Science of the Total Environment 838 (2022) 156422 Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv Increasing resource circularity in wastewater treatment: Environmental implications of technological upgrades Martí Rufí-Salís a,b, Anna Petit-Boix a, Sina Leipold c,d, Gara Villalba a,b, Joan Rieradevall a,b, Eduard Moliné e, ⁎ Xavier Gabarrell a,b, Julián Carrera f, María Eugenia Suárez-Ojeda f, a Sostenipra, Institute of Environmental Science and Technology (ICTA), Unidad de excelencia “María de Maeztu” (MDM-2015-0552), Universitat Autònoma de Barcelona (UAB), 08193 Bellaterra, Barcelona, Spain b Department of Chemical, Biological and Environmental Engineering, Universitat Autònoma de Barcelona, Escola d'Enginyeria, c/de les Sitges s/n, Edifici Q, 08193 Bellaterra, Barcelona, Spain c Department of Environmental Politics, Helmholtz Centre for Environmental Research-UFZ, Permoserstr. 15, 04318 Leipzig, Germany d Chair of Environmental Politics, University of Jena, Bachstr. 18k, 07743 Jena, Germany e Depuración de Aguas del Mediterráneo (DAM), Guglielmo Marconi, 11 piso 2°, 10 despacho 19, Parque Tecnológico, 46980 Paterna, Valencia, Spain f GENOCOV Research Group, Department of Chemical, Biological and Environmental Engineering, Universitat Autònoma de Barcelona, Escola d'Enginyeria, c/de les Sitges s/n, Edifici Q, 08193 Bellaterra, Barcelona, Spain H I G H L I G H T S G R A P H I C A L A B S T R A C T • Rethinking wastewater treatment is key to recover energy and resources in cities. • Increased biogas production, nitrogen re- moval and struvite recovery are studied. • Radical upgrades through nutrient recov- ery decrease the plant's environmental im- pacts. A R T I C L E I N F O A B S T R A C T Editor: Huu Hao Ngo A paradigm shift is needed in wastewater treatment plants (WWTPs) to progress from traditional pollutant removal to resource recovery. However, whether this transformation produces overall environmental benefits will depend on the Keywords: efficient and sustainable use of resources by emerging technologies. Given that many of these technologies are still Life cycle assessment being tested at the pilot scale, there is a lack of environmental assessments quantifying their impacts and benefits. Emergent technologies In particular, an integrated approach to energy and nutrient recovery can elucidate the potential configurations for Circular economy Resource recovery WWTPs. In this study, we conduct a life cycle assessment (LCA) of emergent wastewater treatment technologies Industrial ecology aimed at increasing resource circularity in WWTPs. We focus on increasing energy self-sufficiency through biogas up- grades and a more radical circular approach aimed at nutrient recovery. Based on a case-study WWTP, we compare its current configuration with (1) implementing autotrophic nitrogen removal in the mainstream and deriving most of the organic matter for biogas production, which increases the quality and quantity of biogas available for energy produc- tion; (2) implementing struvite recovery through enhanced biological phosphorus removal (EBPR) as a radical ap- proach to phosphorus management, offering an alternative to mineral fertilizer; and (3) a combination of both approaches. The results show that incremental changes in biogas production are insufficient for compensating for ⁎ Corresponding author. E-mail address: mariaeugenia.suarez@uab.cat (M.E. Suárez-Ojeda). http://dx.doi.org/10.1016/j.scitotenv.2022.156422 Received 9 March 2022; Received in revised form 27 May 2022; Accepted 30 May 2022 Available online 2 June 2022 0048-9697/© 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/). M. Rufí-Salís et al. Science of the Total Environment 838 (2022) 156422 the environmental investment in infrastructure, although autotrophic nitrogen removal is beneficial for increasing the quality of the effluent. Combined phosphorus and energy recovery reduce the environmental impacts from the avoided use of fertilizers and phosphorus and the nitrogen release into water bodies. An integrated approach to resource man- agement in WWTPs is thus desirable and creates new opportunities toward the implementation of circular strategies with low environmental impact in cities. 1. Introduction BNR. This new configuration of the mainstream line of an urban WWTP (refer to Section 2.1 for more details) reduces the energy consumption for The circular economy is becoming increasingly popular in cities to ap- aeration and uses most of the organic matter contained in the wastewater proach the sustainable management of natural resources. A circular urban for biogas production, which enhances the energy recovery potential and metabolism can help “keep products, components, and materials at their reduces the overall operational costs of the facility. highest utility and value at all times” (Bocken et al., 2017) by taking advan- This type of technology produces incremental changes in energy recov- tage of the local resources available in cities. Urban circular strategies target ery based on the existing infrastructure, meaning, in this case, that a larger a large variety of resource flows and urban sectors, from infrastructure to amount of biogas is available. Pilot-plant studies of new technologies, how- consumers. (Petit-Boix and Leipold, 2018). The conception and implemen- ever, tend to focus on process efficiency, whereas the environmental im- tation of strategies requires, however, a paradigm shift in different urban pacts associated with their implementation are often disregarded. sectors to increase the availability of local resources while reducing the en- Upgrading WWTPs to enhance circularity requires an investment in natural vironmental impacts of urban infrastructure. Wastewater treatment plants resources that needs offsetting. We need to ensure that the environmental (WWTPs) are an essential component of urban infrastructure; this paradigm benefits outbalance the impacts, which might not be accomplished with a shift has been discussed but has yet to become a reality. single technological upgrade. Here, nutrient recovery becomes an asset. No- In recent decades, water research has called for a transition from “what tably, the literature highlights the interest in examining the combined ben- must be removed” from wastewater to comply with water quality standards efits of resource recovery approaches, such as onsite energy generation, to “what can be recovered” from wastewater (Guest et al., 2009). The envi- nutrient recovery and water reuse (Mo and Zhang, 2013; Robles et al., ronmental consequences of focusing on removal technologies stress this 2020; Villarín and Merel, 2020; Wang et al., 2018). Additionally, Life need. For