ABSTRACT
Human urine, which is high in nutrients, acts as a resource as well as a contaminant. Indiscriminate urine discharge causes environmental pollution and wastes resources. To elucidate the research status and developmental trajectory of source-separated urine (SSU) treatment and recovery, this study was based on the Web of Science Core Collection (WOSCC) database and used the bibliometric software VOSviewer and CiteSpace to conduct a comprehensive and in-depth bibliometric analysis of the related literature in this field. The findings revealed a general upward trend in SSU treatment and recovery from 2000 to 2023. The compendium of 894 scholarly articles predominantly focused on the disciplines of Environmental Sciences, Environmental Engineering, and Water Resources. China and the USA emerged as the foremost contributors. Keyword co-occurrence mapping, clustering, and burst analysis have shown that the recovery of nitrogen and phosphorus from urine is currently the main focus, with future prospects leaning toward the retrieval of biochemicals and chemical energy. This study systematically categorizes and compares the developmental status, current advancements, and research progress in this field. The findings of this study provide a valuable reference for understanding developmental pathways in this field of research.
HIGHLIGHTS
A total of 894 publications pertinent to source-separated urine (SSU) treatment and recovery was analyzed.
Bibliometric results were visualized using VOSViewer, and Citespace.
The current treatment and resource recovery technologies applied in SSU were reviewed.
The research focus and development status of SSU treatment and recovery were summarized.
This provides prospective avenues and suggestions for SSU treatment and recovery.
INTRODUCTION
The global population is rapidly expanding, leading to an increasing demand for food. To ensure a stable global food supply, there is a growing need for chemical fertilizers. Excessive use of chemical fertilizers not only reduces the quality of agricultural products but also pollutes the soil, groundwater, surface water, and air (Zhang et al. 2023b). As a result, to ensure food security and mitigate environmental damage caused by excessive fertilizer application, it is crucial to limit on-farm chemical fertilizer application and to explore nitrogen and phosphorus substitutes or supplementations for crops. Human urine is a rich source of nitrogen, phosphorus, potassium, and other nutrients (Lahr et al. 2016) making it a natural fertilizer. Adults produce approximately 1,270 g of urine daily, which contains approximately 15 g of nutrients (Liu et al. 2023). Global nitrogen emissions from human urine total 30 million tons annually and are expected to increase as the population grows. According to projections, phosphorus emissions from urine will be 1.3 times higher in 2050 than those in 2009. Urine discharged directly into sewage networks as a pollutant wastes resources and places a burden on sewage treatment plants (Mihelcic et al. 2011). Despite accounting for only 1% of the total wastewater volume, urine exhibits a remarkably concentrated presence of vital nutrients, encompassing approximately 80% of nitrogen, 70% of potassium, and up to 50% of the total phosphorus in wastewater (Huang et al. 2016). Scientists have proposed ‘ecological sanitation,’ which focuses on recovering and concentrating phytonutrients in urine by source separation of human excreta. Human urine contains urea (75–90% nitrogen), (95–100% phosphorus), and potassium ions, which are easily absorbed and utilized by plants, as the main ‘concentrate’ of nutrients in urine and feces (Volpin et al. 2019b). These vital elements substantially contribute to plant growth and can be utilized in the production of organic fertilizers. Additionally, the desiccated solid product derived from urine can function as an effective slow-release fertilizer in agricultural applications. Separating urine not only reduces the strain on tailwater-receiving rivers and municipal wastewater treatment plants but also recovers nutrients such as potassium, phosphate, and nitrogen, which can replace nearly 13% of global agricultural fertilizer requirements (Zuo et al. 2023). Crucially, separating urine into water and organic matter generates reusable water resources which are applicable for agricultural irrigation in regions characterized by aridity.
Currently, there has been a gradual increase in the number of publications in the field of source-separated urine (SSU) treatment and recovery, as numerous researchers have reviewed and summarized the research in this field. Masrura et al. (2021) conducted a comprehensive analysis on the adsorption and removal of nutrients, including nitrogen, phosphorus, and drugs, from SSU using biochar with varying characteristics. Martin et al. (2022) analyzed six SSU treatments in detail: storage, acidification, alkalization, nitrification, mixing with organic substrates, struvite precipitation, and the resulting fertilizers. Larsen et al. (2021) studied the various roles of biological, physicochemical, and electrochemical approaches in SSU stability, volume reduction, nitrogen recovery, phosphorus recovery, nutrient removal, disinfection, and organic micropollutant treatment. However, currently, the depth of analysis on the trend of changes in the quantity of literature on SSU treatment and recovery is insufficient. Additionally, there is a deficiency in the generalization and comprehensive analysis of the field at the macro scale. Bibliometry is a multidisciplinary discipline that analyzes bibliographic data using mathematical and statistical methodologies. Visual mapping converts textual articles into intuitive visual maps, revealing the development history, current state, and hotspots, as well as assisting in the prediction of research trends in a specific field (Jiang et al. 2023). Bibliometrics is now widely used in a variety of research fields, including cobalt extraction and recovery (Zhou et al. 2023), the recovery of value-added products from wastewater (Barragán-Ocaña et al. 2021), and waste heat recovery (Kuah et al. 2023). It has emerged as a crucial tool for predicting trends in a variety of disciplines and research areas.
