylphenol or bisphenol-A with treatment times up to 100 min [31]. In the case of organic matter removal, it has been established that the ozone dose varies depending on the stream flow to be treated between values of 1 to 200 mg O₃ L −1 .The typical parameters of ozonation are presented in Fig. 3B. The electrophilic character of ozone and its high oxidation potential (E 0 =2.07 V) enhance its reactivity towards compounds with low oxidation state such as deprotonated amines, sulfides, and aromatic rings with electron donor groups [29,32,33]. In addition to direct oxidation, there are indirect oxidation pathways in which reactive oxygen species (ROS), most notably •OH radicals, are produced [29,30], leading to the decomposition of water pollutants due to their high oxidation potential (E 0 =2.80 V), as seen in Fig. 3D [34]. The studied pollutant concentrations are in the range of 0.1 ng L −1 and 1.5 mg L −1 for pharmaceuticals and pesticides, and from 1 mg L −1 to 500 mg L −1 for the combination of ozonation with an electrochemical method [25]. Although most authors focus their research on ozone-mediated micropollutant removal, some works have studied degradation pathways as well as the consequences of treatment on the treated effluent in terms of toxicity, estrogenicity or mutagenicity [31,35,36]. Different species have been considered for in vivo studies on the potential mutagenicity and toxicity of the treatment, in particular using Daphnia magna [37], Aliivibrio fischeri [37,38] and Potamopyrgus antipodarum [39]. Furthermore, the formation of intermediates [40] or the influence of ozonation on ARGs and ARBs [38,41] have been studied. As a special case, the application of this process alone or in combination with GAC and sand filter against inactivation of microorganisms provided reductions of up to 4.3 log removal value (LRV) for Enterococci and Escherichia coli bacteria [41–43]. Unlike in chlorination, the formation of by-products affecting water quality such as haloalkanes is prevented [44]. To improve its efficiency and pollutant removal performance, ozonation can be combined with UV irradiation [23,45] and hydrogen peroxide [26,37]. UV light at wavelengths up to 310 nm provokes the dissociation of O 3 into an oxygen molecule and hydrogen peroxide. Similarly, the peroxone process (O₃₃-H₂₂O₂₂) combines the oxidizing power of ozone with the decomposition of H₂O₂ to enhance the generation of •OH radicals and has been shown to be effective in reducing ozone-resistant micropollutants [27]. One of the most important factors to consider in the peroxone process is the ratio of O₃ to H₂O₂, which governs the reaction rate. However, the high capacity of H 2 O 2 regarding the degradation of OMPs allows kinetic improvements even when present in trace amounts [46]. The H₂O₂-mediated generation of •OH can consume up to half the amount of available O₃ [47], therefore, the H₂O₂/ O₃ ratio is generally set between 0.5 and 1.0. At pilot and full scale, the reaction is often carried out in gas-liquid reactors with a configuration similar to ozone bubbling columns. Alternatively, the decomposition of hydrogen peroxide in presence of UV irradiation leads the formation of additional •OH radicals [23,48]. Catalytic ozonation (O₃₃-CAT) makes use of a catalyst to promote the decomposition of ozone and the subsequent formation of ROS. Although many homogeneous catalysts based on transition metal ions, preferably bivalent, have been investigated, precipitation or lack of retention systems have prevented their largescale application [49]. In the late 1990s, the first pilot plants applied heterogeneous catalysts to improve ozonation efficiency in leachate treatment [50]. Applicable materials are iron flakes, metal/metal oxidecoated ceramic membranes or clay minerals such as montmorillonite [49,51]. The application of alumina-based catalysts showed a significant enhancement of micropollutant removal compared to ozonation alone [52]. In contrast, the application of iron-based catalysts for the removal of sulfamethoxazole showed no improvement compared to the results with the results obtained for conventional ozonation [53]. On the other hand, the possibility of integrating ozonation with electrochemical methods aims at the coagulation of dissolved metals [25] and will be applicable for wastewaters of high content in heavy metals. The combination of ozonation with different types of activated carbon such as GAC [54,55], biological activated carbon (BAC) [56,57], biofiltration [38,55] or sand filtration [43,58] has been successfully applied. As an example, ¨ Ostman et al. [59] reported the improvement in benzothiazole removal from 30% to 82% by incorporating a GAC unit after the ozonation stage. However, in this study, the application of sand filtration to this effluent did not show a significant effect [56,58,59]. On the other hand, the use of biological filters has been studied by Knopp et al. [55] and Ternes et al. [38], showing similar results to those obtained by the application of ozonation. 