The main nutrients flowing from households come from the toilet. Our research on sanitation systems aims at developing robust and socially acceptable systems that safely and effectively utilise the resources available in our excreta. Closing the cycle with the toilet involves certain risks due to pathogenic microorganisms potentially being circulated. We evaluate and minimise these risks with improved technologies and good management.
Plant nutrients in urine are readily available and very clean with respect to heavy metals. The simplest approach to utilize source-separated urine as fertiliser is to collect urine in a sealed tank and then use it as liquid fertiliser. The ammonia content, naturally found in the urine, is effective at inactivating any present pathogens during storage (Vinnerås et al., 2008). In a year, we can collect up to 500 liters of urine per person plus any flush water. However, source-separated urine has a low concentration of the nutrients compared to mineral fertilisers. The nitrogen content in the source-separated urine without flushing water is 0.4-0.6%. The function (sorting, stop, etc.) of urine diverting systems have been improved through past research. Now we focus on systems for concentrating urine to produce a dry fertiliser. Most of the nitrogen excreted in the urine is as urea. We raise urine pH and thereby stop the natural enzymatic processes that break down urea into more volatile form, ammonia (Senecal & Vinnerås, 2017). This allows for dehydration of the excess water (urine is approx. 93% water) without loss of nitrogen. The dry product can have similar nutrient content and concentration to manufactured NPK fertilisers, as well as similar form and application. A urine-based fertiliser also has the added benefit of intrinsic micronutrients such as calcium and magnesium. Surveys in India (Simha et al., 2017) have also indicated that cultivators would prefer dry fertilisers manufactured from urine rather than liquid urine. Our goal is to produce a fertiliser with 20% nitrogen and 2% phosphorus by weight, which can be used in the same manner as today's commercial fertilisers.
The handling of faeces in dry-source-separating systems is still neither optimized nor socially well-accepted. Faeces always present a very high risk of infection and it is important that the management system is built so that the chain of infection is broken, and the risk of reinfection is minimized. We work with two alternative concepts. One approach we develop is having a long residence time within the toilet while efficiently reducing the volume of faeces and toilet paper to minimize the accumulation of materials, leading to a low discharge rate. A second approach is based on an efficient and value-adding processing outside the toilet. Here, the faeces container is emptied frequently and the collected material is processed in a value-creating process, eg by composting with fly larvae to produce protein.
Our studies on the use of dry toilets have shown that people are most satisfied with their system when the toilet is emptied often, every one or two weeks. The main reason for this is that it then becomes a simple routine and the handling-weight is minimal – the routine becomes much like taking out the garbage.
In low-flow source separating systems that collect toilet excreta in a closed tank, we cooperate with several Swedish municipalities to safely treat this fraction and return plant nutrients to agricultural systems (see sanitation and organic waste). This is a simple and effective way to improve the circulation of nutrients while reducing infectious and eutrophying emissions. By creating these localized cycles, at municipal level, we can reduce risks towards surface and groundwater as there is no sewage to flow into these water bodies. Such a cyclical system also reduces the use of fossil resources and the emission of greenhouse gases.
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.
Simha, P., Lalander, C., Vinnerås, B., Ganesapillai, M. 2017. Farmer attitudes and perceptions to the re–use of fertiliser products from resource–oriented sanitation systems – The case of Vellore, South India. Science of The Total Environment, 581–582, 885-896.
Vinnerås, B., Nordin, A., Niwagaba, C., Nyberg, K. 2008. Inactivation of bacteria and viruses in human urine depending on temperature and dilution rate. Water Research, 42(15), 4067-4074.