Science of the Total Environment 905 (2023) 167193 Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv Review Literature review on the potential of urban waste for the fertilization of urban agriculture: A closer look at the metropolitan area of Barcelona Verónica Arcas-Pilz a, Xavier Gabarrell a, b, *, Francesco Orsini c, Gara Villalba a, b a Sostenipra Research Group (2021 SGR 00734), Institut de Ciència i Tecnologia Ambientals ICTA-UAB (CEX2019-0940-M), Z Building, Universitat Autònoma de Barcelona (UAB), Campus UAB, 08193 Bellaterra, Barcelona, Spain b Department of Chemical, Biological and Environmental Engineering, Universitat Autònoma de Barcelona (UAB), Campus UAB, 08193 Bellaterra, Barcelona, Spain c DISTAL—Department of Agricultural and Food Sciences, Alma Mater Studiorum—University of Bologna, 40127 Bologna, Italy 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 • Nutrient circularity is key due to losses into the environment and depleting resources. • Urban agriculture can serve as a link to increase nutrient circularity in cities. • Urban waste can be divided into two streams wastewater and organic, bio, food waste. • Existing structures like WWTP can sup­ ply great amounts of key nutrients like N and P. • Social perception and legal constraints are key in the future of nutrient recovery. A R T I C L E I N F O A B S T R A C T Editor: Damia Barcelo Urban agriculture (UA) activities are increasing in popularity and importance due to greater food demands and reductions in agricultural land, also advocating for greater local food supply and security as well as the social and Keywords: community cohesion perspective. This activity also has the potential to enhance the circularity of urban flows, Nutrient circularity repurposing nutrients from waste sources, increasing their self-sufficiency, reducing nutrient loss into the Waste management environment, and avoiding environmental cost of nutrient extraction and synthetization. Urban agriculture The present work is aimed at defining recovery technologies outlined in the literature to obtain relevant Nutrient recovery Wastewater nutrients such as N and P from waste sources in urban areas. Through literature research tools, the waste sources Food waste were defined, differentiating two main groups: (1) food, organic, biowaste and (2) wastewater. Up to 7 recovery strategies were identified for food, organic, and biowaste sources, while 11 strategies were defined for waste­ water, mainly focusing on the recovery of N and P, which are applicable in UA in different forms. The potential of the recovered nutrients to cover existing and prospective UA sites was further assessed for the metropolitan area of Barcelona. Nutrient recovery from current composting and anaerobic digestion of urban sourced organic matter obtained each year in the area as well as the composting of wastewater sludge, struvite precipitation and ion exchange in wastewater effluent generated yearly in existing WWTPs were assessed. The * Corresponding author at: Sostenipra Research Group (2021 SGR 00734), Institut de Ciència I Tecnologia Ambientals (CEX2019-000940-M), Z Building, Uni­ versitat Autònoma de Barcelona (UAB), Campus UAB, 08193 Bellaterra, Barcelona, Spain. E-mail address: xavier.gabarrell@uab.cat (X. Gabarrell). https://doi.org/10.1016/j.scitotenv.2023.167193 Received 18 April 2023; Received in revised form 20 August 2023; Accepted 16 September 2023 Available online 22 September 2023 0048-9697/© 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by- nc-nd/4.0/). V. Arcas-Pilz et al. Science of the Total Environment 905 (2023) 167193 results show that the requirements for the current and prospective UA in the area can be met 2.7 to 380.2 times for P and 1.7 to 117.5 times for N depending on the recovery strategy. While the present results are promising, current perceptions, legislation and the implementation and production costs compared to existing markets do not facilitate the application of nutrient recovery strategies, although a change is expected in the near future. 1. Introduction demanding crops before being discarded or further reused (Rufí-Salís et al., 2020c; Rufí-Salís et al., 2020b). While this can certainly be a 1.1. Urban agriculture: reducing distances and optimizing the use of space plausible solution to minimize fertilizer and water loss into the envi­ ronment, it does not entirely solve the burden of nutrient extraction or In past decades, the increase in population in expanding urban areas generation to further increase agricultural activity inside the city, such has risen the demand for food (United Nations, 2019), putting great as in the case of phosphate rock mining or synthetic nitrogen production pressure on the agricultural industry to supply cities within the existing (Cordell et al., 2009; Cordell and White, 2014). On the other hand, agricultural land. This additional stress builds over the ongoing chal­ additional infrastructure and equipment are installed to close the water lenges in agriculture, including climatic instability and land degradation flow and to ensure both the stability of the given nutrient solution and (Dsouza et al., 2021). These pressures have prompted the need for highly the control of pests and diseases, making the system much more intensified production systems that can further contribute to land complex. degradation, the use of non-renewable resources and greenhouse gas While the notion of UA and the utilization of building rooftops offer emissions (Chojnacka et al., 2020; Dsouza et al., 2021). Agriculture and enticing prospects for augmenting agricultural pursuits within urban the total food system are currently responsible for 21–37 % of all GHG areas, the integration of nutrient flows necessitates thorough investi­ emissions, which are expected to increase by 30 % in 2050 due to gation. Such exploration holds the promise of facilitating a transition population growth, dietary change, income growth and consequentially from conventional linear systems to more sustainable and circular al­ land-use change (IPCC, 2019). One plausible approach to mitigate the ternatives. By delving into the possibilities of nutrient flow integration, considerable pressures associated with sustaining urban populations is cities can potentially embrace more efficient and regenerative practices, to enhance the resilience and self-sufficiency of urban centres. This ushering in a new era of urban agricultural sustainability. approach would foster an environment for food production within the city (Ercilla-Montserrat et al., 2019; Sanyé-Mengual et al., 2018). 1.2. Do we need circular nutrient strategies in cities? The concept of UA has garnered significant attention in recent years and is recognized as a viable means to address food provisioning in The extraction and synthetization of fertilizers has been an ongoing urban regions. Additionally, it serves as a catalyst for fostering com­ activity since the agricultural green revolution, becoming a necessity to munity cohesion and augmenting the availability of green spaces within maintain the great production of goods to feed the world. While earlier cities (Lal, 2020; Wielemaker et al., 2019). The potential benefits of UA methods of fertilization included the use of urine, manure, human also include shortening supply chains and consequently reducing excreta and guano, the modern agricultural industry relies on the mining transportation and food losses (Sanyé-Mengual et al., 2015; Sanyé and synthetization of plant available nutrients (Sun et al., 2018). This Mengual, 2015; Toboso-Chavero et al., 2019). These benefits may practice has greatly shifted the global nutrient pools with the export and change depending on the typology of UA applied, which can greatly vary import of these products to ensure fertilization of large land extensions between soil-based outdoor agriculture and indoor hydroponic vertical (Villalba et al., 2008). farming. This nutrient pool shift has generated several consequences not only UA has different appearances and can present itself in different due to the extraction of nutrients by excessive mining but also via the shapes and forms, as well as motivations and functions. Urban residents emissions generated by their transport and application elsewhere. are often guided by the traditional images of agriculture; therefore, most Fertilization is also responsible for the leakage of excess nutrients to­ family and community gardens, as well as social peri-urban farms, are wards the environment, generating exosystemic problems that include conceived to follow the principles of organic or agroecological farming acidification, eutrophication, or emissions of GHGs into the atmosphere on soil. While UA globally faces issues in accessing fertile soils in highly (Cordell et al., 2009; Liu et al., 2008). The current anthropogenic dense cities, alternative ways for crop production have been suggested, sources of N2O mainly originate from nitrogen over fertilization and including building integrated agriculture solutions (Despommier, 2013) poor management, with soil emissions of 3 Mt. of N2O-N each year or Z-Farming (Thomaier et al., 2015), leading to the colonization of (IPCC, 2019). Today, approximately 90 % of the extracted phosphate often forgotten and underutilized spaces, as for instance, building rock is still used to produce agricultural fertilizer, while 50 % of the rooftops (Appolloni et al., 2021). nitrogen demanded for agriculture is supplied by synthetic N generated The production systems most suitable for rooftop and indoor UA are via the Haber-Bosch process (Chojnacka et al., 2020; Zabaleta and mainly based on soilless systems with the use of alternative substrates to Rodic, 2015). The application of P fertilizers is also regarded as greatly avoid heavy loads. The practices of hydroponic/aeroponic agriculture inefficient with extensive losses due to erosion and leaching, with only and aquaponics have been shown to be great alternatives for building- approximately 10 % of the applied P reaching consumers (Chojnacka based agricultural systems, although downsides can also be observed. et al., 2020). This non-renewable resource is expected to be depleted by Hydroponic technologies can entail a great investment in infrastructure 20–35 % by 2100 under best estimates if its extraction continues at and require technical specialization for their manipulation. Besides, present rates, reaching its peak production rate between 2030 and 2040 these technologies rely on the use of mineral nutrients, commonly dis­ (Möller et al., 2018; Van Vuuren et al., 2010). solved into a nutrient solution supplied to the crop through fertigation. Significant strides in diminishing reliance on non-renewable nutri­ While productivity commonly increases, as compared to soil-based ents and curbing the release of P into the environment can be accom­ agriculture, these practices imply a great environmental burden, being plished through the implementation of recovery and reuse strategies. based on linear systems (Dsouza et al., 2021; Sanjuan-Delmás et al., Despite their potential, these strategies are presently underutilized and 2018). warrant increased attention and implementation (Chojnacka et al., Efforts have been made to reduce the emissions of UA production 2020; Oarga-Mulec et al., 2019). A large share of nutrient losses in urban related to fertilization by closing flows within the crop system, reusing areas is associated with food and human waste. Estimations of global the leachate nutrients for the same crop or as a cascade system on less annual food waste indicate that 931 million tonnes were generated in 2 V. Arcas-Pilz et al. Science of the Total Environment 905 (2023) 167193 2019, approximately 121 kg per capita each year, where approximately being investigated for nutrient recovery in urban settings and define if 60 % originated from households (Forbes et al., 2021). Approximately it’s possible to cover the needs of existing or future UA systems. 