Filtration
Filtration is the final particle barrier before disinfection, and its performance is judged mostly by turbidity and headloss. Rapid-rate granular filters (2–4 gpm/ft2, sometimes higher) use dual media - anthracite over sand - with garnet and gravel support; the coarse-to-fine gradation lets particles penetrate deeper, which increases solids capacity. Slow sand filters run near 0.1–0.3 gpm/ft2 and rely on the biological schmutzdecke. Pressure filters are enclosed versions of rapid gravity filters. Filtration removes particles, not dissolved contaminants.
Backwash typically runs 15–20 gpm/ft2 with about 20–30 percent bed expansion for roughly 8–15 minutes, often followed by air scour; then filter-to-waste or a slow restart. Watch for breakthrough (turbidity rising before terminal headloss), air binding (negative head, vacuum, entrapped air), mudballs, media loss, and gravel displacement. Filter run length ends on turbidity, headloss, or a set run time. The L/d ratio (media depth to effective size) is a design check on bed depth.
Traps: confusing filtration rate units; assuming a filter removes hardness or nitrate; forgetting the combined filter effluent turbidity limits (0.3 NTU in 95 percent of monthly samples, never above 1 NTU); and ignoring the need to recalibrate turbidimeters.
Worked example: 1.5 MGD through 400 ft2. Flow = 1,500,000 ÷ 1,440 = 1,042 gpm. Rate = 1,042 ÷ 400 = 2.6 gpm/ft2, within normal rapid-filter loading.