Blue Diversion Autarky – Wastewater Treatment off the Grid

Autarky at a glance

The Blue Diversion Autarky toilet is a sanitation system which provides hygiene and comfort without relying on water and wastewater infrastructure. Water, urine and feces are collected separately and treated on site in specific modules. The Blue Diversion Autarky toilet recycles water for hand washing and flushing, recovers nutrients for fertilizer production and inactivates pathogens reliably. The treatment modules can be replaced, integrated in other sanitation systems or used as stand-alone units. One example is the hand washing station, which was also developed by the Blue Diversion Autarky team.

Blue Diversion Autarky was launched in the context of the “Reinvent the Toilet Challenge” funded by the Bill & Melinda Gates Foundation. The project is currently in its second project phase. Blue Diversion Autarky is the continuation of the Blue Diversion project.

 

Blue Diversion Autarky toilet
Blue Diversion Autarky toilet
Blue Diversion Autarky handwashing station

Guiding principles

Safety & comfort

The Blue Diversion Autarky toilet offers the safety and comfort of a modern flush toilet without requiring piped water or sewerage.

Safety & comfort

Source separation

Pathogen inactivation, nutrient recovery and water recycling for flushing and hand washing are achieved by the separate treatment of feces, urine and water.

Source separation

Modularization

A modular design allows for a wide range of applications. Single modules can be used alone or combined with other technologies. One example is the stand alone hand washing station.

Modularization

Research at a glance

The Blue Diversion Autarky system is based on the separate treatment of water, urine and feces. The core process for the water treatment is a gravity-driven membrane (GDM) filtration. After post-treatment in the form of an activated carbon filter and an electrolysis unit to remove microorganisms, the same water can be reused for flushing and hand washing. Urine is pretreated to eliminate malodor and pathogens, as well as to avoid the loss of nutrients. The volume of urine is then reduced by evaporating the water. The product is a concentrate of organic and inorganic nutrients, that can be used as fertilizer. The treatment of the feces is based on hydrothermal oxidation (HTO). Using high temperature and pressure, the feces are mineralized  to  carbon dioxide, water and precipitated inorganic solids.

We recently field-tested the Blue Diversion Autarky toilet in a 14-people household in a peri-urban zone of Durban, South Africa.The toilet system functioned well and the recycled water was readily used. As the adoption of new sanitation technology does not depend on the technical functionality alone, the testing was closely accompanied by a team of social scientists. The results of our socio-technical assessment and a detailed evaluation of the water treatment system are available throughopen access publications (see below).

 

Publications

Socio-technical analysis of a sanitation innovation in a peri-urban household in Durban, South Africa

The provision of water and sanitation for all that is safe, dignified, reliable, affordable and sustainable is a major global challenge. While centralized sewer-based sanitation systems remain the dominant approach to providing sanitation, the benefits of non-sewered onsite sanitation systems are increasingly being recognised. This paper presents the outcomes of the testing of the Blue Diversion Autarky Toilet (BDAT), a sanitation system providing hygiene and dignity without relying on water and wastewater infrastructure, in a peri-urban household in Durban, South Africa. The BDAT was used by a single household as their only form of sanitation during three months of technical and social testing. An analysis based on technical data in combination with interpretive, qualitative research methods revealed that the BDAT functioned well and achieved high levels of social acceptance in the test household. The flushing, cleanliness and odour-free nature of the sanitation technology, its functionality, the household's previous sanitation experience, and their experience with and understanding of water scarcity, were the main factors underpinning their positive response to this innovation in sanitation. The testing process resulted in broader developmental benefits for the household, including improved basic services due to the upgrading of the electrical and existing sanitation system, social learning, and improved relationships between household members and the local state. A transdisciplinary research process, which emerged through the assessment, enabled the integration of different forms of knowledge from multiple actors to address the complexity of problems related to the development of socially just sanitation. The benefit of engaging with societal actors in sanitation innovation and assessing its outcomes using both the technical and social sciences is evident in this paper.
Sutherland, C.; Reynaert, E.; Dhlamini, S.; Magwaza, F.; Lienert, J.; Riechmann, M. E.; Buthelezi, S.; Khumalo, D.; Morgenroth, E.; Udert, K. M.; Sindall, R. C. (2021) Socio-technical analysis of a sanitation innovation in a peri-urban household in Durban, South Africa, Science of the Total Environment, 755, 143284 (12 pp.), doi:10.1016/j.scitotenv.2020.143284, Institutional Repository

