Anthroponics: nutrients recovery from human urine and their reuse as fertiliser to grow spearmint (Mentha spicata L.)

1 Introduction

Human urine contains a high concentration of essential nutrients vital for plant growth and biomass production (Larsen et al. 2009; Bonvin et al., 2015; McHunu et al., 2018; Viskari et al., 2018; Pandorf et al., 2019).

Conventional wastewater treatment processes, which are often dominated by the costs and energy demands of converting and eliminating nitrogen (N) and phosphorus (P), contribute significantly to overall environmental burden (Larsen et al., 2016). Simultaneously, the production of mineral fertilisers relies on finite and depleting natural resources, such as rock phosphates for P. Nitrogen fixation in industrial fertiliser production is also determined by the cost and availability of fossil fuels. In response to these challenges, the concept of source separation in urban wastewater management has emerged as a compelling, sustainable, and resource-efficient alternative to traditional end-of-pipe systems (Bisinella de Faria et al., 2015). In recent years, the conversion of urine into fertiliser has gained momentum through various innovative processes and technologies. Human urine contains approximately 0.8–2 g of P and about 3–7 g of N in one L as excreted by humans (Pradhan et al., 2017). Recovering these nutrients from urine is often calculated to be energetically more efficient than their removal from wastewater and subsequent new production from raw resources (Maurer et al., 2003; Udert and Wächter, 2012). Also, nutrient recovery from human urine plays a key role in advancing circular economy principles, particularly by closing the nutrient loop through urban and suburban agriculture, and by reducing waste and environmental pollution via ornamental plant cultivation and green wall systems (Jacquin et al., 2018; Volpin et al., 2020; Sohn et al., 2024).

To move beyond the direct application of raw urine, which is limited by issues such as high-water content, a pungent smell, N losses due to ammonia volatilisation, and the presence of micro-pollutants, new technologies have been developed to transform source-separated urine into safe, transportable, and effective fertiliser products. In Australia, our research group at the University of Technology Sydney proved through the UrVal technology that human urine can be transformed into a viable plant fertiliser, known as UrVal fertiliser (Volpin et al., 2020). The Urval technology is a sustainable fertiliser production process that can be summarised in four key steps: 1) Source separation and hydrolysis of urine: Human urine is collected separately from wastewater and undergoes hydrolysis to stabilise nutrients and prepare the feed stream for treatment; 2) Aerobic membrane bioreactor treatment: The hydrolysed urine is treated in an aerobic ultrafiltration membrane bioreactor coupled with powdered activated carbon (PAC), which removes organic contaminants and improves product quality; 3) Permeate collection: the treated permeate, containing valuable plant nutrients, is collected for further processing; 4) Distillation and concentration: the permeate is concentrated through distillation or membrane distillation to produce Urval, a nutrient-rich liquid fertiliser suitable for agricultural, horticultural, hydroponic, and urban greening applications.

Spearmint is an established specialty crop in Australia, primarily valued for its essential oil, fresh and dried leaf products, and extracts used across food, beverage, personal care, pharmaceutical, and wellness sectors. Within Australia, the spearmint market sits largely within the broader mint essential oils and essential oils markets, which have experienced consistent growth driven by rising demand for natural, plant-based products (Department of Primary Industries, 2007).

This research study aimed to evaluate the effectiveness of UrVal fertiliser on spearmint plant growth in comparison to a commercial mineral fertiliser, and to identify the most suitable potting media for optimal plant development.

2 Materials and methods

2.1 Plant material and experiment timeline

Spearmint (Mentha spicata L.) was selected for this study because it was not tested before with urine derived fertilisers, it has a fast growth rate and it’s tolerant to moderate salinity (Chrysargyrisa et al., 2019). Seedlings were purchased from a local grower and delivered to the Royal Botanic Garden (RBG) nursery in Sydney (33.87° S and 151.22° E, Elevation 15 m). The seedlings were subjected to biosecurity checks to ensure they are free of pathogens and pests as per RBG nursery hygiene protocols. Established spearmint seedlings were pruned to 100 mm height (Supplementary Figure S1) and potted up into 200 mm black plastic pots on the 27^th of July 2023 and were allowed to grow up for 6 weeks under experimental conditions (Figure 1; Supplementary Figure S2). Data were recorded on the 4^th, 11^th, 18^th, 25^th of August and the 1^st and 8^th of September 2023.

