On the Use of Iron Chloride and Starch for Clarification in Drinking Water Treatment

In drinking water treatment plants, chemical reagents are employed to aggregate and remove suspended particles. However, not all reagents are eco-friendly and exists concerns over environmental, economic, and health issues. This study shows features of the sustainability of commercial coagulants/flocculants and presents experimental research on floc characterization and settling of dispersed solids with a combination of Ferric Chloride (FeCl3 ) and gelatinized starch. Bench studies were conducted using kaolin suspensions and results were validated with raw water collected from a river (Rio dos Sinos, Brazil). Flocculation indexes, floc structure, and residual turbidities were compared with Polyaluminum Chloride (PAC), as a reference. All techniques showed that the combination of FeCl3 and starch formed well-structured, larger, and more settleable flocs than those produced with PAC. Superficial loadings, in a continuous separation tank (2 to 4 m.h-1) were studied with and without lamellae. Best results were obtained with 15 mg.L-1 Fe3+ and 10 mg.L-1 starch, with a velocity gradient, G, of 60 s-1 in the slow mixing and with 60° inclined lamellae spaced 1.3 cm apart. Best conditions were applied to the clarification of the raw water and again, due to the rapid settling of flocs with FeCl3 and starch, better results were obtained compared to PAC. A turbidity reduction of 94% and a residual value of 2.5 NTU with superficial loadings of 3 m.h-1 were obtained. Results were discussed in terms of interfacial and operating parameters and a promising potential for the combination of FeCl3 with starch for solid/liquid separation was envisaged.