What is the correct sequence for treatment of polluted surface water ? 1. Disinfection and fluoridation 2. Screening and mixing 3. Flocculation and sedimentation 4. Filtration and sludge processing Select the correct answer using the code given below :

The treatment of polluted surface water follows a standardized sequence to ensure safety and quality. The process begins with screening to remove large debris and mixing (chemical addition) to initiate coagulation [t1][t3]. This is followed by flocculation, where gentle mixing forms larger particles (flocs), and sedimentation, where these flocs settle to the bottom [t1][t4]. The next stage involves filtration to remove remaining suspended particles and sludge processing to manage the settled solids [t3][t5]. Finally, the water undergoes disinfection to eliminate pathogens and fluoridation to adjust fluoride levels for public health before distribution [t2][t7]. This logical progression—moving from physical removal of large objects to chemical aggregation, settling, fine filtration, and finally chemical/biological safety—aligns with the sequence 2-3-4-1.

Sources

1. [1] https://www.sanangelo.gov/DocumentCenter/View/927/Conventional-Municipal-Water-Treatment-Methods-PDF
2. [2] https://www.cdc.gov/drinking-water/about/how-water-treatment-works.html
3. [3] https://www.sciencedirect.com/topics/engineering/drinking-water-treatment
4. [4] https://dec.alaska.gov/media/11475/chapter-4-intro-to-water-treatment.pdf