This study used the network visualization software VOSviewer 1.6.18 and the citation network analysis tool CiteSpace 6.1. R3 to gain a thorough understanding of the research status and development trends in SSU treatment and recovery. A bibliometric systematic analysis of 894 pertinent articles, encompassing the period from 2000 to 2023 in the Web of Science Core Collection (WOSCC) database, was undertaken. The analysis evaluated numerous dimensions, including the volume of publications, individual author contributions, frequency of article citations, affiliated institutions, journals responsible for publishing, countries where publication occurred, and relevant keywords. This study aims to provide scientific references for improvements and advances in the field of SSU treatment and recovery research.
METHODS
Data sources
The data for this study were obtained from the WOSCC database, which contains significant and influential global research literature and is widely recognized as the leading platform for retrieving scholarly information (Huang et al. 2022). The retrieve strategies were as follows: TS = (‘human urine’ OR ‘yellowwater’ OR ‘yellow water’ OR ‘source separated urine’ OR ‘source separated human urine’ OR ‘source separated yellowwater’ OR ‘source separated yellow water’ OR ‘urine source separation’) and (recover* OR recycl* OR reus* recla?m* OR reconcentrat* OR removal* OR fertili* OR fertigat* OR ‘power generation’ OR separat* or excretion) and (organic* OR nutrient* OR phosph* OR struvite OR vivianite OR nitrogen OR potassium OR kalium OR potash OR urea OR hydrogen OR hormone OR biomass OR crh OR enzyme OR urokinase OR ‘chorionic gonadotrophin’) NOT (determination OR detection OR identification OR monitor*). The search period was from 1 January 2000 to 31 December 2023, and the asterisk (*) in the search subject term represents any character group or empty characters, such as, this article recover* can encompass recovery and recoverable, and recycl* can encompass recycle and recyclable. A total of 1,085 results were retrieved. By filtering the search results article by article and removing duplicate articles, patents, book reviews, and other publications, 894 documents that met the criteria for validity were obtained.
Bibliometric analysis
RESULTS AND DISCUSSION
Annual publication trend
The number of publications related to treatment and resource recovery of SSU from January 2000 to December 2023.
The number of publications related to treatment and resource recovery of SSU from January 2000 to December 2023.
WOSCC categories
TOP 10 WOSCC categories of publications related to treatment and resource recovery of SSU from January 2000 to December 2023.
TOP 10 WOSCC categories of publications related to treatment and resource recovery of SSU from January 2000 to December 2023.
Distribution of published journals
TOP 10 journals of publications related to treatment and resource recovery of SSU from January 2000 to December 2023.
TOP 10 journals of publications related to treatment and resource recovery of SSU from January 2000 to December 2023.
Analysis of publication country
TOP 10 countries (regions) of publications related to treatment and resource recovery of SSU from January 2000 to December 2023.
TOP 10 countries (regions) of publications related to treatment and resource recovery of SSU from January 2000 to December 2023.
Country collaboration map of publications related to treatment and resource recovery of SSU from January 2000 to December 2023.
Country collaboration map of publications related to treatment and resource recovery of SSU from January 2000 to December 2023.
Analysis of publication institution
TOP 10 institutions of publications (a) and cooperative networks among institutions (b) related to treatment and resource recovery of SSU from January 2000 to December 2023.
TOP 10 institutions of publications (a) and cooperative networks among institutions (b) related to treatment and resource recovery of SSU from January 2000 to December 2023.
Analysis of publication author
TOP 10 authors of publications (a) and cooperative networks among authors (b) related to treatment and resource recovery of SSU from January 2000 to December 2023.
TOP 10 authors of publications (a) and cooperative networks among authors (b) related to treatment and resource recovery of SSU from January 2000 to December 2023.
