Factors influencing the recovery of organic nitrogen from fresh human urine dosed with organic/inorganic acids and concentrated by evaporation in ambient conditions

Abstract

To feed the world without transgressing regional and planetary boundaries for nitrogen and phosphorus, one promising strategy is to return nutrients present in domestic wastewater to farmland. This study tested a novel approach for producing bio-based solid fertilisers by concentrating source-separated human urine through acidification and dehydration. Thermodynamic simulations and laboratory experiments were conducted to evaluate changes in chemistry of real fresh urine dosed and dehydrated using two different organic and inorganic acids. The results showed that an acid dose of 1.36 g H₂SO₄ L^-1, 2.86 g H₃PO₄ L^-1, 2.53 g C₂H₂O₄·2H₂O L^-1 and 5.9 g C₆H₈O₇ L^-1 was sufficient to maintain pH ≤3.0 and prevent enzymatic ureolysis in urine during dehydration. Unlike alkaline dehydration using Ca(OH)₂ where calcite formation limits the nutrient content of fertiliser products (e.g. <15 % nitrogen), there is greater value proposition in acid dehydration of urine, as the products contain 17.9-21.2 % nitrogen, 1.1-3.6 % phosphorus, 4.2-5.6 % potassium and 15.4-19.4 % carbon. While the treatment recovered all phosphorus, recovery of nitrogen in the solid products was 74 % (±4 %). Follow-up experiments revealed that hydrolytic breakdown of urea to ammonia, chemically or enzymatically, was not the reason for the nitrogen losses. Instead, we posit that urea breaks down to ammonium cyanate, which then reacts with amino and sulfhydryl groups of amino acids excreted in urine. Overall, the organic acids evaluated in this study are promising for decentralised urine treatment, as they are naturally present in food and therefore already excreted in human urine.

Keywords: Acidification; Decentralised sanitation; Fertiliser; Nutrient recycling; Source separation; Wastewater treatment.