# Circular management of human excreta as a pillar of sustainable cities and food systems

Tanguy Fardet

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Tanguy Fardet. Circular management of human excreta as a pillar of sustainable cities and food systems. 2025 International Fertiliser Society Conference, International Fertiliser Society, Dec 2025, Cambridge, United Kingdom. hal-05478504

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# Circular management of human excreta as a pillar of sustainable cities and food systems

Tanguy Fardet

## Abstract

Human excreta (urine and faeces) is a major nutrient deposit; in the context of sustainable agri-food systems, it would in fact constitute the largest source of renewable phosphorus. However, current management through sewer-based sanitation leads to significant degradation of this resource as well as to the loss of the majority of the nutrients to the environment. This paper details the flows of nitrogen throughout the French sanitation system and discusses the levers and obstacles to nutrient recovery in sewer-based sanitation systems.

Using monitoring data from wastewater treatment plants (WWTPs) as well as records on population and dietary intakes, we assessed the flows of nutrients from food, through sanitation systems, and to the environment or environmental parcels. The mass flow diagram showed that 90% of the nitrogen and 50% of the phosphorus entering the system were not returned to agricultural fields but lost to the environment, thus inducing an intrinsic dependence on external nutrient imports.

Though legislation has contributed to reducing water pollution by limiting the amount of nutrients discharged to water, they have had little effect on recycling, especially for nitrogen. Further regulations on WWTPs are unlikely to enable the high nutrient recycling rates that could help secure domestic fertiliser supply chains.

Because of this and given the vulnerability of sewer-based sanitation in the face of climate change and increasing water scarcity, we investigate the potential of excreta source-separation to provide high-quality resources for bio-based fertiliser production. We show that, if secured to fertilise crop production for human consumption, the nutrients in French excreta could cover a third of the nutrient requirements of domestic food consumption.

**Keywords:** nitrogen, phosphorus, source separation, excreta, circular economy

## Contents

1 Introduction 2

1.1 Western sanitation: broken cycles in an unstable geopolitical context 2

1.2 Original articles and methods 2

2 Results 3

2.1 Mass balance of the French sanitation system 3

2.2 Impact and role of regulations 4

2.2.1 Historical trends and future prospects 4

3 Perspectives for fertilizer production from human excreta 5

3.1 Nutrient recovery in sewer-based sanitation 5

3.2 Source separation as a complementary paradigm 7

3.3 The scale and stakes of human excreta as a fertilizer deposit 7

4 Conclusion 9
# 1 Introduction

## 1.1 Western sanitation: broken cycles in an unstable geopolitical context

Circular sanitation is an emerging topic in the broader context of food security and fertiliser supply chains. Indeed, the dependence of EU countries on direct or indirect<fnref n="1" /> imports of fertiliser is increasingly acknowledged as a major geopolitical, and sustainability, issue (Nanda et al. 2020).

There are many ways to increase fertiliser autonomy; first and foremost the increased use of pulses in cropping and diets, to directly capture nitrogen from the atmosphere and increase the nutrient use efficiency (NUE) compared to animal-based products; NUE should also be further increased thanks to adapted inputs (rate, type and timing) and the reduction of nutrient loss throughout the wider supply chain<fnref n="2" />; finally we should aim to increase nutrient circularity by returning recovered nutrients to the fields (Walsh et al. 2023).

In western countries, most large wastewater treatment plants (WWTPs) include what is called a tertiary treatment to protect the water bodies from excess phosphorus (P) and nitrogen (N) discharges. They nitrify, then denitrify nitrogen to send it back to the atmosphere as N₂, which consumes a similar amount of energy as converting N₂ to reactive N in the HaberBosch process (Sutton and UNEP 2013). Recovering and reusing the reactive N instead of destroying it would thus decrease energy consumption and dependency on external fertiliser supplies, increasing food self-sufficiency. Regarding P, there is no destruction of reactive phosphate, but the resource is contaminated by the various pollutants present in sewers<fnref n="3" /> and its circularity remains low. Given that phosphate resources are both finite (Scholz and Wellmer 2013), inhomogeneously distributed (Childers et al. 2011) and may contain high concentrations of heavy metals and radioactive elements, which can pollute agricultural soils (Watson et al. 2014), not taking advantage of the high-quality deposit in human urine and faeces is a missed opportunity. Indeed, studies have consistently found that recycling all P in human excretions could meet approximately 20% of the current global P demand in agriculture (Mihelcic et al. 2011), with variations from 15 to 50% depending on the diets and food systems (Starck and Esculier 2025).