97 % of global food waste is disposed in landfills, where inappropriate management may cause nutrient leaching into the environment, causing 2. Methodology eutrophication and accumulation in soils, as well as methane emission and odour (Chojnacka et al., 2020; Ren et al., 2020). 2.1. State of the art – nutrient recovery and reuse in urban environments With a circular economy mindset, the potential of the reuse and recycling of wastes is explored, and the capacity for self-sufficiency and This work comprises the literature analysis of research on the resilience is enhanced, escaping from linear production approaches that nutrient recovery of urban waste streams for the nutrient supply for UA. entail the need for importing and exporting resources into and out of the For this purpose, an initial literature search was performed via the system (Girotto and Piazza, 2021; de Kraker et al., 2019). Presently, search platform Scopus with the keywords “nutrient recovery” AND urban nutrient cycles predominantly lack a circular approach to effec­ “Urban agriculture” OR “Urban waste”. tively manage and recycle nutrients, resulting in a significant reduction This initial search resulted in 26 scientific articles (Table 1) that in the self-sufficiency of cities. The import of external manure, or other could be further sorted into two main categories established depending organic or synthetic nutrients to pursue UA may seem redundant due to on the waste type or source. These categories were defined as “food-bio- an increase in nutrient loss within the urban ecosystem as well as the organic waste” and “wastewater”. For the literature review, only nu­ already existing potential source of nutrients within urban areas (Martin trients that can be sourced in urban areas have been identified, dis­ et al., 2019; Wielemaker et al., 2019). regarding potential organic material from the agricultural industry or Here is where UA can increase its benefits for sustainable urban other imported material. Notably, only wastes have been regarded, development, not only raising its food security with the local production while products specifically elaborated for fertilization were disregarded. of goods, as well as increasing green spaces and biodiversity, but also From these categories, a second search was elaborated with the key serving as a destination for urban recovered nutrients to further close words “nutrient recovery” AND “Urban” AND “organic waste” AND NOT urban material flows (de Kraker et al., 2019). The increasing interest in “wastewater” (10) “nutrient recovery” AND “food waste” AND NOT UA and hydroponic production with the use of recycled nutrients can “wastewater” (56), “nutrient recovery” AND “bio waste” AND NOT serve as a marker to further encourage nutrient recovery practices that “wastewater” (8) and “nutrient recovery” AND “urban” AND “waste­ can also become profitable in the near future (Martin et al., 2019). water” (81). All literature before 2017 was excluded to avoid outdated The focus on waste disposal and nutrient recovery has been greater in waste treatment methodologies. recent years with the impulse of new EU regulations and development Applying these criteria, the articles obtained for the search of goals striving for better waste management strategies with a reduction nutrient recovery based on food-bio-organic waste yielded a total of 24 of landfilling as well as making P recovery in wastewater treatment results, while 58 results were retrieved for wastewater sourced fertil­ plants (WWTPs) a requisite (Kratz et al., 2019). The primary objective of izers. These articles were then classified into categories corresponding to this study is to comprehensively explore the various avenues currently the recovery technology, as shown in Table 2 for food-bio-organic waste Table 1 Waste type, treatment and target nutrient identified with primary search on urban waste-derived nutrient recovery. Reference Waste type/origin Recovery technology Target Application nutrients 1 (Weidner and Yang, 2020) Organic waste Composting/insect rearing/anaerobic digestion NPK Soil-based agriculture/ hydroponic/aquaponic 2 (Shrestha et al., 2020) Organic waste Composting NP Soil-based agriculture 3 (de Kraker et al., 2019) Kitchen waste/garden residue/ Anaerobic digestion/vermicomposting/ NP Urban agriculture/municipal Urine composting/struvite precipitation green/peri-urban agriculture 4 (Kjerstadius et al., 2017) Centralized and source-separated Struvite precipitation/anaerobic digestion/ NP Agriculture food waste and wastewater biological nitrogen removal/sludge composting 5 (Factura et al., 2010) Bio-waste and excreta Terra Preta NP Urban agriculture 6 (Suthar and Gairola, 2014) Garden waste Vermicompost NPCa Soil-based agriculture 7 (Rufí-Salís et al., 2020c) Nutrient leachate Leachate recirculation, chemical precipitation, and P Hydroponic membrane filtration 8 (Pimentel-Rodrigues and Source-separated urine Urine storage P Green roofs Siva-Afonso, 2019) 9 (Ali et al., 2023) Organic waste Composting N 10 (Akoto-Danso et al., 2019) Domestic wastewater P Soil-based agriculture 11 (Podder et al., 2020) Landfill leachate Anaerobic digestion/struvite precipitation NP 12 (You et al., 2019) Urban wastewater Sorption (synthetic Zeolites Ze-CA) NP Food production 13 (Schröder et al., 2021) Organic waste Compost/vermicompost NP Soil-based agriculture/urban agriculture 14 (Guaya et al., 2018) Urban wastewater Sorption (synthetic Zeolites) NPK Food production 15 (Magwaza et al., 2020) Domestic wastewater NP Hydroponic 16 (Rufí-Salís et al., 2020a) Urban wastewater Struvite NP Urban agriculture 17 (Calabria et al., 2019) Domestic wastewater Sorption (natural Zeolite) N Hydroponic 18 (D’ostuni et al., 2023) Domestic wastewater Moving bed biofilm reactor (MBBR) NPK Hydroponic/building-integrated agriculture 19 (Van Ginkel et al., 2017) Domestic wastewater Struvite NP Hydroponic/aquaponic 20 (Zhao, 2014) Domestic wastewater and organic Anaerobic digestion Urban agriculture waste 21 (John et al., 2023) Urban wastewater Freeze concentration technology N Agriculture 22 (Weidner et al., 2020) Urban wastewater Membrane bioreactor NPK Hydroponic/aquaponic 23 (Tesfamariam et al., 2022) Wastewater and organic waste Thermal hydrolysis/anaerobic digestion NPK Soil-based agriculture/urban agriculture 24 (Brown et al., 2023) Wastewater and organic waste Compost/anaerobic digestion NP Urban agriculture 3 V. Arcas-Pilz et al. Science of the Total Environment 905 (2023) 167193 Table 2 Table 2 (continued ) Waste type and treatment identified with a secondary search on food-bio- Waste type Treatment Reference organic waste-derived nutrient recovery (red) and wastewater-derived nutrient recovery (blue). 2021), (Schröder et al., Waste type Treatment Reference 2021), Food-bio- Agro-food waste/ Anaerobic digestion (Oarga-Mulec et al., (Milinković et al., organic kitchen waste/ 2019), 2019), waste Textile sludge/ (Davidsson et al., (Weidner and Yang, bio-waste 2017), 2020), (Pleissner et al., (Shrestha et al., 2017), 2020), (Ren et al., 2020), (Dsouza et al., (Kumar et al., 2021) 2020), (Manga et al., (Gienau et al., 2022) 2018a,b), (Ali et al., 2023), (Möller et al., (Fang et al., 2023) 2018), (Chojnacka et al., (Wang and Lee, 2022), 2021), (Kacprzak et al., (Ravindran et al., 2022), 2021), (Rana et al., 2022) (Reilly et al., 2021), Onion waste/ Vermicomposting (Pellejero et al., (Weidner and Yang, Organic waste 2020), 2020), (Ravindran et al., (Tian et al., 2019), 2021), (Battista et al., (Schröder et al., 2020), 2021), (Rojo et al., 2021), (Milinković et al., (Mikula et al., 2019) 2023), (Biruntha et al., (Kumar et al., 2020), 2023), (Wongkiew et al., (Feiz et al., 2022), 2023) (Bruno et al., 2022), Organic waste BSLF/ Insect rearing (Magee et al., (Liu et al., 2022), 2021), (Bareha et al., (Weidner and Yang, 2022), 2020) (Guruchandran (Mok et al., 2020) et al., 2022), Wastewater Treated Ion Exchange/ (Guaya et al., (Campos et al., wastewater Adsorption 2018), 2019), (primary & (You et al., 2019), (Toop et al., 2017), secondary (Calabria et al., (Chojnacka et al., effluent) 2019), 2022), (Sheikh et al., (Kacprzak et al., 2023), 2022), (Sánchez and (Eraky et al., 2022) Martins, 2021), Food waste Thermal treatments: (Idowu et al., (Reig et al., 2021), Thermal hydrolysis 2017), (Gowd et al., 2022) (TH), (Rao et al., 2018), Treated Hydroponic (Magwaza et al., Hydrothermal (Gollakota and wastewater agriculture 2020), Carbonization (HTC), Savage, 2018), (primary (Clyde-Smith and Hydrothermal (Wang and Lee, effluent) Campos, 2023) liquefaction (HTL) 2021), Source-separated Green roofs (Pimentel- (Sarrion et al., urine Rodrigues and Siva- 2021), Afonso, 2019), (Sudibyo et al., (D’ostuni et al., 2022) 2023) (Palansooriya et al., Untreated Soil-based agriculture (Akoto-Danso et al., 2023), wastewater 2019) (Sarrion et al., Filtered Photobioreactor (Dalvi et al., 2021), 2023b), untreated /Photo-fermentation (Chatterjee et al., (Wu et al., 2023), wastewater, 2019), (Sarrion et al., Urine, (Karbakhshravari 2023a), Treated et al., 2020), (Chojnacka et al., wastewater (González et al., 2022) (primary 2020), (Kacprzak et al., effluent), (Escudero et al., 2022) Treated water 2020), Food waste/ Composting/ Co- (Mortula et al., (secondary (Sánchez-Zurano Green Waste/ composting/ Compost 2020), effluent) et al., 2021), Sewage sludge/ tea (Awasthi et al., (Robles et al., Urban organic 2020), 2020), waste (Möller et al., (González-Camejo 2018), et al., 2019), (Ravindran et al., (Yulistyorini, 2017), (continued on next page) 4 V. Arcas-Pilz et al. Science of the Total Environment 905 (2023) 167193 Table 2 (continued ) Table 2 (continued ) Waste type Treatment Reference Waste type Treatment Reference (Khandan et al., Wastewater Composting (Oarga-Mulec et al., 2020), sludge, 2019), (Guilayn et al., (Firmansyah et al., 2020), 2021), (de Morais et al., (Chen et al., 2022) 2022), Treated Stripping (Bolzonella et al., (Amaya-Santos wastewater 2018), et al., 2022), (primary (Lorick et al., (Panda et al., effluent), Source- 2020), 2021), separated urine (Macura et al., (Moges et al., 2019), 2020), (Guilayn et al., (Gowd et al., 2022), 2020) (Zhu et al., 2022) Wastewater Microbial (Kashima, 2020), Treated Struvite precipitation (Karbakhshravari Treated water electrochemical (Matar et al., 2022), wastewater et al., 2020), (secondary technologies (METs) (Besson et al., (primary (Kjerstadius et al., effluent) 2021) effluent), Source- 2017), separated urine, (Rufí-Salís et al., Anaerobic 2020a), and wastewater. digester centrate (Rodrigues et al., Additional searches on Scopus, Web of Science and Google Scholar 2019), were conducted for each nutrient recovery process and waste treatment (Lorick et al., to encourage a better description and insight. 