example, heterotrophic biological nitrogen removal (BNR) Cycle Assessments (LCAs) are becoming abundant to capture the environ- would be the classical choice when implementing nitrogen removal tech- mental effects of emergent technologies targeting nitrogen (N) and phos- nologies in WWTPs, but it would entail a large energy consumption due phorus (P) recovery in WWTPs (Roldán et al., 2020; Rufí-Salís et al., to increased aeration needs to convert ammonia nitrogen to nitrite and to 2020; Sena and Hicks, 2018; van Zelm et al., 2020). Nutrient recovery is convert its subsequent oxidation to nitrate (ENERWATER, 2015). Embrac- generally presented as an environmentally friendly strategy due to reduced ing the “circular water economy” has the potential to expand the functions chemical use and the substitution of mineral fertilizer (Lam et al., 2020). of urban wastewater systems, as becomes a valued service (e.g., clean water Nutrient recovery thus constitutes a more radical change in wastewater from sanitation), energy source (e.g., biogas) and carrier of resources treatment, as it shifts from the traditional removal paradigm and provides (e.g., nitrogen and phosphorus) (Brears, 2020). Circular wastewater man- valuable resources that can be utilized by other urban sectors, such as agement aims to reuse, recycle, recover and reclaim the resources embed- local food production (Rufí-Salís et al., 2020). In parallel, enhancing energy ded in water flows, which is the result of rethinking the current system recovery generates a partial increase in a WWTP's existing energy self- (Smol et al., 2020). Examples include water reuse in agriculture sufficiency. The question that remains is what kind of recovery strategy e.g., Almanaseer et al., 2020; Chen et al., 2021) or nutrient recovery in (i.e., incremental, radical or a combination of both) can increase the avail- the form of struvite (e.g., Mavhungu et al., 2020). Earlier in the circularity ability of local resources with the lowest environmental cost. Addressing debate, energy recovery and efficiency dominated the wastewater this question can help redesign WWTPs and prioritize circular strategies industry's efforts, (Guest et al., 2009), whereas current political and scien- from an environmental standpoint. However, while academics call for tific debates encourage nutrient recovery. For instance, the European LCAs integrating both energy and nutrient recovery (e.g., Mo and Zhang, Union's updated Circular Economy Action Plan mentions nutrient recovery 2013), an increasing number of recovery solutions are being developed at and wastewater among the planned measures (European Commission, the pilot level and still lack an evaluation of their impacts and benefits 2020a). Both energy and nutrient recovery offer opportunities for resource (Bohra et al., 2022). self-sufficiency; the question that remains is what kind of circularity ap- To fill this gap, we assessed the environmental impacts and benefits of proach holds the largest potential to minimize the environmental impacts upgrading WWTPs for energy and nutrient recovery based on two emergent of wastewater treatment. technologies. Additionally, we analyze the extent to which increased en- In the case of energy self-sufficiency, process optimization and onsite ergy and nutrient recovery compensate for the environmental investment energy generation through biogas have become widespread circular strate- in infrastructure. Based on a case-study WWTP, we perform an LCA to ana- gies (Jenicek et al., 2012). To date, most self-sufficient WWTPs take advan- lyze scenarios with technological upgrades, including i) implementing au- tage of biogas to heat their digesters and to generate electricity (Gu et al., totrophic BNR allowing an increase in biogas production, ii) recovering 2017), and the literature has long suggested that replacing conventional en- nutrients in the form of struvite through enhanced biological phosphorus ergy sources can reduce a WWTP's environmental impacts (e.g., Foley et al., removal (EBPR) and iii) implementing EBPR and autotrophic BNR allowing 2010; Pasqualino et al., 2009; Stokes and Horvath, 2010). However, the the recovery of nutrients in the form of struvite and simultaneously increas- system often depends on external waste inputs, as many WWTPs apply ing biogas production. codigestion of their sludge with urban organic waste to enhance biogas pro- duction (Mata-Alvarez et al., 2014). Considering that current environmen- 2. Methods tal targets (e.g., CO2 emissions) demand a more stringent implementation of energy self-sufficiency measures, emergent treatment technologies 2.1. Emergent resource recovery technologies might be more beneficial than classical alternatives. An example is the im- plementation of autotrophic (i.e., without organic matter) BNR in the main- The exemplary technologies selected in this study are autotrophic BNR stream WWTPs, i.e., implementation of partial nitrification and anammox and EBPR. Autotrophic BNR can be implemented in the water line (main- for removing nitrogen species from wastewater (Jetten et al., 1997; Kartal stream) of an urban WWTP with several configurations, but the most prom- et al., 2010; Siegrist et al., 2008), instead of the classical heterotrophic ising setup includes two stages (Pérez et al., 2015). The first biological stage 2 M. Rufí-Salís et al. Science of the Total Environment 838 (2022) 156422 aerobically removes organic matter through a high-rate activated sludge do not comply with more stringent treatment, according to article 5 of (HRAS) system operated at high organic loading rates (higher than 1 kg the Urban Wastewater Treatment Directive (European Commission, of chemical oxygen demand (COD) m−3 d−1), low dissolved oxygen con- 1991). In fact, the nitrification process does not take place at SRT of 14 centrations (lower than 1 g O2 m−3) and low sludge retention times days because the plant was limited by the available volume of the reactors (lower than 3 days) (Carrera et al., 2022; Jimenez et al., 2015). Therefore, of the biological treatment. Due to this limitation, the WWTP had an ex- the secondary sludge produced in this stage would have higher anaerobic emption of the Public Administration regarding the nitrogen removal. biodegradability than the secondary sludge produced in the conventional Moreover, the WWTP was operated at a very low dissolved oxygen concen- activated sludge (CAS) system that is available in most urban WWTPs. tration to save energy consumption and to completely stop the nitrification The effluent of this HRAS stage is wastewater with a low COD/N ratio, process at any season of the year. In terms of energy self-sufficiency, this