Highly cited publications
Highly cited publications provide insight into significant authors and major research challenges in a discipline throughout period (Bashir et al. 2021). The top 10 most frequently cited literature in the field of treatment and resource recovery of SSU is shown in Table 1. According to the records shown in Table 1, the literature that has received the highest number of citations is ‘Phosphate and potassium recovery from source separated urine through struvite precipitation’ authored by Wilsenach J A from Delft University of Technology and published in Water Research. This study focused on the design and testing of a laboratory-scale precipitator to optimize the performance of a continuously stirred tank reactor for the recovery of potassium through struvite precipitation. Furthermore, the study investigated the impact of various operating parameters, such as hydraulic residence time, mixing intensity, and pH, on the performance of the precipitator (Wilsenach et al. 2007). The second publication titled ‘Nutrients in urine: energetic aspects of removal and recovery’ authored by Maurer M from EAWAG published in Water Science & Technology, This study conducts a comparative analysis of the energy requirements associated with various methods employed for the extraction and reclamation of nutrients present in urine. The energy expenditure associated with the removal of nitrogen and phosphorus from sewage exceeds the caloric intake required for the retrieval of nitrogen and phosphorus from sewage and urine (Maurer et al. 2003). Third, the article titled ‘Low-cost struvite production using source-separated urine in Nepal’ by Etter B from EAWAG published in Water Research. This study investigates the potential of converting phosphorus extracted from human urine into a concentrated form for agricultural fertilization. This study investigates the potential of converting phosphorus extracted from human urine into a concentrated form for agricultural fertilization. The research used locally sourced magnesium oxide from a magnesite mine as a magnesium source to facilitate the precipitation of struvite from urine. Additionally, a reactor equipped with an external filtration system is employed to effectively eliminate over 90% of the phosphorus in merely 1 h. Consequently, this method results in a minimum 40% increase in overall phosphate recovery (Etter et al. 2011). In conclusion, the aforementioned studies investigated the economic and technological feasibility of recovering nitrogen and phosphorus from human urine via various approaches, which provides crucial theoretical and experimental foundations for the research in the field of treatment and resource recovery of SSU. Meanwhile, the behaviors of heavy metals, drugs and other pollutants in urine in the process of resource recycling were investigated, which provided the direction and ideas for the risk control in the process of urine recycling.
Top 10 papers related to treatment and resource recovery of SSU from January 2000 to December 2023 with high citations
Title . | Author . | Journal . | Institution . | Year . | Citations . |
---|---|---|---|---|---|
Phosphate and potassium recovery from source-separated urine through struvite precipitation | Wilsenach J A | Water Research | Delft University of Technology | 2007 | 334 |
Nutrients in urine: energetic aspects of removal and recovery | Maurer M | Water Science and Technology | EAWAG | 2003 | 266 |
Low-cost struvite production using source-separated urine in Nepal | Etter B | Water Research | EAWAG | 2011 | 250 |
Global potential of phosphorus recovery from human urine and feces | Mihelcic J R | Chemosphere | University of South Florida | 2011 | 209 |
Fate of major compounds in source-separated urine | Udert K M | Water Science and Technology | EAWAG | 2006 | 196 |
Complete nutrient recovery from source-separated urine by nitrification and distillation | Udert K M | Water Research | EAWAG | 2012 | 194 |
Reducing micropollutants with source control: substance flow analysis of 212 pharmaceuticals in faeces and urine | Lienert J | Water Science and Technology | EAWAG | 2007 | 166 |
The behaviour of pharmaceuticals and heavy metals during struvite precipitation in urine | Ronteltap M | Water Research | EAWAG | 2007 | 159 |
Struvite precipitation from urine–influencing factors on particle size | Ronteltap M | Water Research | UNESCO-IHE Institute for Water Education | 2010 | 156 |
Nutrient recovery from human urine by struvite crystallization with ammonia adsorption on zeolite and wollastonite | Lind B B | Bioresource Technology | Göteborg University | 2000 | 154 |
Title . | Author . | Journal . | Institution . | Year . | Citations . |
---|---|---|---|---|---|
Phosphate and potassium recovery from source-separated urine through struvite precipitation | Wilsenach J A | Water Research | Delft University of Technology | 2007 | 334 |
Nutrients in urine: energetic aspects of removal and recovery | Maurer M | Water Science and Technology | EAWAG | 2003 | 266 |
Low-cost struvite production using source-separated urine in Nepal | Etter B | Water Research | EAWAG | 2011 | 250 |
Global potential of phosphorus recovery from human urine and feces | Mihelcic J R | Chemosphere | University of South Florida | 2011 | 209 |
Fate of major compounds in source-separated urine | Udert K M | Water Science and Technology | EAWAG | 2006 | 196 |
Complete nutrient recovery from source-separated urine by nitrification and distillation | Udert K M | Water Research | EAWAG | 2012 | 194 |
Reducing micropollutants with source control: substance flow analysis of 212 pharmaceuticals in faeces and urine | Lienert J | Water Science and Technology | EAWAG | 2007 | 166 |
The behaviour of pharmaceuticals and heavy metals during struvite precipitation in urine | Ronteltap M | Water Research | EAWAG | 2007 | 159 |
Struvite precipitation from urine–influencing factors on particle size | Ronteltap M | Water Research | UNESCO-IHE Institute for Water Education | 2010 | 156 |
Nutrient recovery from human urine by struvite crystallization with ammonia adsorption on zeolite and wollastonite | Lind B B | Bioresource Technology | Göteborg University | 2000 | 154 |
Keyword analysis
Keywords cluster analysis
Distribution of research hotspots related to SSU treatment and resource recovery from January 2000 to December 2023.
Distribution of research hotspots related to SSU treatment and resource recovery from January 2000 to December 2023.
Cluster I (in green) has 72 keywords in total, including ‘source-separated urine,’ ‘membrane distillation,’ ‘microbial fuel cells,’ ‘nanofiltration,’ ‘electrolysis,’ ‘nitrogen recovery,’ and ‘nutrient recovery,’ with a focus on nitrogen recovery in SSU. Nitrogen recovery technology for SSU primarily consists of adsorption, air stripping, ion exchange, membrane separation, and electrochemistry.
