Filter media in water treatment — Which of the following substances are commonly used within a filter bed or cartridge?

Introduction:Water and process-liquid filtration often employs granular or fibrous media to remove particulates and, in some cases, dissolved organics. Sand and activated carbon are among the most common media used in municipal and industrial systems.

Given Data / Assumptions:

• Gravity or pressure filters in treatment trains.
• Objectives: turbidity removal (sand) and adsorption of organics/odors (carbon).
• No specialized membrane processes implied.

Concept / Approach:Sand filters (single or dual media) remove suspended solids via straining and depth filtration. Activated carbon (charcoal) filters adsorb organic molecules and chlorine/chloramine, improving taste and odor and protecting downstream processes. Aluminum chloride is a coagulant added upstream to form flocs, not a bed medium itself.

Step-by-Step Solution:Identify bed media: sand and carbon are standard.Distinguish coagulants (aluminum salts) from media.Select “Both (a) and (b)”.

Verification / Alternative check:Treatment schematics show coagulation–flocculation (aluminum salts) preceding sand/carbon filtration stages, confirming roles.

Why Other Options Are Wrong:

• (a) or (b) alone ignores the other widely used medium.
• (d) Misclassifies a chemical additive as a filter medium.
• (e) Polyethylene pellets are not standard universal media for potable-water filters.

Common Pitfalls:Assuming carbon alone clarifies suspended solids efficiently; in practice, sand handles particulates while carbon adsorbs dissolved compounds.

Final Answer:Both (a) and (b)