Innovation for improved hand hygiene: field testing the Autarky handwashing station in collaboration with informal settlement residents in Durban, South Africa

Safe and accessible water services for hand hygiene are critical to human health and well-being. However, access to handwashing facilities is limited in cities in the Global South, where rapid urbanisation, service backlogs, lack of infrastructure and capacity, and water scarcity impact on the ability of local governments to provide them. Community participation and the co-production of knowledge in the development of innovative technologies, which are aligned with Water, Sanitation and Hygiene (WASH) principles, can lead to more sustainable and socially-acceptable hand hygiene systems. This paper presents the outcomes of the testing of the Autarky handwashing station, a technology that provides onsite treatment and recycling of handwashing water, in an informal settlement in Durban, South Africa. The transdisciplinary research approach adopted enabled the participation of multiple stakeholders with different knowledge systems in the framing, testing and evaluation of the system. The process of co-producing knowledge, as well as the outcomes of the testing, namely high levels of functionality and social acceptability of the technology, supported the WASH principles. The evaluation revealed that the Autarky handwashing station is a niche intervention that improved access to safe and appealing handwashing facilities in an informal settlement. Its novel design, socially desirable features, reliability and ability to save water increased its acceptance in the community. The testing of the system in a real-world context revealed the value of including communities in knowledge production processes for technology innovation. Further work is required to ensure that real-time monitoring of system function is feasible before such systems can be implemented at larger scale.
Sutherland, C.; Reynaert, E.; Sindall, R. C.; Riechmann, M. E.; Magwaza, F.; Lienert, J.; Buthelezi, S.; Khumalo, D.; Dhlamini, S.; Morgenroth, E.; Udert, K. M. (2021) Innovation for improved hand hygiene: field testing the Autarky handwashing station in collaboration with informal settlement residents in Durban, South Africa, Science of the Total Environment, 796, 149024 (13 pp.), doi:10.1016/j.scitotenv.2021.149024, Institutional Repository

More about water treatment

The water treatment system, referred to as Water Wall, recycles hand washing water, toilet flush water (separated from the major part of urine and feces), or both. A multi-barrier approach with four treatment stages ensures that the water is safe for reuse.

  • The first barrier and core of the treatment is an aerated bioreactor, in which microorganisms degrade contaminants such as soap, urine, and feces.
  • As the second barrier, the water is then filtered by gravity through an ultrafiltration membrane. The membrane pores are smaller than the size of bacteria and most viruses, providing water that is microbiologically safe after filtering through the membrane.

Contacts

Prof. Dr. Eberhard Morgenroth Tel. +41 58 765 5539 Send Mail

The combined biological treatment and membrane filtration without cleaning will produce a biofilm on the membrane. This effect is desirable in a gravity-driven membrane (GDM) approach, as the biofilm also contributes to the removal of organic carbon. The GDM system achieves approximately 95% removal of organic carbon.

The filtered water is stored in a clean water tank.  Additional treatment is required to limit regrowth of pathogens and potential contamination.

  • The third treatment barrier, an activated carbon filter, removes remaining organic contamination in the clean water tank by adsorption.
  • Finally, the fourth and last barrier, an electrolysis unit, further reduces organic carbon concentrations and produces a chlorine residual, both of which help to limit pathogen growth during storage.
Water treatment process

Current activities

With this setup, we were able to develop a small series of Water Wall prototypes for treating and recycling hand washing water or toilet flush water. These prototypes were extensively tested under laboratory conditions, under which they reliably removed pathogens, nutrients, malodor, and color from recycled water. However, when moving outside the laboratory, we expect a much higher variability of the number of users, composition of the water and external conditions. This is why we are currently exposing different configurations of the Water Wall to real-life conditions.

Update: our results from field tests in Switzerland and South Africa have now been published. The open access publication can be found here: https://doi.org/10.1016/j.wroa.2020.100051.

Water wall

 

 

A Water Wall as part of a complete toilet system. The biological treatment takes place in the lower tank, with the ultrafiltration membrane sitting on the bottom of the tank. The activated carbon filter and the electrolysis unit are located in the upper tank.

