Study On Enhanced Coagulation Process In Different Source Water Qualities

Enhanced Coagulation is one of the drinking water treatment technologies which are economical and effective. Although there are many academic researches on Enhanced Coagulation so far, these researches fruits could be just adapted to one specific site because of the various raw water qualities and the difference of applied coagulants. In this paper, two raw waters with different characteristics were studied using the Ferric Chloride (FC), Aluminum Sulfate (AS), Poly-Aluminum Chloride (PAC) and Polymeric Aluminum-Ferric Chloride Sulfate (PAFCS) as coagulants. The removal of turbidity and organic matter from water by enhanced coagulation was examined; the optimal dosage and pH condition for each coagulant were also determined in this study. The performance of the coagulant, including its possible effect on operation of filter bed was systematically evaluated; the possible mechanisms in coagulation were qualitatively and quantitatively discussed.Jar-test experiments were used to determine the critical coagulation concentration of ferric chloride: 0.5mg/L. Under this value, feeble action on particulars destabilization was observed. The optimal dosage of FeCl₃ was 12mg/L for Miribel Jonage Lake water and SCD indicated that the solution after coagulation was positive. Good results were observed with the effluent turbidity of 0.2NTU and with the UV254 removal rate of 20%. In the full-scale test, the dosage of 12mg/L was also proved effective, at which the effluent turbidity and residual ferric meet the criteria and the filter bed operated for 25 hours without water quality deterioration.The results of study showed that: the order of turbidity removal capacity is PAFCS>AS>PAC; the order of UV₂₅₄ removal capacity is PAC>AS>PAFCS. In combination of Zeta potentials measuring results, the fact was concluded that sweep coagulation by voluminous ferric hydroxide flocs was the mainly coagulation mechanism of AS while adsorption, bridging, sweep coagulation for PAC and PAFCS. The effective pH region for AS, PAC and PAFCS was 5.8-7.5, 5.5-7.8 and 5-8 respectively.