but the system does not remove ammonia nitrogen. HRAS can be imple- WWTP includes a biogas cogeneration unit for producing electricity and mented as the sole stage for organic matter removal before the autotrophic heat. The biogas produced by anaerobic digestion of a mixture of primary BNR process, or it can be implemented after a primary sedimentation and sludge and secondary sludge has 57% methane (CH4) content based on just before the autotrophic BNR process, as demonstrated like in the study the WWTP records. The selected technologies were assessed in three differ- system. In the selected configuration, organic matter removal is carried ent scenarios. out in the primary sedimentation and HRAS processes. In the first configu- The SAVING-E scenario (SE) (Fig. 1) considers the implementation of ration, which is only an HRAS process, the percentage of COD removal is the autotrophic BNR in the mainstream through an HRAS system for re- approximately 40–60% (Guven et al., 2019). In the second configuration, moving organic matter and two separated reactors (partial nitritation and which comprises the primary sedimentation plus HRAS process, the per- anammox) for removing nitrogen species. SAVING-E technology (www. centage of COD removal can be increased up to 65–80% (Carrera et al., saving-e.eu) has been tested at the pilot scale for three years in the same 2022). The second stage is the autotrophic BNR process, which consists of full-scale WWTP that is considered the baseline scenario. With this config- two biological steps: partial oxidation of ammonium to nitrite (partial uration, biogas production would be increased by 61%, and the CH4 con- nitritation) and anaerobic ammonium oxidation (anammox) to di‑nitrogen tent would reach 75%. In this case, nitrogen species are removed through gas. Autotrophic BNR in the mainstream can be implemented through a sin- autotrophic BNR, whereas P is still precipitated through the addition of gle reactor (Laureni et al., 2016; Lotti et al., 2014) or through two reactors iron chloride. (Isanta et al., 2015; Pérez et al., 2015; Reino et al., 2018; Juan-Díaz et al., The struvite recovery scenario (SR) considers a theoretical CAS-EBPR to 2022). transfer P to the sludge line (Izadi et al., 2020), where it can be recovered From a nutrient recovery perspective, EBPR has been extensively uti- from the digestate in the form of struvite through an Ostara® process. lized in recent years, as it is able to remove P without chemical precipitants From a technical point of view, struvite precipitation also contributes to N (Oehmen et al., 2007). Instead, EBPR uses bacteria that accumulate P and recovery since struvite is composed by a mol of potassium, a mol of ammo- transfer it to the sludge line. EBPR is implemented in current WWTPs nium and a mol of phosphate. However, if we make a mass balance, only with CAS systems by adding an anaerobic reactor to produce volatile fatty around 5–6% of the total inflow N of the WWTP can be recovered in form acids for P-accumulating bacteria (Izadi et al., 2020). If EBPR is imple- of struvite. Considering the number and difficulty of the calculations mented with autotrophic BNR in the mainstream, the HRAS stage should needed in this study, we decided not to consider that small N-recovery also be modified to include an anaerobic reactor. This technology has amount. The total resource recovery scenario (TRR) combines autotrophic been successfully demonstrated at the lab scale by operating the EBPR- BNR and HRAS-EBPR by adding an anaerobic reactor to the HRAS system HRAS system at sludge retention times (SRTs) as low as 3 days (Roots (Roots et al., 2020; Zhang et al., 2021), thus simultaneously enhancing et al., 2020; Zhang et al., 2021). Regardless of how the EBPR is applied the production of biogas and P recovery. (i.e., in a CAS or HRAS configuration), P is recovered in the sludge line after anaerobic sludge digestion. In the digester, P accumulates in the super- 2.3. Life cycle assessment methodology natant or digestate (Pastor et al., 2008), which is the main target of P recov- ery technologies. In this case, we selected Ostara® technology (Ostara, This section includes the main considerations and data for three of the 2019) to recover struvite, which is a promising new P fertilizer with slow- phases of an LCA, namely, goal and scope definition, life cycle inventory release characteristics (Latifian et al., 2012; Rahman et al., 2014). Ostara® analysis, and life cycle impact assessment (ISO, 2006). was selected because of the environmental performance of this technology when compared with other alternatives (Amann et al., 2018; Rufí-Salís 2.3.1. Goal and scope definition et al., 2020). Moreover, Ostara® is one of the few struvite recovery technol- To assess the environmental impacts of the four scenarios, a functional ogies that operates worldwide and produces a commercial struvite fertil- unit needs to be defined to enable a scenario comparison. In this case, the izer. impacts refer to 1 m3 of treated wastewater, which is one of the most com- mon functional units in wastewater-related LCAs (Corominas et al., 2013). 2.2. Upgrade scenarios in a case-study WWTP We used the WWTP's average treated effluent for the period 2014–2016, which was 7,342,338 m3/year. A lifespan of 30 years was assumed for The environmental effects of these technological upgrades are exempli- the WWTP based on previous estimations (Morera et al., 2017; Risch fied through an existing urban WWTP selected for our case study. The et al., 2015). The infrastructure associated with the upgrades has an ex- WWTP is located in the Metropolitan Area of Barcelona and serves pected lifespan of 20 years according to the producer. 