4.2. Ultraviolet treatments Ultraviolet (UV) irradiation is commonly applied as disinfection step after biological treatment. UV units for disinfection usually consist of cylindrical borosilicate modules housing mercury pressure lamps immersed in the wastewater stream (Fig. 4A). Both low pressure (LP) lamps, with a sharp emission peak at about 254 nm, and medium pressure (MP) lamps with a broader emission spectrum in the UV-C region (200–600 nm) are applied. The flow capacity of these full-scale modules can be up to 3000 m 3 h −1 [60] in single-pass mode for a residence time of less than 1 min. The influent treated in the revised studies was characterized by a pH in the range of 7.0–7.6 and a temperature ranging between 18 and 24 ◦C, and comparably low TSS and DOC values (Fig. 4B). Most of the considered studies were performed in Europe, with a concentration to the Mediterranean area, while the largest plants regarding treated flow were located in China, only one study was in Brazil, focusing on pathogen removal. Pathogen removal by UV light typically ranges between 5 and 7 LRV at irradiance levels of 70 W m −2 . These high removal values are necessary to mitigate the effect of bacterial regrowth after UV treatment [61]. This possibility is of particular concern when reclaimed wastewater is stored in buffer tanks prior to use for agricultural purposes. It should be noted that certain species of bacteria show resistance to UV treatment. While the most commonly investigated E. coli bacteria are S. de Boer et al. Journal of Water Process Engineering 46 (2022) 102587 6 effectively removed even within a short residence time, Pseudomonas strains are frequently detected in effluent from UV disinfection units [42]. Regarding the elimination of genetic patterns relevant to the evolution of antibiotic resistances, several studies reported that UV treatment only selectively eliminates ARGs, and reported removals of DNA or gene fragments was lower than for pathogens (ESM 6) [60,62]. Although many authors have studied OMP removal in full-scale modules designed for disinfection [63], these approaches only lead to slight removal efficiencies for applied irradiances up to 500 mJ cm −2 , even an irradiance of 4000 mJ cm −2 only resulted in a removal of 0.77 LRV of micropollutants [64]. Similar removal percentages for endocrine disrupting chemicals (EDCs) were reported by C´ edat et al. [65] at irradiance of 1000 mJ cm −2 . It was also shown that estrogenicity could not be satisfactorily removed, even though the parent compounds were partially transformed [66]. Regardless of the lamp intensity in the photolysis process, it is required that the target OMPs absorb the UV radiation in the range of the lamp spectrum to be abated. However, only a few organic micropollutants present in wastewater undergo photolysis at the applied wavelength of 245 nm. These are, for example, sulfamethoxazole with a removal efficiency between 22 and 70% [67], depending on the applied irradiance [63,68,69] and diclofenac between 45% and 100% [61,68] (Fig. 4D). Even if the target molecule is not susceptible to photolysis, it may undergo secondary radical transformation promoted by light-induced excitation of electrons from compounds present in the matrix. Generally, the studies reviewed were operated at dissolved organic carbon (DOC) values below 15 mg L −1 with few exceptions. Still, most of the investigated OMPs are only degraded to a negligible extent [70] or even with negative removal efficiencies [71,72]. This may be related to several factors such as photolysis of coupling products towards the original compounds, errors in quantitative measurements due to matrix interference or desorption of residual particulate matter [66]. None of the investigated studies consider the fate of heavy metals during UV treatment, as UV irradiation is known to have no effect on their removal. UV-treatment combined with hydrogen peroxide (UV-H₂₂O₂₂) produces reactive hydroxyl radicals as light with wavelengths >254 nm Fig. 4. A) Schematic representation of an ultraviolet treatment unit. B) Worldwide distribution of the analyzed studies (circle area is proportional to the treated flow rate and color represent the type of treatment –blue: UV, yellow: UV +H 2 O 2 , green: UV+other oxidant and red: solar treatment–). C) Overview of the quality requirements of the influent and typical operational parameters and consumables ranges of the analyzed studies. D) Removal efficiencies for the most investigated compounds (n >2). The acronym list is available in Supplementary Information (ESM 3). Grey columns stand for UV +H 2 O 2 and white columns represent standalone UV treatment results. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) S. de Boer et al. Journal of Water Process Engineering 46 (2022) 102587 7 induces photodissociation of H₂O₂ [70], resulting in enhanced micropollutant removal [73–75]. Typically, a H₂O₂ dose of 10 mg L −1 is applied in photoreactors equipped with dosing-mixing systems that are already commercially available for large-scale applications [65,76]. Various reactor geometries have been evaluated to maximize mixing and irradiation [68]. However, the benefits of combining sunlight or UV with hydrogen peroxide are less prominent for antibiotic removal and disinfection [67]. Although H 2 O 2 is the most used oxidant, it has been shown that the use of other oxidants considerably minimizes bacterial regrowth compared to UV treatment alone [61]. Regarding OMP, removal efficiencies are highly dependent on the type of contaminants, so variable energy efficiency values were obtained, ranging from 0.16–18 kW m −3 for ciprofloxacin and clarithromycin, respectively [10]. It is therefore necessary to evaluate and model the degradation kinetics of the expected micropollutants both in laboratory and at pilot scale [66,67,77]. Other oxidants such as ozone, chlorine, chlorine dioxide and persulfate in combination with UV treatment have been also investigated for OMP removal [73–75]. Compared to •OH radicals, Cl and SO₄ − . radicals have been reported to be less prone to deactivation by matrix constituents such as HCO₃ − or NO₃ − and to have less affinity towards natural organic matter [61,77]. The underlying mechanisms of radical formation and reaction with OMP are described in detail elsewhere [78,79]. 