The minimum and maximum temperatures and daily rainfall data from the Royal Botanic Garden Sydney weather station are presented in Table 1.

TABLE 1. Climate data for the period of the experiment from SYDNEY BOTANIC GARDENS weather station (Station Number: 066,006; State: NSW; Opened: 1885; Status: Open; Latitude: 33.87°S; Longitude: 151.22°E; Elevation: 15 m) (Bureau of Meteorology, 2023).

2.2 Experimental design and treatments

The potted experiment was carried out for 6 weeks at RBG Sydney Nursery in full sun in the nursery top yard and pots were placed on a raised mesh bench (720 mm from ground). The experiment was laid out in a completely randomised design with 5 replications per treatment combination (Supplementary Figure S3). Three different potting media treatments were used for the experiment (Figure 1):

• Vermiculite (Grade 4 Coarse, Ausperl PTY LTD, Australia),
• Coco chips (Growrite, Coir Cocochip), and
• Peat (Growrite, Cocopeat)/Perlite (P400, Ausperl PTY LTD, Australia) at a ratio of 1:1.

Two fertiliser treatments, Urval and Cal-Mg Grower (ICL fertilisers) (Table 2), were applied as diluted nutrient solutions drench at a rate equivalent to 200 ppm of nitrogen (N) per pot, once per week. The pH of the nutrient solutions was recorded between 6.2 and 6.4. The selection of 200 ppm was based on previous unpublished trial and recommendations by Chrysargyrisa et al., 2017. Pots were watered as required based on rainfall events and the standard nursery watering program.

TABLE 2. Elemental composition of fertilisers used in this study.

2.3 Data collection and analysis

On weekly basis, data was collected on plant height, and total number of stalks for the period of the experiment. At the end of the experiment, stems and leaves of each plant were weighed to assess the fresh biomass (Supplementary Figure S4). To measure the dry biomass, leaves and stems samples were placed in paper bags then kept at 100 ̊C for 24 h in a Memmert GmbH oven (Model 400, Germany) and then weighted again (Meskelu et al., 2014). Data was recorded manually on data sheets then entered to excel and analysed using Origin pro® graphing and analysis software.

A regression analysis was performed to evaluate the effects of potting media and fertiliser type on plant height and the total number of stalks, and to determine the strength and direction of the linear relationships between these variables and plant growth responses overtime. Linear regression models were fitted to assess trends in plant height and stalk production across the different treatment combinations, with coefficients of determination (R²) used to evaluate the goodness-of-fit of the models. A two-way analysis of variance (ANOVA) was performed to evaluate the effects of potting media and fertiliser type, as well as their interaction, on biomass production expressed as fresh weight and dry weight. The two-way ANOVA assessed the main effects of potting media and fertiliser type and determined whether the response to fertiliser varied depending on the potting medium used. Statistical significance was evaluated at the 95% confidence level (p < 0.05), and further comparisons among treatment means were conducted using Tukey post hoc test.

