Optimization and Kinetic Study of Phosphorus Dissolution from Primary Settled-Nightsoil Sludge

In this study, chemical extraction using different acid concentrations, solids concentrations, and reaction time with subsequent interactions mechanism were carried out to evaluate the potential of phosphorus (P) recovery from primary settled-nightsoil sludge (PSNS). The response surface methodology (RSM) with Box-Behnken experimental design and one-way ANOVA analysis were also employed to establish optimal P leaching conditions. The extraction efficiency relied mainly on acid and solids concentration. The second-order polynomial model was successfully developed for extracting process designs. Approximately 93% of P could effectively be extracted from PSNS of 20,000 mg/L with 0.5 M of H2SO4 at reaction time of 45 min (optimum condition). Kinetic studies showed that the pseudo-second order was fit to describe leaching of P and metals. Moreover, the rate of kinetic constants (k2) of the P, Fe, Mg, and Ca under optimum condition were found to be 0.1607, 0.1099, 0.0317, and 0.0053 g/mg·min, respectively. The 99% leaching of maximum extracted P concentration at the equilibrium (9.6673 mg/g) took place in less than one hour. The findings of a suitable simple and low-cost method P dissolution from PSNS not only provides understanding of leaching kinetics, but also helps to pave a way of recovering P from a renewable resource in the field of waste utilization.