Abstract
Fresh human urine, after it is alkalized to prevent the enzymatic hydrolysis of urea, can be dehydrated to reduce its volume and to produce a solid fertilizer. In this study, we investigated the suitability of MgO to alkalize and dehydrate urine. We selected MgO due to its low solubility (<2 g·L−1) and relatively high saturation pH (9.9 ± 0.2) in urine. Using a laboratory-scale setup, we dehydrated urine added to pure MgO and MgO mixed with co-substrates (biochar, wheat bran, or calcium hydroxide) at a temperature of 50°C. We found that, dehydrating urine added to a mixture of MgO (25% w/w), biochar, and wheat bran resulted in a mass reduction of >90% and N recovery of 80%, and yielded products with high concentrations of macronutrients (7.8% N, 0.7% P and 3.9% K). By modeling the chemical speciation in urine, we also showed that ammonia stripping rather than urea hydrolysis limited the N recovery, since the urine used in our study was partially hydrolyzed. To maximize the recovery of N during alkaline urine dehydration using MgO, we recommend treating fresh/un-hydrolysed urine a temperature <40°C, tailoring the drying substrate to capture NH4+ as struvite, and using co-substrates to limit the molecular diffusion of ammonia. Treating fresh urine by alkaline dehydration requires only 3.6 kg MgO cap−1y−1 and a cost of US$ 1.1 cap−1y−1. Therefore, the use of sparingly soluble alkaline compounds like MgO in urine-diverting sanitation systems holds much promise.
Highlights
In the decentralized sanitation sector, there is a growing body of research focusing on urine-diversion based systems that treat and recycle human urine as crop fertilizer (Larsen et al, 2013; Martin et al, 2020)
We modeled the urine with a reduced ammonia N concentration (400 mg·L−1) as against the composition measured in this study, 1,700 mg·L−1, to understand the effect ammonia N has on the solubility of magnesium oxide (MgO)
This study explored the use of MgO for treating human urine
Summary
In the decentralized sanitation sector, there is a growing body of research focusing on urine-diversion based systems that treat and recycle human urine as crop fertilizer (Larsen et al, 2013; Martin et al, 2020). Several promising technologies at various scales are being developed across the world for the on-site treatment of urine (Harder et al, 2019). One among these technologies is alkaline dehydration (Simha et al, 2020a; Simha et al, 2020b; Simha et al, 2020c), where urine is dried to produce a solid fertilizer with 10–30 times higher concentrations of plant nutrients than what is originally present in freshly excreted urine. Urease-producing bacteria are widespread in the environment (Mobley and Hausinger, 1989), and are commonly found in urine-diverting sanitation systems (Udert et al, 2003). The motivation to alkalize urine is to inhibit enzymatic ureolysis, as this allows evaporating water from urine and recovering N as urea, a widely used crop fertilizer (Dampney et al, 2003)
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