135,000 population equivalents (p.e.). This WWTP is representative of The system boundaries are shown in Fig. 2. For the purpose of our anal- large facilities (urban agglomerations >100.000 p.e.), which represent ysis, the operation stage will elucidate the short-term variations in the envi- 52% of the EU's wastewater load (European Commission, 2020b). The anal- ronmental impacts of WWTPs resulting from technological upgrades. The ysis compares four scenarios that result in various modifications of the orig- construction of the whole WWTP was excluded from the analysis, as it is as- inal WWTP configuration (Fig. 1). These modifications lead to incremental sumed to remain constant before and after the upgrades (Guven et al., changes in the quantity and quality of biogas production and/or radical 2018; Hadjimichael et al., 2016). However, we did consider the additional changes in nutrient management, i.e., from removal to recovery (Table 1). impacts related to the construction of new infrastructure required for the The baseline scenario (BS) (Fig. 1) corresponds to the current configura- technological upgrades. The end of life of the new infrastructure was not tion of the WWTP. The facility removes organic matter through a CAS sys- modeled as it is still uncertain whether the tanks could be reused or tem operated at an SRT of 14 days, while P is chemically precipitated by recycled for similar purposes. Sludge final disposal and solid waste manage- adding iron chloride. This WWTP does not remove nitrogen species, ment were also excluded. In our case, sludge final disposal undergoes ther- being representative of 34% of the WWTPs in the European Union that mal drying, which is an energy-intensive process (Bennamoun et al., 2013) 3 M. Rufí-Salís et al. Science of the Total Environment 838 (2022) 156422 Fig. 1. Original configuration of the wastewater treatment plant (left column) and modifications to the treatment units in the selected scenarios (SE, SR and TRR). Autotrophic BNR: autotrophic biological nitrogen removal; EBPR: enhanced biological phosphorus removal. that might overshadow the actual operational energy use of the WWTP. explained in Section 2.3.1, waste management was excluded. The con- Discussing the best sludge treatment processes is under debate (Hospido sumption of electricity and chemicals was also available in the WWTP re- et al., 2005) and is beyond the scope of this analysis. ports and databases provided by facility managers. To address the changes in biogas production, we applied the average electricity and chem- 2.3.2. Life cycle inventory ical values for the period 2014–2016. In the case of electricity, the WWTP is Table 2 shows the life cycle inventory for each scenario. Facility man- connected to the low voltage power grid, which we modeled using the agers provided detailed flow charts that helped us to identify the main Spanish electricity mix (Red Eléctrica de España, 2014). The WWTP's mon- flows involved in the operation of the baseline scenario, which are electric- itoring system already provided data on the amount of cogenerated and ity, chemicals, and direct pollutant emissions to air and water. Each flow purchased electricity, and no assumptions were needed. The chemicals was quantified for the main treatment blocks depicted in Fig. 1, comprise iron chloride and polyelectrolyte, which are utilized in the pri- i.e., wastewater pretreatment and treatment and sludge treatment. As mary settler and sludge management, respectively. Background inventory Table 1 Features of the scenarios applied to study technological upgrades in the WWTP. Modeling scenarios Organic Nitrogen management Phosphorus management Biogas features matter removal Baseline scenario (BS) CAS Not implemented Removal: Chemical 57% of CH4 content precipitation SAVING-E (SE) HRAS Removal: Autotrophic Removal: Chemical 61% production increase with respect to BS scenario BNR precipitation 75% of CH4 content Struvite recovery (SR) CAS-EBPR Not implemented Recovery: Struvite 57% of CH4 content, no increment in the biogas production compared to BS precipitation scenario Total resource recovery HRAS-EBPR Removal: Autotrophic Recovery: Struvite 61% production increase with respect to BS scenario (TRR) BNR precipitation 75% of CH4 content 4 M. Rufí-Salís et al. Science of the Total Environment 838 (2022) 156422 Fig. 2. System boundaries of the WWTP under study, including the flows and processes involved in its current configuration and with the application of technological upgrades. data were retrieved from Ecoinvent v3 (Wernet et al., 2016) (refer to Section 2.3.1. Second, we have a function of energy provision from biogas Table 1 in the Supplementary Material). cogeneration, either to increase the energy self-sufficiency of WWTPs or to The new infrastructure required for the emergent technologies provide burden-free energy to the electricity market. Last, we have a func- (i.e., autotrophic BNR and EBPR) was included in the assessment (lifespan tion of phosphorus fertilizer provision to the regional agricultural market. of 20 years). We assumed that the current treatment capacity would be cov- To account for the main function of treating wastewater and the benefits ered with these additional or substitutive technologies, per the technical as- of nutrient recovery through struvite and energy production via biogas co- sessment of the pilot plant (Section 2.2). We did not account for space generation, we conducted a system expansion and considered the substitu- limitations in the installation of the new reactors to show the full potential tion of mineral fertilizer, considering stochiometric ratios and the market of these upgrades. As a result, several airlift reactors (ALR) for the partial for electricity considering direct substitution, and accounted for this prod- nitritation process (Isanta et al., 2015) and Upflow Anammox Sludge Blan- uct displacement as environmental credits. Diammonium phosphate ket (UAnSB) for the anammox process (Reino et al., 2018) were designed (DAP) was chosen as the baseline fertilizer, as it is the most prevalent P fer- using steel for the SE scenario, and steel and concrete reactors were consid- tilizer worldwide (IPNI, 2007). To model the effects of electricity cogenera- ered for the HRAS-EBPR unit. Regarding the operation, the SE scenario de- tion through biogas, we accounted for the credits to energy production, mands less energy due to decreased oxygen consumption and a slight i.e., avoided consumption of energy from the grid. We made a theoretical increase in polyelectrolyte consumption. These data were obtained from assumption in scenarios with enhanced biogas production (SE and TRR). the SAVING-E pilot plant. In terms of P recovery, we accounted for the elec- Ideally, increased biogas generation implies that all the gas is consumed tricity and chemicals (magnesium chloride and sodium hydroxide) re- to boost energy production. For this reason, we assumed that the capacity quired in the Ostara© process as detailed by (Amann et al., 2018), of the cogeneration turbines would be adapted, but the additional infra- considering a P recovery efficiency of 25% with respect to the P content structure required in these cogeneration units was not modeled due to a in the influent. Iron chloride is thus not needed. We included the transport lack of data. The substitution process for each scenario is provided in of chemicals from the actual suppliers to the WWTP. Eq. (1) for the BS and SE scenarios and in Eq. (2) for the SR and TRR As resources, both in terms of energy and nutrients, are recovered from scenarios. WWTPs, the system becomes multifunctional. First, we have a function