The adsorption method entails adding solid adsorbents to SSU, which allows to be transferred from the solution to the surface of the solid adsorbents, resulting in nitrogen recovery. It has several advantages, including ease of use, a large adsorption capacity, excellent removal efficiency, low cost, and reusability. Biochar, activated carbon, zeolites, hydrotalcite, and metal oxides are commonly used adsorbents (Cheng et al. 2017). Because of their remarkable water-retention capabilities, natural zeolites, in particular, can increase nutrient levels in infertile soils and are widely used as soil conditioners. Previous research concentrated on the use of natural zeolites for
removal. Maurer et al. (2006) were the first to report the use of zeolites to recover nitrogen from urine. Tarpeh et al. (2017) examined the effectiveness of various adsorbents, including zeolites, biochar, and synthetic cationic resins, for nitrogen removal from separated urine. Cationic resins (Dowex Mac 3 and Dowex 50; Dupont, Wilmington, DE, USA) were more efficient at removing nitrogen, but also more expensive than zeolite and biochar. In addition to zeolites, biochar is widely used as a nitrogen recovery adsorbent. Otieno et al. (2021) examined the adsorption of ammonium nitrogen (
-N) in human urine using pineapple peel biochar and red soil adsorbents. Pineapple peel biochar has a larger surface area and porosity than red soil, resulting in a higher adsorption capacity for
-N (13.40 mg/g), while red soil only adsorbed 10.73 mg/g. Zhang et al. (2020b) developed a porous organic polymer (POP) and tested its ability to remove
from urine against natural zeolites. POP achieved a removal rate of 74.96 mg/g, which was approximately five times higher than that of zeolite (15.42 mg/g). Nonetheless, the adsorption method exhibits limited adsorption capacity and selectivity for nitrogen in SSU. The adsorbent at a certain time after contact with urine becomes saturated, adsorption capacity is reduced, and may at the same time the adsorption of other beneficial or harmful substances in the urine. Future endeavors should prioritize the development of highly efficient, selective, and effortlessly regenerable adsorbents to augment nitrogen recovery. Concurrently, exploring novel approaches for adsorbent reuse and regeneration is vital to minimize treatment expenses and prolong adsorbent lifespan.
The air-stripping technique takes advantage of the volatile nature of ammonia gas under alkaline conditions. By adjusting the pH of the wastewater to alkaline conditions, is converted into free ammonia, which is then blown off by air or steam to achieve nitrogen removal (Zhu et al. 2017). It primarily consists of the processes of air-stripping-acid absorption and hot steam vaporization–condensation. Contrasted with alternative nitrogen recovery methodologies in SSU, the air-stripping technique markedly enhances the nitrogen recovery rate and demonstrates superior stability. Nonetheless, the requisite conditions of heating, stirring, and ventilation inherently contribute to elevated energy consumption. Researchers are actively pursuing the development of more effective and energy-efficient removal equipment, along with adjusting operational parameters, such as temperature and gas flow rate. These advancements aim to maximize the efficacy of nitrogen recycling and minimize energy consumption. Liu et al. (2015) studied the efficiency and cost of ammonia recovery from an SSU using the air-stripping method combined with a modified dual-membrane model. According to the findings, increasing the airflow rate and temperature could reduce operational costs; however, adjusting the pH value increased operational costs. The desorption efficiency of ammonia nitrogen is 80% at a liquid-to-air flow rate of 14 L·L−1 min, a temperature of 50 °C, an operation time of 2.2 h, and a pH value of 9.3.
The ion-exchange method involves transferring to ion-exchange materials such as zeolites, resins, and siliceous ash. Saturated adsorbents can be reused as solid fertilizers in agricultural fields or as reusable adsorbents (Guida et al. 2021). Tarpeh et al. (2017) compared the adsorption capacity of four ion-exchange agents for
: clinoptilolite, biochar, Dowex 50, and Dowex Mac 3, and found that Dowex Mac 3 had the highest adsorption capacity. Clark & Tarpeh (2020) coated polymer cation exchange resins with amino-complexed metals. The metal-ligand exchange adsorbent outperformed other commercial ion-exchange resins in terms of
adsorption capacity and selectivity (Clark & Tarpeh 2020). The ion-exchange technique has numerous applications; however, the regeneration of ion-exchange materials necessitates the use of large amounts of chemicals, impeding the low-carbon development of this technology. Analogous to the challenges associated with the adsorption method, the ion-exchange technique exhibits limited selectivity for nitrogen and becomes saturated upon extended exposure to urine, thereby diminishing its nitrogen adsorption capacity. Future research should focus on devising more efficient, selective, and readily regenerable ion-exchange materials to enhance nitrogen recovery efficacy.
