Filtration
What you must understand
Filtration is the final particle barrier before disinfection. Know filter types-rapid sand (granular media), dual-media (anthracite over sand), mixed-media, slow sand, pressure filters, and membrane processes. Understand the mechanisms: straining, sedimentation, interception, adsorption, and biological action (important in slow sand and biologically active filters).
Master the operating concepts: filtration rate (gpm/ft²), head loss, filter run time, backwash (rate, duration, expansion, air scour), and filter-to-waste after backwash. Know ripening and breakthrough, and why turbidity and particle counts rise at the end of a run. Understand the filter profile and the importance of media L/d ratio and support gravel. Recognize that coagulation quality largely determines filter performance-poor floc escapes filters.
Traps
Frequent confusion: head loss (pressure drop through media) vs. turbidity (particles in effluent)-a filter can have high head loss and good effluent, or low head loss and breakthrough. Another trap: thinking backwash should be maximally vigorous-excess rate washes out media and sorts it poorly. Watch units: gpm/ft² vs. m/h, and the difference between terminal head loss and maximum run time as run-ending criteria.
Rule of thumb
Rapid sand filters typically run at about 2–4 gpm/ft²; slow sand filters run far lower (around 0.05–0.15 gpm/ft²) but achieve excellent pathogen removal. Backwash should expand media roughly 15–25% for sand. A worked check: a 100-ft² filter at 3 gpm/ft² passes about 300 gpm; verify against design flow and other units in service before declaring capacity.