Enhanced coagulation under changing alkalinity-hardness conditions and its implications on trihalomethane precursors removal and relationship with UV absorbance

Abstract

This study evaluates the effectiveness of aluminium sulphate (alum) and polyaluminium chloride (PACl) as coagulants for the treatment of surface water with low DOC (2.1–2.4 mg/L) and at two alkalinity–hardness levels. Effectiveness was evaluated by measurement of DOC, UV absorbance at 254 nm (UV 254), and different trihalomethane formation potential (THMFP) tests. PACl showed higher removal efficiency for all the analyzed variables and under both high and low alkalinity–hardness conditions. Results also showed that alum coagulation is more sensitive to operational conditions, especially pH. Maximum differences between coagulants were observed in the removal of final THMFP with average values ( n = 48) being 48 and 31%. This difference was maximum when coagulation–flocculation was carried out at pH 8.0. Comparison of full scale Plant data and jar test data revealed that optimization based on DOC and UV 254 removal does not ensure optimum results for THMs precursors removal. Solution pH above 6.0 units ensures acceptable residual levels of dissolved aluminium (<0.2 mg Al 3+/L). Precursor removal optimization did not compromise residual aluminium requirements. After treatment a noticeable increase in the contribution of low molecular weight molecules to the THMFP was observed. Under high alkalinity–hardness conditions bromine incorporation factor (BIF) of treated water is much more sensitive to the type of coagulant used. Results of BIF suggest that at higher alkalinity–hardness levels hydrophilic natural organic matter (NOM) is better removed. Finally, the relationship between removal of UV absorbance and removal of THMFP was investigated. Results suggest that this relationship is more accurate and more reliable when PACl is used.