Modeling the treatment of drinking water to maximize dissolved organic matter removal and minimize disinfection by-product formation

Surface waters used for drinking purposes can vary markedly in their organic and inorganic content. High levels of variation occur in a range of water quality parameters such as turbidity, alkalinity, colour, natural organic matter, algae and micro-organisms. The removal of organic matter using inorganic coagulants is impacted by the character and concentration of the organics and the turbidity and alkalinity of the raw water. Mathematical models that relate the character and concentration of dissolved organic matter in raw water to inorganic coagulant dosing that maximize removal of dissolved organic carbon (DOC) have been developed. These models were used to predict alum doses that were subsequently applied to treat waters from two Australian drinking water sources (Googong and Middle River reservoirs) under jar test conditions and in pilot plant trials. Percentage removals of DOC were ~50–60% with application of model predicted alum doses for maximizing removal of DOC when coagulation was performed at pH 6. Much higher coagulant dosing at similar pH resulted in comparatively minor additional removal of DOC. Trihalomethane formation potential (THMFP) under standard laboratory conditions was found to be proportional to the residual DOC concentrations and appeared to be linearly related. Formation of individual THMs was consistent in each water source but differed between the two sources.

Models have been developed that relate coagulant dose to the concentration and character of organics present in natural raw waters. These models enable prediction of inorganic coagulant doses that maximize removal of organics at a particular coagulation pH. For two waters studied, the residual DOC concentration in alum treated water was directly related to the THMFP. Formation of individual THMs in the THMFP test was found to be highly consistent for each water but varied markedly between the two waters. The results of this study indicate that the application of enhanced coagulation to maximize removal of organics may not necessarily result in attainment of stringent levels of THMs in drinking water, where chlorine is used as disinfectant. This is due to the concentration of residual DOM that is recalcitrant to removal by coagulation.