The saying ‘we are what we eat’ is only part of the story. What we eat is what we excrete, and this means plant nutrients. Human excreta contain the same nitrogen, phosphorus and potassium (N-P-K) as the fertilisers used to produce the food consumed (Winker et al., 2009). However, human excreta are considered unwanted waste throughout the world, creating humanitarian and environmental problems (Baum et al., 2013). In order to replace the nutrients removed from the fields during harvesting, more fertilisers are manufactured in industrial processes that are contributing to environmental changes at global level (Rockström et al., 2009). Recycling human excreta back to agricultural fields would reduce the current dependence on fossil fuel-derived fertilisers (Ramírez & Worrell, 2006). It would also improve crop yields in e.g. sub-Saharan Africa, where fertiliser application is low (FAO, 2015), and protect marine ecosystems in the Baltic Sea by limiting the flow of excess nutrients to surface waters (Rockström et al., 2009).
The limitation of human urine as a fertiliser is its low nutrient concentration compared with commercial fertilisers. Urine is mostly water (97%), meaning that the concentration of nutrients is low. For example, the N concentration in urine is 0.6% (Vinnerås et al., 2006), whereas that of the manufactured fertiliser urea is 46%. Lower nutrient concentrations require larger quantities of urine to be applied per hectare as fertiliser, which creates logistics problems in terms of storage (as approx. 550 L of urine are produced per person and year) and increases the costs of transportation and application. Hence urine, as excreted, is not a competitive fertiliser. To better utilise the nutrients, the excess water in urine needs to be removed.
Concentrating the nutrients in urine while retaining N is challenging. Approximately 85% of the N in urine is initially present in non-volatile form, as urea (CO(NH2)2). Once excreted, the urea is quickly hydrolysed to volatile form, ammonia (NH3), in a reaction that is catalysed by urease enzymes. The volatility of NH3 means that urine cannot simply be dehydrated, but stopping the urease enzyme is also a challenge.
This study developed a technique to limit the urease enzymes by applying high pH and, through dehydration, increase the N concentration, from 0.6% to > 6%, to produce a dry fertiliser of monetary value and avoid the need for liquid disposal from the toilet. The technique is intended for a container-based sanitation system that collects, contains, treats and reduces the volume of urine within the container. In tests, fresh human urine was added at various intervals to wood ash (initial pH >12) at 35 °C and 65 °C, to alkalise and thus inhibit the enzyme urease from catalysing hydrolysis of urea to ammonia. Mass balance calculations demonstrated a 95% reduction during dehydration, while preserving up to 90% of the N. Such a system would greatly simplify the logistics and costs of storage, transportation and application of urine as a fertiliser. The truly innovative feature is the final product: a dry powder with 7.8% N, 2.5% P and 10.9% K by weight, i.e. equivalent to commercial fertiliser.
Welcome to check out our latest publication on the subject in the journal Science of The Total Environment.
Senecal, J., Vinnerås, B. 2017. Urea stabilisation and concentration for urine-diverting dry toilets: Urine dehydration in ash. Science of The Total Environment, 586, 650-657.