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.