Water Treatment > Quality Water Solutions > Operational Issues > Evaluating Treatment Costs

Water utilities work hard to balance several treatment goals at once, often while coping with hard-to-treat waters. Coagulant regimens evolved through trial and error may accomplish these goals, but may not be as cost efficient as possible. Economic issues concerning coagulant use affect many other plant factors, including other chemicals used (e.g., filter aids and those for pH control and alkalinity) and residue outlays.

In evaluating coagulant cost, consider all relevant cost factors, including coagulant and co-coagulant dosage, pH adjustment, cationic and anionic polymer, other chemicals used, sludge disposal and backwash water.

Here are examples of how several plants lowered treatment costs and improved performance by altering their coagulation programs.

1. A Midwestern utility treating river water wanted to hold filtered water turbidities below 0.1 NTU and reduce chemical usage. To do so, it switched coagulants from alum to ferric sulfate and then to a high basicity PACl/polymer blend. The last option lowered treatment costs by $12.88/MG, held filtered turbidities below 0.08 NTU on average, and eliminated the need for lime and cationic polymer. Filter run lengths also increased 30%.
   
   
2. A plant accessing both lake and river water treated the lake water with a PACl/polymer blend and the river water with a combination of ferric sulfate, PACl and polymer. In seeking to simplify operations and reduce cost, it switched to a high basicity PACl. The new PACl performed at least as well as the other coagulants on both source waters, met the goal for easy operation, and reduced cost by 35% for river water and 31% for lake water by sharply cutting coagulant and soda ash usage.
   
   
3. A plant treating river water changed coagulant from ferric sulfate to an alum/ polymer blend and then to a PACl in order to lower residual processing and disposal costs and reduce caustic consumption. The new coagulant halved chemical residue volume, cutting sludge disposal costs 44%, and reducing caustic consumption more than 80%. Total treatment costs dropped 55%.
   
   
4. A plant using alum to treat river water could not meet its clarification goals when water temperature dropped to 45? F in the fall. It switched to a PACl, which eliminated cold water problems. Although PACl dosage was 85% less than that of alum (liquid basis), settled turbidity was halved, filtered water turbidity decreased, and filter run lengths improved by 12 hours. Cold water treatment costs fell by 12%.