Publications

Practical implementation of true on-site water recycling systems for hand washing and toilet flushing

On-site wastewater reuse can improve global access to clean water, sanitation and hygiene. We developed a treatment system (aerated bioreactor, ultrafiltration membrane, granular activated carbon and electrolysis for chlorine disinfection) that recycles hand washing and toilet flush water.
Three prototypes were field-tested in non-sewered areas, one in Switzerland (hand washing) and two in South Africa (hand washing, toilet flushing), over periods of 63, 74 and 94 days, respectively.
We demonstrated that the system is able to recycle sufficient quantities of safe and appealing hand washing and toilet flush water for domestic or public use in real-life applications. Chemical contaminants were effectively removed from the used water in all prototypes. Removal efficiencies were 99.7% for the chemical oxygen demand (COD), 98.5% for total nitrogen (TN) and 99.9% for phosphate in a prototype treating hand washing water, and 99.8% for COD, 95.7% for TN and 89.6% for phosphate in a prototype treating toilet flush water. While this system allowed for true recycling for the same application, most on-site wastewater reuse systems downcycle the treated water, i.e., reuse it for an application requiring lower water quality. An analysis of 18 selected wastewater reuse specifications revealed that at best these guidelines are only partially applicable to innovative recycling systems as they are focused on the downcycling of water to the environment (e.g., use for irrigation). We believe that a paradigm shift is necessary and advocate for the implementation of risk-based (and thus end-use dependent) system performance targets to evaluate water treatment systems, which recycle and not only downcycle water.
Reynaert, E.; Greenwood, E. E.; Ndwandwe, B.; Riechmann, M. E.; Sindall, R. C.; Udert, K. M.; Morgenroth, E. (2020) Practical implementation of true on-site water recycling systems for hand washing and toilet flushing, Water Research X, 7, 100051 (13 pp.), doi:10.1016/j.wroa.2020.100051, Institutional Repository

Disruptions in loading and aeration impact effluent chlorine demand during biological greywater recycling

Greywater recycling systems designed for high-quality applications, such as hand washing, must deliver microbially safe and aesthetically acceptable water under the challenging operating conditions present where such systems are needed most urgently. As chlorination is the most popular strategy for reducing bacterial concentrations in greywater, understanding chlorination in the context of disruptive and challenging operation is essential to designing robust treatment. In this study, we have examined how disruptions through overall increased loading, interrupted aeration and increased ammonia loading have impacted the chlorine demand of the water produced by a greywater recycling system. We also presented concentrations of significant chemicals that contributed to this chlorine demand. The results indicate that a 1 d period with 8 times (8x) the normal design loading produced a peak chlorine demand of 0.74 mg Cl2/L, which is approximately double the baseline value. While this chlorine demand can be overcome by adding more chlorine, tests involving disruptions in aeration or feeding additional ammonia into the bioreactor produced much greater increases (>30x). The risks of increased chlorine demand on microbial safety can be overcome by limiting ammonia inputs to the system, providing backup systems to ensure sufficient aeration, or through additional anti-bacterial measures that do not depend on maintaining residual chlorine.
Ziemba, C.; Sharma, P.; Ahrens, T.; Reynaert, E.; Morgenroth, E. (2021) Disruptions in loading and aeration impact effluent chlorine demand during biological greywater recycling, Water Research X, 11, 100087 (10 pp.), doi:10.1016/j.wroa.2020.100087, Institutional Repository

Linking transformations of organic carbon to post-treatment performance in a biological water recycling system

Ozone, electrolysis and granular activated carbon (GAC) were examined as potential post-treatments to follow a household-scale biologically activated membrane bioreactor (BAMBi), treating a wash water containing trace urine and feces contamination. Each post-treatment was evaluated for abilities and reaction preferences to remove or transform dissolved organic carbon (DOC), chemical structures that contribute color, and assimilable organic carbon (AOC), which can support bacterial regrowth. Batch treatment with each technology demonstrated an ability to remove ≥95% DOC. Ozone demonstrated a reaction selectivity through increased reaction rates with larger compounds and color-contributing compounds. Electrolysis and GAC demonstrated generally less-selective reactivity. Adding post-treatments to full-scale systems reduced DOC (55-91%), AOC (34-62%), and color (75-98%), without significant reaction selectivity. These reductions in DOC and AOC were not linked to reduction of bacterial concentrations in treated water. Reductions in bacterial concentrations were observed with ozone and electrolysis, but this is credited to oxidation chemicals produced in these systems and not the removal or transformations of organic materials.
Ziemba, C.; Larivé, O.; Reynaert, E.; Huisman, T.; Morgenroth, E. (2020) Linking transformations of organic carbon to post-treatment performance in a biological water recycling system, Science of the Total Environment, 721, 137489 (8 pp.), doi:10.1016/j.scitotenv.2020.137489, Institutional Repository

