Coal fly ash for the recovery of nitrogenous compounds from wastewater: Parametric considerations and system design

Abstract

The use of coal fly ash for nutrient recovery in wastewater treatment offers a potential win–win scenario as it simultaneously utilizes and re-channels two wastes – coal fly ash and wastewater. This study investigated the adsorptive recovery of urea from synthetic urine using fly ash. Equilibrium experiments revealed that 1.5g ash loading, initial urea concentration of13.5gL−1, urine pH=6.0, sorption temperature=30°C and 150rpm shaker speed were the optimal process parameters and maximum adsorption capacity was 410mgg−1. Proof of concept to the use of synthetic urine was conducted by performing adsorption with real human urine which validated the experimental findings. Comparative analysis and non–linear optimization of nine isotherm models through comprehensive error analysis revealed that Flory–Huggins and Redlich–Peterson equations best described the adsorption. Process thermodynamics suggested that Van der Waal’s and electrostatic interactions occur between urea molecules and the surface of fly ash particles; besides, the sorption was found to be exothermic, spontaneous and physical in nature. Kinetic studies pointed toward a pseudo–second–order kinetic fit with contributions from intra–particle diffusion. Further, the rate of mass transfer was seen to be controlled and limited by film diffusion of urea which featured stronger than its pore diffusion. To design a multistage batch adsorber, a mathematical model unique to the sorption system was derived that minimized the total amount of fly ash required for 90% removal of urea from different volumes of influent synthetic urine solutions.