Phosphorus removal and recovery from secondary effluent in sewage treatment plant by magnetite mineral microparticles

Abstract

This study investigated the applicability and chemical stability of magnetite mineral microparticles (Mag-MM) (average particle size: 34μm) for phosphorus (P) removal and recovery from actual secondary effluent (SE) of a sewage treatment plant. The maximum P adsorption capacity of Mag-MM was 0.52–0.83mgP/g at 5–45°C. The values of three thermodynamic parameters, standard entropy ΔS0 (137.74–138.04J/(Kmol)), standard enthalpy ΔH0 (7.32kJ/mol), and standard Gibbs free energy ΔG0 (−30.97 to −36.57kJ/mol), indicated that P adsorption on Mag-MM was endothermic and spontaneous in nature and involved both physisorption and chemisorption. The coexisting cations, anions, and dissolved organic matter in the actual SE had little or no effect on P adsorption on Mag-MM. Approximately >40g/L Mag-MM and >200g/L Mag-MM were individually required to produce quality effluent (<0.1mgP/L) from the actual SE of anaerobic-anoxic-oxic (1.0–2.0mgP/L) and conventional activated sludge (~8mgP/L) processes, respectively. During 30 adsorption–regeneration cycles, the amount of reloaded P Mag-MM varied over a range of 0.023–0.039mgP/g, which stably produced quality effluent from the actual SE. The P in exhausted regeneration solutions was finally recovered as a precipitate of calcium phosphate by the addition of CaCl2, with an optimum Ca/P (mol ratio) of 2.0. Compared with NaAc solutions, other alkaline regeneration solutions (i.e., NaOH, KHCO3 or Na2CO3) can produce quality effluent with lower P or organic concentration. These results demonstrated that Mag-MM has a remarkable potential for cost-effective P removal and recovery from actual SE.