Comparing the anti-bacterial performance of chlorination and electrolysis post-treatments in a hand washing water recycling system

Innovative solutions are necessary to enable the decentralized recycling of greywater for applications requiring high-quality water, such as hand washing. While physical barriers such as ultrafiltration membranes effectively prevent the passage of bacteria, and chemical and biological treatments can effectively reduce the carbon content of the treated water, there exists a knowledge gap regarding the application of anti-bacterial strategies to prevent the growth of harmful bacteria following treatment. In this study, the effluent water from a household-scale greywater treatment system was fed to seven parallel experimental post-treatment tanks: three receiving direct chlorination with free chlorine residuals of 0.2, 1 or 5 mg Cl2/L, three with chlorine produced through electrolysis at the same residual concentrations, and one control with no chlorine added. For increasing concentrations of direct chlorination, the median total cell count (TCC) values were 9 × 104, 2.9 × 104 and 1.8 × 103 cells/mL, respectively. Electrolysis treatment produced very similar TCC concentrations, 8.8 × 104, 1.1 × 104 and 2.3 × 103 cells/mL. The TCC concentrations were lower than the concentration of the water entering each tank (~3 × 105 cells/mL). Intact cell count (ICC) measurements indicated that the viable cell concentrations, were less than 10% of the TCC values. Though electrolysis treatment can produce powerful oxidants, such as hydroxyl radical, there was no evidence that electrolysis in this system provided additional benefits beyond chlorine production for control of total or intact cell counts. Oxidation of bacteria by chlorine was the dominant anti-bacterial mechanism in our system. Monitoring of dissolved organic carbon (DOC) and assimilable organic carbon (AOC) did not suggest that carbon-limitation significantly impacted cell counts when chlorination or electrolysis treatment was applied. This work demonstrates that either direct chlorination or electrolysis treatment are able to reduce bacteria concentrations over long-term operation of a hand washing water treatment system. We recommend selecting chlorine residual targets such that a chlorine residual is maintained during periods of challenging operating conditions. We observed that a target residual of 1 mg Cl2/L, in our system, maintained the TCC below the concentration found in Zürich drinking water.
Ziemba, C.; Larivé, O.; Deck, S.; Huisman, T.; Morgenroth, E. (2019) Comparing the anti-bacterial performance of chlorination and electrolysis post-treatments in a hand washing water recycling system, Water Research X, 2, 100020 (10 pp.), doi:10.1016/j.wroa.2018.100020, Institutional Repository

Chemical composition, nutrient-balancing and biological treatment of hand washing greywater

On-site biological hand washing water treatment can improve global access to safe hand washing water, but requires a thorough understanding of the chemical composition of the water to be treated, and an effective treatment strategy. This study first presents a detailed characterization of the individual inputs to hand washing water. We demonstrate (1) that soap is likely the most significant input in hand washing water, representing ∼90% of mass loading, and (2) that inputs to hand washing water have low concentrations of biologically-essential macro- and micro-nutrients (nitrogen, phosphorus, potassium, copper, zinc, molybdenum and cobalt) with respect to carbon, which may impair biological carbon removal. This study next formulates a recipe that recreates a representative composition of hand washing water and develops a procedure to identify and supplement nutrients in which this recipe is estimated to be deficient. Batch testing of the nutrient-supplemented hand washing water with an inoculum of planktonic bacteria demonstrated improved assimilable organic carbon removal (99% vs. 86% removal) and produced lower final DOC concentrations (1.7 mgC/L vs. 3.5 mgC/L) compared to realistic (nutrient-deficient) washing water. Supplementing nutrients did promote cell growth (50x higher final total cell count). Full-scale testing in a biologically activated membrane bioreactor (BAMBi) system treating 75 L/day of nutrient-supplemented hand washing water showed that long-term operation (100 days) can deliver effective carbon removal (95%) without detrimental fouling or other disruptions caused by cell growth. This work demonstrates that biological treatment in a BAMBi system, operated with appropriate nutrient-balancing offers an effective solution for decentralized treatment of light greywater.
Ziemba, C.; Larivé, O.; Reynaert, E.; Morgenroth, E. (2018) Chemical composition, nutrient-balancing and biological treatment of hand washing greywater, Water Research, 144, 752-762, doi:10.1016/j.watres.2018.07.005, Institutional Repository

