Raw-water temperature that approaches the freezing point has what effect on coagulation and flocculation processes?

• A. Increases clarity of water
• B. Speeds up floc settling
• C. Increases viscosity, slowing floc settling
• D. Decreases coagulant effectiveness

Answer: (C)

When raw-water temperature approaches the freezing point, it significantly affects the physical and chemical properties of the water, particularly its viscosity. As the temperature drops, the viscosity of water increases, which directly influences the coagulation and flocculation processes.

Higher viscosity means that the water is thicker, making it more difficult for the floc particles to settle quickly. The increased resistance to movement impedes the natural settling velocities of the flocs, thus slowing down the overall floc settling process. This can hinder the efficiency of the water treatment operation, as slower settling may lead to a longer retention time in sedimentation basins and potentially less effective removal of suspended solids.

In this context, the other options do not accurately describe the effects of low temperatures on these processes. For instance, the clarity of water does not necessarily improve in colder temperatures due to slower settling. Similarly, while the settling process gets impeded, it does not speed up, and coagulant effectiveness may be reduced, but the direct impact of increased viscosity is a more critical factor concerning the effectiveness of floc settling in cold conditions.