By analysing the fate of nutrients in human excreta, in particular how much is recycled to agriculture, and where losses occur, we can identify the main levers to further increase recycling in the context of current sanitation systems as well as the potential of alternative paradigms. In particular we investigate the potential of source-separation to reduce the dilution and contamination of the nutrients and its possible contribution in the transition towards sustainable food and sanitation systems capable of providing high nutrient recycling rates.

## 1.2 Original articles and methods

The main results of this proceeding come from two peer-reviewed articles: one on nitrogen (Starck et al. 2023) and the other on phosphorus (Starck et al. 2024); please see these articles for more details about the data, methods and uncertainties.

The assessments detailed here were made using data from the French water agencies, detailing around 20,000 WWTPs in metropolitan France. We looked at the flows in and out of the WWTPs and at data about individual autonomous systems in areas without collective sanitation. Discharges from industries and losses throughout the sanitation system were also included. The rates of nutrient recycling were estimated using data on sludge management practices and nutrient content of the sludge.

To better assess the sources of nutrients, we coupled this data with estimates of the nutrient inputs stemming from French residents. The data for residents was taken from the national census and the nutrients they excreted were approximated from the amounts that they ingested, based on data from a national food consumption survey.

<footnote n="1">1. e.g. gas imports, to produce nitrogen fertilizer, or feed for cattle.</footnote>

<footnote n="2">2. through runoff, leaching, volatilization or food waste.</footnote>

<footnote n="3">3. such as metals (copper, zinc, lead...), microplastics, PFAS and other reactive organic molecules.</footnote>
# 2 Results

## 2.1 Mass balance of the French sanitation system

From the data that was gathered, we could deduce a complete mass balance analysis of the sanitation system, as shown on Figure 1. This showed that the main source of N and P to the sanitation system was human excreta and that the majority of nutrients were lost to the environment (50% for P and 90% for N).

In terms of inputs to the sanitation system, the contribution from industries was very small at the national scale. In the case of nitrogen, greywater was also a very small contributor, compared to the excreta contained in the blackwater, while it provided a more noticeable flow for P, because of the detergents used in dishwasher and laundry machines.

Once the nutrients reach the sewers, around 10% is lost through leaks, incorrect connections, or overflows during rainy events. Once they reach the WWTP, N and P are handled in significantly different ways. To understand this difference, one must remember that the original purpose of WWTPs is not to recover nutrients or produce fertilizer, but to protect water quality by preventing the discharge of nutrients in large amounts. The processes it implements are thus aimed at removing as much nutrients as possible from the treated effluent before it is sent back to the aquatic medium. To do so, the nutrients must be moved to another phase (solid or gas) or separated from the main effluent through a membrane.

<figure><img src="image_2.png" /></figure>

Figure 1: Sankey diagrams of the average yearly flows of phosphorus (P, on the left) and nitrogen (N, on the right) in French sanitation systems. Most of the P and N (respectively 29 ktP and 297 ktN out of 40 ktP and 318 ktN) enter the sanitation system from human excretions. The flows highlighted in red are those who contribute to the agricultural reuse, which represents a bit less than 50% of the incoming flow for P and only 10% for N. The IAS are individual autonomous systems used in areas that are not directly connected to sewers. Both figures are adapted from Starck et al. (2023, 2024), please see the original articles for more details about the precise numbers and associated uncertainties.