Controlling bacterial pathogens in water for reuse: treatment technologies for water recirculation in the Blue Diversion Autarky Toilet

The Blue Diversion AUTARKY Toilet is a urine-diverting toilet with on-site treatment. The toilet is being developed to provide a safe and affordable sanitation technology for people who lack access to sewer-based sanitation. Water used for personal hygiene, hand washing, and flushing to rinse urine- and feces-collection bowls is treated, stored, and recycled for reuse to reduce reliance on external water supplies. The system provides an opportunity to investigate hygiene of water for reuse following treatment. Treatment in the toilet includes a Biologically Activated Membrane Bioreactor (BAMBi) followed by a secondary treatment technology. To identify effective secondary treatment, three options, including granular activated carbon (GAC) only, GAC+chlorine (sodium hypochlorite), and GAC+electrolysis are considered based on the bacterial inactivation and growth inhibition efficiency. Four different hygiene-relevant bacteria are tested: Escherichia coli, Enterococcus faecalis, Pseudomonas aeruginosa, and Salmonella typhimurium. Our evaluation demonstrates that—despite treatment of water with the BAMBi—E. coli, P. aeruginosa, and S. typhimurium have the potential to grow during storage in the absence of microbial competition. Including the indigenous microbial community influences bacterial growth in different ways: E. coli growth decreases but P. aeruginosa growth increases relative to no competition. The addition of the secondary treatment options considerably improves water quality. A column of GAC after the BAMBi reduces E. coli growth potential by 2 log10, likely due to the reduction of carbon sources. Additional treatments including chlorination and electrolysis provide further safety margins, with more than 5 log10 inactivation of E. coli. However, reactivation and/or regrowth of E. coli and P. aeruginosa occurs under in the absence of residual disinfectant. Treatment including the BAMBi, GAC, and electrolysis appear to be promising technologies to control bacterial growth during storage in water intended for reuse.
Nguyen, M. T.; Allemann, L.; Ziemba, C.; Larivé, O.; Morgenroth, E.; Julian, T. R. (2017) Controlling bacterial pathogens in water for reuse: treatment technologies for water recirculation in the Blue Diversion Autarky Toilet, Frontiers in Environmental Science, 5, 90 (13 pp.), doi:10.3389/fenvs.2017.00090, Institutional Repository

On-site treatment of used wash-water using biologically activated membrane bioreactors operated at different solids retention times

Biologically activated membrane bioreactors (BAMBis) were operated at suspended solids retention times (SRT) of 7 and 102 days and at full solids retention. The effect of these different approaches of operation on substrate and nutrient conversion, and on permeate flux was investigated. Variations in organic loads and aeration intensities were also studied. Permeate flux stabilized during long-term operation independently of suspended SRT. Removal of organic substrate was independent of solids concentrations and remained stable over the long term. Microorganisms colonizing the surface of particles were found as the main mechanism responsible for degradation of organic substrate in the particulate form. BAMBi appeared to be a robust technology, adapted to on-site treatment of used wash-water, as it can be operated without control of suspended SRT. Thus BAMBis can be operated for long periods without any control of biofouling and sludge formation, leading to low maintenance needs. When BAMBis were operated at low aeration, the formation of anoxic zones led to combined nitrification and denitrification and thus significant nitrogen removal.
Ravndal, K. T.; Künzle, R.; Derlon, N.; Morgenroth, E. (2015) On-site treatment of used wash-water using biologically activated membrane bioreactors operated at different solids retention times, Journal of Water Sanitation and Hygiene for Development, 5(4), 544-552, doi:10.2166/washdev.2015.174, Institutional Repository

An energy-efficient membrane bioreactor for on-site treatment and recovery of wastewater

