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Abstract
Chemical phosphorus (P) removal during aerobic wastewater treatment induced by iron
salt addition in sewer systems for sulfide control is investigated. Aerobic batch
tests with activated sludge fed with wastewater containing iron sulfide precipitates
showed that iron sulfide was rapidly reoxidised in aerobic conditions, resulting in
phosphate precipitation. The amount of P removed was proportional to the amount of
iron salts added, and for the sludge used, ratios of 0.44 and 0.37 mgP/mgFe were obtained
for ferric and ferrous dosages, respectively. The hydraulic retention time (HRT) of
iron sulfide in sewers was found to have a crucial impact on the settling of iron
sulfide precipitates during primary settling, with a shorter HRT resulting in a higher
concentration of iron sulfide in the primary effluent and thus enabling higher P removal.
A mathematical model was developed to describe iron sulfide oxidation in aerated activated
sludge and the subsequent iron phosphate precipitation. The model was used to optimise
FeCl(3) dosing in a real wastewater collection and treatment system. Simulation studies
revealed that, by moving FeCl(3) dosing from the WWTP, which is the current practice,
to a sewer location upstream of the plant, both sulfide control and phosphate removal
could be achieved with the current ferric salt consumption. This work highlights the
importance of integrated management of sewer networks and wastewater treatment plants.
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