Using Coagulants to remove organic matter

Abstract

This study examined removal of organic matter (OM) by coagulation in drinking water treatment. First, the ability of three coagulants to simultaneously reduce turbidity and remove OM from water was determined. The coagulants tested were the commercial products alum and polyaluminum chloride (PACl) and a polyaluminum hydroxysulfate (PAHS) synthesized in the authors' laboratory. Tannic, humic, lignosulfonic, and salicylic acids (SAL) were used as model organic compounds in cold and warm water. The effect of the concentration of tannic acid was also examined, and semiquantitative relationships between OM concentration and coagulant dosage required were determined. In cold water, PACl and PAHS were efficient coagulants for all organic compounds except SAL (which was not itself removed, although it did not inhibit turbidity reduction). However, alum was a poor coagulant in the presence or absence of OM. In warm water, alum and PACl were the more efficient coagulants. OM had approximately the same effect on alum at both temperatures, but its effect on PACl was somewhat increased in warm water. Although PAHS effectively reduced turbidity in the absence of OM or in the presence of SAL, the presence of the other organic substances greatly increased the coagulant demand in warm water.