Passive evaporation of source-separated urine from dry toilets: a lab study David N. Bethune, Angus Chu and M. Cathryn Ryan ABSTRACT A methodology for evaporating human urine from dry toilets using gravity-drainage through vertically stacked plastic ‘cafeteria-type’ trays was tested. A thin layer (500 g) of medium-grained sand on the bottom of each tray enhanced evaporation, ammonia stabilization and solid product removal. A prototype laboratory unit initially evaporated up to 8.5 L m 2 d 1 but decreased to 1.5 L m 2 d 1 over time as salinity increased. The evaporation process produces a dark, highly saline, brine solution before drying to a solid product. The solid product has almost no odor and is mostly comprised of Na, Cl, N, P and K. Nitrogen loss, primarily by ammonia volatilization, significantly decreased the amount of N relative to P and K in the brine and solid product. About 90% of the NH 4 /NH 3 initially present in the input urine was lost in the evaporator system. David N. Bethune (corresponding author) Angus Chu Civil Engineering, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada, T2N 1N4 E-mail: bethuned@ucalgary.ca M. Cathryn Ryan Geoscience, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada, T2N 1N4 Key words | diversion, evaporation, sanitation, source-separation, urine INTRODUCTION Rural sanitation systems based on the separation of urine and feces from the toilet onward (a.k.a. urine diversion [UD] or urine source separation; Larsen & Gujer ; Hanæus et al. ; Hellstrom & Johansson ) offer some very distinct advantages over traditional urine-feces mixed (a.k.a. blackwater) systems. Urine and feces have several important differences that make their separation advantageous to each. Urine has a much higher water con- tent than feces (96% vs 70%) which, when combined with the annual rates of per capita production (500 L of urine vs. 50 L of feces), makes the onsite disposal or transport of urine challenging (Del Porto & Steinfeld ). Nutrient recovery from urine is also safer than from feces (which contain relatively high numbers of pathogens; Schonning & Stenstrom ), although some pathogen cross-con- tamination to urine can occur at the UD toilet (Schonning et al. ; Höglund et al. ). Finally, source-separated feces are drier, less odorous, and are more easily stabilized or composted than feces mixed with urine (Hill et al. ). Urine is a saline (3.5%), normally sterile solution con- taining soluble salts, nutrients and organic compounds dominated by the nitrogen-rich urea. Immediately after urine exits the human body it enters a non-sterile environ- ment and undergoes a spontaneous, irreversible process called urea hydrolysis (Warner ; Andrews et al. ). This conversion of organic urea nitrogen to inorganic ammonia leads to a dramatic pH increase from about 6 to 9 (Ciba-Geigy ; Kirchmann & Pettersson ; Jonsson et al. ; Udert et al. ) leading to high levels of ammo- nia gas producing a strong odor, and a substantial increase in alkalinity. The high ammonia levels naturally hygenize or sterilize urine over time (Höglund et al. ; Höglund et al. ) eliminating any potential pathogens within 4–6 months (Höglund ; Schonning & Stenstrom ). Urine contains most of the nutrients excreted by humans (85–90% nitrogen, 50–80% phosphorus, and 80–90% potass- ium; Larsen & Gujer ). On average, each person produces about 15 g d 1 of combined nutrients (N, P, and K) in urine, and only about 3 g d 1 in feces (Del Porto & 654 © IWA Publishing 2014 Journal of Water, Sanitation and Hygiene for Development | 04.4 | 2014 doi: 10.2166/washdev.2014.058 Downloaded from https://iwaponline.com/washdev/article-pdf/4/4/654/384974/654.pdf by guest on 21 May 2020