3 Results and discussion

3.1 Plant height

Spearmint plant height increased progressively over the six-week growing period across the various fertiliser and potting media treatment combinations (Figure 2). However, the magnitude and rate of growth varied clearly among treatments. CMV (Cal-Mg Grower with Vermiculite) consistently produced the tallest spearmint plants throughout the 6-week growth period increasing from approximately 16.3 cm at week 1–30.8 cm by week 6. This was followed by CMPP (Cal-Mg Grower with Peat/Perlite) with plant height increasing from about 16.0 cm to 24.6 cm over 6 weeks and CMC (Cal-Mg Grower with Coco chips) reaching nearly 20.0 cm by week 6, compared with an initial height of about 14.1 cm. The results indicated that the commercial fertiliser was generally more effective in promoting vertical growth compared to Urval-based treatments. In contrast, UV and UC treatments exhibited more moderate increases in plant height. Plants under UV grew from approximately 12.5 cm at week 1–17.8 cm at week 6, while UC demonstrated a comparable trend, increasing from 11.7 cm to 17.8 cm over the same period. The UPP treatment resulted in the lowest overall plant height, with growth from about 12.2 cm at week 1–16.4 cm at week 6. Regression lines for all treatments showed positive linear trends, with relatively narrow 95% confidence bands, indicating consistent growth patterns and low variability among replicates within treatments. The relatively narrow confidence intervals further suggest reliable treatment effects and low experimental variability.

The results demonstrated that treatment composition had a pronounced effect on spearmint vegetative growth, as reflected by differences in plant height. CMV consistently promoted the greatest height increase, suggesting a balanced nutrition. The CMPP and CMC treatments also enhanced plant height compared to UV, UC, and UPP. This suggests that the commercial fertiliser offered complete nutrient provision to the plants as compared to Urval which lacks essential micronutrients. This was demonstrated by UV, UC, and UPP treatments which showed slower growth rates, resulting in shorter plants by week 6. While steady increases were still observed, the lower overall height indicates that these treatments may not have supplied sufficient nutrients to optimise vegetative growth.

Plant growth parameters are widely recognised as sensitive indicators of plant stress and toxicity and have been extensively used to assess the effects of adverse environmental conditions on plant performance. Previous studies have evaluated toxicity responses using measurements such as plant height, biomass accumulation, leaf area, root growth, and chlorophyll content, with reductions in these parameters typically indicating growth inhibition caused by salinity, nutrient imbalances, or heavy metal stress. For example, plant height has been used as a key indicator of crop growth and plant health throughout the growing season (Jamil et al., 2022), while reductions in plant height, leaf area, and stem diameter have been associated with salinity-induced stress in tomato (Madugundu et al., 2023). The heavy metal toxicity study by Mohamed et al. (2025) reported decreases in root length, shoot growth, biomass, and chlorophyll content as evidence of phytotoxic effects. Similar findings were reported by Elik and Gul, 2025, who evaluated the effects of lead (Pb) and cadmium (Cd) toxicity on maize growth. Their study demonstrated that plant height, biomass production, stem diameter, leaf number, and leaf area declined significantly with increasing heavy metal concentrations. In the present study, the generally linear growth trends observed across all treatments indicate that spearmint maintained normal growth over time, suggesting the absence of growth suppression or toxicity effects.

3.2 Number of stalks

The total number of stalks per spearmint plant increased steadily over the six-week experimental period for all treatments (Figure 3), although clear differences were observed in both the rate of stalk initiation and final stalk number among treatments. The CMV treatment produced the highest number of stalks throughout the study, increasing from approximately 14.4 stalks at week 1–31.6 stalks by week 6. Similarly, CMPP stalk number have increased from about 11.2 to 27.6 stalks over the 6 weeks period. The CMC treatment also resulted in an increase in stalk production, reaching approximately 23.6 stalks at week 6, compared with 7.2 stalks at week 1. The CMV treatment showed a positive effect on stalk initiation producing more stalks than all other treatments throughout the experiment. This enhanced tillering response likely reflects improved nutrient availability, particularly nitrogen, which is known to promote axillary bud development and shoot branching in mint species. The rapid and sustained increase suggests that CMV supported continuous vegetative growth rather than early growth saturation. The CMPP and CMC treatments also increased stalk number, although to a lesser extent than CMV. These treatments likely provided adequate nutrient supply to stimulate branching, but possibly with slower nutrient release or lower overall availability compared to CMV. In contrast, UV and UC treatments exhibited moderate stalk proliferation. Under UV, the number of stalks increased from around 4.2 to 16.6, while UC increased from approximately 3.4–10.4 stalks between week 1 and week 6. The UPP treatment consistently produced the lowest number of stalks, increasing from about 2.6 stalks at week 1–7.8 stalks at week 6. The consistently low stalk numbers under UPP suggest insufficient or imbalanced nutrient provision, which can restrict lateral growth and reduce canopy density. While UV and UC showed gradual increases over time, their final stalk numbers remained below those of CMV, CMPP, and CMC.

