Chemical stress from Fe salts dosing on biological phosphorus and potassium behavior

In simultaneous precipitation, interaction between chemical and biological P removal could not be ignored. This work investigated effects of ferrous sulfate and Fe precipitates on metabolic behavior of bio-P and its counter cation of potassium. After dosing, mixed liquid suspended solids (MLSS) increased 9%, pH decreased from 7.35 to 7.00, sludge volume index (SVI) decreased, electrical conductivity increased. Chemical oxygen demand (COD) and NH₃ removal was not affected. Fe dosing initially showed synergistic effect, and then inhibition appeared at accumulative dose above 10 mgFe/gMLSS. Both precipitate FePO₄ and Fe(OH)₃ deteriorated effluent P. FePO₄ dissolved 35% in anaerobic phase which failed to be totally reprecipitated in oxic phase, resulting in increased effluent P. FePO₄ inhibited K uptake rather than bio-P uptake. Fe(OH)₃ caused reduction of bio-P release, meanwhile, its inhibition on K and bio-P uptake was greater than FePO₄. Phosphorus metabolism was inhibited when sludge contained 0.15 mM FePO₄ or 0.10 mM Fe(OH)₃. Increased K/P molar ratio and coefficient b could be indicators for Fe residual in sludge. Intermittent dosing was suggested for wastewater treatment plant (WWTP) operation.