Membrane separation uses the pressure, concentration, and potential differences of the membrane to remove nitrogen, dehydrate urine and recover nutrients and water resources. It can use different membrane processing techniques, including ultrafiltration, nanofiltration, reverse osmosis, membrane distillation (MD), or a combination of multiple membrane processes. Employing the membrane separation technique enables the effective segregation of nitrogen from other constituents present in urine, culminating in an efficient and continuous nitrogen recovery process. Gau et al. (2022) used a hybrid system of forward osmosis (FO) and MD to recover nutrients from human urine. Cation, anion, and dissolved organic carbon recovery rates in the FO-MD system and FO side exceeded 93.7 and 79.5%, respectively, indicating that approximately 85% of the nutrients in the feed solution were recovered. Volpin et al. (2019a) used FO as a pretreatment for MD and found that by modifying the porosity and thickness of the membrane, a higher membrane permeate quality could be obtained, resulting in the recovery of additional water resources. As operational duration increases, the membrane materials may experience aging, potentially impacting nitrogen recovery efficiency. Researchers have devised an array of innovative membrane materials exhibiting anti-pollution and anti-aging properties to enhance the effectiveness of nitrogen recovery through membrane separation. Zhang et al. (2023a) discovered that under pH 12 conditions the liquid hollow fiber membrane contactor technology recovered ammonia nitrogen from urine efficiently (recovery rate > 93%), and the ammonia nitrogen transmembrane flux remained constant over a two-month operation, with only mild membrane fouling observed. Moreover, the efficacy of nitrogen recovery from SSU via membrane separation can be enhanced by optimizing process parameters, including pressure, flow rate, and temperature. Further improvements can be achieved through refining process flows and implementing strategies such as pretreatment and routine cleaning measures.
Electrochemical techniques use specific electrochemical reactors to directly or indirectly electrolyze pollutants in wastewater using the electrochemical behavior of the electrolyte and the electrode under the influence of an electric current. These techniques also possess denitrification, mineralization, and decontamination functions (Kuntke et al. 2017). Electrochemical techniques facilitate the transformation of nitrogen present in urine into recyclable forms, such as ammonia nitrogen, urea, or ammonia, via anodization, electrolysis, and other advanced processes. These methods circumvent the need for chemical reagents, thereby reducing treatment costs and mitigating risks associated with secondary pollution. Fe, Ti, and Pt electrodes are currently the most extensively used electrodes for extracting nitrogen and phosphorus from SSU. Furthermore, researchers have focused on enhancing the nitrogen recovery performance of electrochemical technique by developing high-performance electrode materials with degradation and corrosion resistance properties, optimizing reactor structure (including electrolytic cell shape, electrode configuration, and arrangement), and fine-tuning operational parameters (such as electrolytic duration, current, and voltage). Gao et al. (2018) designed a self-powered bio-electrochemical system with a U-power configuration that included an anode, a cathode, and nitrogen and phosphorus recovery chambers. Microorganisms on the anode degrade organic substances in urine to generate electricity, whereas the electric field between the anode and cathode moves the ionized nitrogen and phosphorus ( and
) from the urine to the recovery solution. El Gheriany et al. (2022) used a novel water-oriented electrode design that contained a built-in cooler to remove excess heat generated during the electrolysis process and used H2 bubbles on the cathode surface for self-stirring. Depending on the operating conditions, such as current density, pH, NaCl concentration, and initial urea concentration, urea removal rates can reach 55–90%. Urea removal increases as current density and NaCl concentration increase, whereas it decreases when the pH of the solution and initial urea concentration increase. Despite the extensive adaptability, ease of operation, and efficient and swift nitrogen recovery capabilities of the electrochemical method, positioning it as a prominent green and low-carbon technology for nitrogen recovery in SSU, it is characterized by relatively high energy consumption. Additionally, the majority of this method's applications are primarily in the experimental phase in laboratory settings.
Cluster II (in blue) includes 42 keywords such as ‘phosphorus,’ ‘phosphate,’ ‘struvite precipitation,’ and ‘crystallization.’ This cluster focuses on the recovery of phosphorus from SSU. According to estimates, phosphorus recovered from urine can supply 22% of the global phosphorus demand (Barbosa et al. 2016). Chemical precipitation has proven to be the most effective and cost-efficient method of recovering phosphorus. When calcium, magnesium, or iron are added to urine, compounds such as magnesium ammonium phosphate (MAP) or magnesium potassium phosphate (MPP) are formed, causing crystals of nitrogen, phosphorus, and potassium to precipitate. The efficiency of chemical precipitation in recovering phosphorus is predominantly dictated by the precipitant dosage, reaction conditions (such as pH value and reaction time), and the effectiveness of solid–liquid separation. It is crucial to identify the optimal precipitant and refine its dosage, as well as to optimize treatment parameters and process flow, to enhance the recovery efficiency of struvite. Struvite, which is a white crystalline substance with low water solubility, a high-grade phosphorus ore, a valuable slow-release fertilizer, and which has a P2O5 content of approximately 58.0%, is formed as a result of this process (Anawati & Azimi 2020). The struvite precipitation and crystallization process provides the benefits of fewer impurities during the recovery process, good economic benefits, and the potential for use as a slow-release soil fertilizer. This is considered a long-term and efficient method of phosphorus recovery.
