Why Sedimentation + Filtration Makes River Water Drinkable

The sedimentation → filtration sequence is the classic conventional water treatment train for surface water. Each step targets specific impurity classes:

Sedimentation (Clarification)

• Mechanism: Gravity settling in large basins (2–6 hr detention). Flocculants (alum, ferric, polymers) aggregate colloids into settleable flocs.
• Removes: Suspended solids (clay, silt, algae, organic debris) — 80–95% of turbidity.
• Benefits: Reduces load on filters; lowers chemical demand; produces clearer water for disinfection.

Filtration (Polishing)

• Media: Rapid sand (0.5–1 mm), dual media (anthracite/sand), or GAC (granular activated carbon).
• Mechanisms: Straining, sedimentation in pores, adsorption, biological activity (schmutzdecke in slow sand).
• Removes: Fine particles, residual floc, cysts (Crypto, Giardia), bacteria, some viruses, dissolved organics (GAC).
• Critical: Achieves <0.3 NTU turbidity—required for effective disinfection (chlorine, UV, ozone).

Why Both?

Sedimentation alone leaves fine colloids, pathogens. Filtration alone clogs rapidly, short runs, high backwash waste. Together: robust, economical, meets EPA Surface Water Treatment Rule (SWTR) and WHO guidelines.

Modern Enhancements

• Plate/Tube settlers → smaller footprint.
• Membrane filtration (UF/MF) → replaces granular media; absolute barrier.
• Pre-oxidation, PAC, ion exchange for specific contaminants.

This duo remains the global workhorse for municipal surface water treatment.