Removal of aquatic organic matter and humic substances by selected water treatment processes

Abstract

The characteristics of dissolved aquatic organic matter present in four natural water sources, as well as corresponding treated waters derived from eight water treatment plants, is described. An assessment is made of the performance of direct filtration, conventional treatment, and lime softening in removing trihalomethane (THM) precursors from a diverse array of water sources. A comparison is made between conventional treatment and direct filtration in removing THM precursors from a common natural water source, the Colorado River. In addition, the effect of selected initial conditions and operating parameters on the direct filtration process are evaluated by using a synthetic water/bench-scale apparatus. Important characteristics of the organic matter, including molecular weight (MW) distribution, carboxylic acidity and humic substances content, appear to affect the removal of this organic material. As a general rule, THM reactivity or yield (ug THM/mg C) increases with MW. The <10,000 MW range was found to be the most consistent reactive fraction of aquatic organic matter. All of the various treatments preferentially removed the most reactive fraction of precursor present in each MW range. None of the various treatments proved to be very effective in removing precursor material below a MW of <500. Humic molecules, with the highest carboxylic acidity and hence highest charge density, are generally more difficult to remove by alum coagulation. All of the various treatment processes studied preferentially removed hydrophobic over hydrophilic aquatic organic matter. None of the source related conditions (fulvic acid, kaolinite, pH) nor process-related conditions (flocculation velocity gradient, mixing time, prechlorination, preozonation) evaluated under the same experimental conditions exerted a major impact upon the performance of the bench-scale direct filtration process. Larger sized particles may be easier to remove by bench-scale treatment, but removal of the smaller particles may result in better removals of organic matter as quantified by NVTOC and UV Absorbance.