Cluster III (in yellow) includes 31 keywords including ‘adsorption,’ ‘activated carbon,’ ‘biochar,’ ‘zeolite,’ ‘fertilizer,’ ‘plant availability,’ and ‘toxicity,’ with the focus on the agricultural utilization of nitrogen, phosphorus, and other nutrients in urine. The nitrogen, phosphorus, and potassium content of urine accounts for approximately 88, 67, and 73% of human excreta, respectively. The majority of nitrogen (90–100%) exists as urea or ammonium, with a small proportion as uric acid, amino acids, and other compounds. The nitrogen content of urine is closely related to that of urea or ammonium fertilizers, which accounts for approximately 90% of all chemical fertilizers. Almost all phosphorus and potassium (95–100%) exist in inorganic forms. Fertilizers made from processed urine can be used in agricultural production because most plants can absorb and use nitrogen, phosphorus, and potassium (Maurer et al. 2003). Human urine also contains trace elements, such as B, Cu, Zn, Mo, Fe, Co, and Mn, which are essential for plant growth. Numerous studies have shown that using urine as a substitute fertilizer in agricultural production can minimize reliance on chemical fertilizers. This not only contributes to reducing environmental pollution caused by fertilizer production and transportation but also benefits human beings. Tang & Maggi (2016) found that 2 L/m2 of SSU exceeded the growth requirements of barley (Hordeum vulgare) and soybeans (Glycine max), partially replacing chemical fertilizers. Roy et al. (2022) discovered that lime-treated urine significantly increased overall sunflower plant growth by 85%; plant growth by 151%, leaf area by 137%, and number of leaves per plant by 2.5. Furthermore, found that mixing urine with compost products improved their fertilizing qualities (Shrestha et al. 2013).
The primary components of fresh urine are inorganic salts such as , Na+, K+, Cl−, and
, and organic compounds such as uric acid and urea. Prolonged fertilization with urine can increase soil electrical conductivity and salinity by converting
into H+ and NH3, posing environmental hazards such as soil salinization and acidification (Simha et al. 2023). To limit the negative effects of urine reuse in agricultural areas, it is essential to consider factors such as the volume of urine applied, crop type, nutrient requirements, and soil properties before applying urine. Furthermore, regular groundwater monitoring should be conducted in specified agricultural areas to prevent groundwater contamination.
Cluster IV (in red) comprises 86 keywords including ‘decomposition,’ ‘degradation,’ ‘hydrolysis,’ ‘life cycle assessment,’ ‘storage,’ ‘sustainability,’ ‘human urine,’ and other keywords for urine storage and utilization. Urine storage refers to the containment of urine in source separation toilets or transfer stations before being used. This serves a dual purpose: first, it ensures the harmless disposition of pathogenic bacteria in urine, and second, it stabilizes urine by decomposing detrimental pollutants through the putrefaction process. Pathogenic microorganisms found in human urine include Leptospira interrogans, Escherichia coli, Salmonella, and minute sporozoan cysts. The highest concentrations of pathogenic bacteria and viruses in urine contaminated with fecal matter were 106 and 109 PFU/mL, respectively (Höglund et al. 2002a, 2002b). Therefore, the inactivation of microorganisms in urine is critical. Studies have indicated that the concentrations of pathogenic bacteria and viruses in urine decrease to below 102 and 102 PFU/mL, respectively, after 6 months of storage at room temperature. This ensures that it can be used safely in a variety of crops (Udert & Wächter 2012). Simultaneously, during storage, urea is degraded into ammonia and carbonate by urease, accompanied by the volatilization of ammonia gas. Improper storage methods might result in nitrogen losses ranging from 60 to 90%. To prevent the hydrolysis of fresh urine and the volatilization of NH3, alkaline materials, such as Ca(OH)2, CaO, and wood ash, must be added to stabilize fresh urine (Lv et al. 2020).
Keywords burst analysis
The top 20 keywords with the strongest citation bursts related to the treatment and resource recovery of SSU from January 2000 to December 2023.
The top 20 keywords with the strongest citation bursts related to the treatment and resource recovery of SSU from January 2000 to December 2023.
