Blue Diversion AUTARKY – Wastewater Treatment off the Grid

Project short description

We develop reactors for the separate treatment of urine, feces and water directly in the toilet. The overall aim of the project is to establish a self-sustaining grid-free (autark) toilet for the world's poorest. Through the separation of urine, feces and water at the source, the three streams can be treated according to their specific properties, which allows a maximum recovery of resources such as nutrients or water.


Research at a glance

The AUTARKY team develops reactors for the treatment of urine, feces and water. Urine will first be stabilized by dosing calcium hydroxide and then concentrated by evaporating the water. The feces treatment is based on the supercritical water oxidation (SCWO). The final products are carbon dioxide, water and precipitated inorganic solids. The core process for the water treatment is a gravity driven membrane (GDM) filtration. In this process, a biofilm grows on the membrane and ensures a constant water flow. After some advanced treatment to remove microorganisms, the filtered water can be used for flushing and hand washing.

Read more about water treatment

The gravity driven membrane of the Blue Diversion Toilet has a permeability of approximately 3-4 L/h respectively 1-1.5L/h/m2. It filtrates water for handwashing and flushing for about 50 uses per day (10 users).

The gravity driven membrane achieves approximately 95% removal of organic carbon entering the system. Additional treatment is required to maintain microbial stability within the clean water tank. Microbial stability refers here to water with limited potential for contamination and growth of pathogens. The electrolysis unit in the previous version of the toilet has demonstrated an ability to reduce organic carbon concentrations and produce a chlorine residual, both of which help to limit pathogen growth.

Treatment technologies such as activated carbon, ozone and UV light are being considered as alternatives to the electrolysis. These alternatives may be less expensive to install and maintain, and they may produce superior water quality given considerations such as their impact on biology of the membrane. The advantages and disadvantages of each technology will be first evaluated in batch; promising technologies will then be integrated into full-scale toilet systems. A quantitative microbial risk assessment will be performed on the entire module. In this regard, pathogen surrogates are used to investigate passage through the membrane and in competitive experiments with the natural bacteria community which forms in the clean water reservoir.

Further information on gravity driven membranes

Project: Gravity-Driven Membrane (GDM) technology

The preceding project Blue Diversion, achieved tremendous progress towards developing gravity driven membrane technology (GDM) into a practical toilet water recycling system for urban developing world applications. The goal of AUTARKY is to better understand and verify the safety of our system with respect to the user and to explore alternative technologies which may reduce capital and energy costs. We are also examining how the same membrane technology functions in systems representing higher and lower organic and nutrient loading.


Derlon, N.; Mimoso, J.; Klein, T.; Koetzsch, S.; Morgenroth, E. (2014) Presence of biofilms on ultrafiltration membrane surfaces increases the quality of permeate produced during ultra-low pressure gravity-driven membrane filtration, Water Research, 60, 164-173, doi:10.1016/j.watres.2014.04.045, Institutional Repository
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
Chomiak, A.; Traber, J.; Morgenroth, E.; Derlon, N. (2015) Biofilm increases permeate quality by organic carbon degradation in low pressure ultrafiltration, Water Research, 85, 512-520, doi:10.1016/j.watres.2015.08.009, Institutional Repository

Read more about urine treatment

Two processes are necessary to treat the source-separated urine in the toilet: urine stabilization and water removal. Stabilization is necessary to avoid malodour which occurs during storage, to inactivate pathogens and to conserve the valuable nutrient urea. Once the urine is stabilized the volume of the urine will be reduced by evaporation, the remaining end-product is a mix of salts. After further external treatment steps these salts can be used as fertilizer in agriculture.


Prof. Dr. Kai UdertTel. +41 58 765 5360Send Mail

Urine stabilization in detail

The main goal of this treatment step is to prevent urea hydrolysis, which is a process that converts urea to ammonia and carbon dioxide. Through the addition of calcium hydroxide to fresh urine the pH increases to values above 12 and prevents microbial urea hydrolysis. An additional benefit of the high pH value is the killing of pathogens and the prevention of biological processes that produce malodour.