2020), With the initial literature search, two main nutrient sources for urban (Macura et al., 2019), nutrient recovery were identified: wastewater-based residues as well as (Sánchez and food wastes and other organic and bio wastes produced in urban eco­ Martins, 2021) systems that mainly originated in households, food processing and (Mayor et al., catering industries or green areas such as gardens and parks (Möller 2023), (Sánchez, 2020), et al., 2018). All recovery technologies identified in Tables 1 and 2 are (Podder et al., depicted in Fig. 1 for a better understanding of the waste flows and 2020), possible combinations of methodologies within each waste type as well (Gowd et al., 2022), as different combinations of waste types. (Besson et al., 2021) Treated water AnMBR (Jiménez-Benítez 3. Results (secondary et al., 2020a), effluent) (Jiménez-Benítez In this section the two main sources for nutrient recovery identified et al., 2020b), in literature will be described (Sections 3.1 and 3.2), followed by the (González-Camejo et al., 2019), recovery technologies identified (Section 3.3). (Mullen et al., 2019), 3.1. Nutrient recovery from food and biowaste (Zhou et al., 2022) Wastewater Membrane filtration: (Vecino et al., City landscapes and households produce a large amount of biomass Treated water Reverse Osmosis, 2019), (secondary Membrane (Volpin et al., throughout the year, up to 400 to 800 g daily per person (Ahmed et al., effluent) Contractors, 2019), 2019), which could be increased with the integration of agriculture Membrane Bioreactor (Sheikh et al., within urban areas (Dsouza et al., 2021; Manríquez-Altamirano et al., 2023), 2020). Biomass and food waste are often underused sources of nutrients, (Reig et al., 2021), (Samarina and although there is significant potential for the recovery of non-renewable Takaluoma, 2020), and energy-intensive nutrients such as P and N (Idowu et al., 2017; (Deemter et al., Zabaleta and Rodic, 2015). The waste generated in the household is of 2022), great importance, but the overproduction and further disposal of food (Yao et al., 2017) surplus is also part of the problem. Previous work has identified that up Wastewater Thermal treatment: (Jellali et al., 2021), sludge Pyrolysis, (Tomasi Morgano to 40 % of produced food is wasted, and savings from up to 25 % of Eutectic freeze et al., 2018), mined P can be achieved by the reduction of these food wastes (Drangert crystallization (EFC) (Guilayn et al., et al., 2018). The landfilling and poor management of food and biowaste 2020), can lead to large amounts of anaerobic decomposition that causes (John et al., 2023), Wastewater Anaerobic digester (Kjerstadius et al., greenhouse gas emissions, which needs to be avoided and processed in a sludge, 2017), controlled way (Dsouza et al., 2021). Increasing efforts to reduce the Source-separated (Srivastava et al., landfilling of organic waste advocate for effective source separation and urine 2020), further treatment of municipal waste to recover and recycle nutrients (de Kraker et al., from the organic fraction (Davidsson et al., 2017). This appears to be 2019), (Firmansyah et al., highly relevant, considering that in 2018, only 30 % of the European 2021), organic fraction is currently source-separated and further recycled (Guilayn et al., (Möller et al., 2018), while the remainder is landfilled or incinerated 2020), (Sun et al., 2018). (Lehtoranta et al., 2022), (Brown et al., 2023) 5 V. Arcas-Pilz et al. Science of the Total Environment 905 (2023) 167193 Vermicompost Soil amendment, Soil based fertilizer 3.3.13 3.3.12 Soil amendment, Thermal treatment Soil based fertilizer 3.3.11 3.1 Compost tea Liquid based fertilizer 3.3.10 Organic/ Food Waste Compost/ Co-compost Soil amendment, Soil based fertilizer Soil based fertilizer/Soil Urban waste Solid amendment 3.3.5 Anaerobic Liquid based fertilizer 3.2 digestion 3.3.6 Liquid Ammonia stripping Liquid based fertilizer Wastewater-based 3.3.7 NPK precipitation Struvite Fertilizer Sludge Photobioreactor 3.3.4 3.3.8 3.3.1 WWTP NPK Adsorption NP soil amendment Ef uent 3.3.3 3.3.9 AnMBR Membrane Liquid based fertilizer ltration 3.3.2 Liquid based fertilizer Source separated Urine in hydroponic Fig. 1. Main flows of nutrients from urban waste to agricultural fertilizer identified from the studies compiled in Tables 1 and 2. 3.2. Nutrient recovery from wastewater denitrification process (Ostace et al., 2013; Vilanova et al., 2017). Further tertiary processes can be added for further nutrient removal, in The perception of WWTPs has been shifting and evolving in recent which chemical removal or ultrafiltration processes are usually installed years, transitioning from waste carrying and removal technologies to (You et al., 2019). resource recovery plants for nutrients and for energy (You et al., 2019). The sludge from the first clarifier, second clarifier, and nutrient Currently, wastewater and processed sewage sludge and effluent are the removal processes is treated in parallel by mixing, dewatering, anaer­ main P carriers that are further recycled in agriculture in some European obic digestion, or composting systems (Vilanova et al., 2017). countries, although environmental regulations often do not enable the WWTPs have great potential to house nutrient recovery technolo­ direct use in conventional agriculture and therefore are often just being gies. These installations are the main collectors of domestic wastewater incinerated (Möller et al., 2018). Better management of wastewater in urban areas and have the sole purpose of bringing the water streams is endorsed in European regulations, pursuing greater circu­ composition below thresholds established by environmental regula­ larity in urban areas with larger water and nutrient recovery as well as tions. The reduction of nutrients in the water is a requisite that can the reduction of energy consumption and GHG emissions (Marinelli usually be implemented via biological or chemical treatments (Guisa­ et al., 2021; You et al., 2019). The proximity of WWTPs to urban areas sola et al., 2019; Vilanova et al., 2017). Biological treatments have been gives them a great advantage to supply nutrients for UA, avoiding great considered more reliable and effective methods for achieving a sub­ transportation or potential storage problems. The potential of this stantial reduction in nutrients. However, as thresholds for nutrient circularity has been discussed in previous studies: in a neighbourhood in emissions into the environment are reduced, chemical treatments are Munich, Germany, to reach savings of 26 % of freshwater resources gaining more popularity in water treatment processes (Hospido et al., while promoting UA to produce 66 % and 246 % of fruit and vegetable 2004). demand, respectively (Marinelli et al., 2021). Chemical treatment mainly consists of the reduction of soluble nu­ trients (mainly P) to particulate nutrients by their binding and precipi­ tation, which can be achieved with the addition of metal salts such as Ca 3.3. Nutrient recovery technologies in the form of lime, Fe and Al. Once precipitated, nutrients can be further disposed in the sedimented sludge, or repurposed as fertilizers like the The following Sub-sections 3.3.1 to 3.3.13 detail the technologies case of struvite. The main constraint for chemical treatments is the and facilities identified in the literature (Tables 1 & 2). amount of chemicals needed to effectively remove the soluble P, which must be in a 1:1 relation to the present P. This method involves a great 3.3.1. Nutrient recovery technologies in WWTP investment not only for the metal salts themselves but also for the The outline of a WWTP can incorporate certain steps depending on required storage infrastructure (Crini and Lichtfouse, 2019; Foley et al., the purpose of the treated water, but wastewater is commonly pre- 2010). On the other hand, the excessive application of these chemicals treated to remove any potential solid waste and oils. Then, the waste­ can lead to unwanted chemical reactions. For the chemical removal of N, water is sent to a primary clarifier to settle the primary sludge and obtain several techniques are available, including air stripping, ion exchange or a clearer wastewater that can then undergo primary treatment with membrane filtration. To increase the sustainability of the process, bio­ activated sludge. While simpler layouts may involve the addition of a logical treatment has recently assumed the form of a required step for secondary clarifier after the primary treatment, other WWTPs incorpo­ initial nutrient removal. This step consists of the removal of N and P with rate a secondary treatment, which is usually the nitrification and 6 V. Arcas-Pilz et al. Science of the Total Environment 905 (2023) 167193 microbial activated sludge (Vilanova et al., 2017). The removal of N is nutrient removal and serving as liquid fertilizer (Yang et al., 2015; Yao based on the decomposition of incoming organic N into ammonia (NH+ 4) et al., 2017). Other technologies can entail membrane filtration or through aerobic and heterotrophic bacteria, which can undergo further reverse osmosis, ammonia stripping and ion exchange through resins or steps for successful ammonia removal. Most commonly, the produced sorbents (Federico Volpin et al., 2018). Source-separated urine can also ammonia can be released in an aerobic environment for a nitrification serve as feedstock for other recovery technologies, such as photo­ process generating nitrate (NO−3 ) with the help of autotrophic bacteria. bioreactors, being used as a nutrient source for algal growth to ensure A denitrification step can be added to generate N in the form of gas (N2) nutrient recovery (Tuantet et al., 2019; Zhang et al., 2014). Overall, the that can be exhausted into the environment (Hou et al., 2021). This step use of source separated urine as feedstock for other recovery processes is performed under anaerobic conditions by heterotrophic bacteria that has shown a reduction in GHG emissions, especially due to a reduced require the addition of organic matter as a carbon source (Vilanova emission of N2O from nitrifying processes and from and avoided N fer­ et al., 2017). tilizer production (Marinelli et al., 2021). The biological removal of P, on the other hand, is based on the addition of phosphate-accumulating organisms (PAOs), which also 3.3.3. Anaerobic membrane bioreactor - AnMBR require anaerobic and aerobic stages. These bacteria can release P in the In recent years, waste treatment has undergone several innovations form of phosphate (PO4) under anaerobic conditions, while P can be to reduce the impact of management while obtaining cleaner water and captured in aerobic environments (Close et al., 2021; Guisasola et al., nutrients. One promising solution for wastewater management is the 2019; Hou et al., 2021; Poh et al., 2021; Vilanova et al., 2017). This combination of anaerobic digestion with a membrane bioreactor system. process can be combined with the activated sludge and nitrification and This combination is referred to as an anaerobic membrane bioreactor denitrification stages for organic N removal (Hou et al., 2021; Sarvajith (AnMBR) and has shown promising results for the generation of high- and Nancharaiah, 2022), obtaining biomass that can be further disposed quality effluent and greater energy recovery than anaerobic treat­ with the settled sludge or further rereleased into an aqueous phase to be ments in WWTPs (Jiménez-Benítez et al., 