Biologically Induced Variations in the Nature and Removability of THM Precursors by Alum Treatment

A study was conducted to examine whether biological activity in a reservoir, which is partly responsible for seasonal variations in finished water trihalomethane (THM) concentrations, could affect the efficiency of alum coagulation in removing THM precursors. Alum coagulation reduced THM precursors by about 50 percent, and its effectiveness did not appear to be altered by variations in algal or bacterial population densities in the lake. Yields of THMs from the dissolved organic carbon that remained after treatment, however, were generally greater when biological activity in the reservoir was high. Diurnal variations caused by intense biological activity could affect the ability of a water treatment plant to produce water with THM levels consistently less than the maximum contaminant level.