Efficient removal of phosphorus from turbid water using chemical sedimentation by FeCl3 in conjunction with a starch-based flocculant

Phosphorus removal is an important issue in the field of water and wastewater treatment. In this study, a starch-based flocculant (St-CTA), as an assisting agent, was fed after the application of traditional inorganic precipitant of ferric chloride (FeCl3) to remove inorganic and organic phosphorus from their respective simulated turbid wastewaters. The effects of various influencing factors, including CTA content of St-CTA, dosage, initial pH, initial turbidity, and initial total phosphorus (TP), were investigated systematically. This modified chemical sedimentation process assisted by St-CTA not only showed high efficient in removing TP and turbidity but also evidently reduced the required dosage of FeCl3. This combination exhibited better efficiency in removing organic TP than in removing inorganic TP from water. The synergistic mechanisms of FeCl3 and St-CTA were discussed in detail by combination of apparent removal effects and floc properties. St-CTA exhibits strong charge neutralization and bridging flocculation effects and can thus efficiently aggregate and precipitate various phosphorus and iron complexes previously formed through chemical sedimentation, adsorption, and chelation. Besides, the validation of this combination usage was further confirmed by treating a real wastewater, and half of the required FeCl3 dosage and one third of the total cost were saved after addition of only 0.4 mg/L St-CTA when 90% TP and 95% turbidity were reduced. This study thus provided a novel technique for the diminution of TP and turbidity in turbid wastewater, considerable reduction of FeCl3 dosage and final cost, and lowering of secondary pollution risk.