4.3. Adsorption on activated carbon Although advanced materials such as zeolites or carbon nanotubes are gaining interest as adsorbents in wastewater polishing, activated carbon is used as the predominant material, depending on the grain size, as powdered activated carbon (PAC) or granular activated carbon (GAC). Powdered activated carbon is applied in grain sizes of 50–100 μ m with a BET surface area between 900 and 1300 m 2 g −1 [80]. PAC is usually added to the effluent of the biological treatment stage in a [81–83] loading range between 10 and 20 mg L −1 . One of the constraints in the design of different process configurations must ensure the retention of spent PAC [84], achieved by (1) the implementation of a sedimentation unit [85,86] or by (2) sand, anthracite or expanded shale bed Fig. 5. A) Schematic representation of typical configurations for 1) GAC and 2) PAC processes. B) Worldwide distribution of the analyzed studies (circle area is proportional to the treated flow rate and color represent the continent). C) Overview of the quality requirements of the influent and typical operational parameters and consumables ranges of the analyzed studies. D) Removal efficiencies for the most investigated compounds (n >2). The acronym list is available in Supplementary Information (ESM 3). Grey columns stand for GAC, and white columns stand for PAC treatment results. S. de Boer et al. Journal of Water Process Engineering 46 (2022) 102587 8 filter columns or, in some cases, by (3) membrane filtration units (Fig. 5A). In the latter two options, PAC recycling can be achieved by backwashing to maximize the carbon usage rate. However, in the first case, it is necessary to improve PAC retention by coagulationflocculation, which requires the addition of Fe 3+ -based coagulants [83]. In some cases, an anionic polymer is additionally applied as a flocculant [85]. Alternatives where PAC is directly applied in membrane bioreactors (MBR) may pose problems in sludge management [87] as spent PAC is mixed with the sludge matrix and cannot be regenerated after use. A promising advance in process simplification is the development of μ GAC, which is coarser and thus easier to separate and regenerate [88,89]. GAC units are mainly applied continuously in packed bed filter columns (Fig. 5A), which have to be replaced after the breakthrough threshold is reached. Several columns are applied in series or in parallel, while each configuration has drawbacks and advantages [90,91]. The same applies to the filtration direction, either downflow or upflow, which results in different head loss profiles, while the removal efficiency was reported only slightly higher in the upflow configuration [90]. In all cases, regular backwashing is required to impede pressure buildup and can increase the operation time of the filter columns. Based on experience during one year of pilot scale operation, Kårelid et al. [87] estimated the maintenance time to be 30–60 min per day. The most critical factors of the wastewater matrix that affect the stable performance of GAC filters are DOC and pH value, these values are between 6 and 11 mg L −1 (Q1-Q3) and 7.0–7.6 (Q1-Q3) respectively. TSS are considered to have a minor effect on the removal performance of GAC filters, but high levels require increased backwashing and range from 3 to 6 mg L −1 (Q1- Q3) in the reviewed studies [40,92] (Fig. 5B). Fig. 5C shows the geographical distribution of the revised studies based on activated carbon, indicating a focus of implementation in central and Western Europe and the USA. This is explainable by the stricter legislation regarding the presence of OMP in WWTP effluents implemented in Switzerland and local initiatives as in some German federal states (North Rhine Westphalia and Baden Wurttemberg). Both PAC and GAC can be considered as a strong barrier against most micropollutants [87,93]. Due to the numerous GAC products available, variable wastewater composition, and complex interaction mechanisms, a quantitative prediction of treatment capacity and GAC breakthrough values is not straightforward and must be performed on a case-by-case basis [89]. Another factor that adds complexity to the description of GAC processes and modeling of breakthrough behavior is the development of a biofilm in the GAC bed over a prolonged operating time, which ultimately turns them into BAC filters [55]. While adsorption-related removal efficiency will decrease during microbial evolution, biodegradation becomes more relevant [40,94,95]. As expected in relation to the larger surface area, a smaller GAC particle size is usually favorable for OMP removal [59]. The data obtained from the revised studies indicates that the GAC process can achieve higher maximal removals for all selected indicator compounds, but the data is more scattered than in the PAC process. Most critical compounds were diazepam, valsartan, acesulfame K and sulfamethoxazole, while atenolol, bezafibrate and metoprolol were removed >80% in most studies (Fig. 5D). In particular, higher pK a values and hydrophobicity of OMP favor adsorption onto negatively charged activated carbon [22,94]. Depending on the treatment goals and quality of treated water, GAC processes can be integrated into treatment trains, e. g. GAC is often used as a polishing step after ozone/AOP treatment, combining the benefit of low DOC content and the efficient removal of ozonation/oxidation byproducts. 