Optimizing enhanced coagulation with PAC: A case study

Issam Najm, Carol Tate, and Douglas Selby he upcoming Disinfectants/Disinfection ByProducts (D/DBP) Rule recognizes that a greater reduction in the total organic carbon (TOC) concentration of a natural water supply may appreciably reduce the concentrations of known and unknown DBPs formed during chlorination. Thus, the D/DBP Rule requires that conventional surface water treatment plants use enhanced coagulation (EC) to achieve specific reductions in the TOC concentration.1 This goal can be met by increasing the chemical coagulant dosage, by modifying the coagulation pH to the optimum range for the specific coagulant used, or both. This study used Colorado River water (CRW), which is not Enhanced coagulation can remove only a finite fraction of the natural organic matter (NOM) in water. Supplementing enhanced coagulation with powdered activated carbon (PAC) increases the removal of NOM and disinfection by-product (DBP) precursors. The removal of total organic carbon (TOC) and DBP precursors from Colorado River water was evaluated in an experimental matrix of jar tests using four ferric chloride dosages and four PAC dosages. On the basis of the chemical dosages used, the cost of each combination of coagulant and PAC addition was estimated. For a given TOC removal goal, a particular combination of ferric chloride and PAC dosages resulted in the lowest chemical costs. The analysis also indicated that this same dosage combination would produce the least sludge at the water treatment plant. This study shows that the combination of a chemical coagulant and PAC can be more cost-effective than a chemical coagulant alone for meeting the requirements of Stage 1 of the Disinfectants/DBP (D/DBP) Rule. In addition, the combination of a chemical coagulant and PAC may be a cost-effective alternative to granular activated carbon for meeting the requirements of Stage 2 of the D/DBP Rule. For executive summary, see page 181.