The observed increase in stalk number across all treatments indicates that spearmint responded positively to all nutrient regimes; however, the marked differences among treatments highlight the importance of treatment composition in stimulating shoot proliferation. Stalk number is a critical yield component in spearmint, as it directly influences biomass production and harvest potential. The linear growth trends and narrow confidence intervals across treatments suggest stable growth conditions and reliable treatment effects. Importantly, no decline or plateau in stalk number was observed, indicating that none of the treatments induced stress or excessive vegetative suppression during the experimental period. Overall, these results demonstrate that CMV is the most effective treatment for enhancing stalk production in spearmint, followed by CMPP and CMC, while UPP, UC, and UV were less effective. Given the strong relationship between stalk number and harvestable biomass, treatments that maximise shoot proliferation are likely to offer clear agronomic and commercial advantages in spearmint cultivation.

3.3 Fresh and dry biomass

The final biomass measurements provided a comprehensive view of treatment efficacy (Table 3). A two-way ANOVA showed that both fertiliser type and potting media had significant effects on the fresh and dry biomass production of spearmint, while the interaction between these two factors was not significant (fresh biomass: F = 178.23, p < 0.0001 for fertiliser; F = 16.28, p < 0.0001 for potting media; interaction p = 0.078; dry biomass: F = 175.99, p < 0.0001 for fertiliser; F = 16.83, p < 0.0001 for potting media; interaction p = 0.104) (Supplementary Figures S5, S6). Overall, the CM fertiliser produced significantly greater fresh and dry biomass (86.37 g and 17.81 g, respectively) than U (40.05 g and 7.95 g, respectively). Among the potting media, V resulted in the highest fresh biomass (76.88 g) and dry biomass (15.92 g), significantly outperforming both PP and C, which did not differ significantly from each other. Although the fertiliser × potting media interaction was not statistically significant, the treatment combination CMV produced the greatest fresh and dry biomass, reaching 102.61 g fresh weight and 21.63 g dry weight, followed by CMPP (85.44 g fresh weight and 16.90 g dry weight) and CMC (71.07 g fresh weight and 14.91 g dry weight). In contrast, all U treatments produced lower biomass, with fresh weights ranging from 32.59 to 51.14 g and dry weights from 6.30 to 10.21 g. These results indicate that CM fertiliser was more effective than U in promoting spearmint growth and that V was the most suitable potting medium for maximizing biomass production.

TABLE 3. Fresh and dry biomass of Spearmint in response to fertiliser and potting media treatments.

Means in the same column and level (1, and 2) followed by the same letter are not significantly different (P < 0.05), the interaction fertiliser and potting media was found to be insignificant.