When Ca(OH)2 is added to the urine  only about the amount dissolves, which is needed to reach the necessary high pH value. This allows providing a depot of Ca(OH)2 in the stabilization tank; therefore no expensive and complicated dosage mechanisms are required. Moreover, calcium hydroxide is a cheap reagent and readily available worldwide. An additional benefit of Ca(OH)2 dosage is the precipitation of calcium phosphate. This mineral could be recovered apart from the other urine compounds and used for fertilizer production.

Sedimentation of calcium phosphate in stabilised fresh urine
Sedimentation of calcium phosphate in stabilised fresh urine
Filtration of stabilised fresh urine
Filtration of stabilised fresh urine


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
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

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  

Urine water removal in detail

The direct application of human urine as fertilizer is a common practice in many rural areas worldwide. However, the high water content of urine – no matter if stabilized or not – requires significant storage capacity and can make the 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 temperatures (distillation) or pressures (reverse osmosis).


Christophe BonvinTel. +41 58 765 6401Send Mail
Prof. Dr. Kai UdertTel. +41 58 765 5360Send Mail

We are currently focusing on several evaporation techniques, which do not require any heating and pressurizing processes. One example is the evaporation pipe (see picture 2) where urine is trickled down a pipe, which is aerated with ambient air that takes up water. Our goal is to recover a hygienically safe end-product that concentrates valuable nutrients that will be applied to cropland.


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.

Evaporation of stabilized urine

First lab tests: Urine trickles down through the inner surface of the evaporation pipe while air is forced upwards causing urine to dry progressively.
First lab tests: Urine trickles down through the inner surface of the evaporation pipe while air is forced upwards causing urine to dry progressively.

Read more about feces treatment

In the Autarky toilet, feces are separated from urine and collected in a container at the bottom of the toilet. They may contain harmful compounds such as pathogens and must be inactivated quickly to avoid anaerobic decomposition and the emission of malodorous gases.


Frédéric Vogel (PSI/ FHNW)

In our approach the organic matter of the feces 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 and can be safely vented to the atmosphere. The aqueous stream may be utilized as a fertilizer.

Feces mineralization in detail

When mixed with air and heated above ca. 400°C under high pressure, the feces sludge decomposes and is oxidized completely to carbon dioxide and water. This process is called “hydrothermal oxidation” or HTO. 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.

We are developing a comprehensive computer model of the HTO reactor to be built for the Autarky toilet. 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 ca. 300°C and runs to completion within a few minutes at 400°C. The next step is to build a small laboratory-scale prototype to perform experiments under more realistic conditions and to test the model predictions. With the validated model a full-scale prototype will be designed and built in 2016.

Feces sludge before (left) and after HTO treatment (right).


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 

Hübner, T. (2015) Hydrothermal Oxidation of Faecal Sludge, Master thesis, TU Dresden and Paul Scherrer Institut.

Vogel, F.; Smith, K.A.; Tester, J.W.;Peters, W.A. (2002) Engineering Kinetics for Hydrothermal Oxidation of Hazardous Organic Substances, AIChE J. 48(8), 1827-1839.

Read more about FMECA

By applying the Failure Mode, Effects and Criticality Analysis (FMECA), we will identify failure modes with relatively high probability and severity for the three treatment modules (urine, feces and polluted water). The analysis will be iterated three times at different stages of the project to account for adaptations. Established standard procedures will be applied for the analysis. After performing the FMECA, we will formulate technical recommendations to reduce the consequences of critical failures. This may include selecting components with higher reliability, reducing the stress level at which a critical item operates, or adding redundancy or monitoring to the system.


PD Dr. Heiko GebauerGroup leader: Business Innovation for Sustainable Infrastructure ServicesTel. +41 58 765 5484Send Mail
Mirella HaldimannPhD StudentTel. +41 58 765 5283Send Mail

About the project

AUTARKY was launched in the context of the “Reinvent the Toilet Challenge” funded by the Bill & Melinda Gates Foundation with the goal to develop a truly aspirational “next-generation” toilet that does not require a sewer, water connection or electricity, costs less than 5 cents per user per day with a design to meet people’s needs to enable universal access to sustainable sanitation services. Blue Diversion AUTARKY is the continuation of the project.

Blue Diversion


AUTARKY brings together an international team of scientists from the Eawag, the Paul Scherrer Institute, the FHNW, the EPFL, the Swedish University of Agriculture and designers from the Austrian design office EOOS.

Autarky Team

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