Complete Nutrient Recovery
Urine contains most of the nutrients excreted from human
metabolism: close to 90% of nitrogen and potassium, about 60% of phosphorus and
nearly 100% of sulfur. Recycling those nutrients to agriculture would have environmental
and possibly economic benefits, but a direct recycling of urine is only
sensible for some rural areas: the high water content renders transport costly
and the high concentration of ammonia (NH3) makes urine handling
unpleasant and cause nitrogen losses. Urine treatment processes are needed to
concentrate and stabilize the nutrients. Struvite precipitation (STUN) is an easy and efficient way to recover
phosphorus in a dry powder, but apart from a small fraction of ammonia, the
recovery is restricted to one nutrient. A complete recovery of all nutrients is
possible in an evaporation reactor.
Fertilizer product derived from stabilized urine after 98% of the water was evaporated. (photo: Stefan Kubli, Zurich)
Salt production by evaporation is a well-known and rather
simple process, but urine has to be pretreated. If stored urine is treated with
evaporation, a viscous mass is produced because of the high content of organic
substances and ammonia is lost by volatilization. These problems can be
prevented if urine is stabilized with nitrification (Nitrogen Stabilization): half of the ammonium is oxidized to nitrate,
the pH is between 6 and 7 and 90% of the organic substances are degraded.
With laboratory experiments, we could show that evaporation allows to concentrate practically all nutrients from nitrified urine in a dry powder; less than 3% of the initial dissolved nitrogen was lost by volatilization. Due to the high temperature during evaporation, the product is free of pathogens. The theoretical energy is about 700 Wh/L, though this can be reduced by up to 90% with vapor compression. Further research efforts focus on the thermal stability of the product (ammonium nitrate) and the reduction of the energy demand.
Presentations
Udert, K.M., Waechter, M. (2010) Complete Nutrient Recovery from Source-Separated Urine. 7th IWA Leading-Edge Conference on Water and Wastewater Technologies, 2-4 June 2010, Phoenix, Arizona.