2020a). AnMBR has only been chemically precipitated (Anders et al., 2021). applied at pilot scales, but previous work on this innovative treatment Many of the technologies described in the upcoming sections can be has shown the potential to combine wastewater with food and organic added to the WWTP outline for further nutrient removal, and most waste, obtaining lower environmental impacts than other anaerobic- importantly, recovery. based treatments (Jiménez-Benítez et al., 2020a). The effluent quality has been reported to be high for application in agriculture since its N and 3.3.2. Source-separated urine P contents are elevated. While this effect has been considered a Human urine has been generally considered highly suitable for fer­ constraint in AnMBR technology, it can be considered an opportunity for tilizer production due to its high N and P contents, being the greatest fertigation purposes reducing N and P mineral fertilizer application by nutrient contributor in wastewater (80 %, 50 % and 55 % for N, P and K, 71 % and 39 %, respectively (Jiménez-Benítez et al., 2020b). The ob­ respectively) while being only 1 % of the fraction detected in the total tained effluent from AnMBR can be further processed for nutrient re­ wastewater (Chatterjee et al., 2019; Federico Volpin et al., 2018). covery and concentration through ion exchange technology (Mullen To increase waste treatment efficiency, the separation of waste et al., 2019) and microalgal biomass production (González-Camejo streams at the domestic level has been suggested to reduce separation et al., 2019). and nutrient recovery processes. This idea for source separation of household waste streams has already been regarded as an upcoming 3.3.4. Biological treatment (photobioreactor) reality in some countries, such as China and Sweden, enabling new An alternative biological treatment to that commonly observed in nutrient recycling regulations (Kjerstadius et al., 2017; Pimentel- WWTPs is nutrient capture via algal biomass growth (Guilayn et al., Rodrigues and Siva-Afonso, 2019). These countries have already 2020; Tuantet et al., 2019). This process is regarded as a better solution established separation technologies with urine diverting toilets for bet­ for wastewater treatment than secondary activated sludge or even sec­ ter management and fertilizer production (Pimentel-Rodrigues and Siva- ondary nutrient removal technologies (Mennaa et al., 2019; Muna­ Afonso, 2019). Other ways to increase circularity would not only be singhe-Arachchige et al., 2020). Nutrient capture is a suitable treatment separation in households but also the direct application of urine in due to its cost-efficiency for pathogen, BOD, N and P removal and the building green roofs, rooftop agriculture or green facades, directly generation of oxygen for organic N breakdown via photosynthesis, avoiding nutrient loss due to storage and transport (D’ostuni et al., 2023; therefore avoiding costly aeration mechanisms (Mennaa et al., 2019). Pimentel-Rodrigues and Siva-Afonso, 2019). The direct application of The resulting biomass can then be utilized in several ways, from biomass urine in agricultural production has shown promising results compared for biofuel production, for fertilization purposes and even as animal to wastewater treatment with increased N and P recovery rates as well as feedstock, being a more circular approach to N dissipating technologies a reduced impact on surrounding environments due to eutrophication employed in nitrifying and denitrifying processes (Nagarajan et al., (Malila et al., 2019). 2020). Algal production using wastewater as a nutrient source can be However, while the application of human urine in agriculture has performed in open air ponds with natural light conditions or in photo­ been performed it has also shown several constraints in present times, bioreactors, with or without continuous illumination, to further enhance containing greater N concentration compared to other nutrients, the algal growth. Open air production can be less costly but greatly sub­ potential content of chemicals and pharmaceuticals but also the general jected to natural temperature and light conditions, while exterior or negative perception of human urine application by producers and con­ indoor photobioreactors can provide more stable environments sumers (Ikeda and Tan, 1998; Simha et al., 2017; Simha and Ganesa­ throughout the year (Nagarajan et al., 2020). Work has been performed pillai, 2017; Federico Volpin et al., 2018). To avoid these constraints, on the combination of WWTP processes with algal production, using several technologies have been applied to enhance nutrient recovery different wastewater stages as potential feedstock for biomass growth, while reducing the content of potential impurities in urine (Calabria from untreated wastewater, primary clarified, anaerobically digested, et al., 2019; Guaya et al., 2018; Rodrigues et al., 2019; Rufí-Salís et al., tertiary treated wastewater to even source-separated urine (Amaya- 2020a). These same technologies can also be applied to WWTP and Santos et al., 2022; Samorì et al., 2013; Tuantet et al., 2019; Zhang et al., anaerobic digestion effluents to further remove nutrient content for 2014). The removal rate for N and P varies depending on the algal better water recovery or disposal into water bodies, as further explained species and growth conditions (Panda et al., 2021). Previous work has in the upcoming sections (Sections 3.3.3 to 3.3.9). The processes that reported recovery rates up to 52 % and 38 % in N and P, respectively, via have been mostly explored consist of nutrient precipitation, obtaining microalgal growth (Chatterjee et al., 2019), but higher rates can be mineral fertilizer such as struvite or urine concentration to enhance reached in photobioreactors with up to 80 % nitrogen and total P 7 V. Arcas-Pilz et al. Science of the Total Environment 905 (2023) 167193 removal in urine (Tuantet et al., 2019; Zhang et al., 2014) or even 99 % incoming feed, with suggested C/N ratios of 15–30 (Guruchandran et al., N and P removal levels in the case of Chlorella sorokiniana in source- 2022). Previous work on anaerobic digestion treatments of food waste separated blackwater (Moges et al., 2020). Even with these extensive showed C/N ratios of 49, although the content of oils and spices in food positive traits of algal production for nutrient removal, the application wastes can be detrimental to methanogenic activity, slowing its progress of this technology as a sole large-scale wastewater treatment is still not (Kumar et al., 2020), co-digestion processes with textile waste has effective and can be highly energy consuming (Gowd et al., 2022). Po­ shown C/N of 12.2 (Kumar et al., 2020), while Deinking sludge co- tential unsuccessful removal of toxins as well as bacterial contamination digestion showed C/N over 100 (Bareha et al., 2022). make a previous wastewater sterilization necessary, which primarily To obtain a digestate of the required quality for UA, the organic increases the treatment costs, placing photobioreactors as tertiary biomass must be collected free of impurities, which is difficult to achieve treatment stages. On the other hand, bacterial contamination can be even through selective organic waste collection (Naroznova et al., avoided with the use of extremophile microalgal species or with the co- 2016). Additional pre-treatment options can be evaluated to increase the culture of beneficial or symbiotic bacterial strains (Rashid et al., 2019), digest value and specially to avoid heavy metal contamination for its use making organic matter removal possible (Robles et al., 2020). Algal as a nutrient source without risk. Previous work on biowaste pre- biomass sampling can also be crucial, adding an additional step and cost treatment options detailed three technologies, namely, “biopulp”, to nutrient removal. This sampling can be performed by centrifugation, “screw press” and “disk screen”, to reduce impurities in the digestate. filtration or chemical precipitation, which can be expensive and energy- The most environmentally favourable technology seems to be the “bio­ consuming (Mennaa et al., 2019; Robles et al., 2020). The produced pulp” technology, which allows for increasing the digestion value with algal biomass can further contribute to nutrient recovery processes with greater biogas production as well as nutrient recovery (Khoshnevisan the coupling of other technologies like anaerobic digestion for addi­ et al., 2018). Although biogas digestate is a very attractive option to tional biogas production (Panda et al., 2021) or its application as slow provide readily available and consistent recovered nutrients for UA release fertilizer in the form of green biomass (Moges et al., 2020). (Chojnacka et al., 2022), the need to enforce pre-treatment technologies is necessary to avoid potential contamination into the food production 3.3.5. Nutrient recovery from anaerobic digestion system (Davidsson et al., 2017; Kjerstadius et al., 2017). In the case of The recycling of nutrients from biowaste via anaerobic digestion has wastewater streams source separation of both blackwater and urine can largely been analysed and considered to have great potential in urban increase nutrient recovery through anaerobic digestion processes (Leh­ waste management it’s a mature technology used globally which is toranta et al., 2022). capable of generating economic surpluses (Rojo et al., 2021) due to the While solid biogas digestate can be applied without processing as generation of methane to meet local energy requirements, as well as well as in a composted or co-composted form as soil amendment and high nutrient recovery in the digestate with a small fraction loss of growing bed substrate (Brown et al., 2023; Czekała et al., 2022; phosphorous and nitrogen (approximately 0 to 10 %) (Oarga-Mulec Dimambro et al., 2015), work has also been conducted in the application et al., 2019). Apart from macronutrients contained in the digestate, of liquid digestate in hydroponics using the nutrient film technique other compounds such as micronutrients, hormones and other organic (Martin et al., 2019; Bergstrand et al., 2020; Weidner and Yang, 2020). elements can have a positive effect on plant and soil microorganisms These studies have shown the potential of organic fertilization based (Ren et al., 2020). on biogas digestate but urge for a better control of nutrients for balanced The process of anaerobic digestion has four stages that involve fertilization as well as heavy metal content (Bergstrand et al., 2020; different key microbial communities in an oxygen-deprived environ­ Ezziddine et al., 2021; Wang and Lee, 2021). ment. During the first stage, hydrolysis and breakdown of the feed Through life cycle analysis, the environmental performance of component polymers occurs, being reduced to monomers by microbial anaerobic digestion of food waste has been analysed, showing favour­ secreted hydrolases (Rojo et al., 2021; Sikora et al., 2017). Acidogenesis able results in the case of nutrient recovery (Bruno et al., 2022) and is the second phase of anaerobic digestion, where the hydrolysed com­ overall better performance than incineration and composting processes pounds undergo acidic fermentation, followed by acetogenesis with the (Feiz et al., 2022) as well as economic profitability (Liu et al., 2022). formation of acetate, hydrogen