of treating wastewater that aligns with the functional unit described in F TW ¼ F TWþEP  F EM (1) 5 M. Rufí-Salís et al. Science of the Total Environment 838 (2022) 156422 Table 2 Average life cycle inventory data of the WWTP in its current configuration (baseline scenario) and with technological upgrades. Values per 1 m3 of treated wastewater. km for the flows of chemicals refers to the distance between the factory and the WWTP under study. These values were converted to tons per km (tkm) to perform the LCIA. The lifespan of “Construction” flows is 20 years. TSP: total suspended particles. Flows Units Scenarios Baseline (BS) SAVING-E (SE) Struvite recovery (SR) Total resource recovery (TRR) Inputs Construction Steel kg – 4.53E-03 2.61E-05 4.55E-03 Concrete kg – – 1.36E-03 1.36E-03 Transport km – 5.00E+01 5.00E+01 5.00E+01 Energy Wastewater pretreatment kWh 6.00E-02 6.00E-02 6.00E-02 6.00E-02 Wastewater treatment kWh 1.80E-01 1.20E-01 1.80E-01 1.20E-01 Sludge treatment kWh 4.60E-02 4.60E-02 4.60E-02 4.60E-02 Struvite recovery kWh – – 9.63E-03 9.63E-03 Chemicals (wastewater treatment) FeCl3 kg 6.50E-02 6.50E-02 – – km 3.09E+01 3.09E+01 – – Polyelectrolyte kg 2.25E-03 2.48E-03 2.25E-03 2.48E-03 km 1.88E+01 1.88E+01 1.88E+01 1.88E+01 Chemicals (struvite recovery) MgCl2 kg – – 1.36E-02 1.36E-02 km – – 1.88E+01 1.88E+01 Na(OH) kg – – 3.52E-04 3.52E-04 km – – 1.88E+01 1.88E+01 Outputs Emissions to water WWTP effluent kg N 6.30E-02 1.00E-02 6.30E-02 1.00E-02 kg P 1.00E-03 1.00E-03 3.35E-04 3.35E-04 Emissions to air WW degradation kg CH4 5.95E-06 5.95E-06 5.95E-06 5.95E-06 kg N2O 1.26E-03 1.26E-03 1.26E-03 1.26E-03 Biogas cogeneration kg CH4 7.93E-08 1.38E-07 7.93E-08 1.38E-07 kg CO 1.74E-07 3.02E-07 1.74E-07 3.02E-07 kg N2O 8.92E-10 1.55E-09 8.92E-10 1.55E-09 kg NOx 9.42E-10 1.64E-09 9.42E-10 1.64E-09 kg PM10 3.92E-10 6.80E-10 3.92E-10 6.80E-10 kg PM2.5 3.92E-10 6.80E-10 3.92E-10 6.80E-10 kg TSPP 3.92E-10 6.80E-10 3.92E-10 6.80E-10 Energy and nutrient recovery Energy production kWh 1.75E-01 2.31E-01 1.75E-01 2.31E-01 Energy self-sufficiency % 61% 102% 59% 98% Struvite recovery g – – 1.33E-02 1.33E-02 Avoided diammonium phosphate kg – – 7.14E-03 7.14E-03 F TW ¼ F TWþEPþSP  F EM  F FM (2) 2.3.3. Life cycle impact assessment We conducted the mandatory classification and characterization stages of the impact assessment using the ReCiPe (H) method (Huijbregts et al., where F is a specific function, TW is treating wastewater, EP is energy pro- 2016) and SimaPro 9 (PRé Consultants, 2017). As this paper addresses en- vision (from cogeneration), EM is energy from the market, SP is struvite ergy self-sufficiency, the results focus on the global warming indicator (in provision and FM is fertilizer from the market (diammonium phosphate). kg CO2 eq) and cumulative energy demand (in MJ). Global warming is an This substitution process was performed in the current attributional LCA easily recognizable indicator in the context of decision-making, whereas to avoid allocation that otherwise had to be made to a service (treating the cumulative energy demand gives an indication of the energy consump- wasteater), a mass-based product (struvite) and an energy-based product tion throughout the life cycle of the WWTP. Moreover, because the changes (energy from biogas). Additionally, we opted for a simple substitution applied in the wastewater treatment may affect the quality of the effluent, (without accounting for specific market characteristics, contrary to conse- we also included the categories freshwater eutrophication (in kg P eq) quential LCA) over just a system expansion to allow the comparability of and marine eutrophication (in kg N eq). Additionally, due to the toxicity po- our study with published literature (mainly on resource-intensive tential associated with steel, we also included the ecotoxicity impact cate- WWTPs) and future research (potentially focused on converting WWTPs gory (in kg 1,4-DB eq), which consists of the product of the sum of to resource-recovery facilities). terrestrial, freshwater and marine ecotoxicity. Additionally, we estimated the direct pollutant emissions to air and water, which are associated with three different processes, i.e., wastewater treatment, treated water discharge, and biogas cogene- 2.4. Impact payback times and eutrophication net environmental impact ration. First, wastewater degradation is a known source of significant CH4 and N2 O emissions at different stages of the treatment process Impact payback times (IPTs) indicate the amount of time a module or a (Corominas et al., 2013; Eijo-Río et al., 2015; Lorenzo-Toja et al., technology must produce environmental credits to compensate for the envi- 2016a). CH4 emissions were calculated based on the emission factors re- ronmental impacts generated to produce the module or technology. For up- ported by Lorenzo-Toja et al. (2016a), who conducted experimental grades in WWTPs, the IPT for every impact category is estimated based on studies in a similar area. N2O emissions were calculated based on emis- Eq. (3), which is an adaptation to wastewater management of existing pro- sion factors reported by Reino et al. (2017) for partial nitritation under posals in renewable energy literature (Alsema and Phylipsen, 1995; Sumper mainstream conditions. Second, water quality tests provided informa- et al., 2011). The investment in natural resources and environmental im- tion about the amount of N and P discharged into the river, which are pacts is depicted in the nominator, where IP and IT are the impacts gener- the main drivers of eutrophication. Last, the use of biogas for energy co- ated by the production and transport, respectively, of the infrastructure generation produces direct air emissions. Biogas cogeneration emis- required to implement technology i. In our case, i represents the scenarios, sions were calculated using the methods suggested for WWTPs by the i.e., SE, SR and a combination of both (TRR). The denominator includes the Catalan Office for Climate Change (2015). This method uses data from avoided impacts (AI) resulting from technology i in terms of electricity from Spanish air emission inventories (Ministry of Agriculture Food and the grid (E) and mineral fertilizer (F). Since many WWTPs already include a Environment, 2014). biogas cogeneration unit to produce electricity, avoided impacts might 6 M. Rufí-Salís et al. Science of the Total Environment 838 (2022) 156422 already occur in the baseline scenario. To account for the actual benefits of apparent in marine eutrophication. Scenarios SE and TRR score six times the incremental or radical changes to resource recovery, the initial avoided better than the baseline and SR scenarios due to the