The performance gap between urine-based fertiliser and a conventional mineral fertiliser (Yara CAN N27 + Mg) was also reported by Viskari et al. (2018), who observed lower total grain yields for the barley variety Wolmari when fertilised with urine. In their study, the urine was collected during summer 2015, stored over winter in 1 m³ IBC containers, and applied in spring 2016. The urine was characterised by a strong odour, necessitating deep soil injection during application to avoid N losses, a high pH of approximately 9, and relatively low total nitrogen (N) content. In contrast, Urval offers several advantages, including a pleasant odour, a pH of approximately 6.2 after dilution, and a substantially higher total N concentration in its concentrated form. Treadwell et al. (2011) reported that nutrient management significantly influenced spearmint productivity and that the highest yields were achieved using conventional hydroponic nutrient solutions or poultry-litter-based fertilisers. Their results showed higher spearmint yield compared to ours, but the experimental conditions were significantly longer (up to 107 days after transplanting). On the other hand, Akpan-Idiok et al. (2012) reported that human urine significantly increased nutrient uptake, growth, and yield of okra compared with an unfertilised control, with the highest urine application rate (20,000 L ha^-1) producing superior growth and yield compared with NPK 15:15:15 fertiliser. Furthermore, the 15,000 L ha^-1 urine treatment produced growth and yield responses comparable to the commercial fertiliser treatment, indicating that urine can effectively supply crop nutrient requirements. Similarly, Sridevi et al. (2016) found that human urine applied to French bean and maize crops produced yields that were comparable to or greater than those obtained with chemical fertilisers. Treatments combining human urine with farmyard manure achieved the highest crop yields, while urine alone generally outperformed urea-based fertilisation. The authors attributed these responses to the readily available nitrogen and other nutrients present in urine, which supported improved crop growth and nutrient uptake throughout the growing season.

Our findings are consistent with those reported by Sohn et al. (2024), who evaluated the performance of Urval fertiliser across a range of horticultural species including parsley, rhipsalis, coriander, and basil. Similar to the present study, commercial fertilisers generally produced greater biomass and vegetative growth than Urval; however, plants fertilised with Urval maintained healthy growth and achieved substantial biomass production without visible signs of nutrient toxicity or growth suppression. In parsley, Urval produced approximately half the biomass of the commercial hydroponic fertiliser, but stem numbers remained comparable among treatments, indicating that the urine-derived fertiliser could support normal plant development despite lower overall productivity. Likewise, rhipsalis grown with Urval showed growth responses, root-to-shoot ratios, and tissue nutrient concentrations that were largely comparable to those obtained with a commercial Ca-Mg fertiliser, suggesting that some plant species can effectively utilise nutrients supplied by Urval. In coriander and basil, Urval achieved biomass yields that were approximately 65%–75% of those obtained with commercial fertilisers, again demonstrating its potential as an alternative nutrient source. These observations support the results of the current spearmint study, where Urval fertiliser promoted continuous increases in plant height, stalk production, and biomass throughout the six-week experiment, although growth remained lower than that achieved with the commercial Ca-Mg fertiliser. The superior performance of the commercial fertiliser across both studies is likely attributable to its more balanced nutrient profile and greater concentrations of micronutrients. Sohn et al. (2024) tissue analysis results revealed that despite substantially lower phosphorus, potassium, calcium, magnesium, and micronutrient concentrations in Urval formulations, plants were still able to maintain acceptable nutrient status and growth. Nevertheless, the highest yields were consistently associated with fertilisers containing adequate calcium, magnesium, iron, and trace elements. Similarly, the Urval fertiliser used in the present study contained markedly lower concentrations of calcium, magnesium, iron, boron, and manganese than the commercial fertiliser, which likely contributed to the reduced plant height, stalk production, and biomass observed in the UrVal treatments. The overall consistency between these studies suggests that the principal limitation of current urine-derived fertiliser formulations is not the supply of nitrogen, but rather the availability of certain macro- and micronutrients required to maximise plant productivity. An important finding emerging from both studies is that plant species and growing conditions strongly influence the performance of urine-derived fertilisers. Sohn et al. (2024) demonstrated that rhipsalis responded particularly well to Urval, while basil exhibited a larger productivity gap relative to commercial fertilisers. Similarly, the present study showed that spearmint performance was highly dependent on the growing medium, with vermiculite significantly enhancing biomass production under both fertiliser regimes. The superior performance of vermiculite may be related to its high water-holding capacity, aeration characteristics, and ability to retain and gradually release nutrients, thereby improving nutrient availability from Urval. These findings highlight that optimisation of both fertiliser formulation and cultivation practices will be essential for maximising the effectiveness of anthroponic production systems. Despite lower productivity than commercial fertilisers, Urval consistently supported healthy plant growth across multiple crops, reinforcing its potential as a sustainable fertiliser within circular economy frameworks aimed at recovering and reusing nutrients from source-separated human urine.