4.4. Pressure-driven membrane filtration treatments In reverse osmosis (RO) and nanofiltration (NF) processes, water is pumped through a variable set of pressurized membrane elements, yielding a treated permeate stream and a concentrated retentate stream. The percentage of the permeate flux regarding the influent flux is defined as recovery, ranging between 50 and 90% [18]. High recoveries can be achieved by applying two consecutive stages, feeding the second stage with retentate from the first stage, while higher permeate quality can be reached by a second pass through another stage (Fig. 6A). The most commonly applied polymeric membranes have a lifetime of about 5 years, depending on the quality of the treated water [96]. Recently, ceramic membranes are gaining interest despite higher manufacturing costs due to their longer lifetime, which can reach a duration 20 years [97,98]. In the reviewed studies, the operation of spiral wound polymeric modules in crossflow configuration is preferred over flat sheet configuration, while membrane areas between 2.2 and 14.0 m 2 for pilot scale [99,100] and between 1000 and 3000 m 2 for full scale were investigated. In both NF and RO processes, pressures in the range of 7–15 bar are applied, necessary to overcome the membrane resistance and the osmotic pressure between permeate and concentrate [18]. Pressure must be carefully monitored, as its sudden decrease reflects membrane damage, while continuous increase in pressure indicates membrane fouling/scaling due to the deposition of salts, colloids and organic matter clogging the pores [101–103]. Despite some exceptions where TSS concentrations were up of 1800 mg L −1 for pharmaceutical effluents [104] and 390 mg L −1 for primary treated municipal wastewaters [105], TSS and DOC levels are lower with maximum concentrations of 15 mg L −1 [96] and 30 mg L −1 [106] respectively. Considering the susceptibility to fouling, RO and NF systems are usually preceded by an ultrafiltration (UF) module or a membrane bioreactor (MBR). High water temperatures (>30 ◦C) usually lead to a decrease in membrane performance [107,108] (Fig. 6B) Despite the limitations imposed by influent quality, the implementation of RO/NF as an alternative to secondary biological treatment has already been applied as a decentralized treatment solution to produce high quality effluent [105]. Nevertheless, the main application at large scale is water reclamation in the range of 150,000–450,000 m 3 d −1 in regions with high water scarcity [109]. The studies considered in this review were performed at smaller scale plants which ranged from 50 to 180 L h −1 (pilot) [96,110] and 24–168 m 3 d −1 (full scale) [105,111]. A relevant share (39%) of the studies reviewed in our work were conducted in Spain, while studies were distributed worldwide, including studies from South Africa, Brazil, and India (Fig. 6C). Focusing on the group of target contaminants, the most studied compounds in the reviewed publications are carbamazepine (CBZ), caffeine (CAF), diclofenac (DFC), ibuprofen (IBU) and sulfamethoxazole (SMX) are presented in Fig. 6D. The removal efficiency for both technologies is similar for the analyzed pollutants, except for carbamazepine, for which RO proved more effective. Retention of OMP is mainly governed by size exclusion, however several factors such as hydrophobicity, surface loading of contaminants, or biofilm formation can positively influence retention [111]. Although most studies report excellent removal of OMP (e.g., sulfamethoxazole, diclofenac, ibuprofen and atenolol) below the detection limit, this is not the case for some compounds, especially of small molecular weight [112]. For example, the small endocrine disruptor bisphenol-A or nitrosamines, such as N- nitrosodimethylamine (NDMA), and other ozonation/AOP byproducts are frequently found in RO permeates [111,113]. In this regard, it may be beneficial to apply RO/NF stages prior to AOP processes, as it has been shown that NDMA precursors such as ranitidine can be efficiently removed by NF and RO [114]. It should be noted that lower concentrations of OMP are more difficult to remove possibly due to slower reaction rates and/or matrix effects [112], which also emphasizes the need for investigations at environmental concentrations in order not to overestimate the efficiency of the process. Research studies rarely focus on pathogen removal by RO and NF stages, as they are usually preceded by an ultrafiltration unit to avoid biofilm growth. Although ultrafiltration membranes are a sufficient barrier against bacteria, viruses and OMPs are only sufficiently retained by NF and RO membranes [96,115]. NF and RO are also effective for heavy metal removal, but this feature is only relevant in special applications, e.g. highly contaminated landfill S. de Boer et al. Journal of Water Process Engineering 46 (2022) 102587 9 leachate or construction site effluents [116,117], since heavy metals in municipal effluents are mostly successfully retained in secondary biological treatment. 4.5. Catalyst-based processes Instead of directly using photon energy to decompose micropollutant structures, ROS are formed during photocatalytic processes using materials with semiconducting properties or based on light-driven decomposition of hydrogen peroxide (photo-Fenton). Due to the higher energy requirements of UV light compared to solar light, recent work focuses on the search for new materials capable of exploiting the advantages of this type of irradiation with a focus on reducing energy consumption resulting in greater technological feasibility for large-scale implementation (Fig. 7A). In photocatalysis processes, approximately half of the studies have used solar light and provide irradiance between 20 and 40 W m −2 in lower latitude areas [118–120]. The most applied technologies were pilot raceway pond reactors (RPR) or compound parabolic reactors (CPR). While the studied RPR reactors have an illuminated area of less than 0.5 m 2 and a depth of 5 to 15 cm to cope with a working volume of about 20–100 L [120–122], the largest pilot CPC plants for wastewater treatment have a treatment capacity around 100 L in batch operating mode [123–125]. The most frequent parameters studied with these techniques were depicted in Fig. 7B. Heterogeneous photocatalysts (PC) in wastewater treatment are based on the use of solid-state metal oxides with semiconducting properties. In these materials, the valence and conduction bands (VB and CB) are separated by an energy band gap. Upon