Effects of Coagulant Type and Coagulation Conditions on NOM Removal from Drinking Water

Abstract

Removal of natural organic matter (NOM) from soft and low alkalinity raw waters was studied in a coagulation-dual media anthracite-sand filter pilot plant. Three raw waters with colour levels of 15, 30 and 50 mg Pt/L, and five different coagulants were tested: alum (ALG), two poly-aluminium chlorides with different Ca-contents (PAX 14 and Ca-PAX), iron chloride sulphate (JKL), and chitosan (Chi). The tests were run at water temperatures of 6 – 11 °C. Coagulation performance at low water temperatures (1–2 °C) using ALG and 3 different on-line static mixers was studied in a specific set of experiments. In terms of head loss and turbulence level, the 3 tested mixers were considered to represent the upper and lower extremes, as well as a more practically applicable unit. Optimum coagulant dose and pH levels were identified for the different combinations of coagulants and raw waters. The experiments demonstrated that coagulant dose requirements when using metal-based coagulants were controlled by the metal residues in treated water rather than colour or organic carbon. Thus, common problems related to excess metal content in treated water is normally caused by sub-optimum coagulant doses and improper pH control. On the other hand, coagulant overdosing adversively affects filter run length and head loss development. The results showed that the type of mixer did not affect ALG dose requirements and overall process performance to any great extent. Low water temperatures increased the pH-sensitivity of the process, especially with respect to residual aluminium. The natural cationic biopolymer chitosan was able to remove colour efficiently. With respect to organic carbon removal however, chitosan was less effective than metal-based coagulants, even at relatively high doses.