and carbon dioxide, which are further transformed into methane in methanogenesis during the last step 3.3.6. Ammonia stripping (Guruchandran et al., 2022). Ammonia stripping is an easy process that has been incorporated in The resulting digestate is mainly applied in agricultural fields due to wastewater treatment plants for ammonia remediation, favouring the its high nutrient content, especially for nitrogen, phosphorous and po­ formation of gaseous ammonia (NH3) through an increase in pH, which tassium (Gienau et al., 2018b). The liquid and solid fraction of the is usually made with the addition of lime (Kinidi et al., 2018). While remaining digestate contain mineral and organic forms of nitrogen high concentrations of ammonia can be toxic for bacteria and therefore greatly available for plants, while P is mainly recovered in the form of not recommended for biological treatment, ammonia stripping has a phosphate (Vögeli et al., 2014; Wang and Lee, 2021; Zabaleta and Rodic, high removal success of up to 90 %, being tested already in municipal 2015). While the liquid fraction has a greater nitrogen (in the form of waste, landfill leachate and wastewater effluent (Kinidi et al., 2018; dissolved ammonia) and potassium content, the solid fraction has Zangeneh et al., 2021). The recovery and further use of the stripped greater amounts of total nitrogen and phosphorous. Compared to sludge ammonia gas can be achieved by adsorption to acid, obtaining ammo­ or dairy, food waste anaerobic digestate contains greater amounts of nium sulphate fertilizer (Lorick et al., 2020). Ammonia stripping can nitrogen in the form of ammonia and a greater N:P ratio (Dutta et al., present difficulties with the presence of organic and inorganic fine 2021). particles, generating problems of fouling and clogging of the stripping Additional treatments can be added to the solid and liquid fraction to column (Bolzonella et al., 2018). Other methods of ammonia stripping further recover P and N, such as thermal treatment of the digestate into have been developed over the years to avoid high energy and chemical biochar as well as fertilizer production through precipitation processes, use for this process, combining ammonia stripping processes with ion exchange/adsorption processes, ammonia stripping and membrane electrodialysis and membrane stripping (Federico Volpin et al., 2018). filtration, (Campos et al., 2019; Eraky et al., 2022; Gienau et al., 2018b; Based on its comparatively steady performance and capability, Guilayn et al., 2020; Vaneeckhaute et al., 2017). ammonium stripping is currently the industry standard technology for While the generated digestate proves to be a good source of nutrient highly effective ammonium removal in China. However, due to the high recovery, the generation of volatile nitrogen compounds such as NH3 expense of employing caustic soda, hot steam, and sulfuric acid, as well can increase the impact of this waste. The composition of the digestate, as the creation of the secondary pollutant, the application is restricted ammonia release and potential methanogenesis greatly depend on the economically (Zhu et al., 2022). 8 V. Arcas-Pilz et al. Science of the Total Environment 905 (2023) 167193 3.3.7. Struvite precipitation Because the nutrient-rich sorbent/exchange media may be used The process of P precipitation has been largely analysed and directly as a nutrient product in agriculture, adsorption/ion-exchange considered a valuable approach to recover P, N and Mg from wastewater technologies can be thought of as hybrid nutrient accumulation- and human urine. Precipitation occurs when the struvite components nutrient recovery technologies (Mehta et al., 2015). Mg2+:NH+ 3− 4 :PO4 are present with a molar ratio of 1:1:1 and a pH of Adsorption has shown to be adequate for the recovery of nutrients in approximately 8.5–9.5 (de Kraker et al., 2019; Uysal et al., 2014). The low concentrations as well as a great design flexibility, although some amount of Mg in wastewater and urine is usually insufficient to ensure disadvantages have been reported, especially on the complexity of total precipitation and therefore is usually added, although other pre­ adsorbent regeneration and the additional waste generation due to cipitation techniques have been developed with the addition of sea adsorbent replacement, generating extra costs (Zhou et al., 2023). water (Shaddel et al., 2020). The precipitated crystalline mineral with the composition MgNH4PO4⋅6H2O is referred to as struvite or magne­ 3.3.9. Membrane filtration sium ammonia phosphate (MAP) (Simha and Ganesapillai, 2017) and Membrane-based separation methods for nutrient recovery entail a has been considered a valuable slow release fertilizer due to its low great variety of filtration methods, such as nanomicro- and ultrafiltra­ solubility, being also generally regarded as a pollutant and heavy metal- tion (NF, MF, and UF), which are usually followed by reverse osmosis free crystal (de Kraker et al., 2019). Although it has been mostly applied (RO) in treated wastewater and in anaerobic digestion liquid fractions on laboratory scale (Lorick et al., 2020), the precipitation of P has been (Gienau et al., 2018a). The use of MF and UF is common in WWTP, and endorsed in WWTP installations due to the great recovery capacity of P further application of NF and RO can be made to obtain non-drinkable reaching up to 90 % or even a complete recovery under the right con­ clean water (Hube et al., 2020). NF and RO are pressure-driven filtra­ ditions (Simha and Ganesapillai, 2017; Federico Volpin et al., 2018) tion processes that can incur high overall costs as well as more opera­ while also enabling for recovering N, although in smaller proportions tional complications compared to other previously explained (Podder et al., 2020). This process can be added as a treatment for pri­ techniques, such as ammonia stripping (Gienau et al., 2018a; Volpin mary or secondary effluent as well as source-separated urine and sludge et al., 2018). Membrane filtration-based processes such as NF and RO effluent (Sánchez, 2020). The addition of struvite precipitation in can also cause urea and ammonia losses that can lead to poor N recovery WWTPs with enhanced biological phosphorous removal treatment in (Simha and Ganesapillai, 2017; Federico Volpin et al., 2018). place does not entail great modifications (Rufí-Salís et al., 2020a), being More recently, membrane technology has been further developed a further source of nutrient recovery and direct application in soil and with the application of membrane contractors (MC) which differ from hydroponic agriculture (Arcas-Pilz et al., 2021; Carreras-sempere et al., the previously explained pressure-based membrane filtration. This 2021; Liu et al., 2011). technology enables the transfer of a substance from one phase to the other through a non-selective membrane without the dispersion of the 3.3.8. Ion exchange/adsorption phases. The diffusion occurs through a defined gradient or pressure on The processes of NH4-N and PO4-P capture via ion exchange or the receiving phase which also defines de separation selectivity (Rong­ adsorption have been investigated and used for nutrient removal. The wong and Goh, 2020). MC can be used for the diffusion in different principle of this removal technology is the fixation of the pollutant into a phases, liquid-gas, gas-liquid, liquid-liquid, gas-gas. In the case of solid surface through a physicochemical interaction. The ion exchange wastewater recovery of nitrogen in the form of ammonia, liquid-liquid process involves bringing the solution in contact with the resin, and as MC has been used. This process requires the increase of the liquid ions in the solution come into contact with the resin, they bind to the phase pH to obtain ammonia gas to then diffuse through the membrane resin while releasing other ions into the solution. Adsorption is a process pores into the gaseous phase (Licon Bernal et al., 2016; Noriega-Hevia in which molecules or ions from a liquid or gas adhere to the surface of a et al., 2020; Rongwong and Goh, 2020). On the other side of the solid material, known as an adsorbent. The interaction is typically due to membrane sorbents, usually strong acids like sulfuric and nitric acids, weak van der Waals forces or other intermolecular forces (Mehta et al., can be the used to dissolve the ammonia gas from the membrane into the 2015; Zhou et al., 2023). Previous studies focused on nutrient removal liquid phase (gas-liquid). Stable liquid fertilizers can be obtained through ion exchange technology from wastewater mainly used natural through this process like ammonium sulphate which can be applied in or synthetic zeolites for N and metal-loaded chelating resins, iron-based fertigation (Gonzalez-Salgado et al., 2022; Licon Bernal et al., 2016; hydroxide compounds and hydrotalcites for P as the captor or exchange Noriega-Hevia et al., 2020; Vecino et al., 2020). This technique has been surface (Kuntke et al., 2016; Williams, 2013). The ion exchange process seen to have great recovery potential as well as a better environmental is a simple exchange between the wastewater flow and an exchange performance with the reduced energy needs compared to air stripping material-containing column, and while NH4-N or PO4-P is attached to and pressure-bound membrane separation. On the other hand the high the media column, other cations are released into the wastewater content of suspended solids in the liquid can cause clogging and mem­ (Williams, 2013). Loaded zeolites can be then regenerated with a NaOH brane fouling (Licon Bernal et al., 2016; Rongwong and Goh, 2020) solution for a consequent use (Reig et al., 2021; Sheikh et al., 2023). This therefore it’s combination with pre-treatments is important to ensure method shows a great recovery (between 43 and 80 % and 90–97 %) for longer membrane lifetime and operation (Deemter et al., 2022). Previ­ both N and P and can then be reversed with salt water, which can then be ous studies have applied this technology for the production of liquid precipitated as struvite to be used as fertilizer (Gowd et al., 2022; Loh­ fertilizers from the aqueous streams obtained from the regeneration step man et al., 2020; Mullen et al., 2019; Federico Volpin et al., 2018; Zhou of zeolites used in WWTP tertiary treatment (Mayor et al., 2023; Sancho et al., 2023). Work has also been done on the recovery of P from et al., 2017; Sheikh et al., 2023) as well as adsorption processes anaerobic digest from municipal solid waste, sewage sludge and sewage (Samarina and Takaluoma, 2020). This combination of emerging tech­ sludge ash through electrodialysis showing promising results (up to 81 nologies has proven to be promising, ensuring a greater ammonia con­ % P recovery in municipal solid waste). This process consists of the se­ centration through the loaded zeolites and using NaOH for zeolite lective separation of P anions through an ion exchange membrane regeneration creating a closed loop process, able to recover up to 92 % of charged with an electric field (Oliveira et al., 2021). Although ion ex­ N in the form of ammonia (Reig et al., 2021; Sheikh et al., 2023). change technologies are promising, their implementation has only been in small scale and laboratory settings (Sánchez and Martins, 2021). 