Contents lists available at ScienceDirect Environmental Research journal homepage: www.elsevier.com/locate/envres Rift Valley Lake as a potential magnesium source to recover phosphorus from urine Awoke Guadie a,b,∗ , Abaynesh Belay c , Wenzong Liu b , Asamin Yesigat d , Xiaodi Hao e , Aijie Wang b,∗∗ a Department of Biology, College of Natural Sciences, Arba Minch University, Arba Minch 21, Ethiopia b Key Laboratory of Environmental Biotechnology Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, PR China c Department of Civil and Environmental Engineering, Norwegian University of Science and Technology, Trondheim, Norway d Department of Environmental Engineering, College of Biological and Chemical Engineering, Addis Ababa Science and Technology University, Addis Ababa, 16417, Ethiopia e Sino-Dutch R&D Centre for Future Wastewater Treatment Technologies/Key Laboratory of Urban Stormwater System and Water Environment, Beijing University of Civil Engineering and Architecture, Beijing, 100044, China ARTICLE INFO Keywords: Phosphorus recovery Urine Magnesium source Rift valley lake Struvite ABSTRACT Phosphorus recovery from urine is a sustainable approach. However, the challenge of this process is the ac- cessibility of economically feasible magnesium sources. This study aimed to investigate the potential of low- cost Rift Valley Lake magnesium (RVL-Mg) source for phosphorus recovery from urine, where data is defcient in the source area. The efect of various operational conditions such as storage, Mg:P ratio (0.8–2.0), pH (5.5–10.5), mixing speed (30–180 rpm), urease enzyme addition (50–700 μL), urine dilution (0.11–9.0) and Ca:Mg ratio (0.3–2.5) was investigated. Under optimum operating conditions (M:P = 1.6, pH = 9, urease enzyme = 500 μL, mixing speed = 120 rpm, 60 min precipitation), the phosphorus removal efciencies were >96% for actual and >98% for synthetic urine. During storage, spontaneous phosphorus losses were observed from synthetic (24.0%) and actual (32.0%) hydrolyzed urine due to precipitation with calcium and magne- sium. The phosphorus recovery efciency was reduced at higher (0.11:1) and lower (9:1) urine to RVL-Mg dilution, which is related to lower supersaturation of phosphorus and magnesium ions, respectively. Addition of calcium did not afect phosphorus removal efciency, but the efect was signifcant on crystal product. With low (<1.0) Ca:Mg ratio, the crystal chemical analysis showed that the product has to be pure struvite (>99%), which was further identifed by scanning electron microscope and X-ray difraction to be quality struvite that might be used for agricultural purpose. Overall, low-cost magnesium ion collected from Rift Valley Lake can be a potentially candidate for sustainable phosphorus recovery from urine and any other phosphorus containing waste stream. 1. Introduction Phosphorus is an essential element for life and agricultural pro- duction. To enhance crop production and meet the demand for global food security, a large amount of phosphorus is demanded by fertilizer industries. The world population which increases annually by 1.1% (UN, 2019) has also an increasing annual phosphorus consumption/ demand (2.2%) (FAO, 2015). Industries are producing phosphorus containing mineral fertilizer using fossil, which is a limited non-re- newable resource (Daneshgar et al., 2018). As a result, pure phosphate rock used for this purpose is depleting (Krugera and Adam, 2017; Li et al., 2018). It has been predicted that phosphorus reserve will run out in the next 50–100 years (Cordell et al., 2009; Daneshgar et al., 2018; Li et al., 2018). Previous study revealed that phosphorus and nitrogen can be re- covered from urine (Etter et al., 2011; Mehta and Batstone, 2013). Urine which accounts 1% of the volume of wastewater contains a large amount of phosphorus (>90%) and nitrogen (>70%) (Berndtsson, 2006). Study showed that urine replaces the synthetic fertilizer con- taining nitrogen (19%), phosphorus (20%) and potassium (29%) (Vinneras et al., 2008). Phosphorus recovery from urine in the form of struvite (MgNH 4 PO 4 .6H 2 O) is well documented in the literature (Doyle and Parsons, 2002; Etter et al., 2011; Guadie et al., 2014). The crystal https://doi.org/10.1016/j.envres.2020.109363 Received 16 January 2020; Received in revised form 6 March 2020; Accepted 8 March 2020 ∗ Corresponding author. Department of Biology, College of Natural Sciences, Arba Minch University, Arba Minch 21, Ethiopia. ∗∗ Corresponding author. E-mail addresses: awokeg@yahoo.com (A. Guadie), ajwang@rcees.ac.cn (A. Wang). Environmental Research 184 (2020) 109363 Available online 12 March 2020 0013-9351/ © 2020 Elsevier Inc. All rights reserved. T