In the case of nitrogen, the conversion of the reactive N to inert $N_2$ was chosen by the sanitation sector, meaning that, in terms of circularity, all N “successfully” processed by the plant is lost to the atmosphere, which is around 50% of the whole inputs in France and more generally in large cities with sewer-based sanitation and advanced treatment. As N is not easily trapped in the sludge, most of the non-denitrified N ends up in the water. Added to the other losses, this amounts to 40% of undesired reactive N ending up in the environment, in France. This leads to significant eutrophication and contributes to algae blooms that can have strong negative impacts on the aquatic environment or even on human health. Overall, only 10% of the reactive nitrogen is recovered and recycled to agricultural fields in France. The loss-rate of nitrogen is thus 90%, despite the fact that 75% of French
sewage sludge is spread on agricultural parcels, with or without a composting step.
The choice of denitrification, stemming from choices in an industry that has limited ties to agriculture, not only led to a quasi-absence of N recovery, but also to very high energy consumption: the aeration of the wastewater, necessary for the biological process, consumes 4% of the worlds electricity and the treatment generates around 3% of the worlds GHGs according to the IEA (in part because of undesired $N_2O$ emissions). From an environmental perspective, the destruction of the reactive nitrogen is thus generating as much emission as its original synthesis.

In the case of P, as it has no gaseous phase and can more effectively be trapped as a solid or in the sludge, almost 50% is recycled to agricultural fields. A greater fraction is actually trapped within the sludge but is lost from the 25% being landfilled or incinerated without P recovery. Yet, 35% of the P still ends up in the water or as diffuse loss to the environment, contributing to almost half of the P-related eutrophication.

For both nutrients, the sanitation system thus leads to significant losses. It is also worth noting that what is returned to the fields is not simply nutrients but sewage sludge, which means that there is additional contamination by a wide array of other compounds such as metals (notably Cu and Zn), microplastics, various organic compounds (PFAS, PAH...), pathogens and pharmaceuticals. While the situation may appear less problematic for P in terms of recycling, the sludge-based paradigm itself is thus an intrinsic limitation in terms of quality.

## 2.2 Impact and role of regulations

Despite this significant loss to the environment, the situation today constitutes a marked improvement compared to what it was a couple of decades ago, not only in terms of water quality but also in terms of resource management. In this section, we will discuss some of the regulations that led to these improvements.

### 2.2.1 Historical trends and future prospects

So far, two main types of regulations have been put in place to tackle water pollution from sanitation systems. The first type target wastewater treatment plants (WWTPs), either by mandating the to increase their removal efficiency (the fraction of nutrients that they remove from the stream) or by regulating the absolute amount of nutrients that they are allowed to discharge into the water. The second type of legislation is focused on upstream sector and targets the products that will mix with the water.

Let us start with the second type. In France, phosphates in laundry detergents were banned in 2007, then, in 2013, the European Union banned consumer laundry detergents with standard doses >0.5 gP, and in 2017 automatic dishwater detergents with standard doses >0.3 gP. This was years in the making and led to a progressive but significant decrease of the P content of wastewater. Overall, between 1990 and 2020, the amount of P entering the WWTPs was halved (see Figure 2).

In parallel, there was the creation of legislation targeting wastewater treatment plants, based on the fact that their basin was classified as sensitive to nutrient discharge or not. Starting in 1994, particularly in the north-east of France, some basins were classified as nutrient sensitive. Starting in 2006, the regulation made the distinction between nitrogen and phosphorus sensitive areas, with the northern part of France quickly declaring that all basins were sensitive to both nitrogen and phosphorus, which led to stricter requirements for the large WWTPs in these areas. In the South, though, most basins were not classified as nitrogen-sensitive and only half were progressively classified as phosphorus-sensitive until 2024. This discrepancy between basins lead to striking differences between the yield of WWTPs in sensitive and "non-sensitive" areas, as can be seen on Figure 3.

At the EU level, the upcoming urban wastewater treatment directive, is also going to target WWTPs and will eventually mandate that all plants larger than 10,000 PE should remove 90% of the phosphorus that enters the sewers and 80% of the nitrogen. In terms of nutrient discharge to the water, this will obviously have a major impact for WWTPs that are in basins currently considered "non-sensitive".
<figure><img src="image_3.png" /></figure>

Figure 2: For each of the 6 French water agencies basins: on the left, the N/P molar ratio of incoming flows in WWTPs, showing an relative increase in the N content compared to P; on the right the incoming P in WWTPs, normalized by WWTPs capacity, in population equivalent (PE) showing an absolute decrease in the P load. Original figure from Starck et al. (2024).

