Increasing alkalinity can improve treatment at the best coagulant dosage. Runoff can rapidly change coagulant demand and produce water that is difficult to treat by conventional coagulation processes. In cases in which alkalinity dropped below about 30 mg/L as CaCO3, adding supplemental OH– (using lime or sodium hydroxide)—and thereby markedly increasing alkalinity before adding coagulant—sharply reduced turbidity after sedimentation but increased coagulant demand. Coagulant demand was a linear function of increased alkalinity and raw water dissolved organic carbon, and it was not strongly influenced by turbidity or calcium concentration below about a pH of 7.8.

A Langmuir model providing good prediction of full‐scale TOC removal during enhanced coagulation may help utilities comply with forthcoming regulations. A Langmuir model provides the best available prediction of full‐scale removal of total organic carbon (TOC) during enhanced coagulation. Because jar tests allow more transfer of carbon dioxide gas than treatment plants, the model can even correct certain errors arising during such tests. When the model was adapted to predicting TOC removal in two raw water sources mixed together in different proportions, standard error was only 0.15 mg/L for two years of jar‐test data at one utility. Case studies of 27 full‐scale utilities showed accurate prediction of TOC removal by coagulation (1) at a range of utilities using alum, ferric, or polyaluminum chloride coagulants; (2) over long time periods without a need for frequent recalibration; (3) with use of a predicted rather than measured coagulation pH; and (4) for two raw water sources mixed in various proportions.

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