Open access publications in the SLU publication database

Abstract

Over four billion people are discharging untreated human excreta into the environment without any prior treatment, causing eutrophication and spreading disease. This eutrophication is caused by nutrients found predominantly in urine. If managed adequately, urine can be used as a fertiliser because it contains the same plant nutrients as the fertilisers used to produce the food that people eat. Currently to replace the nutrients removed from fields during harvesting, more fertilisers are being manufactured and applied and ultimately more are being leached into the environment.

The use of human urine as a fertiliser is limited by its low nutrient concentration compared with commercial fertilisers. This study sought to increase the nitrogen (N) concentration (from 0.6 % to >6 %) through dehydration to produce a dry fertiliser of monetary value and where no liquid disposal from the toilet is required. The objective of this thesis was to evaluate a treatment that could stabilise urea and concentrate the urine while retaining >80 % of the NKP. Fresh human urine was added at various intervals to wood ash or biochar to first alkalise and thus inhibit the enzyme urease which catalyses the hydrolysis of urea. The urine was then dehydrated at temperatures of between 35 and 65 °C. A hygiene assessment was undertaken to observe the inactivation of five microorganisms (three indicators: Enterococcus faecalis, MS2 bacteriophage and ΦX 174 bacteriophage; and two pathogens: Ascaris suum and Salmonella enterica sub enterica Typhimurium) at the end of the alkaline dehydration process.

Urine mass was reduced by 95 % during dehydration, while preserving up to 90 % of the N and all the P and K. Ascaris inactivation data was fitted to a non-linear regression model, which estimated that 325 days of storage would be required for a 3 log10 reduction at 20 °C and 9.2 days of storage at 42 °C. The bacteria and bacteriophages were below the detection limit within four days at 20 °C. Just collecting urine separately from faeces provides a 5.2 log10 reduction. The material is concentrated during dehydration which results in a 3.5 log10 reduction overall just from urine-diversion.

This alkaline dehydration system installed in new or already existing toilets would greatly simplify the logistics and costs of storing, transporting and applying 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 on dry weight, i.e. equivalent to commercial fertilisers. After just four days of storage, the dehydrated medium would meet WHO (2006) and USEPA (1994) guidelines for unrestricted fertiliser use.