removal of N species impacts (if any) need to be subtracted (AIE,BS). in the autotrophic BNR process, which is present in both scenarios. Struvite precipitation offers the possibility of obtaining a valuable re- I P,i þ I T,i source at a low environmental cost (Rufí-Salís et al., 2020), as the reconfig- IPT i ¼ (3) AI E,i þ AI F,i  AI E,BS uration of the WWTP has several associated benefits. First, the lower impacts of SR and TRR compared with BS and SE are related to the EBPR Additionally, we calculated the Eutrophication Net Environmental Impact process (either operated as CAS or HRAS), which is capable of removing (ENEI). The ENEI is an equation proposed by Lorenzo-Toja et al. (2016b) up to 99% of the influent P under optimal conditions (Izadi et al., 2020; adapted from the originally proposed Net Environmental Benefit by Roots et al., 2020; Zhang et al., 2021). Furthermore, given that the technol- Godin et al. (2012). With the goal of complying with legislation removal ogy recovering struvite precipitates P from the digestate, the quantity of P standards, this indicator accounts for the net improvement in water quality in the effluent in the SR scenario is equivalent to that in the TRR scenario from a removal/recovery technology plus the thresholds imposed by cur- (ca. 0.34 mg/L), as are the emissions to water in freshwater eutrophication rent legislation (Eq. (4)). Current emission limits in Spain are 1 and 10 (3.4·10−4 kg P/m3). mg/L of P and N for WWTPs with more than 100,000 p.e. (Royal Decree Second, the TRR scenario generates lower impacts on ecotoxicity and 509/1996.OE, 1996). To adapt to the ENEI, which is based on the Eutrophi- cumulative energy demand than the BS scenario due to the avoided use cation Potential impact category from the CML-IA impact method (Guinée, of iron chloride in the HRAS-EBPR. In contrast, the BS and SE scenarios 2002) in Lorenzo-Toja et al. (2016b), we quantified the Freshwater ENEI are able to remove P through chemical precipitation, leaving an effluent (FENEI) and Marine ENEI (MENEI) replacing the Eutrophication Potential concentration of ca. 1 mg/L. Third, struvite is a substitute fertilizer with from CML-IA by Freshwater Eutrophication and Marine Eutrophication im- proven feasibility and environmental benefits at different scales (Gell pact categories from the ReCiPe (H) impact method (Huijbregts et al., et al., 2011; Latifian et al., 2012; Liu et al., 2011; Rufí-Salís et al., 2020). 2016). In terms of interpretation, values equal to 1 mean that N or P are re- In our case, the avoided impacts due to fertilizer substitution in the SR moved according to legislation. Values greater than 1 mean that the WWTP and TRR scenarios are approximately 3 times higher than the impacts of is performing better than what the legislation imposes, whereas values less struvite recovery on global warming and cumulative energy demand. than 1 mean that the WWTP is underperforming. Enhanced energy recovery entails increased environmental burdens for certain indicators, although the net balance is positive for other indicators. EPinfluent þ EPprocess ðindirectÞ þ EPeffluentðrealÞ The construction of the autotrophic BNR infrastructure in the SE and TRR ENEI ¼ (4) EPinfluent þ EPeffluent ðlimt by  lawÞ scenarios represents a major impact on ecotoxicity (45%) due to the ecotoxicity potential of steel. Moreover, because of this new infrastructure, the SE scenario generates slightly larger impacts on freshwater eutrophica- 3. Results tion than the BS scenario. Nonetheless, if the avoided impacts of increasing biogas cogeneration are subtracted from the total impacts (Fig. 4), the final 3.1. Illustrating the potential environmental impacts of technological upgrades performance of SE is slightly better than BS in this impact category because the avoided impacts obtained through biogas cogeneration are 16% higher Our impact assessment results highlight the environmental benefits of in SE and TRR than in BS. struvite recovery (SR scenario) and a combination of energy and nutrient The aggregated credits obtained in the TRR scenario could help mitigate recovery technologies (TRR scenario) (Fig. 3; refer to Table 2 in the Supple- 24%, 67% and 102% of the plant's impacts on global warming, ecotoxicity mentary Material for detailed results). In particular, the TRR scenario gen- and cumulative energy demand, respectively; these values are much higher erates lower environmental impacts than the baseline in all impact than those quantified for the BS scenario in the same categories (15%, 16% categories except for global warming, even without accounting for the and 37%, respectively). As reported by Lorenzo-Toja et al. (2016a), direct avoided impacts. The benefits of the emergent strategies are particularly emissions to air comprise the main contributor to global warming in Fig. 3. Impacts of the baseline (BS) and upgrade (SE, SR and TRR) scenarios in the assessed impact categories. Data per m3 of wastewater. The “struvite recovery” component includes the chemicals, energy and infrastructure required to recover struvite. “Cogeneration infrastructure” includes the infrastructure (concrete and steel) required to implement the improvement in the cogeneration process as described in Sections 2.1 and 2.2. GW – global warming (kg CO2 eq); FE – freshwater eutrophication (kg P eq); ME – marine eutrophication (kg N eq); ET – ecotoxicity (kg 1,4-DB eq); CED – cumulative energy demand (MJ). 7 M. Rufí-Salís et al. Science of the Total Environment 838 (2022) 156422 Fig. 4. Net environmental impacts of the BS and upgrade (SE, SR and TRR) scenarios in the assessed impact categories after subtracting the avoided burdens. Data per m3 of wastewater. GW – global warming (kg CO2 eq); FE – freshwater eutrophication (kg P eq); ME – marine eutrophication (kg N eq); ET – ecotoxicity (kg 1,4-DB eq); CED – cumulative energy demand (MJ). WWTPs. For this reason, the percent contribution of the credits is lower in biogas cogeneration, which greatly benefits the payback times of the sce- the global warming indicator, as the direct emissions to air contribute 0–2- narios that consider fertilizer substitution (SR and TRR). 