irradiation of photons with an energy equal to or higher than this energy, electrons in the VB are promoted to the CB. The formation of electron-hole pairs provides sites for the generation of ROS by oxidation and reduction reactions, acting as direct oxidant when an organic molecule serves as an electron donor, or as indirect oxidant by generation of •OH which in turn attacks organic molecules [126]. In theory, organic micropollutants could be completely mineralized as the radical reactions are non-specific and are capable of degrading a wide range of compounds, however, the generation of by- Fig. 6. A) Schematic representation of different configurations of membrane filtration unit: two-stage (green line boundary) and two-pass configuration (blue line boundary). B) Worldwide distribution of the analyzed studies (circle area is proportional to the treated flow rate and color represent the continent). C) Overview of the quality requirements of the influent and typical operational parameters of the analyzed studies D) Removal efficiencies for the five most investigated compounds for nanofiltration and reverse osmosis studies. Grey columns stand for RO and white columns stand for NF treatment results. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) S. de Boer et al. Journal of Water Process Engineering 46 (2022) 102587 16 alternative post-treatments to a PAC process, showing that the process costs with the implementation of nanofiltration comprised 0.80 € m −3 while sand filtration accounted only for 0.16 € m −3 . Echevarría et al. [238] reported an OPEX of 0.31 € m −3 for a plant size of 15,000 m 3 d −1 while assuming a membrane lifetime of 6 years and energy consumption of 1.3 kWh m −3 . The CAPEX accounted for 662 € m −3 d −1 (translating to 0.044 € m −3 at peak flow). When treatment goals are less stringent, a common option for cost reduction is to divert the flow of wastewater, mixing with RO treated water. By producing a 50% blend, the OPEX could be reduced to 0.18 € m −3 on the expense that removal of selected OMP is reduced to 50%. Furthermore, since the energy demand of pumping scales almost linearly with membrane area and applied pressure, the size of the plant does not significantly reduce operating costs. Garcia et al. [96] calculated OPEX reaching 0.24 € m −3 while CAPEX decreased from 0.48–0.31 € m −3 comparing treatment capacities between 1 and 1000 ML d −1 . Conversely, Hube et al. [239] argued that the rather simple configuration of membrane filtration can lead to low capital costs compared to other treatments. Thirty-nine percent of the reviewed studies investigating membrane treatments were conducted in Spain, while the rest of the studies were distributed worldwide, including publications from South Africa, Brazil and India. In contrast, only one pilot-scale study was presented for Germany [240]. This can be explained by the discrepant energy costs [241], as in 2020 household electric energy prices were 0.08 $ kWh −1 in India, the price in Germany was 0.38 $ kWh −1 . Recently, the possibility of using energy-intensive but flexible processes, such as reverse osmosis, as a buffer in energy grids increasingly based on extractable energy has been proposed. Flexible operation patterns of WWTPs could compensate for the fluctuating production capacity of wind and solar energy [242]. Electrochemical treatments are usually regarded as energy intensive treatments. Although electrochemical treatments, such as electrooxidation, present high energy consumptions [76], the use of electrochemical-assisted techniques presents competitive operational costs to be implemented in WWTPs. The use of electrodialysis is optimized by its extensive use in desalination and can be an effective method to remove heavy metals. For instance, a pilot plant with a productivity of 285 m 3 h −1 integrating electrodialysis in a treatment train with ultrafiltration and reverse osmosis attained excellent reduction of copper concentration with an estimated total cost of 0.3–0.4 € per m 3 of treated water [243]. The use of electrochemical technologies for the electrogeneration of H 2 O 2 is also reported to reduce the costs compared to dosage. For instance, Wang et al. [100] estimate that the energy required for the in-situ generation of H₂O₂ accounts for additional costs of between 0.4 and 0.8 USD kg −1 , while the dosage of H₂O₂ involves costs of 1.2–1.5 USD kg −1 . For other innovative treatments, there is a lack of available data for cost calculations, as only laboratory-scale data are available. The energy consumption of UV-photocatalysis, ultrasound and microwave photocatalysis treatments have been reported to exceed even 100 kW m −3 , however, calculations on this basis tend to overestimate energy consumption and thus costs [229]. Increasing energy efficiency with increasing plant size is a common concept in studies considering primary or secondary treatment plants. In the short-term perspective, these treatment options have potential to be further applied and investigated for the treatment of highly contaminated industrial wastewater, landfill leachate and RO/NF retentates. For example, the higher overall treatment costs of landfill leachate, even with conventional treatments, and the typically lower volumes treated, open up more options for the application of innovative treatment processes. In terms of the costs and impacts of each technology, users and regulators must carefully balance the amount of investment, both economic and environmental, to achieve the goal of safer water resources. Mu˜ noz et al. [244] propose an integrated approach to assess the ecoefficiency of a technology by weighing environmental and economic burdens. Based on their reasoning, ozonation was less favorable than photo Fenton or solar photo Fenton treatment. However, the costs assumed in