3.3.10. Composting/co-composting Previous studies using natural zeolites for the recovery of both A classic and commonly practised nutrient recovery from household ammonium and phosphate, applied the charged sorbent as a slow biomass is the process of composting, which has been widely employed releasing soil amendment, showing promising results for its application not only on a larger scale with municipal green and organic waste but in agriculture (Guaya et al., 2018; You et al., 2019). also on smaller scales in neighbourhoods and even private gardens 9 V. Arcas-Pilz et al. Science of the Total Environment 905 (2023) 167193 (Dsouza et al., 2021; Shrestha et al., 2020; Ulm et al., 2019). Not only production has shown great yields when substituting chemical for 30 % can urban green and food waste be destined for composting sites, but of the N requirements, showing better results than the crops fertilized anaerobic digestion as well as sewage sludge can also be composted, as with 100 % N chemical fertilizer (Fang et al., 2023). commonly happens with 60–90 % of sewage sludge produced in the UK, Ireland, Spain, France, or Luxemburg (Bastida et al., 2019). 3.3.11. Compost tea Finland, Denmark, Ireland, The Netherlands, Sweden, and Compost and vermicompost tea, originating during the composting Switzerland prohibit the usage of municipal solid waste-derived and vermicomposting processes or by the addition of water, have also compost. While in Austria, UK, Hungary and Italy the municipal solid been considered important nutrient sources and have been used in hy­ waste compost is landfilled; France, Spain, and Portugal also use this droponic production systems, showing promising results in the nutri­ compost for agriculture. (Kacprzak et al., 2022). tional content within the plants, although yield could be compromised The composting process is divided into three stages: an initial mes­ (García-Villela et al., 2020; Pérez et al., 2012; Preciado-Rangel et al., ophilic stage, a thermophilic phase, and a maturation stage. The names 2015; Santiago-López et al., 2016). The compost tea quality greatly of the stages correspond to the temperatures reached in the compost pile depends on the compost composition and feed origin, as compost tea and, therefore, the corresponding bacteria and fungi that are active in from municipal waste is a great source of necessary nutrients for plant each phase (Ravindran et al., 2021). The decomposition of organic waste growth and is applicable to hydroponic systems via the irrigation sys­ mostly occurs during the thermophilic phase, where oxygen is consumed tem. The increase in bacterial activity by the application of compost tea by microorganisms, while carbon dioxide and ammonia are released. can also increase pathogenic suppression in the plant substrate (Stewart- This phase is also crucial for good compost quality since the high tem­ Wade, 2020). peratures reached in this phase enable the elimination of potential pathogens (Babu et al., 2021). 3.3.12. Thermal treatment In urban areas, compost has not only been regarded to recover nu­ Waste thermal treatments such as incineration, pyrolysis, and trients but also as a method for urban soil remediation (Heyman et al., emerging technologies like hydrothermal carbonization (HTC) and hy­ 2019; Kranz et al., 2020; Schwartz et al., 2017). In some cities, compost drothermal liquefaction (HTL) (Sarrion et al., 2021) are processes that generated in urban and peri-urban areas is mostly used for landscaping can be utilized with all kinds of organic residue, entailing the use of high inside the urban area, but only a small fraction is destined for agricul­ temperature and pressure to produce ashes in incineration processes, tural purposes (Eldridge et al., 2018). Although composting might be the referred to as biochars in the case of pyrolysis, hydrochars in the case of most common way to treat biomass, food waste and sewage sludge for HTC (Möller et al., 2018) or energy dense biocrude in the case of HTL nutrient recovery, it also presents some downsides. During the process of (Gollakota and Savage, 2018). While incineration also produces ashes composting, a loss of N can occur due to ammonia volatilization and with inorganic P (Fang et al., 2020; Hartmann et al., 2020; Kirchmann even emissions in the form of N2O and N2, which added to the CH4 et al., 2017), this process requires high energy inputs while generating emissions can cancel out the carbon credit for fertilizer substitution high carbon losses. However, incineration is a common practice in waste (Chen et al., 2022). Usually, good P mineralization is observed, gener­ disposal and the process allows for recovering energy and P (Li et al., ating low N:P ratios, although this also results in the leaching of P with 2020), as in the case of Sweden for municipal waste (Ahmed et al., greater compost applications (Shrestha et al., 2020; Small et al., 2019; 2019). Wielemaker et al., 2019). Further N and P losses may also be experi­ The principle of biochar production is the use of pyrolysis to enced when inappropriate management is provided, e.g., when the breakdown and reorder the minerals and substances in organic biomass, compost pile features too much moisture, high aeration, alkaline pH and and while other elements disperse during the process, P and K remain a low C/N ratio (Jiang et al., 2011; Kashima, 2020). To achieve a good retained (Guilayn et al., 2020). The P retention in the produced biochar compost, an initial moisture in the range of 50–60 % and a pH value depends on the retention time and temperature, with greater retention between 5.5 and 8 must be ensured (Ali et al., 2023). Usually a long at 450 ◦ C to 600 ◦ C (Sun et al., 2018). Other processes to produce bio­ process is needed, guaranteeing the elimination of potential pathogens char can be made with lower temperature requirements of approxi­ and nutrient availability. This last part might be crucial since the mately 440–500 ◦ C (low temperature pyrolysis) or even 180–250 ◦ C, as mineralization process of N via composting can be very slow (Zabaleta in the case of HTC (Dutta et al., 2021; Sun et al., 2018). To generate and Rodic, 2015). While traditional composting can be time-consuming, biochar with great nutrient content, it is important to give a high in-vessel composting systems can be a good way to shorten composting nutrient-containing feedstock. When exploring the potential of munic­ periods while also having overall better control over the composting ipal organic waste and sewage sludge, high concentrations of P were conditions (Ravindran et al., 2021). observed (Sun et al., 2018). The high temperatures achieved during The closing of nutrient loops by composting has been regarded as pyrolysis are favourable for the combustion of pathogens and are also having great potential, Dsouza et al. (2021) even proposing a direct able to immobilize heavy metals when appropriate management and benefit from compost and plant production with CO2 enrichment from processes are ensured (Sun et al., 2018; Xia et al., 2020). These char­ compost exhaust, compost itself and leached nutrients (Chojnacka et al., acteristics make thermal combustion a suitable process to be applied in 2022). By the addition of amendments such as bulking agents or other WWTP (Wang et al., 2020; Zheng et al., 2020), where sludges with high urban-sourced materials for co-composting, it is possible to reach P content can be dried and combusted. In HTC and HTL processes, on the optimal pH, particle size, moisture and C/N ratios and to serve as bio­ other hand, no previous drying is required, and these processes are filters for potential GHG emissions (Kaudal and Weatherley, 2018; potentially more energetically efficient (Rao et al., 2018). Food waste Asquer et al., 2019; Awasthi et al., 2020) like in the case of faecal sludge and food waste digestate contain great moisture and are good candidates co-composting with sawdust as bulking agent which has shown prom­ for HTC and HTL. ising results to avoid nitrogen losses during the composting process Hydrochar also generates a nutrient-rich water as a process by- (Manga et al., 2022). product, which can be further reused for fertilization purposes (Dutta The final compost composition and quality are also highly dependent et al., 2021; Wang et al., 2020; Zabaniotou and Stamou, 2020; Zheng on the incoming feed, and great differences between two sources are et al., 2020) through recovery techniques like struvite precipitation observed. Mechanical separation of organic waste from green wastes (Sarrion et al., 2023b). To increase the nutrient content of the process that can originate in urban settings is crucial, especially with reference water the resulting hydrochar of the HTC process can be washed with to the composition and impurity content, as well as heavy metal con­ acid to encourage the N, P and K leaching, with up to 100 % and 98 % of centrations (Smith, 2009). N and P transference respectively (Sarrion et al., 2021; Sarrion et al., The use of compost obtained from food waste in vegetable 2023a). 10 V. Arcas-Pilz et al. Science of the Total Environment 905 (2023) 167193 On a different note, high temperatures and significant amounts of prompted the inclusion of agricultural activities inside the urban area, acetic acid are needed under HTL working conditions to regulate the especially focusing on rooftop agriculture systems (Appolloni et al., feedstock pH, which may not be economically feasible. Therefore, in 2021; Zambrano-Prado et al., 2021c). This interest has led to extensive order to reduce the operational cost of HTL, future research should find research and educational activities from Higher Education and Research substitutes for acetic acid (e.g., other affordable organic acids, or het­ institutions (e.g., ICTA-UAB, UPC, IRTA) as well as organizations (e.g., erogeneous solid acid catalysts) that can maximize energy and nutrient Replantem) focusing on the integration and application of agriculture in recovery for digestates utilizing HTL under more accommodating the city of Barcelona. The city council has also promoted these activities operating circumstances (Sudibyo et al., 2022). with the creation of 7 hydroponic installations on building rooftops and The slow P release of biochar and hydrochar products makes them patios for social and community integration purposes (IMPD project) favourable fertilizers that could avoid further P losses into the soil and (Biel, 2019) and hosts an annual green roof contest to endorse projects water bodies, as in the case of commercial fertilizers or manure (Möller that propose the creation of rooftop gardens (Ajuntament de Barcelona, et al., 2018; Sun et al., 2018). 2020). The objective is the creation of 34,100 m2 of green roofs by 2030 Further benefits from the use of biochar and hydrochar are the (Zambrano-Prado et al., 2021c). promotion of carbon sequestration and plant growth and the increase in The potential of the metropolitan area of Barcelona to implement microbial communities in the soil (Dutta et al., 2021; Ijaz et al., 2020; rooftop open air agriculture and rooftop greenhouses has already been Zabaniotou and Stamou, 2020). identified for several urban and peri-urban areas. Such work has been Alternative to the previously mentioned thermal treatments, the developed with the help of GIS rooftop databases and remote sensing natural freeze concentration uses the freezing point of water and salt approaches that mainly focus on larger roof extensions to host these concentrated solutions as a natural separation method. This has been installations, with industrial and retail parks and large social housing also used on different wastewater streams emulating cold climate con­ neighbourhoods being the best candidates (Fig. 2) (Nadal et al., 2017, ditions for the concentration of nitrate water (John et al., 2023). 