The potential use of human urine as a solvent for biogas upgrading

Abstract

Biogas upgrading through chemical absorption is an energy-intensive and costly process, but it is essential for the higher-value utilization of biogas as biomethane. The loss of absorbents and the energy consumption for their regeneration lower the economic viability of the process. This study investigated the potential of human urine as an economical and sustainable source of solvent for biogas upgrading. Hydrolyzed urine (HU) is rich in ammonia (>4.4 g N/L) and alkaline (approximately pH 9), which makes it attractive as an absorbent to capture CO2. The hydrolysis characteristics of urine were assessed with and without urease addition at different temperatures (4, 25, and 35 °C). Urease induced immediate urine hydrolysis, and temperature had a more significant effect on the hydrolysis rate when urease was not added. Conductivity, but not pH, proved to be a reliable indicator for monitoring urine hydrolysis. The CO2 capture capacity observed in the biogas upgrading experiments with simulated biogas (60% CH4 and 40% CO2, v/v) was 0.41–0.53 mol CO2/mol total ammonia nitrogen (TAN). The experiments using real biogas (71% CH4) showed that CO2 was effectively captured by HU with complete removal of H2S (2440 ppmv). This study is the first to demonstrate the direct use of HU as a sole solvent for biogas upgrading and suggest the possibility of recovering fertilizer material (e.g., ammonium bicarbonate, NH4HCO3) from the spent HU. The results suggest interesting opportunities to improve the economic feasibility of biogas upgrading and to valorize human urine.