The prominent keywords from 2000 to 2007 were ‘ecological sanitation’ and ‘systems,’ clarifying the forefront and hot topics regarding the role of ecological sanitation engineering in the separation, collection, and treatment of urine. The core concept of ecological sanitation is to separate, divert, collect, and treat different types of domestic wastewater individually, emphasizing the highly resourceful and polluting characteristics of urine. The separation and individual treatment of urine minimizes the ammonia nitrogen load of standard sewage treatment, allowing for the recovery of nutritious elements from urine and the transformation of urine into a renewable fertilizer. This process creates a closed-loop nutrient circulation system connecting farmlands, crops, food, and the human body (Patel et al. 2020; Koulouri et al. 2023). The increase in newly discovered phrases from 2007 to 2018 implies an increase in research intensity, making it a hub of exploration. Key research terms during this phase included ‘phosphate,’ ‘struvite crystallization,’ ‘phosphorus recovery,’ and ‘struvite precipitation.’ As implied by these phrases, the production of MAP through chemical precipitation is a primary strategy for phosphorus recovery from SSU effluent. Precipitation crystallization technology extracts several nutritional elements from urine, such as phosphorus and ammonia nitrogen, resulting in a purer product that serves as a controlled-release, high-quality fertilizer (Li et al. 2022). Although the frequency and duration of burst keywords have decreased since 2018, the intensity of these changes has increased, indicating a shift in the research hotspot toward nitrogen recovery. Throughout the process of urine separation, collection, and storage, the propensity for urine hydrolysis, which causes ammonia volatilization, not only leads to nitrogen loss but also emits odors, ultimately affecting nutrient recovery and air quality (De Paepe et al. 2020). Diverse techniques for nitrogen treatment and recovery from urine include adsorption, stripping, ion-exchange adsorption, membrane separation, and electrochemical methods are all used to treat and recover nitrogen from urine. As a result, adopting suitable processes is pivotal for realizing the sustainable recycling of SSU.
In summary, using keyword analysis and CiteSpace software for visualization, one can observe sudden and sustained recurrence of specific keywords at various time intervals. This represents the academic community's focus on evolutionary hotspots of research in a specific topic. In its early phases, the topic of eco-health systems drew significant research interest; however, in recent years, scholars have focused on aspects such as phosphorus and nitrogen recovery. These findings present the chronological distribution of emergent keywords in urine treatment and recovery research, identifying research trends and thereby offering guidance and insights for further exploration and study.
CHALLENGES AND FUTURE PERSPECTIVES
Development of recycling technologies
The treatment and resource recovery of SSU fulfill two purposes. On the one hand, it improves the quality of effluent from sewage treatment plants while lowering operational costs. In another hand, it also aids in the recovery of valuable nutrients and latent energy trapped in urine. Urine separation is currently difficult owing to the monolithic nature of nitrogen and phosphorus recovery techniques. The simultaneous recovery and utilization of various substances and energies remains challenging. High retrieval costs, suboptimal efficiency, and impurities in reclaimed substances are all issues. Existing research is primarily at the laboratory or pilot scale, necessitating investigation into the development of a comprehensive process for the simultaneous retrieval of nutritious elements from urine. This pursuit aims to minimize the loss of essential elements, increase the commercial viability of urine separation byproducts, and contribute to energy conservation, emission reduction, and the promotion of sustainable development.
Biochemical drug removal and extraction
Most antibiotics, hormones, and other drugs consumed by humans are excreted in feces and urine. Therefore, the direct application of urine to farmlands may result in the accumulation, migration, and transformation of antibiotics in soil and groundwater, ultimately harming human health through the food chain (Hu et al. 2022). Most research currently focuses on the recovery or removal of nutrients such as nitrogen and phosphorus, with less research focusing on the removal of pharmacological components and pollutants from SSU. Adsorption, electrodialysis, nanofiltration, and advanced oxidation are all common approaches. For example, Zhang et al. (2020a) proposed a biocarbon-based heterogeneous catalytic system to extract sulfamethoxazole and its primary human metabolite, N4-acetyl-sulfamethoxazole, from urine. Almuntashiri et al. (2022) investigated the removal of drugs such as naproxen, carbamazepine, ibuprofen, paracetamol, and metamizole from human urine using activated carbon, with carbamazepine having the highest adsorption capacity (56.1 mg g−1) and metamizole having the lowest (32.2 mg g−1). Activated carbon can significantly affect drug removal from human urine.
In addition to recovering nutrients such as nitrogen, phosphorus, and potassium, other compounds including urokinase, human chorionic gonadotropin, ulinastatin, and anticancer substances can also be extracted from human urine. These substances have received widespread attention for their use in the preparation of biochemical drugs. Urokinase can be used in the clinical treatment of a variety of thrombotic diseases and myocardial infarction and can also be used with anticancer medications to enhance their efficacy (Fasoli et al. 2015). Currently, the main methods for extracting urokinase from urine include silica gel adsorption, ultrafiltration, and ion exchange; however, these methods have drawbacks such as long processing times, complex processes, high consumption, and low recovery rates. For example, Xu et al. (2019) developed a foam separation method to improve silica gel adsorption efficiency for urokinase in urine, achieving an activity recovery rate of 89.5% and a purification factor of 56.8 times for urokinase, which were 25.3 and 79.2% higher, respectively, than those of silica gel adsorption. Urokinase has a high commercial value and has become a valuable resource in the field of human health globally.
