In the present study, phosphorus removal was studied using as coagulant spent alum sludge from a water treatment plant of EYDAP (Athens Water Supply and Sewerage Company) and compared to alum (Al2(SO4)3•18H2O), iron chloride (FeCl3•7H2O), iron sulfate (Fe2(SO4)•10H2O) and calcium hydroxide (Ca(OH)2) at a constant pH (equal to 6).The comparison was based on their efficiency to remove phosphorus in synthetic wastewater consisting of 10mg/L P as potassium dihydrogen phosphate and 50mg/L N as ammonium chloride. The experiments were carried out using a jar-test apparatus and the measurements were performed according to the Standard Methods for the Examination of Water and Wastewater. Pure alum, iron chloride and iron sulfate were much more efficient in phosphorus removal than the spent alum sludge but in the case of calcium hydroxide, phosphorus removal was very low in pH=6. Specifically, orthophosphate were totally removed by alum using 15mg/L as Al, by alum sludge using 75mg/L as Al and by FeCl3•7H2O or Fe2(SO4)• 10H2O using 30mg/L of Fe while in the case of calcium hydroxide P removal was actually zero. pH measurements showed that the uptake of phosphates is associated to the release of OH ions in the solution and that the end of P uptake is accompanied by the stabilization of pH. Finally this spent alum sludge was tested on municipal wastewater and proved to be effective as apart from phosphorus it was shown to remove turbidity and COD.

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