Urine-enriched biochar: Coupling sustainability in sanitation and agriculture

Abstract

Linear models of fertilizer production and application are environmentally harmful. Predominant approaches to waste management treat human excreta as a pollutant rather than a source of nutrients for agriculture. Container-based sanitation (CBS) systems safely contain and transport excreta for treatment and reuse, though urine is often contained but not treated. A major challenge of urine-nutrient recovery is the shift in nitrogenous species in urine during storage, from urea to ammonia (NH3) and ammonium (NH4+), due to urease activity. This can lead to gaseous NH3 losses from urine that depletes its fertilizer potential. Urine-enriched biochar (UEBC) may act as a slow-release fertilizer of urine nutrients. We quantified the adsorption of nitrogen in fresh, stored, and CBS-style urine to wood waste, sewage sludge, and walnut shell biochars. These UEBCs were compared to urine-only treatments and fertilized and unfertilized controls in a greenhouse growth experiment. We found that the <500-µm biochar size fraction retained significantly more nitrogen than larger particles across biochars. Urine-nitrogen adsorption to biochar and uptake into plant tissue varied across biochar type and urine condition. The quantity of urine applied in urine-only treatments, regardless of type, was positively correlated with plant nitrogen uptake. Plant biomass did not differ significantly across treatments. These findings emphasize the need to optimize UEBC application for different urine and biochar conditions, particularly for CBS and other urine-diverting operations.