Energy recovery
The high concentrations of organic compounds present in urine have abundant chemical potential. This inherent energy can be captured using technologies such as microbial fuel cells (MFCs) for energy recovery and plant cultivation for biomass fuel production. Urine has a high conductivity and contains natural electrolytes such as ammonium, carbonate, and calcium ions. Common urine-based energy conversion approaches include MFCs for electrical energy recovery and plant cultivation for biomass fuel production. Because of its copious organic content and excellent conductivity, urine is considered an ideal substrate for the growth of electroactive microorganisms. MFCs use microorganisms to directly convert the chemical energy within organic compounds into electricity. Ieropoulos et al. (2013) pioneered the use of urine to fabricate multiple sets of small-scale MFCs for successful energy conversion. A comparison of MFCs of varying sizes revealed only minor differences in power output, with energy conversion rates typically exceeding 50%. Santoro et al. (2013) further demonstrated that MFCs may generate electricity while also removing pollutants. The integration of MAP crystallization and ammonia stripping techniques with MFCs by Zang et al. (2012), You et al. (2016), and Merino-Jimenez et al. (2017), enables the simultaneous recovery of MAP/NH3 and electrical energy, thereby improving urine resource utilization efficiency.
Aquatic plants such as lettuce, cabbage, and algae can be cultivated to absorb and store N, P, and K nutrients as plant body components. The plants can then be converted into biomass fuel to achieve indirect energy recovery from urine. Currently, research has primarily focused on the cultivation of algae in urine. Microalgae have received considerable attention because of their rapid growth rate and large amounts of fat and carbohydrates that can generate biomass energy. According to Jaatinen et al. (2016), when human urine was diluted 1:25 as the sole source of nutrients, the growth of the microalga Chlorella vulgaris reached 0.52 gVSS L−1 after 21 days of culture. Algae can remove nutrients from urine, promote their own growth, and can be used as biofuels to produce biomass energy, thus realizing the recovery and utilization of nutrients in urine.
CONCLUSION
Separating and recycling nitrogen, phosphorus, and potassium in urine can relieve the pressure on centralized sewage treatment plants to handle nitrogen and phosphorus, reduce the need for industrial fertilizers in agricultural production, and effectively alleviate the eutrophication of water bodies. This study conducted a literature review and summarized 894 articles on the treatment and resource recovery of SSU published between 2000 and 2023 in the WOSCC database using VOSviewer and CiteSpace software. A comprehensive bibliometric analysis was conducted, covering aspects such as publications in the research field, countries involved, author and institution contributions, relevant keywords, and highly cited scholarly papers. The outcomes are as follows:
(1) The standard of publications in the field of SSU treatment and resource recovery has steadily increased annually. Moreover, the level of attention gradually increased, indicating a positive developmental trend. The primary academic disciplines involved in this research are Environmental Sciences, Environmental Engineering, and Water Resources. Furthermore, there is a growing interdisciplinary integration with fields such as Engineering Chemical and Agricultural Engineering, illustrating the diversity of research in this field. China and the USA were the most prolific contributors, accounting for 21.03 and 14.65% of the total publication output, respectively. The top three journals in terms of the number of publications were Water Research, Water Science & Technology, and Science of the Total Environment. The two most-cited authors in this research field were Udert KM from EAWAG and Boyer TH from Arizona State University, indicating their significant involvement in the field. Although the Swedish University of Agricultural Sciences, EAWAG, and Chinese Academy of Sciences had the highest frequency of publications, collaborative scientific research among these institutions remains limited. Therefore, strengthening international collaborative capabilities is necessary.
(2) Using keyword clustering and burst analysis, we found that the fields of treatment and resource recovery of SSU were primarily focused on aspects such as nitrogen recovery, phosphorus recovery, urine utilization in agriculture, and urine storage. Within the field of SSU treatment and recovery strategies, nitrogen recovery is primarily achieved through adsorption, air-stripping, ion exchange, membrane separation, and electrochemical methodologies. In contrast, the principal approach for phosphorus recovery in SSU is chemical precipitation. Despite this, the existing stand-alone techniques face limitations in concurrently addressing and utilizing diverse constituents or energy modalities, resulting in suboptimal recovery efficiency and costs that do not meet practical application requisites. Further exploration is warranted in order to devise a proficient and consistent integrated system for the treatment and recovery of SSU.
(3) Based on the current state of research and the analysis of research hotspots, future developments in this field should emphasize the development of integrated and efficient techniques for the stable separation and treatment, extraction of biochemical drugs, and chemical energy recovery from SSU. Aside from serving as an organic fertilizer resource, SSU provides a basis for obtaining high-value biochemical substances, which include antibiotics, urokinase, and human chorionic gonadotropin, etc. Additionally, emerging biological electrochemical systems, such as MFCs, offer innovative methods for converting urine's organic chemical energy into functional power. Also, by utilizing algal biomass as a biofuel feedstock, these approaches contribute to the sustainable use of SSU-derived resources.
(4) In the present investigation, bibliometric analysis was employed to conduct a quantitative analysis of the literature pertaining to SSU treatment and resource recovery in the WOSCC database. Relying exclusively on WOSCC could constrain the range of studies examined in this database and potentially overlook pertinent research in alternative databases. To achieve a more comprehensive overview of the current research status and development trend of SSU treatment and recovery, future bibliometric analyses should consider the utilization of multiple databases, such as Scopus, PubMed, and Google Scholar, which can contribute to heightened accuracy and a widened scope of coverage.
ACKNOWLEDGEMENTS
This project was supported by the National Key Research and Development Project of China (No.2023YFE0113103).
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.