However, for wastewater treatment plants that are already regulated, this is unlikely to make much of a difference, and even in unregulated area, the impact on N recycling will be limited:

* for nitrogen, one must remember that increased removal efficiency does not increase recycling but simply reduces the amount that goes to the water by denitrifying the reactive N and sending it back to the atmosphere,

* for phosphorus, most large plants already have removal efficiencies around 85% so except for a few areas that were still considered as phosphorus-insensitive, the change from 85 to 90% yield will not significantly affect recovery as most P loss either occurs before the WWTP or after if the sludge is incinerated or landfilled.

This means that a significant number of areas in Europe are now getting close to the limit of what WWTP-centric regulations can achieve.

# 3 Perspectives for fertilizer production from human excreta

## 3.1 Nutrient recovery in sewer-based sanitation

There is still room for innovation in wastewater treatment plants to increase nutrient recovery. However, conventional recovery, through sludge and sludge digestate or compost, may not be allowed for long (in the countries where it is not already prohibited) because these streams are contaminated by a wide variety of pollutants, as mentioned in the previous sections. This section will thus focus on other ways of recovering the nutrients.

The first and perhaps most well-known alternative recovery pathway is Reuse: reusing treated wastewater to simultaneously fertilize and irrigate crops (Al-Hazmi et al. 2023). As conventional nutrient recovery through sludge, this pathway is also submitted to a large amount of regulations to limit the risk of soil and water pollution and is not always well-received by the general public. When possible, it might enable to recover some additional amounts of nitrogen, though this is limited to the N
<figure><img src="image_4.png" /></figure>

Figure 3: Comparison between the nitrogen removal efficiency — also simply called “yield” — of individual WWTPs in sensitive (colored) and non-sensitive (grey) areas. As the regulation applies to large WWTPs (> 10,000 population equivalent or PE), these two categories are those where the difference between regulated and unregulated plants can be seen, with most regulated plants having yields greater than or equal to 80% whereas large unregulated plants often have yields lower than 50%. Adapted from Starck et al. (2023).

that is neither denitrified nor trapped in the sludge. For most wastewater treatment plants in Western Europe, this is going to be less than 20% of the initial N entering the sanitation system. Furthermore, since the water is not returned to the river, Reuse may significantly increase water consumption in some areas. In the case of large WWTPs, this can become a significant issue in summer as their outflow can account for a large fraction of the total flow of the river; in the case of Paris, for instance, the WWTP on the western outskirts of the city may account for up to half of the river Seines flow rate when said flow rate is at its minimum. Sending all this water to the fields, instead of the river is thus not possible if one wants to preserve the rivers flow rate. Given that these large WWTPs treat most of the N from sanitation, this issue is likely to significantly limit N recovery.

A second pathway for nutrient recovery is to couple the anaerobic digestion of the sludge to a nutrient extraction process (Gonzalez-Salgado et al. 2022). This enables to recover more N compared to direct sludge use as it limits the amount of N that is circulated back to the stations entry point to go through the denitrification step again. Such processes have already been implemented on semi-industrial scales in pilot plants and can achieve roughly 20% of N recovery, but they are costly and require additional reactants.

Added, together, these two potential pathways for nutrient recovery are unlikely to reach more than 30% of N recovery, which means that alternative pathways should be explored if we want to recover more significant amounts of nitrogen. At the moment, other alternatives — such as direct N recovery from wastewater using membrane-based processes — are still at low TRLs and require additional research and investment to assess whether they could be deployed at scale in large plants.

However, the largest issue with sewer-based sanitation is that it was designed to flush solids given an average water use of 150L per capita. Given the increasing water scarcity due to climate change, the objectives of reducing domestic water use may prove incompatible with well-functioning sewers: insufficient water flow will prevent faeces and other solids from being transported to the WWTP, thus leading to clogging and sanitary issues. Reducing domestic water use at scale without clogging sewer networks will probably require to avoid sending these solids to conventional sewers and thus to use alternative sanitation systems for human excreta.

## 3.2 Source separation as a complementary paradigm

Because of the limitations of sewer-based sanitation in the face of global changes, the second part of the two academic papers focused on the potential of source separation as a complementary paradigm to
sewers. Though often unknown to non-specialists, source separation is far from being a new technique
and has been implemented at various scales throughout the world. This section will focus on large-
scale implementations and a subset of processes suitable for industrial-scale applications.