The present study describes the development of a new type of aerated membrane bioreactor referred to as a biologically activated membrane bioreactor (BAMBi) for on-site treatment of high-strength wastewater. The treated wastewater is reused for flushing and personal hygiene. BAMBi is an adaptation of a gravity-driven membrane reactor, originally developed for the purpose of treating river water to drinking water quality. Initially, a series of reactor configurations were tested and it was found that the simplest possible configuration could treat the wastewater to an acceptable standard, provided that a polishing step for color removal and disinfection was introduced. A commercial electrolysis unit was utilized for polishing. The energy consumption of BAMBi is 0.8 kWh/m3 of water treated, which can be considered low for an on-site membrane bio reactor application.
Künzle, R.; Pronk, W.; Morgenroth, E.; Larsen, T. A. (2015) An energy-efficient membrane bioreactor for on-site treatment and recovery of wastewater, Journal of Water Sanitation and Hygiene for Development, 5(3), 448-455, doi:10.2166/washdev.2015.116, Institutional Repository

More about urine treatment

Two processes are necessary to treat the source-separated urine in the toilet: urine stabilization and water removal.

The main goal of the Autarky urine stabilization is the prevention of urea hydrolysis, a process converting urea to volatile ammonia and carbon dioxide. Through the addition of calcium hydroxide to fresh urine, the pH increases to values above 12 and thus prevents microbial urea hydrolysis. 

Contacts

Michel Riechmann Tel. +41 58 765 5748 Send Mail
Prof. Dr. Kai Udert Tel. +41 58 765 5360 Send Mail

Additionally, the high pH kills pathogens and prevents biological processes that produce malodor. When calcium hydroxide is added to the urine only about the amount dissolves that is needed to reach the necessary high pH value. This allows providing a depot of the reagent in the stabilization reactor; thus, no expensive and complicated dosage mechanisms are required. Moreover, calcium hydroxide, also known as hydrated lime, is a cheap reagent and readily available worldwide.

The direct application of human urine as fertilizer is a common practice in many rural areas around the globe. However, the high water content of urine – no matter if stabilized or not – requires significant storage capacity and can make the collection and transport to the agricultural fields very costly. Volume reduction does not only reduce costs for storage and transport, but could also facilitate field application of the concentrated fertilizer. Standard volume reduction techniques are most often energy intensive processes, because they require high temperature (distillation) or pressure (reverse osmosis).

Offering an alternative to these processes, our approach uses forced convection to reduce the volume of the urine. The evaporation reactor consists of a stacked tray system to create a large surface area. Fans generating a high air stream accelerate the evaporation of the the water from the incoming urine. The offgas is filtered through an activated carbon filter, preventing the emission of organic contamination or malodour.

Once a month, the system needs to be serviced. Tasks are to refill the calcium hydroxide depot and to harvest the trays. The remaining end-product is a concentrate of inorganic and organic nutrients that can be used as fertilizer in agriculture.

Current activities

The stabilization and evaporation reactors have been subject to extensive testing. These tests were supported by lab tests of specific parts of the reactors and a computer model simulating the system. Based on the collected information, the two reactors underwent a major redesign, allowing for an increased evaporation efficiency along with reduced overall size. Another focus of the redesign was the integration of service appliances, e.g. for harvesting of the produced nutrient concentrate.

To validate the technic in real-life settings several field tests of one to four months were conducted. The urine module was tested as part of the Autarky toilet system at Eawag campus in Dübendorf (Switzerland), as well as in a large family household in a Durban Township (South Afrika). As an individual entity, field tests of the urine module took place attached to a mobile Tinihouse (Au, Switzerland) and on a mountain hut (Leglerhütte, Switzerland).

