Filtration Rate (Filter Loading Rate)

Filter loading is the same flow-over-area velocity as an overflow rate, but the trade quotes it in gallons per minute per square foot: 2 gpm/ft² is the classic rapid sand rate, equal to 5 m/h or 120 m/d. Slow sand filters, the technology that ended cholera in Europe, run a hundred times gentler at 0.1–0.2 m/h, which is why they need acres of land; high-rate dual-media anthracite-over-sand beds routinely take 4–6 gpm/ft². A 50 L/s flow spread over a 30 m² bed is 144 m/d, or 2.4 gpm/ft² — squarely conventional.

Rate is only half the story; the other half is what happens when you change it. The chronic field error is closing a filter for backwash and letting the remaining filters absorb the flow in one step, which drives a slug of previously captured particles straight through the bed — the "filter ripening" and rate-change turbidity spikes that show up in every Cryptosporidium outbreak post-mortem, Milwaukee 1993 included. Bring filters back into service slowly, filter to waste for the first few minutes, and watch individual filter effluent turbidity rather than the combined header, which will happily average a failing filter into compliance.