First Step in River-to-Lake Water Treatment: Filtration

The first major treatment step for conveying river water to a lake (or reservoir) is typically filtration—specifically, granular media filtration using sand, anthracite, and/or garnet in single-, dual-, or multi-media configurations.

Why Filtration First?

River water carries suspended solids (turbidity), organic matter, algae, microbes, and adsorbed contaminants. Filtration removes these before disinfection because:

• Particles shield microbes from disinfectants (chlorine, UV, ozone).
• Organics react with disinfectants to form disinfection byproducts (DBPs) (e.g., trihalomethanes)—regulated carcinogens.
• Turbidity interferes with UV transmittance and chlorine demand monitoring.

Typical Treatment Train (River → Lake/Reservoir)

1. Intake screening — coarse debris removal (bar screens, traveling screens)
2. Pre-treatment: Coagulation/flocculation (alum, ferric chloride, polymers) → sedimentation/clarification (plate settlers, DAF, ballasted floc) — removes 80–95% of solids
3. Filtration (first major barrier): Rapid granular media filters (anthracite/sand/garnet) — removes residual particles to <0.3 NTU
4. Disinfection: Chlorine, chloramine, ozone, or UV — primary pathogen inactivation
5. Post-treatment: pH adjustment, corrosion control (orthophosphate), fluoridation
6. Storage/discharge to lake

Filter Media Details

• Anthracite (top): 0.8–1.2 mm, low density — captures large floc, longer runs
• Sand (middle): 0.4–0.6 mm — primary filtration zone
• Garnet/ilmenite (bottom): 0.2–0.3 mm, high density — polishes fines, prevents mixing during backwash

Modern Alternatives

Membrane filtration (MF/UF) increasingly replaces granular media for smaller footprint, absolute pathogen removal, automation. Slow sand filters (biological) used in small systems. Bank filtration (wells near river) uses aquifer as natural filter.