Update: Technical details and everything about the field tests can now be found in our latest publication (open-access): https://doi.org/10.1016/j.wroa.2021.100124

 

 

Urine module
The urine module built-in the Blue Diversion Autarky toilet

Publications

On-site urine treatment combining Ca(OH)2 dissolution and dehydration with ambient air

We present the results of three field tests and three laboratory tests of a new physical-chemical urine treatment system, which can recover all nutrients, while pathogens are inactivated. The system consists of two steps. In the first reactor, biological processes including urea hydrolysis are prevented by mixing fresh urine with calcium hydroxide (Ca(OH)2). Due to the high pH value and the high availability of calcium, phosphate can be recovered by precipitation. The high pH value also fosters the inactivation of microorganisms, including pathogens. In the second reactor, water is evaporated at low energy consumption by blowing unheated ambient air over the stabilized urine. Stabilization in the first reactor was successful in all field and laboratory tests. The pH value remained between 12 and 13, except for short dips due to shortages of Ca(OH)2. Nearly all phosphorus (92-96%) precipitated and could be recovered as calcium phosphate in the first reactor, while nitrogen and potassium overflowed with the urine into the evaporation reactor. The efficiency of the second treatment step was very different for field and laboratory experiments and depended on the duration of the experiment. During a four-day laboratory test, nitrogen recovery was 98%. In contrast, nitrogen recovery was only around 20% in the long-term field experiments. The high nitrogen losses occurred, because biological urea hydrolysis was not inhibited anymore, when the pH value in the second reactor decreased due to the dissolution of high amounts of carbon dioxide from the ambient air. Potassium was not subject to any significant loss, and the measured recovery in the solid evaporation product was 98%. Evaporation rates ranged between 50 g m-2 h-1 (RH = 82±13%, T = 12±6°C) and 130 g m-2 h-1 (RH = 60±19%, T = 24±5°C) in the three field tests. Apart from some disturbances due to low supply of Ca(OH)2, the urine module functioned without any substantial failures and was simple to maintain. The minimum consumption of Ca(OH)2 at full capacity was 6 g·L-1 urine and the electricity demand was 150 Wh kg-1 water evaporated from urine, resulting in operational costs of 0.05 EUR pers-1 d-1.
Riechmann, M. E.; Ndwandwe, B.; Greenwood, E. E.; Reynaert, E.; Morgenroth, E.; Udert, K. M. (2021) On-site urine treatment combining Ca(OH)2 dissolution and dehydration with ambient air, Water Research X, 13, 100124 (12 pp.), doi:10.1016/j.wroa.2021.100124, Institutional Repository

A novel approach for stabilizing fresh urine by calcium hydroxide addition

In this study, we investigated the prevention of enzymatic urea hydrolysis in fresh urine by increasing the pH with calcium hydroxide (Ca(OH)2) powder. The amount of Ca(OH)2 dissolving in fresh urine depends significantly on the composition of the urine. The different urine compositions used in our simulations showed that between 4.3 and 5.8 g Ca(OH)2 dissolved in 1 liter of urine at 25 °C. At this temperature, the pH at saturation is 12.5 and is far above the pH of 11, which we identified as the upper limit for enzymatic urea hydrolysis. However, temperature has a strong effect on the saturation pH, with higher values being achieved at lower temperatures. Based on our results, we recommend a dosage of 10 g Ca(OH)2·L−1 of fresh urine to ensure solid Ca(OH)2 always remains in the urine reactor which ensures sufficiently high pH values. Besides providing sufficient Ca(OH)2, the temperature has to be kept in a certain range to prevent chemical urea hydrolysis. At temperatures below 14 °C, the saturation pH is higher than 13, which favors chemical urea hydrolysis. We chose a precautionary upper temperature of 40 °C because the rate of chemical urea hydrolysis increases at higher temperatures but this should be confirmed with kinetic studies. By considering the boundaries for pH and temperature developed in this study, urine can be stabilized effectively with Ca(OH)2 thereby simplifying later treatment processes or making direct use easier.
Randall, D. G.; Krähenbühl, M.; Köpping, I.; Larsen, T. A.; Udert, K. M. (2016) A novel approach for stabilizing fresh urine by calcium hydroxide addition, Water Research, 95, 361-369, doi:10.1016/j.watres.2016.03.007, Institutional Repository