2018; Sanyé-Mengual et al., 2018; Toboso-Chavero et al., 2019, 2021; Zambrano-Prado et al., 2021a; Zambrano-Prado et al., 2021b). 3.3.13. Vermicomposting While several areas have been identified as potential UA sites due to To increase and stabilize the process of composting, the use of the rooftop material and rooftop extension, these have only been earthworms can be encouraged. The derived compost, also referred to as contemplated in a theoretical way. However, other rooftop UA areas vermicompost, is the bio-oxidation and stabilization of organic matter have been implemented, mainly through the social project “Horts al by earthworms and other microorganisms (Suthar, 2007). For this pro­ terrat” from the municipality of Barcelona and the green roof competi­ cess, some earthworms have been identified as detritus feeders. Some tion. The total potential identified in the literature was 44.44 ha, and the examples of the earthworms most characterized in organic waste recy­ area of existing sites was 0.77 ha, comprising a total of 45.21 ha cling are Perionyx excavates (Perrier), Eisenia fetida (Savigny) and (452’100 m2) within the metropolitan area of Barcelona (Table 3). Other Eudrilus eugeniae (Kingberg) (Biruntha et al., 2020; Gupta and Garg, typologies of UA, such as indoor or soil-based UA, were not included. 2009; Pattnaik and Reddy, 2010). These species have also been cate­ If this area is dedicated to tomato production with an estimated gorized as fast debris feeders and are capable of reducing hazardous productivity of up to 16.5 kg m− 2 year− 1 (Sanyé-Mengual et al., 2018), waste material (Ahadi et al., 2020; Ravindran et al., 2021). Although it is the potential production could entail up to 7459 t of tomato, which is a very eco-friendly and mostly cost-free addition to composting, ver­ equal to 13.3 % of the tomato consumption within the metropolitan area micomposting can entail some different management skills. While the (Table 1 in the Supplementary material). production of compost can entail 6 to 9 months, the process of vermi­ From 2017 to 2021 the LIFE project ENRICH (standing for composting can be much faster, ranging between 1 and 4 months Its end “Enhanced Nitrogen and phosphorus Recovery from wastewater and product can be more homogenous than that of thermophilic compost, Integration in the value Chain”) was developed through the collabora­ and its nutrient content is also enhanced (Ravindran et al., 2021; tion of institutes, not only dedicated to the recovery of nutrients from Schröder et al., 2021). Previous studies on the vermicomposting of wastewater streams in the AMB but also its valorisation in agriculture. sewage sludge and green waste have reported an increase in nutrient This project enabled the creation of a demonstration plant and has availability and therefore greater yield production as well as the content defined several outcomes through its lifetime. The applied recovery of humic substances and plant growth-promoting hormones (Biruntha strategies defined during this project were the struvite precipitation for et al., 2020; Hanc and Pliva, 2013; Tognetti et al., 2005). On the other slow-release solid fertilizer, zeolite adsorption and membrane contrac­ hand, the production of vermicompost needs greater monitoring and tors to produce liquid fertilizer, which were further used in the culti­ skill for its production, maintaining certain conditions to ensure good vation of tomatoes (Carreras-sempere et al., 2021). The results obtained living conditions for earthworms. The specifications and conditions that from the pilot production and application determined that the envi­ have to be maintained are an initial C/N range below 40:1, a tempera­ ronmental benefits and economic viability of struvite crystallization, ion ture range of 18–67 ◦ C, a pH range of 5.9–8.3 and a moisture content of exchange and liquid-liquid MC are promising for future nutrient re­ approximately 10 % (Ahmed et al., 2019; Stewart-Wade, 2020). To covery and valorisation (Mayor et al., 2023). A similar project that has achieve these conditions, a previous composting phase is often encour­ taken place in the same area is the Waste2Product, dedicated to the aged (Ravindran et al., 2021). It should be noted, however, that tem­ recovery of nutrients of WWTP through sorbents and nutrient-enriched peratures lower than those achieved in the thermolysis stage in the zeolites to be further applied as slow-releasing soil amendments (Guaya composting process entail a reduced effectiveness to ensure a pathogen- et al., 2018, 2020; You et al., 2019). free final product (Biruntha et al., 2020; Hanc and Pliva, 2013; Tognetti On the other hand, projects for the nutrient valorisation of organic et al., 2005). The vermicomposting process can generate a by-product wastes have also been developed in this region. In 2004, the AMB began known as vermicompost leachate which contains beneficial microbial a program to promote composting of domestic waste with the collabo­ communities that can enhance the nutrient uptake by plants when used ration of the Catalan Waste Agency. Data from 2014 states that 35 in soilless production systems (Wongkiew et al., 2023). municipalities joined the composting program, with the participation of 4250 citizens. 4. Case study of nutrient recovery application: exploring the The Horizon 2020 project Desicive, focused on the recovery of nu­ potential of NPK recovery in the AMB trients through the composting and anaerobic digestion of bio, organic and food wate based on of its pilot scale demonstration sites in the Barcelona is a densely populated city in the Mediterranean area with campus of the Universita Autónoma de Barcelona, within the AMB. 16,420 inhabitants km− 2, with limited land availability that has Through this project the campus waste production was characterized 11 V. Arcas-Pilz et al. Science of the Total Environment 905 (2023) 167193 Fig. 2. Map of the Metropolitan Area of Barcelona with identified locations for rooftop agriculture. Table 3 Areas identified within the metropolitan area of Barcelona for UA on rooftops. Area/project name Building type Area Comment Reference Potential areas found Zona Franca Industrial park 13.06 Approximately 14 % of tomato imported (Sanyé Mengual, 2015) in literature ha (128,000 people demand year− 1) Sant Boi Retail park 5.58 ha Urban self-supply 3.8 % Sanyé-Mengual et al., 2018 Montigalà Retail park 5.22 ha Urban self-supply 3.5 % Sanyé-Mengual et al., 2018 Montbau Neighbourhood 0.06 ha Up to 37 % of tomato self-supply (Toboso-Chavero et al., 2019) Badia del Vallés Neighbourhood 20.52 Self-sufficiency for tomato 210 % and (Zambrano-Prado et al., 2021a; Zambrano-Prado Barberà del Vallés Neighbourhood ha lettuce 21 % in the neighbourhood et al., 2021b; Zambrano-Prado et al., 2021c) Implemented areas IMPD Project “Hort Municipality 0.02 ha 3590 kg year− 1 of vegetables (Biel, 2019) al terrat” buildings (Ajuntament de Barcelona, 2020) Green Roof Private rooftops 0.75 ha Vegetable and urban green (Ajuntament de Barcelona, 2020) Competition and a micro anaerobic digestor was implemented for the generation of area is divided into three categories: household waste, organic waste energy and fertilizer in the form of composted digestate. from large producers and green waste. The total organic waste collected Furthermore, emerging businesses like “Abono km0” are developing from the classified sorting bin in 2020 was 184,000 tonnes, of which 78 a commercial strategy to recovery food waste from restaurants and % was household waste, while only 6 % and 16 % originated from large kitchens to further produce vermicompost (Website: https://abonokm0. producers and green waste, respectively. This differentiated waste is com/), although there is no available data on the recovered amount of then transported to two specialized installation typologies: compre­ waste nor compost production. hensive waste handling stations called Ecoparcs (1, 2 and 4) and com­ posting sites. The handling of this waste fraction is similar in Ecoparc, 4.1. Organic waste generation and treatment with a pre-treatment to prevent impurities and the mixing of the three organic waste categories for digestion and further production of While a waste recovery system is generally implemented with clas­ compost. The composting sites, on the other hand, do not entail an sified sorting bins, only approximately 36 % of the waste is separately anaerobic digestion step to produce compost. Although the waste is recovered, with a greater fraction of “rest” or “unclassified” waste. The collected in separated bins, the percentage of impurities is still high current goal is to increase the separated fraction up to 50 % in all mu­ depending on the municipality of origin. Ecoparcs 1, 2 and 4 presented nicipalities, which was achieved by only 16 % of all municipalities by impurity percentage ranges of 8.7–23 %, 5.4–30.3 % and 1.7–36.6 %, 2018. The generation of separated organic waste in the metropolitan respectively, while the composting sites showed ranges of 2.5–19.2 %. 12 V. Arcas-Pilz et al. Science of the Total Environment 905 (2023) 167193 The locations of the Ecoparcs and composting sites, which are shown ultrafiltration and reverse osmosis to retrieve and regenerate water. in Fig. 3, are mostly located within the metropolitan area of Barcelona Apart from the WWTP in Gavà I Viladecans, no other WWTP has water except for Ecoparc 4. The destination of the collected organic waste from regenerating processes. the differentiated bins in 2020 was mostly in Ecoparc 2 with up to 44 % In conventional WWTPs, approximately 30 % of the influent waste­ of the generated waste, while Ecoparc 1, Ecoparc 4 and the composting water nutrients are removed through active sludge separation in the sites received 32 %, 18 % and 7 %, respectively, of the generated waste. primary treatment, while further nitrification and denitrification pro­ The yearly production of compost almost reaches 30,000 tons, which cesses in the secondary water treatment can reach a removal of up to 70 can be further employed in gardens and surrounding agriculture. The %. This nutrient removal is applied in 4 WWTPs in the metropolitan “rest” or “unclassified” fraction produced in 2020 was greater than area, namely, the WWTP in el Prat de Llobregat, in St. Feliu de Llobregat, 800,000 tons, and it was also processed in Ecoparcs 1, 2, 3 and 4, with a in Gavà I Viladecans and in Begues. Although this can be considered a distribution of 21 %, 23 %, 23 % and 33 %, respectively, of the total. The good removal rate, approximately 1200 t of N and 160 t of P are still processes this “unclassified” waste fraction undergoes is a mechanic released every year into the Mediterranean Sea, only considering the separation treatment as well as biologic treatments and a biowaste sta­ WWTP in El Prat de Llobregat. Therefore, the potential for additional bilization phase for the annual production of more than 70,000 tons of nutrient removal is great. stabilized biowaste for soil amendment and landfilling. Work on the reduction of P and N in the wastewater effluent in El Prat de Llobregat and Besòs WWTP’s has already been done, proposing 4.2. Wastewater generation and treatment struvite precipitation or zeolite adsorption. These works consider the recovery of these nutrients a success, obtaining 5000 t year− 1 of loaded In the metropolitan area of Barcelona, 7 WWTPs (Fig. 3) are zeolite with a 15 % PO3− 4 content or otherwise a range of 43–368 t responsible for the treatment of approximately 270,000 Mm3 of waste­ year− 1 of P in the form of struvite in the El Prat de Llobregat WWTP water each year. The technologies of all plants vary for both sludge and when precipitation is considered (depending on the recovery effluent treatments, between sludge anaerobic digestion and composting technology). and between sludge dewatering and field application, to secondary or tertiary water treatments (Table 4). 