The correct sequence for the treatment of polluted surface water is as follows:

1. Screening and mixing: This step involves the removal of large debris and particles from the water by passing it through screens. Mixing is done to ensure an even distribution of chemicals and additives.

2. Flocculation and sedimentation: In this step, chemicals known as coagulants are added to the water to form floc, which binds together smaller particles. The water is then allowed to settle, and the floc settles to the bottom as sediment.

3. Filtration and sludge processing: The water is passed through various types of filters to remove any remaining particles and impurities. These filters can include sand, activated carbon, or membranes, depending on the level of purification required. Sludge, which is the solid waste collected during the treatment, is also processed and disposed of properly.

4. Disinfection and fluoridation: This final step involves the addition of disinfectants such as chlorine or UV radiation to kill any remaining harmful microorganisms. Fluoridation, if required, is done to add fluoride to the water for dental health benefits.

Therefore, the correct sequence is option 3: 2-3-4-1. Each step plays a crucial role