Urea hydrolysis and precipitation dynamics in a urine-collecting system

Blockages caused by inorganic precipitates are a major problem of urine-collecting systems. The trigger of precipitation is the hydrolysis of urea by bacterial urease. While the maximum amount of precipitates, i.e. the precipitation potential, can be estimated with equilibrium calculations, little is known about the dynamics of ureolysis and precipitation. To gain insight in these processes, we performed batch experiments with precipitated solids and stored urine from a urine-collecting system and later simulated the results with a computer model. We found that urease-active bacteria mainly grow in the pipes and are flushed into the collection tank. Both, bacteria and free urease, hydrolyse urea. Only few days are necessary for complete urea depletion in the collection tank. Two experiments with precipitated solids from the pipes showed that precipitation sets in soon after ureolysis has started. At the end of the experiments, 11% and 24% of urea was hydrolysed while the mass concentration of newly formed precipitates already corresponded to 87% and 97% of the precipitation potential, respectively. We could simulate ureolysis and precipitation with a computer model based on the surface dislocation approach. The simulations showed that struvite and octacalcium phosphate (OCP) are the precipitating minerals. While struvite precipitates already at low supersaturation, OCP precipitation starts not until a high level of supersaturation is reached. Since measurements and computer simulations show that hydroxyapatite (HAP) is the final calcium phosphate mineral in urine solutions, OCP is only a precursor phase which slowly transforms into HAP.
Udert, K. M.; Larsen, T. A.; Biebow, M.; Gujer, W. (2003) Urea hydrolysis and precipitation dynamics in a urine-collecting system, Water Research, 37(11), 2571-2582, doi:10.1016/S0043-1354(03)00065-4, Institutional Repository

Decrey, L., and Kohn, T. (2017) Virus inactivation in stored human urine, sludge and animal manure under typical conditions of storage or mesophilic anaerobic digestion.  Environmental Science: Water Research & Technology.

Antonini, S., et al. (2012). "Solar thermal evaporation of human urine for nitrogen and phosphorus recovery in Vietnam." Sci Total Environ 414: 592-599.

Pahore, M. M., et al. (2010). "Rational design of an on-site volume reduction system for source-separated urine." Environ Technol 31(4): 399-408.

More about feces treatment

In the Blue Diversion Autarky toilet, the feces are separated from the flush water and collected in a container at the bottom of the toilet. They may contain pathogens and must be inactivated quickly to avoid anaerobic decomposition and the emission of malodorous gases. 

Contact

Frédéric Vogel (PSI/ FHNW) frederic.vogel@psi.ch

In our approach, the organic matter of the fecal sludge is completely mineralized to carbon dioxide, water and minerals such as phosphate salts. The remaining streams are off-gas and a mixture of water and minerals. The off-gas contains mainly nitrogen, carbon dioxide and oxygen. This mixture can be safely vented to the atmosphere. The aqueous stream may be utilized as a fertilizer.

The process used to treat the feces is called “hydrothermal oxidation” or HTO. When mixed with air and heated above around 400°C under high pressure, the fecal sludge decomposes and is oxidized completely to carbon dioxide and water. The water in the sludge does not evaporate but mixes with the air and provides a reaction environment for an efficient conversion of the organic matter within a few minutes.

Current activities

For a better understanding of the HTO process, we are developing a comprehensive computer model of the reactor. This computer model is fed with data from experiments carried out in small autoclaves with real fecal sludge. We determined that the oxidation reaction is rapid above around 300°C and runs to completion within only a few minutes at 400°C.

We are currently improving the third prototype generation of the so-called FOX reactor (short for feces oxidation). This reactor will be field tested with real users soon. We are also working on the design of a more compact version of the FOX reactor, so it can be tested as part of the complete Blue Diversion Autarky toilet in the future.

Publications

Hübner, T.; Roth, M.; Vogel, F. (2016).  Hydrothermal oxidation of fecal sludge: experimental investigations and kinetic modeling, Ind. Eng. Chem. Res. 55 (46), pp.11910-11922 

Mangold, F.; Pilz, St.; Bjelić, S.; Vogel, F. (2019). Equation of state and thermodynamic properties for mixtures of H2O, O2, N2, and CO2 from ambient up to 1000 K and 280 MPa. The Journal of Supercritical Fluids, Volume 153, 104476

Team

The Blue Diversion Autarky team consists of researchers and experts from the Swiss Federal Institute of Aquatic Science (Eawag), the Paul Scherrer Institue (PSI), the University of Applied Sciences and Arts Northwestern Switzerland (FHNW) and from EOOS. The project is supported by an advisory board of six international commercial partners. Funding is provided by the Bill and Melinda Gates Foundation (BMGF).

Partner projects

Blue Diversion Toilet - an attractive, safe and affordable sanitation solution

VUNA - nutrient recovery from human urine

Nest - exploring the future of buildings