Urea stabilisation and concentration for urine-diverting dry toilets: Urine dehydration in ash

Human excreta contain the same nitrogen, phosphorus and potassium (N-P-K) as the fertilisers used to produce the food consumed. However, human excreta are considered unwanted waste throughout the world, creating humanitarian and environmental problems. In order to replace the nutrients removed from fields during crop harvesting, more fertilisers are manufactured, in processes contributing to environmental changes at global level. The limitation of human urine as a fertiliser is its low nutrient concentration compared with commercial fertilisers. This study developed a technique to increase the N concentration (from 0.6% to >6%) through urine dehydration 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 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.