this study were almost ten orders of magnitude higher than in the studies reviewed above. The centralized and distributed application of tertiary treatments was also studied for several established technologies. According to the results, centralized treatment is preferred for the disposal of pharmaceuticals [184], but more research would be needed to confirm this. In addition, social aspects will play a considerable role in the final steps of decision-making. In Switzerland, the public supported the decision to legally oblige WWTPs to guarantee the removal of certain indicator pollutants at 80%. This measure will increase treatment costs by 6% across the country. In addition, energy consumption is estimated to increase by around 0.1% with the most advanced technologies [237,245]. However, developing countries will not have the resources to implement such programs in the near future, as the prior objective is still to provide at least adequately disinfected water. Despite this difficult situation, the lack of infrastructure networks could also lead to the development of simple decentralized treatment options based on renewable energy sources more quickly than in developed countries. 7. Benchmarking of the technologies As far as safe water reuse strategies are concerned, the tertiary treatment processes applied must fulfil several framework conditions. The treatment trains must be adaptable in size to the wastewater flow and pollutant load at the respective application site, while operation has to be reliable and provide redundancy in case of failure of individual units. Therefore, reproducible measurement schedules for indicator substances should be supported by fast online monitoring techniques such as fluorescence excitation emission matrices (FEEM), as well as by effect-based assays on (acute) toxicity, estrogenicity, mutagenicity or antibiotic susceptibility [246]. The integration of reasonable and standardized protocols in a “whole effluent” approach is currently an important research topic [247]. A sound assessment of the (avoided) risks of each technology is the basis of a comprehensive LCA study. Furthermore, regionalization of characterization factors was suggested as a key issue to represent an accurate toxicity impact in local environments [248]. It is also reported that the currently used assessment methods could provide different results and may not include some micropollutant factors [249]. In the following we summarize and qualitatively discuss the main strengths and weaknesses of each advanced treatment technology under technological, environmental and economic points of view, based on the findings of the studies selected. The technologies reviewed can be classified into (advanced) oxidation processes and physical retention treatments. While oxidation processes comprise traditional ozone treatment and emerging ancillary technologies, conventional UV treatment and enhancement technologies, as well as electrochemical treatments, physical treatments include adsorption processes mainly based on activated carbon and membrane filtration (Table 1). One of the main drawbacks of ozonation is that, due to its high reactivity and instability, ozone must be generated on-site from oxygen by electrical discharge, with a yield of about 10%, which leads to high electrical energy consumption in the plant. In addition to the low ozone yield, the effectiveness of ozone treatment is limited by its slow dissolution rate and rapid decomposition. However, modifications to air diffusers that form micro-nano bubbles increase the treatment efficiency [46]. Furthermore, the high toxicity of ozone requires the design of ozone destruction units which remove unreacted ozone from the exhaust gas stream after treatment [250]. Trained personnel and strict safety protocols are necessary to minimize the risk of accidental release. In addition, the formation of toxic by-products such as bromates, which are potential human carcinogens, has been associated with this process [251]. Increased effluent toxicity after ozone treatment was reported by several studies and contributed to worsen environmental impacts from an LCA perspective. Despite these drawbacks, disinfection capacity and low operational costs are the main advantages of ozonation [252]. The S. de Boer et al. Journal of Water Process Engineering 46 (2022) 102587 17 detrimental effects of induced toxicity can be circumvented when an additional filtration/adsorption step is applied after ozonation. In terms of applicability to wastewater matrix conditions, DOC is the most critical constituent that needs to be controlled prior to ozonation, while other characteristics such as TSS or turbidity have a minor influence as they are not related to DOC. The lowest removal rates were obtained for ibuprofen, oxazepam and sucralose, and their removal could be analyzed in the evaluation of operational conditions to increase the sensitivity of the method. Further research should focus on investigating effluent toxicity, elimination of antibiotic resistance genes, and decisionmaking based on operational costs and environmental impact. Conventional UV treatment for pathogen removal requires irradiances of <500 mJ cm −2 and short residence times, resulting in relatively low cumulative energy doses that minimize operating costs. However, these configurations are not efficient for OMP removal, not only because of the low transferred energies but also because of the intrinsic recalcitrance of most OMPs to direct UV photolysis. The first obstacle has been overcome with the development of high intensity photoreactor modules. To address the second problem, the dosing of additional oxidants has proven effective. While hydrogen peroxide in combination with high intensity UV treatment is already applied on a large scale, persulfate and chlorine