Which method in water treatment does NOT involve the removal of chemicals?

Slow sand filtration primarily functions as a biological treatment process rather than a chemical removal process. In this method, water is passed through a bed of sand that supports a layer of microorganisms which effectively break down and assimilate organic matter and pathogens present in the water. This biological action helps in reducing pollutants without the direct removal of chemical substances like in coagulation or chlorination.

In contrast, methods such as chlorination and chemical coagulation involve direct chemical interactions aimed at either disinfecting the water or aggregating suspended particles for removal. Rapid sand filtration, while also removing some impurities, primarily relies on physical filtration through sand at a much faster rate than slow sand filtration, and it may also involve some chemical treatment in practice.

Thus, the correct option highlights a treatment method that is centered on biological processes, effectively rendering it distinct from the other methods that actively involve chemical intervention.