The present study demonstrated that Urval could support plant growth and yield, but a key limitation of the current study is that it was conducted under a specific set of environmental and management conditions and therefore may not fully represent the performance of Urval across different crops, soils, climates, and production systems. A review of the available literature was recently published by Mustaffa et al. (2026), and the authors have shown that the agronomic effectiveness of urine-derived fertilisers varied considerably depending on nutrient recovery technology, application rates, fertiliser formulation, soil properties, salinity levels, and crop nutrient demand. In several field trials, urine-based fertilisers produced yields comparable to mineral fertilisers when applied at equivalent nitrogen rates (Amoah et al., 2017; Pandorf et al., 2019), whereas in other cases yield reductions occurred because nutrient release patterns (Martin et al., 2022) did not adequately match crop requirements or because salt accumulation affected plant growth (Lin et al., 2024). One promising pathway for improving performance is the development of enhanced Urval fertiliser formulations. Recent studies have demonstrated that combining urine-derived nutrients with biochar (Otieno et al., 2021), zeolites (Beler-Baykal et al., 2011), controlled-release matrices (Valle et al., 2022), or other nutrient carriers can improve nutrient retention, mitigate salinity effects, and enhance crop productivity. Safety remains another important consideration for large-scale implementation. Studies have shown that storage, stabilisation (Lahr et al., 2016), membrane treatment (Almuntashiri et al., 2021), adsorption, and activated carbon filtration (Li et al., 2018) can substantially reduce contaminant concentrations prior to agricultural reuse. Furthermore, several risk assessments have concluded that the uptake of pharmaceutical residues into edible crop tissues is generally very low and often comparable to concentrations associated with reclaimed wastewater irrigation (Rodriguez et al., 2025). Public acceptance may represent an equally significant barrier to adoption. Despite growing interest in circular economy approaches, consumers and farmers frequently express concerns regarding hygiene, odour, food safety, and the use of products derived from human waste. Future research should focus on understanding the mechanisms underlying the reduced yield observed in this study. Priorities include evaluating different application rates, testing supplementary nutrient additions, assessing nutrient release dynamics, examining salinity management strategies, and conducting multi-season field trials across different potting media types and climatic conditions. Such investigations will provide a stronger foundation for developing best-practice recommendations and optimising Urval fertiliser products for commercial agriculture. Overall, while Urval represents a highly promising circular fertiliser that contributes to nutrient recovery and resource conservation, the substantial yield reduction observed in the current study indicates that further optimisation is necessary before it can be considered a direct replacement for conventional mineral fertilisers.

4 Conclusion

This experiment has shown that spearmint can be grown using Urval fertiliser without showing any signs of toxicity or decline in growth over 6 weeks period. It was clear from the results that Urval alone cannot outperform a complete commercial mineral fertiliser due to the lack of several micronutrients. Future experiments should attempt to optimise the growing process by adding chelated micronutrients to Urval from the start of the experiment, to check if this will affect the overall growth of plants. Given the environmental and sustainability advantages of urine-derived fertilisers, further trials are warranted. Future studies should explore: Process optimisation of Urval production to enhance nutrient availability, mixing Urval with organic matter, alternative potting media that may better complement Urval’s nutrient profile, and long-term trials to assess cumulative effects on plant health and yield. These steps could help unlock the full potential of Urval as a sustainable fertiliser for herb production, contributing to both ecological and economic benefits.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fenvs.2026.1872561/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: Kai M. Udert, Swiss Federal Institute of Aquatic Science and Technology, Switzerland

Reviewed by: Franziska Häfner, Agroscope, Switzerland

Ajay Jaishi, Kathmandu University School of Science, Nepal

Radjali Amin, Yogyakarta Institute of Technology, Indonesia