Water Treatment > Quality Water Solutions > Treatment Issues > Cold Water Coagulation

Many plants have a tough time meeting treatment goals during the colder months when chemical reaction rates fall and water viscosity rises, (water at 77 °F has essentially half the viscosity of water at 38 °F). These effects can lead to ineffective coagulation, higher chemical demand, slower settling rates, increased turbidities on top of the filters, and other problems.

Plants can use chemical and mechanical means to cope with cold water, including supplemental chemicals, a change in the primary coagulant, a shift in feed points and enhanced mixing. Many plants combine these methods.

Organic polymers or flocculants can offset slower settling rates in cold water by agglomerating fines that coalesce slowly, adding weight to the floc. The molecular weight and type of flocculant used — cationic (positively) charged, anionic (negatively) charged, or nonionic (neutral) — depends on treatment and other considerations.

Plants can consider other options if an additional flocculant is not desired. Some plants successfully treat cold water by switching to an alum-polymer blend or other blended coagulant to speed settling. Many facilities find that blended products are more effective than traditional metal salts used with a supplemental polymer added separately.

Other plants follow a seasonal treatment strategy and change primary coagulant for cold water treatment, e.g., to a polyaluminum hydroxychloride, which has gained a reputation as a good cold weather performer. They switch back to traditional metal salts during the warmer months. Many plants that began with seasonal treatment discover additional benefits from their cold water coagulant and have made them year-round coagulants. These benefits include less need for pH adjustment chemicals, a reduction in chemical solids, and added treated water stability.

Feed point location and mixing can be as important as chemical change in cold water treatment. For example, moving a feed point upstream to enhance mixing by giving chemicals more time to react has improved treatment at some locations. Installing mechanical mixing, such as use of in-line or submersible chemical inductors, can also help. One plant decreased the use of their primary coagulant by up to 15% in the colder months by enhancing mixing at critical locations with inexpensive chemical inductors.