Explanation

Coagulation often precedes rapid sand filtration to aggregate fine particles into larger clumps for easier removal during the filtration process.

However, if coagulation is not required due to the absence of significant turbidity

it may be skipped, making it the step that doesn't belong in the sequence.

Steps in the purification of water using a rapid sand filter

1. Coagulation

2. Rapid mixing

3. Flocculation

4. Sedimentation and THEN

5. Filtration: using the rapid sand filter