National Council of Examiners for Engineering and Surveying (NCEES) Fundamentals of Engineering (FE) Environmental Practice Exam

Coagulants such as aluminum ions (Al³⁺) or ferric ions (Fe³⁺) are typically added during the rapid mixing phase of the water treatment process. This initial step is crucial because it allows for the effective dispersion of these coagulants throughout the water being treated. Rapid mixing creates conditions that facilitate the coagulation process, whereby these charged particles neutralize the negative charges of suspended particles and promote their aggregation into larger flocs.

Once the coagulants have been added and thoroughly mixed, the subsequent steps in the treatment process, such as sedimentation and filtration, can effectively remove these larger particles from the water. The timing and method of adding coagulants are vital for optimizing the overall efficiency of the treatment process, making rapid mixing an essential stage for the coagulation to be effective.