Large-scale implementations of source-separating toilets usually rely on a few specific types of toilets and urinals that work with little to no flush to limit the dilution of the nutrients. Suitable toilets include urine-diverting flush-based toilets and urine-diverting or combined vacuum toilets. Waterless urinals include male, female and mixed urinals, suitable for buildings with shared sanitation.

In the case of source-separated urine, the resource can be transformed into a wide range of urine-
based fertilizers that include nitrified and concentrated urine (associated to a market authorization for
the product called “aurin”), solid compounds from precipitated struvite or dessicated urine, various
liquid fertilizers (ammonium sulfate or nitrate, etc). Urine-diversion is currently the dominant form of
large-scale source-separation in France, Switzerland and the US, with multiple installation at the scale
of single buildings, such as the headquarters of the European Space Agency in Paris, the EAWAG in
Zürich or the PAE Living building in Portland. In France, an entire neighbourhood of Paris (Saint-
Vincent-de-Paul) will be equipped with urine-diverting flush toilets and will produce enough fertilizer
to cover the consumption of all the cities green spaces and horticultural production.

In other European countries, recent neighbourhood-scale implementation have focused on combined
vacuum toilets and send the blackwater (combined urine and feces) to an anaerobic digestion plant
where they are generally co-treated with green or biowaste. Such neighbourhoods include Sneek, in
the Netherlands, Jenfelder-Au in Germany, and Oceanhamnen in Sweden. For the latter, both struvite
precipitation and pelletisation as well as N stripping to produce ammonium sulfate are implemented
on the liquid phase of the digestate (Abuzir et al. 2025).

In terms of environmental impacts, source separation provides multiple benefits compared to sewer-
based sanitation. The first obvious one is water savings: flushes represent roughly a quarter of domestic
water use in Western countries and can significantly contribute to depletion of the water resources in
very populated areas — in the region of Paris, for instance, 200 million cubic meters are flushed out
each year whereas irrigation only represents 30,000 cubic meters.

By isolating urine and faeces from greywater and the sewers, source separation also limits water
pollution by nutrient (eutrophication), which is a significant issue in most Western countries. Since
WWTPs no longer need to handle this pollution, this also contributes to savings in terms of energy
and reactive consumption.

In fact, the savings from the avoided treatment at the WWTP, combined with the avoided fertil-
izer production are significant enough that most excreta-based fertilizer also lead to lower energy
consumption and GHG emissions over their life-cycle (Martin et al. 2023).

3.3 The scale and stakes of human excreta as a fertilizer deposit

The reasons why people are starting to implement these new solutions for circular sanitation are thus numerous and can vary significantly from one region to another, from protection of the water quality, to a reduction of water usage, as described above. But many of them also understand the long-term potential of excreta as a fertilizer.

Globally the amount of nitrogen and phosphorus in human excreta is equivalent to what is currently
recycled from cattle in agriculture respectively 25 MtN vs 37 MtN<fnref n="4" />. Even in France, where there is a
higher than average proportion of industrial farming (where most of the effluent is recovered), human
excreta still accounts for as much as half of the total N recycled through cattle effluent — 300 ktN
vs 640 ktN<fnref n="5" />. As for cattle effluent, human excreta (either urine or mixed urine and faeces) contains
all the necessary compounds for the plants and is well suited for most plants (with the exception of
fruits and leafy vegetables) because it has the right nitrogen to phosphorus ratio and it contains all
the other oligoelements that are needed by the plants.

<footnote n="4">4. Respective sources: personal calculation and extrapolation from Sutton and UNEP (2013) and FAOStat</footnote>

<footnote n="5">5. Respective sources: personal calculation from Agence nationale de sécurité sanitaire de l'alimentation, de l'environnement et du travail (2017) and UNIFA</footnote>
Besides the conventional market, these excreta-based fertilizers also have significant potential for the organic market, which is significantly limited in terms of nitrogen sources. While only struvite is currently explicitly authorized at the moment, the possibility of the inclusion of other recovered fertilizers in the organic whitelist is currently being discussed.

a

Diet

<figure><img src="image_5.png" /></figure>

b

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

Figure 4: Phosphorus Use Efficiency (PUE) of plant- and animal-based products for human consumption. a) The PUE of plant-based products is much higher than that of animal-based products which incurs both losses from crops in the field and even more significant losses due to the animal metabolism. b) The contribution of P in excretions to food supply can be improved by prioritizing the fertilization of crops for human consumption. For instance, with a diet composed of 30% plant-based and 70% animal-based products, a crop PUE of 100% and an animal-products PUE of 10%, the resulting average PUE is 14%. The use of excreta to grow crops for human consumption thus covers a larger fraction of the P in food supply than indiscriminate use including animal feed. Original figure from Starck et al. (2024).

