Linear anthropogenic nutrient flows [Hader et al, 2021] perturbate the natural geochemical Phosphorus and Nitrogen cycles, one  of nine planetary boundaries, which together outline a  a safe conditions for the human population on earth  [Steffen et al, 2015]. The transgression of the planetary boundary for geochemical cycles predominantly arises  from fertilizer application from agriculture [Steffen et al, 2015], which is also strongly impacting at least four other planetary boundaries  [Campbell et al, 2017]. The effects of Nitrogen and Phosphorus pollution, even though defined as a planetary boundary are primarily on regional and local level [de Vries et al, 2013, Steffen et al. 2015]. This does not neglect the concept of an planetary boundary for geochemical cycles, but calls for aggregated regional boundaries as inorganic or chemical fertilizer application differs widely across world regions [de Vries et al, 2013] and thus action on the nutrient recycling and  management need to be though regionally  [van der Weil, 2019] and inter-regionally [Harder et al 2021]. The reduction of the regional nutrient throughput through the local recycling fertilizers will foster the transition to a sustainable regional circular economy [Harder et al, 2021].

Even though the human Nitrogen metabolism is about 5 % of the total productive reactive nitrogen, there is an resource perspective an energy and economical incentive to recover these nutrients [Larsen 2007]. The long-term sustainability of the conventional water based sanitation system has been increasingly been called into question, for both high and low-income countries. High costs for infrastructure and treatment, high water and energy demand, as well as the a more resource focused view from municipal solid and liquid waste – demand change of the conventional sanitary system.  The source separation and separate treatment of human excrements reduced dilution and contamination of nutrient while providing better options for recourse recovery.  [Harder, 2019]

There reasons for a sanitary- and nutrient-revolution have been brought forward by the a group of researcher and practitioners (nährstoffwende.de) [Krause et al, 2021]. 

1.  Resource recovery –  Instead of disposing of our feces, we must the recyclable materials it contains must be recyclable materials in the sense of the should be the focus.
2. Water scarcity – Against the backdrop of climate change and water scarcity, the demand for water for for the transport of feces must be reduced and the water pollution with nutrients and pollutants. pollutants must be prevented.
3. Technical feasible – Nutrient recycling from human feces from human feces with simultaneous removal of pollutants is possible, and thus in harmony with human and environmental health.

The technical feasibility, along with the resource circularity and socio-economy perspective are two very important aspects of the project zirkulierBAR and crucial for the evaluation and success of the project.

The Method 

Explorative material flow analysis is a well-established research method to investigate and explore flow regimes of system i.e. a first approximation of the flows, stocks and processes governing the throughput of the system [Brunner & Rechberger 2016, MinFuture 2018].  The methodology of material flow analysis is profoundly described in Brunner & Rechberger (2016) handbook of material flow analysis.  

A material flow analysis follows the consequential steps of goal or problem definition, system description, data acquisition, material balancing, and scenario building followed by the subsequent interpretation. All the steps are iterative, thus in case of inconsistencies or deviation of the defined goal, the modelling process may revisit and redefine or refine to any previous step [Rechberger & Brunner, 2016].  Material flow can give insights in the total material throughput and pathways of goods. Linking these material flows with substance mass fraction  will reveal the substance specific sources and sinks and allow the specific recommendations to improve substance flows.  

The System 

The system model living-laboratory comprises the dry toilets with serial or parallel urine separation, an aggregated process of hygienisation, composting and post-treatment comprising an thermophilic hygienisation, passive aerated windrows and sieving, an aggregated process of urine processing containing stabilization & purification and distillation as well as a external WWTP. 

Additional input to dry toilets are toilet paper and straw powder. For the composting process bio char, shredded branches and twigs, green waste, and clay minerals as well as a part of mature compost are added to the fresh windrow. The windrow is turned and waters regularly, also stabilized and purified urine is added to increase the nitrogen content and reduce fresh water consumption. After 6-8 weeks the mature compost is sieved, oversize material and mature compost are returned in a fresh compost windrow, impurities are disposed. 

The separated urine,  disregarding  the collection system (in parallel or serial urine separation) shall be stabilized and purified in the VUNA-process (https://vuna.ch/en/urin-recycling-technologie/). Half of the treated urine is used to  further processed in distillation process to concentrated liquid fertilizer and distillate which is discharged to the waste water treatment plant. The other half of the treated urine is used for watering of the compost to increase nitrogen (nitrate) content in the compost product. Excess urine can not be treated as the living-laboratory at the current stage. In the future, additional treatment capacity shall prevent the discharge of valuable and nutrient  to the waste water treatment plant. 

Data collection

Data on mass flows the composting an urine processing was provided by Finizio – future sanitation. Material composition for urine, feces, green waste, toilet paper were obtained from literature [Boldrin 2008, Bauhaus-Universität Weimar 2009, Krogmann et al 2010]. Data from straw, clay minerals and biochar were taken from pre-test laboratory analysis. All values were considered to have an 10 % standard uncertainty. 

System modeling

The material flow analysis is compiled with STAN 2.6.  Mass flows were based on the information by the compost producers.  No data reconciliation was performed. Result are rounded to two significant figures.   

The mass flow are normalized to 1000 population equivalents („Einwohnergleichwerte“) used for the design of waste water treatment plants in the German-speaking area [Gujer 2007]