dioxide are still under investigation, especially regarding the formation of toxic by-products. In addition, the environmental impact of their production and the possible risks associated with their unintentional release must be considered. All UV-based processes require water with low turbidity to maximize transmittance, so UV is usually applied after a clarification/filtration step. Among the catalyst-based technologies, Fenton-based methods are the most studied. However, there is agreement that the working pH of Fenton-based methods is a clear drawback, as chemicals are used both for acidification of the effluent and for pH correction before discharge or reuse. This drawback should be solved by improving the reaction at neutral pH to see its real use in WWTPs. From an environmental point of view, although the operational phase has worse impacts, Fenton-based methods could have less impacts if the construction phase is considered. Heterogeneous processes are favorable compared to homogeneous processes due to the easy recovery of the catalyst and the avoidance of changes in water composition. In addition, research focused on the development of new materials capable of improving light utilization, combined with a reusable catalyst operating under circumneutral conditions, should be the main priority for further research work in this field. In relation to the high environmental impact of mercury pressure lamp manufacturing, alternative irradiation sources should be investigated. Although UV-LED irradiation is promising [253], it does not yet reach the energy efficiency of conventional lamps and cannot yet be considered an economically viable alternative. Moreover, according to available data, direct use of sunlight is more likely to remain a prospect for decentralized plants in the medium term. However, the indirect use of solar energy through photovoltaic plants could be an alternative worthy of consideration in remote areas [254]. Both GAC and PAC adsorption stages are effective against organic micropollutants, however, they do not achieve reliable removal of heavy metals, pathogens and ARGs. In direct comparison of GAC with RO, a similar range of OMP was removed by GAC, but RO was more efficient in removing heavy metals and volatile organic compounds [115]. In terms of on-site energy demand, GAC systems are preferable compared to energy-intensive processes such as membrane filtration and ozonation. However, when production is included in the assessment, GAC can be more energy intensive than ozonation, especially when adequate recycling systems are not applied. End-of-life scenarios also have a high environmental impact [256]. Depending on the treatment goals, GAC processes can be integrated into treatment trains to efficiently exploit the benefits of this treatment stage, for example, when targeting higher OMP removal in secondary effluents with high DOC loading, GAC can be used as a polishing step after ozone/AOP treatment [40,92]. In the case of PAC, recycling of spent adsorbent is currently not possible in most applications. In terms of improving the economic and environmental impact of PAC, not only efficient recirculation and recycling schemes need to be further investigated, but the materials applied must be renewable and not of fossil origin. In the short term, recycling of Table 1 Qualitative classification matrix of the investigated tertiary treatments regarding different feasibility criteria. Categories Oxidation Electro Adsorption Filtration ■Superior performance ■Baseline technology ■Poor performance O3 UV/solar light Elox EC GAC PAC NF RO O3 Cat UV H₂O₂ UV PC F/PF H₂O₂ OMP removal Heavy metal removal Pathogen removal Additives Byproducts Waste Turbidity pH TSS DOC Energy demand a a Operational risks Operational costs a On-site energy demand. S. de Boer et al. Journal of Water Process Engineering 46 (2022) 102587 18 industrial waste such as fly ash from coal plants could also be a possibility [257]. Spent activated carbon sludge must be properly handled to prevent soil and surface water contamination. Researchers are currently investigating the factors influencing OMP removal by activated carbon processes, as well as the application of control strategies, in order to increase the reliability of these processes. Reverse osmosis and nanofiltration processes an efficient barrier to retain most OMP, pathogens (including ARGs) and heavy metals. Depending on the configuration, desalination can be achieved, which is an important treatment objective, especially in reuse applications, and results in a clear benefit of this technology compared to others under review from which only electrodialysis is a considerable alternative for salt removal. The produced water can be provided with a reliable high quality, provided that a proper control and maintenance protocol is applied. Maintenance ensures the preservation of the expensive membrane modules against fouling and scaling, but requires backwashing procedures with antifoulants and antiscalants, which contributes negatively to the environmental impact. Although the RO/NF process is well established in regions with high water stress and comparatively low energy costs, from an LCA perspective, reverse osmosis is not recommended by any of the articles reviewed in this study, as the high energy consumption strongly impacts environmental indicators such as greenhouse gas emissions. Research should be directed towards the development of more durable and efficient membranes. This could lead to lower membrane and energy costs, as well as lower consumption of cleaning agents and thus lower environmental impact. Furthermore, the generation of a waste stream in the range of 10–20% of the originally treated stream increases the environmental burden and decreases the treated water yield [258]. Possible solutions for