Of course, recycling urine and faeces will not automatically render France autonomous in term of food production: its efficiency will vary strongly depending on how these fertilizers are used. Thus, if used naively, excreta-based fertilizers would only cover around 10% of the domestic consumption in Western countries. This comes from the fact that current diets contain high fractions of animal proteins, which have a low nutrient use efficiency (respectively abbreviated NUE and PUE for N and P; for instance, the French diet has a PUE of 11%). This means that recovering urine and faeces to grow crops to feed the animals eaten by humans cannot cover a significant fraction of the total consumption from French citizens. However, prioritizing human excreta to fertilize crops dedicated to direct human consumption would drastically increase French autonomy, covering 34% of the domestic P consumption, simply because the PUE of crops is roughly 100%. See for an illustration. A study by Billen et al. (2021) showed that, provided a twofold decrease in animal-based protein consumption (replaced by plant-based proteins from pulses), combining agroecological practices with the recycling of human excreta could enable the EU to feed its entire population without relying on nutrient or feed imports.

# 4 Conclusion

To achieve sustainable food systems, recovering the nutrients present in human urine and faeces is a necessity, both from a simple mathematical perspective (phosphorus and potassium being finite abiotic resources) and from a geostrategic standpoint, in an increasingly unstable context where many countries cannot guaranty the security of their energy and fertilizer supply chains.
In parallel, flush-and-sewer-based sanitation is also showing its limits to limit water pollution but most of all in the face of climate change and increasing water scarcity. While there is still space for innovation and improvement in the processes implemented at wastewater treatment plants to reduce pollution and increase nutrient recovery, alternative paradigm are necessary to ensure the resilience of sanitation systems in the long run.

Sewer-based sanitation currently seems structurally unable to achieve recovery rates that significantly exceed 50% of the nutrients entering the sewers. Reaching higher recycling rates to secure these local and renewable nutrient deposits thus requires the progressive implementation of new sanitation paradigms. Such systems already exist at the neighbourhood-scale in several European countries, showing that these new fertilizer supply chains are possible. However, these solutions still require investment and political support to make sure that the technologies, supply chains and economic models are ready to scale up and provide locally-sourced fertilizers when traditional supply chains start failing due to natural or artificial scarcity.

## References

Abuzir, Saleh, Lisa Gren, Marta Domini, Giorgio Bertanza, Åsa Davidsson, and Hamse kjerstadius. 2025. "Performance of a Full-Scale Anaerobic UASB Digester Treating Blackwater from an Urban City District in Helsingborg, Sweden." *Cleaner Water* (May 12, 2025): 100082. https://doi.org/10.1016/j.clwat.2025.100082.

Agence nationale de sécurité sanitaire de l'alimentation, de l'environnement et du travail, ed. 2017. *Étude individuelle nationale des consommations alimentaires: avis de l'ANSES, rapport d'expertise collective*. Éd. scientifique. Maisons-Alfort: ANSES, Agence nationale de sécurité sanitaire, alimentation, environnement, travail.

Billen, Gilles, Eduardo Aguilera, Rasmus Einarsson, Josette Garnier, Simone Gingrich, Bruna Grizzetti, Luis Lassaletta, Julia Le Noë, and Alberto Sanz-Cobena. 2021. "Reshaping the European Agro-Food System and Closing Its Nitrogen Cycle: The Potential of Combining Dietary Change, Agroecology, and Circularity." *One Earth* 4, no. 6 (June 18, 2021): 839–850. https://doi.org/10.1016/j.oneear.2021.05.008.

Childers, Daniel L., Jessica Corman, Mark Edwards, and James J. Elser. 2011. "Sustainability Challenges of Phosphorus and Food: Solutions from Closing the Human Phosphorus Cycle." *BioScience* 61, no. 2 (February 1, 2011): 117–124. https://doi.org/10.1525/bio.2011.61.2.6.

