Challenges and opportunities of nutrient recovery from human urine using biochar for fertilizer applications

Although urine constitutes less than 1% volume of the total wastewater, it adds up 50-80% of the nutrient load [Phosphates (PO43-) and Nitrates (NO3-), Ammonium (NH4+)] in wastewater treatment plants (WWTPs). Recovery of Phosphorous (P) and Nitrogen (N) from urine diminishes the eutrophication impact as well it could act as a sustainable source of P by lessening dependence on finite phosphorite reservoirs. Due to its environmental (carbon-negative process) and economic (produced from waste) value, biochar is gaining the researchers attention in recent years. Many studies are ongoing to introduce novel structures and modifications in the surface properties of biochar for efficient adsorption of P and N. Direct application of urine as fertilizer has been discouraged due to various hygienic pitfalls further forced to explore the alternative nutrient recovery methods. This review article explores the nutrient recovery paradigms from source-separated human urine for abridging the nutrient gap between wastewater treatment and agricultural productivity using pristine/engineered biochar. The influencing factors for P and N's adsorption process by biochar and its detailed mechanisms have been outlined. Further, it deliberates the possible strategies of engineering the biochar to enhance the recovery of P and N from human urine. Applications of nutrient-loaded biochar in an agricultural field could act as a slow-release fertilizer to releases P and N in addition to soil conditioning. This review emphasized the challenges of storage, transportation, contamination of urine, and possible difficulties in the adsorption process by pristine/engineered biochar. (C) 2021 Elsevier Ltd. All rights reserved.