0.4 kg CO2 eq/m3, representing 48% (BS), 64% (SE), 61% (SR) and 75% In terms of freshwater eutrophication payback time, Table 3 shows the (TRR) of the impact in the analyzed scenarios. The greatest impact ob- expected results as SR and TRR considered that the P recovered in the form served in global warming in the SE and TRR scenarios is attributed to the of struvite will be used to displace fertilizer from the market. However, we presence of the BNR process. The nitrogen removal achieved by these tech- cannot appreciate a distinct benefit of applying BNR processes in SE and nologies is key to complying with nutrient discharge legislation but in- TRR in marine eutrophication payback time. Since nitrogen in BNR is re- creases the amount of N2O emitted, which radically increases the WWTP's moved but not recovered, thus not displacing another product from the impact on global warming due to the high radiative force of this greenhouse market, the benefits of cleaning wastewater cannot be observed with pay- gas (Vasilaki et al., 2019). back time quantification. Based on this limitation, we quantified the FENEI and MENEI (Table 3), which aim to quantify the degree of improve- 3.2. Quantifying the infrastructure payback time ment of a technology aiming to remove nutrients from wastewater consid- ering the thresholds imposed by national legislation (Lorenzo-Toja et al., The avoided impacts of struvite recovery (SR scenario) or a combination 2016b). of nutrient and energy recovery (TRR scenario) set all payback times below Since the SE scenario only considers an improvement in terms of nitro- the lifespan of the new required infrastructure (20 years), except for gen removal and increased biogas production, its FENEI value is equal to ecotoxicity (Table 3). However, the benefits of the SE scenario alone 1.00, which means that the P removal in this technology strictly complies would be insufficient for compensating for the environmental investment with the legislation. This removal is achieved by the same means employed in the short term. For example, we would need an extra 12% of energy pro- in the baseline scenario: application of iron chloride to chemically precipi- duced by cogeneration to obtain a carbon payback time lower than the tate P. On the other hand, SR and TRR have a FENEI value of 1.12, meaning infrastructure's lifespan. Enhanced biogas recovery alone represents an that these technologies are outperforming compared with the legislation. ecotoxicity payback time that is 8.5 times higher than the infrastructure's This behavior is related to the implementation of EBPR, which increases lifespan, mainly due to the production of steel for the reactors. This effect P removal compared with chemical precipitation processes. is mitigated in the TRR due to the benefits of fertilizer substitution. On The methodological potential of the ENEI is observed when analyzing the other hand, SR presents the lowest payback times because the benefits the MENEI values. For this indicator, the SE and TRR scenarios have a related to fertilizer substitution substantially outperform the additional im- value of 1.17 (Table 3), meaning that they remove nitrogen above the pacts generated by the required infrastructure. This finding is attributed to value imposed by the legislation. This increased removal in SE and TRR not only to the need for less steel infrastructure compared with SE but also compared with SR is attributed to the addition of BNR processes. the lack of P recovery technologies in the WWTP under study in contrast to 4. Discussion Table 3 4.1. Incremental and radical changes in wastewater management Impact payback times and ENEI of each technological upgrade. Indicator Impact category SE SR TRR Shifting from a removal approach to a recovery approach in WWTPs un- veils the environmental effects of focusing on individual strategies toward a Payback time (years) Global warming (GW) 29.5 7.2 10.9 Freshwater eutrophication 27.7 6.2 8.7 circular water economy. The baseline scenario already pursued energy re- (FE) covery through biogas, but it focused on a removal approach when address- Marine eutrophication (ME) 30.3 7.5 9.2 ing nutrients such as P. Indeed, the WWTP had a 61% self-sufficiency Ecotoxicity (ET) 169.4 4.8 26.8 potential and already avoided 37% of the cumulative energy demand Cumulative energy demand 15.0 6.1 7.3 (CED) through energy self-supply. Upgrading the plant through the implementa- Eutrophication Net Freshwater eutrophication 1.00 1.12 1.12 tion of autotrophic BNR in the mainstream allowed for approximately Environmental Impact (FENEI) 30% additional self-supply, thus reaching energy self-sufficiency under Marine eutrophication 1.17 1.07 1.17 optimal conditions. Nevertheless, the environmental investment in (MENEI) infrastructure outbalances the benefits when accounting for payback 8 M. Rufí-Salís et al. Science of the Total Environment 838 (2022) 156422 times. The environmental impacts of producing more energy for self-supply only increases the circularity of the system but also improves its envi- are not paid off within the expected lifetime of the infrastructure ronmental performance. These findings can help elucidate the potential (i.e., approximately 20 years). This limitation is related to two specific pathways of resource recovery that reduce the environmental impacts of reasons. large WWTPs (> 100.000 p.e.) such as the WWTP examined in this anal- First, the assumed benefits of upgrading biogas production depend on ysis. the initial status of the WWTP. This is a recurrent issue when conducting The selected technologies are meant to be exemplary, and therefore, LCAs, as the results are case-specific and highly determined by the baseline other configurations are possible, which is a limitation of this analysis. Fur- scenario (Corominas et al., 2013). In this sense, the implementation of au- thermore, their economic feasibility needs to be assessed in future studies to totrophic BNR in the mainstream might score better when the baseline sce- ensure that strategies are prioritized from a holistic standpoint. A cost– nario does not include a cogeneration unit or when planning a new WWTP benefit analysis will determine the investment costs, externalities and profit from scratch. To show an example, the WWTP used as a case study was af- associated with the selected technologies, which is needed to complement fected by a national policy on renewable energy self-supply (Ministerio de the literature on the economic evaluation of wastewater treatment (Lin Industria Energía y Turismo, 2015) and stopped the production of electric- et al., 2016). Such analyses need to account for the externalities of N re- ity for several months. If we compare the benefits of upgraded biogas pro- moval, which can be estimated through shadow prices (Molinos-Senante duction with a scenario without electricity generation, the environmental et al., 2010), as well as the uncertainties of a slowly developing struvite investment in the implementation of autotrophic BNR in the mainstream market. is compensated for in a shorter period (7 years). Nevertheless, biogas pro- duction is a common practice in moderate- to medium-sized WWTPs 4.2. Perspectives and prospects: toward a circular water economy in cities (Jenicek et al., 2012), and thus, this scenario only seems realistic in the con- struction of new WWTPs in small municipalities aiming to increase energy The combination of upgrades can constitute a radical change to waste- self-sufficiency. water management by taking stock of the circular perspective of water as Second, the benefits of increasing the quality of the effluent are not a service, energy source and nutrient carrier (Brears, 2020). The benefits reflected in the payback times if the implemented technology is aimed at re- of this integration span beyond the boundaries of WWTPs, which aligns moving instead of recovering nutrients. Since the removal of nitrogen does with the interconnected nature of urban systems and the effects of circular not substitute for any other marketable product (such as biogas cogenera- strategies at different scales (Kirchherr et al., 2017; Nika et al., 2020). tion for primary energy or struvite for chemical fertilizers), credits cannot Upscaling the effects of these incremental and radical changes at the be given to this improvement, and thus, the implementation of autotrophic urban or regional level will provide further insights into the environmental BNR increases the impact on global warming and cumulative energy de- effects of resource recovery. Notably, in addition to the energy and nutrient mand. To exploit the potential of implementing this process, we adapted values associated with water flows, the integration of carbon capture tech- the ENEI proposed by Lorenzo-Toja et al. (2016b), although other authors nologies into WWTPs could turn the water industry into a global source of suggested adding complementary water quality-related functional units negative carbon emissions (Lu et al., 2018). (Rodriguez-Garcia et al., 2011). Given the local value of WWTPs, the interconnections between sanita- In this case, a removal approach seems to be more attractive when tion systems and the urban environment need to be further explored to un- assessing the benefits of autotrophic BNR, as the SE and TRR scenarios derstand the extent to which a circular water economy can be beneficial for integrate a new N removal strategy. Although this nutrient was man- other urban sectors. These localized innovations might trigger changes at aged from a removal standpoint, it succeeded in dramatically reducing larger scales due to the interconnections among food, energy, water, mate- the plant's impacts on marine eutrophication because the baseline sce- rials and land (Bleischwitz et al., 2018). The relationships between these nario includes an urban WWTP without N removal. This is not the small-scale upgrades and the metabolism of a city are worthy of future re- case in urban WWTPs, where N removal is implemented through classi- search, especially given the influence of economic incentives and social cal heterotrophic BNR systems. Previous LCAs confirm the benefits of awareness on the implementation of circular strategies (Robles et al., implementing autotrophic BNR processes to the environmental perfor- 2020). For instance, there seems to be a direct relationship between waste- mance of WWTPs, although tradeoffs may arise in terms of larger air water and local food production (Mehta et al., 2015; Massey et al., 2007; emissions and electricity use (Hauck et al., 2016). This was the case in Rufí-Salís et al., 2020), whereas excess energy could be shared with other our study, in which direct N 2 O emissions substantially contribute to industries to generate a symbiosis. In this case, we did not model the effects the final value of global warming. However, most studies have focused of distributing struvite as a fertilizer to local farmers and its spreading on on the implementation of autotrophic BNR in the sidestream, i.e., for agricultural soil, but it could also enhance urban food self-sufficiency and treating the reject water generated from the dewatering of the digested reduce the impacts of local food production (Rufí-Salís et al., 2020). How- sludge. Only a few studies have explored the implementation of autotro- ever, “nexus thinking” seems to remain a conceptual approach that does phic BNR in the mainstream, highlighting their benefits compared with not translate to actual strategies, which means that a transformation into heterotrophic BNR removal (Arias et al., 2020; Cho et al., 2019). How- “nexus doing” is necessary to take stock of radical actions and transform ever, this paper clearly shows the synergistic effect of modifying them into real implementation projects (Simpson and Jewitt, 2019). In par- existing WWTPs by implementing autotrophic BNR in the mainstream allel, it is imperative that “nexus doing” in a circular water economy not and P recovery as struvite. This study also opens a research avenue to only aligns with environmental and economic goals but also ensures that understand whether N recovery is environmentally friendlier than N re- “exporting” recovered resources from wastewater to other sectors does moval. not generate environmental injustice toward certain urban sectors and so- From a resource recovery standpoint, our study shows that the cial groups (Cox, 2020). incremental benefits of enhanced biogas production might become more attractive when combining this technology with additional tech- 5. Conclusions nological upgrades. Existing LCA studies point out the environmental benefits of nutrient recovery and reuse, as they avoid the consumption Our analysis shows that incremental changes in biogas production of mineral fertilizer and reduce the WWTP's demand for chemicals are insufficient for compensating for the environmental investment in (Lam et al., 2020). Indeed, the payback time of combined energy and infrastructure, although autotrophic nitrogen removal is beneficial for nutrient recovery is much shorter and within the lifetime of the infra- increasing the quality of the effluent. In contrast, combined phosphorus structure. Additionally, it also scores better than the baseline in most and energy recovery reduces the environmental impacts from the categories. For this reason, combining an incremental change in an avoided use of fertilizers and phosphorus and nitrogen release into existing strategy with a “radical” change in an unexplored flow not water bodies. An integrated approach to resource management in 9 M. Rufí-Salís et al. Science of the Total Environment 838 (2022) 156422 WWTPs (i.e., upgrading existing facilities with a set of emergent tech- References nologies) is thus desirable and creates new research avenues toward the implementation of circular strategies. 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