4.3. Nutrient recovery potential in the metropolitan area of Barcelona The yearly production of sludge is approximately 57,000 tons of dry matter (2020), which is then directly employed in agriculture (24 %) or Considering all previously collected information, an estimate of the composting (68 %). The water treatment in the WWTP from El Prat de potential of nutrient recovery with the existing infrastructure of the Llobregat follows five main steps, starting with a pre-treatment for solid AMB can be determined (Table 5 and Tables 2, and 3 from the sup­ and oil separation, a primary treatment where the sludge is removed, a porting material). This estimation is again based on the assumption that secondary treatment with nitrification and denitrification processes for all the defined area are dedicated to tomato production. nitrogen removal, tertiary treatment, denitrification processes and The existing generation of food/bio/organic waste and the produced Fig. 3. Map of the metropolitan area of Barcelona with the locations of the currently active WWTPs (blue), Ecoparcs (green) and composting sites (orange). 13 V. Arcas-Pilz et al. Science of the Total Environment 905 (2023) 167193 Table 4 Description of the WWTPs identified in the metropolitan area of Barcelona, yearly treated amount (* obtained from the official website of the metropolitan area of Barcelona: https://www.amb.cat/c/portal/download_file?type=xls&name=aigua_tractada_edar&jsons=aigua_cabal_tractat_edar-0-0), and percentage in relation to the total wastewater treated in the area (** obtained from the technical sheets of the EDAR installations of the official website: https://www.amb.cat/es/web/ecologia /aigua/instalacions-i-equipaments). Estimation of incoming N and P (*** based on You et al., 2019); CAnD – Composted anaerobic Digestion, DW – Dewatering. WWTP Incoming flow (Mm3 y− 1) % from total Sludge treatment Effluent treatment Incoming N (ton y− 1) Incoming P (ton y− 1) (2019)* ** ** ** *** *** 1 El Prat de Llobregat 92.1 36 % CAnD Secondary and tertiary treatment 5525 t 626 t 2 Besòs 120.4 45 % DW Secondary treatment 7225 t 819 t 3 St. Feliu de Llobregat 18.5 7% CAnD Secondary and tertiary treatment 1114 t 126 t 4 Montcada I Reixac 18.8 6% – Secondary treatment 1129 t 128 t 5 Gavà I Viladecans 14.8 5% CAnD Secondary and second decanter 892 t 101 t 6 Begues 0.3 0.7 % – Secondary treatment 21 t 2t 7 Vallvidrera 0.2 0.5 % DW AnMBR 15 t 2t Table 5 OM = Organic Material, (* taken from the technical sheet of Ecoparcs and composting site official website: https://www.amb.cat/es/web/ecologia/residus/instalac ions-i-equipaments/llistat), compost generated in Ecoparcs 1 to 4 was destined to a digestion process; WWS = wastewater sludge (** obtained from the technical sheets of the EDAR installations of the official website: https://www.amb.cat/es/web/ecologia/aigua/instalacions-i-equipaments), WWE = wastewater effluent, Min = minimum removal of 7 % P with AirPrex technology, and Max = maximum removal of 60 % P with RemNut Technology; ** obtained from Rufí-Salís et al. (2020a), *** obtained from You et al. (2019). Waste type Amount (ton DW/year) (2020) N content in waste (kg) P content in waste (kg) Times N demand is met Times P demand is met OM Compost* 987.7 21,235 7802 1.8 2.7 OM Digest* 19,250 525,525 171,325 46 58 WWS- Compost** 57,000 1,328,100 1,117,200 117.5 380.2 WWE - Struvite Min*** 341.3 19,829 43,000 1.7 14.6 WWE - Struvite Max*** 2920.6 169,962 373,000 14.5 125.2 WWE – Zeolite**** 5000 – 750,000 – 255.2 compost and digest in the centralized Ecoparcs and composting sites can that could be achieved in all WWTPs (Table 4 from the supporting in­ produce up to 550 t of N‑nitrogen and 170 t of P-phosphorous each year, formation). If all WWTPs had the capacity for struvite precipitation meeting approximately 48 and 60 times the N and P demand, between 7 and 60 % of the incoming P, a potential production of 126 to respectively. 1082 t of P and 52 to 489 t of N could be recovered yearly. Sludge compost generation in all WWTPs can be a great source of nutrients, especially P, with a yearly production greater than 1000 t, 5. Constraints and obstacles to fulfilling the nutrient recovery which can cover 117 and 380 times the UA requirements of N and P, potential respectively. For both recovery strategies no additional logistics would be needed, The capabilities to recycle and reuse nutrients such as N and P are taking in account the existing infrastructure and existing waste trans­ clear, but why are the advances in the application of these processes so port. The centralized recovery in the local facilities shown in the slow? The use of organic waste from urban areas is generally perceived metropolitan area would only require an additional transport to the as having a poor quality or containing great amounts of unwanted ele­ urban gardens. ments that could be toxic or polluting (Gímenez Lorang et al., 2005). The generation of struvite can vary between recovery technologies, This finding is also true for sewage sludge quality, which was deter­ as defined by Rufí-Salís et al. (2020a), ranging from the lowest recovery mined to contain great amounts of potentially toxic elements (PTEs) in of 7 % of P for incoming wastewater P in the AirPrex technology to P long-term experiments that could be traced to World War II (Möller recovery of 60 % for incoming P in the RemNut technology. In this case, et al., 2018). On the other hand, the quality of organic waste and sewage only the WWTP in El Prat de Llobregat is considered due to the existing sludge has been increasing for the past decade, making it a great nutrient necessary installation. The generation of struvite only in this plant can resource. Nevertheless, due to public or private legislation standards, meet the N demand 1.7 to 14.5 times and the P demand by 14.6 to 125.2 most of these recovery sources are not permitted, e.g., in organic times. farming (Awasthi et al., 2020; Möller et al., 2018). Furthermore, the The work of You et al. (2019) encourages the possibility of ion ex­ production costs of recovered P can be a constraint to implementing change processes with loaded zeolites in the El Prat de Llobregat WWTP these processes compared to the existing extraction chain of mined due to the existing filtration installations. If the struvite precipitation is phosphate rock (Chojnacka et al., 2020; Oarga-Mulec et al., 2019). not considered, yearly recovery of 5000 t of loaded zeolite is defined Higher initial investment in large scale application and the maintenance with an approximate content of 750 t of P, meeting the P demand by 255 costs can also influence the application of technologies like nutrient times. While the simultaneous nutrient obtention is possible in the recovery systems in existing WWTP (Environmental Protection Agency, defined OM and WWS scenarios, in this case the WWE can undergo 2021). Therefore, most of the emerging recovery technologies remain on different processes that are not considered to happen at the same time, the laboratory or pilot scale with limited market uptake (Cordell et al., like struvite precipitation and ion exchange process. 2021). To encompass most barriers on the up-scaling and application of Although other technologies could be applied to the nutrient cycle in P recovery technologies Barquet et al., 2020 defines four key points, this area, it would be necessary to further install additional infrastruc­ being 1) the current insufficient knowledge on the technology applica­ ture, which can also generate further environmental impacts on the tion and performance (economic, agricultural and environmental); 2) nutrient recovery systems (Rufí-Salís et al., 2020a). On the other hand, the slow change in existing infrastructure for the implementation of the potential of struvite as a slow-release fertilizer and the generally recovery systems, such as WWTP with long-term financial investments, positive characteristics to be used in UA due to the lack of smell, low defining lower applicability of decentralized systems such as source- content of heavy metals and potential pathogens suggests the production separated systems; 3) the lack of coordination between livestock and 14 V. Arcas-Pilz et al. Science of the Total Environment 905 (2023) 167193 crop farms, making the application of mineral and synthetic fertilizer a Data availability more economically viable alternative 4) the lack of awareness on the current nutrient dependency and policy implementation. Barquet et al., Data will be made available on request. 2020 further point out the dissociation between the nutrient reduction and recovery strategies, defining that while most policy implementa­ Acknowledgements tions on nutrient management are directed to the reduction of load thresholds, this does not specifically encourage nutrient recovery for its The authors are grateful to the Spanish Ministry of Economy, In­ valorisation as agricultural fertilizer. Although the application of re­ dustry and Competitiveness (Spain) for the grant awarded to V. Arcas- covery technologies is still not fully considered in waste treatment Pilz (FPI-MINECO 2018). This work was supported by the “María de processes, this state of mind is slowly changing and pushed towards Maeztu” program for Units of Excellence in R&D [CEX2019-0940-M]. recovery in response to the foreseen P shortages in the coming years as The research leading to these results has received funding from the well as the environmental impact of untreated waste. Since 2021 Eu­ European Research Council under the European Horizon2020 research ropean legislation has considered the use of struvite as an agricultural and innovation program under grant agreement No 862663 (FoodE) as fertilizer and therefore encouraged its recovery and application. An in­ well as, the ERC grant agreement n◦ 818002 URBAG, awarded to Gara crease in biowaste-derived fertilizers is expected and estimated to Villalba and the Interreg NWE grant agreement NWE 474 GROOF. replace up to 30 % of inorganic fertilizers (Chojnacka et al., 2020) and Finally, we would like to thank the Ministerio de Ciencia y Innovación up to 50–60 % of phosphate rock imported into Europe and utilized in (MCIN/AEI/10.13039/501100011033/FEDER, UE) and the EU found­ agriculture (Möller et al., 2018). ing “Next Generation” for the founding of the projects MOVE4EDU (PID2021-126845OB-C21) and BINAFET (TED2021-130047B-C21). The 6. Conclusions publication reflects the author’s views. 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