Filtration
The last physical barrier
Filters capture the floc and particles that settling left behind and are the plant's most important barrier against Giardia and Cryptosporidium. Candidates must know media design - sand, anthracite, dual-media, mixed-media, GAC, and support gravel - and why coarse-over-fine, deeper beds produce longer runs and better particle capture.
Key operating numbers: filtration rate = flow ÷ media area, typically 2–6 gpm/ft² for rapid gravity filters; terminal headloss, the point at which a filter is backwashed; and a backwash rate giving roughly 25–50% bed expansion (often 15–20 gpm/ft²), preceded by air scour where provided. A filter-to-waste or slow-start step prevents the turbidity spike at the beginning of a run.
Know the failure modes: breakthrough (turbidity climbs before headloss - media exhausted or short-circuited), air binding from negative pressure, mudballs, media loss to the wash trough, gravel upset, and short runs from polymer or coagulant carryover. Filter profile sampling shows where in the bed the problem sits.
Regulatory benchmarks to memorize: combined filter effluent ≤0.3 NTU in at least 95% of monthly samples and never above 1 NTU, with individual filters held to 1 NTU (and 2 NTU action-level frameworks in some programs), including right after backwash.
Worked example: a 600 ft² filter handles 3.0 MGD = 2,083 gpm → 2,083 ÷ 600 ≈ 3.5 gpm/ft², within the typical band. Backwashing at 15 gpm/ft² requires 600 × 15 = 9,000 gpm.
Traps: using total plant flow for a single filter; confusing filter rate with backwash rate; ignoring the ripening turbidity spike; assuming low headloss means clean media when it may mean short-circuiting.