Enhanced coagulation: US requirements and a broader view

Enhanced Coagulation is a new regulatory requirement in the United States a imed at removing TOC by coagulation thereby controlling formation of disinf ection byproducts. Coagulation principles are summarized for alum coagulati on of natural organic matter (NOM). Negatively charged NOM creates a coagul ant demand for positively charged Al species resulting in a stoichiometric relationship between the alum dosage and the raw water DOC that is pH depen dent. The paper addresses coagulation with a broader view than Enhanced Coa gulation, termed multiple objective coagulation. In general the objectives include: 1) to maximize particle and turbidity removals by downstream solid -liquid separation, 2) to maximize TOC and DBP precursor removals, 3) to mi nimize residual coagulant, 4) to minimize sludge production, and 5) to mini mize operating costs. Optimum coagulation conditions are those that maximiz e pathogen removals, produce low turbidities and particle counts, and minim ize residual Al. it is shown, for treatment of waters of low alkalinity, th at the optimum alum dosage selected to minimize UV absorbance with strict p H control produced excellent treatment for turbidity, pathogens, and NOM. F ull scale plant data are used to demonstrate a dual coagulation strategy of alum and cationic polymer that reduces sludge production and overall opera ting costs compared to alum alone. (C) 1999 Published by Elsevier Science L td on behalf of the IAWQ. All rights reserved.