Gonzalez-Salgado, Irene, Christelle Guigui, and Mathieu Spérandio. 2022. "Transmembrane Chemical Absorption Technology for Ammonia Recovery from Wastewater: A Critical Review." *Chemical Engineering Journal* 444 (September 15, 2022): 136491. https://doi.org/10.1016/j.cej.2022.136491.

Al-Hazmi, Hussein E., Ali Mohammadi, Aleksander Hejna, Joanna Majtacz, Amin Esmaeili, Sajjad Habibzadeh, Mohammad Reza Saeb, Michael Badawi, Eder C. Lima, and Jacek Mkinia. 2023. "Wastewater Reuse in Agriculture: Prospects and Challenges." *Environmental Research* 236 (Pt 1 2023): 116711. https://doi.org/10.1016/j.envres.2023.116711.

Martin, Tristan M. P., Joël Aubin, Enola Gilles, Julie Auberger, Fabien Esculier, Florent Levavasseur, Jennifer McConville, and Sabine Houot. 2023. "Comparative Study of Environmental Impacts Related to Wheat Production with Human-Urine Based Fertilizers versus Mineral Fertilizers." *Journal of Cleaner Production* 382 (January 1, 2023): 135123. https://doi.org/10.1016/j.jclepro.2022.135123.

Mihelcic, James R., Lauren M. Fry, and Ryan Shaw. 2011. "Global Potential of Phosphorus Recovery from Human Urine and Feces." *Chemosphere*, The Phosphorus Cycle, 84, no. 6 (August 1, 2011): 832–839. https://doi.org/10.1016/j.chemosphere.2011.02.046.
Nanda, Madhuri, Arun Kansal, and Dana Cordell. 2020. "Managing Agricultural Vulnerability to Phosphorus Scarcity through Bottom-up Assessment of Regional-Scale Opportunities." *Agricultural Systems* 184 (September 1, 2020): 102910. https://doi.org/10.1016/j.agsy.2020.102910.

Scholz, Roland W., and Friedrich-Wilhelm Wellmer. 2013. "Approaching a Dynamic View on the Availability of Mineral Resources: What We May Learn from the Case of Phosphorus?" *Global Environmental Change* 23, no. 1 (February 1, 2013): 11–27. https://doi.org/10.1016/j.gloenvcha.2012.10.013.

Starck, Thomas, and Fabien Esculier. 2025. "Fertilizing Potential of Nitrogen in Human Excreta across Diverse Countries and Food Systems." *Environmental Research Letters*, https://doi.org/10.1088/1748-9326/ae0cdf.

Starck, Thomas, Tanguy Fardet, and Fabien Esculier. 2023. "Fate of Nitrogen in French Human Excreta: Current Waste and Agronomic Opportunities for the Future." *Science of The Total Environment* 912:168978. https://doi.org/10.1016/j.scitotenv.2023.168978.

Starck, Thomas, Tanguy Fardet, and Fabien Esculier. 2024. "Phosphorus Recycling from Human Excreta in French Agroecosystems and Potential for Food Self-Sufficiency." *Nutrient Cycling in Agroecosystems* (June 28, 2024). https://doi.org/10.1007/s10705-024-10367-4.

Sutton, Mark A., and UNEP, eds. 2013. *Our Nutrient World: The Challenge to Produce More Food and Energy with Less Pollution* ; [Global Overview on Nutrient Management]. Edinburgh: Centre for Ecology & Hydrology.

Walsh, Michael, Gerhard Schenk, and Susanne Schmidt. 2023. "Realising the Circular Phosphorus Economy Delivers for Sustainable Development Goals." *npj Sustainable Agriculture* 1, no. 1 (October 4, 2023): 2. https://doi.org/10.1038/s44264-023-00002-0.

Watson, Ingrid, Peter Van Straaten, Tobias Katz, and Louw Botha. 2014. "Mining and Concentration: What Mining to What Costs and Benefits?" In *Sustainable Phosphorus Management*, edited by Roland W. Scholz, Amit H. Roy, Fridolin S. Brand, Deborah T. Hellums, and Andrea E. Ulrich, 153–182. Dordrecht: Springer Netherlands. https://doi.org/10.1007/978-94-007-7250-2_3.