Filtration
Filters polish clarified water and are the final particle barrier. Know granular media types (sand, anthracite, dual-media, mixed-media, granular activated carbon) and why graded, larger-on-top media allow deeper solids penetration and longer runs. Understand filtration mechanisms-straining, sedimentation, interception, and attachment (including charge effects)-and that most removal happens in the upper media, not at the surface alone.
Master the operating cycle: filter run, headloss, effluent turbidity, breakthrough, terminal headloss, backwash (with air scour or water-only), filter-to-waste, and ripening. Know that filter loading rate = flow ÷ filter area, and that exceeding design rate degrades effluent quality. Traps: treating backwash as optional (poor backwash causes mudballs, media loss, and short runs); confusing headloss (resistance) with turbidity (particle passage); forgetting that a filter can pass particles while headloss is still low (breakthrough before terminal headloss). Also watch questions on filter-to-waste during ripening and on media loss from excessive backwash rate.
Rule of thumb: a filter is judged by effluent turbidity and particle counts, not by headloss alone-watch both.
Worked example: A filter 20 ft × 30 ft treats 3.0 MGD. Area = 600 ft². Loading rate = 3,000,000 gpd ÷ 600 ft² = 5,000 gpd/ft² ≈ 3.5 gpm/ft². If effluent turbidity rises while headloss is still moderate, suspect breakthrough and check media condition and backwash effectiveness.