the management of retentate are its recirculation to WWTP headworks, evaporation ponds or AOP treatments [259]. For smaller volumes and higher intrinsic conductivity, electrochemical oxidation seems a promising approach [141]. Electrochemical methods have great potential to reduce the environmental impacts associated with micropollutant removal, as they mainly use electricity, which can be considered as a clean chemical. However, pilot-scale research is still scarce, and the different technologies need to reach larger scales of implementation in a significant number of publications to consolidate this potential. As highlighted in the LCA section, energy and chemical production are responsible for most of the environmental impacts in tertiary treatment. Electrochemical treatments could therefore be one of the keys in the quest to reduce chemical consumption. For example, electrooxidation does not use any chemical reagents, while electro-Fenton processes produce H₂O₂ by electroreduction of oxygen. As for energy consumption, electrochemical systems can be easily combined with renewable energy sources, but more studies are required to optimize the coupling [260]. The integration of renewable energy sources in combination with electrochemical wastewater treatments is proven to reduce the environmental impacts produced by energy consumption [222]. 8. Concluding remarks One of the drawbacks of the presented study is that peer-reviewed research articles are a limited source of operational data, given their specific focus and basic process parameters such as unit dimensions, critical conventional wastewater parameters and flow rates are not always reported. Regarding the rapidly evolving research on tertiary treatment schemes, more effort should be made in publishing reliable process-related data in the peer-reviewed literature. Nevertheless, the overview presented on the state of the art of tertiary treatments clearly identifies the key points of both full-scale processes already operated and innovative treatment processes based on these configurations and presents the solutions currently under development. To meet the requirements of water safety standards, a combination of different processes (multi-barrier approach) is often essential for each individual case. In addition to economic and technological considerations, environmental constraints identified by a life cycle assessment must be incorporated into decision-making. Many authors point out that the key issues for most tertiary treatments are the electrical energy and chemicals consumption. More effort needs to be devoted to the characterization of the transformation products produced in the treatments to accurately assess the impact of the methods. The variability of influent quality needs to be monitored to take advantage of the high adjustability of chemicals and added energy in the AOP, which will reduce process costs. Risks to personnel and the environment should be assessed prior to implementation. For a complete risk assessment, which is necessary for a sound life cycle assessment, more attention should be paid to process evaluation based on toxicological data than to degradation efficiency alone. However, since so far, the application of effect-based effluent control is still under investigation, treatment efficacy is regulated and compared based on indicator substance removal targets. Many authors argue that the low concentrations of micropollutants and the lack of systematic assessment of their long-term effects when released into the environment mean that the environmental impact of new treatment trains outweighs the impacts of micropollutant discharge into the environment. Faced with this problem, more environmental studies are needed to better understand the environmental profiles of tertiary treatments, preferably under real wastewater conditions and on a larger scale, to optimize construction and operation data, especially energy and chemical consumption. In addition, more work is needed on the analysis of innovative tertiary techniques, as the analyzed evaluations provided some contradictory results. In parallel, the characterization factors of micropollutants need to be revised and validated, to improve the accuracy of LCA results for effluents containing these pollutants. Supplementary data to this article can be found online at https://doi. org/10.1016/j.jwpe.2022.102587. Availability of data and material All data generated or analyzed during this study are included in this published article [and its supplementary information files]. Funding This research was supported by HP-NANOBIO (PID2019-111163RB- I00) and SPOTLIGHT (PDC2021-121540-I00) projects, granted by Spanish Ministry of Science and Innovation. CRediT authorship contribution statement Literature search, data analysis and original draft preparation: Sabrina De Boer, Jorge Gonz´ alez-Rodríguez and Julio J. Conde; Idea and critical revision: Maria Teresa Moreira. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements S.d.B. thankfully acknowledges funding by the European Union's Horizon 2020 research and innovation program under Marie Sklodowska-Curie grant agreement No 81288. J.G.-R. thanks Xunta de Galicia Counseling of Education, Universities and Vocational Training (ED481A-2019/172) and Spanish Ministry of Science, Innovation and Universities (FPU19/00461) for his predoctoral fellowships. J.J.C. was supported by a postdoctoral fellowship from Xunta de Galicia (ED481B- S. de Boer et al. Journal of Water Process Engineering 46 (2022) 102587 19 2021/015). The authors belong to the Galician Competitive Research Group (GRC) ED431C-2021/37. References [1] J. Popp, Z. Lakner, M. Harangi-R´ akos, M. F´ ari, The effect of bioenergy expansion: food, energy, and environment, Renew. 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