13.2 Filtration, Disinfection, and CT Logic

Filtration as the Barrier After Settling

After sedimentation, a conventional plant still needs filtration because small floc, cyst-sized particles, and residual turbidity remain. Granular-media filtration captures particles by straining, interception, attachment, and depth filtration through sand, anthracite, garnet, or dual/multi-media beds. The exam rarely asks microscopic filtration theory; it asks whether you can check filter rate, read headloss and turbidity trends, and link upstream treatment to filter performance.

A filter's hydraulic loading rate is Q/A, usually in gpm/ft^2. Typical conventional rapid sand and dual-media rates run about 2-6 gpm/ft^2; the Surface Water Treatment Rule (SWTR) combined-filter-effluent turbidity standard is <=0.3 NTU in at least 95% of monthly readings and never above 1 NTU for conventional plants. Use active filter area - if one unit is in backwash or out of service, that area does not count. A run begins at low (clean-bed) headloss and ends when headloss reaches the terminal limit, effluent turbidity breaks through, or policy forces a backwash.

Filtration Checks and Upstream Diagnosis

High filter effluent turbidity is frequently an upstream problem. If coagulant dose, pH, floc energy, or sedimentation is wrong, the filter is just the first visible failure point. Raising chlorine dose never fixes particle breakthrough. Backwash clears accumulated solids, but if every run is short, suspect coagulation or hydraulic overload, not the media itself. Backwash typically needs 15-23 gpm/ft^2 (rate sufficient to fluidize and expand the bed 20-50%) for several minutes.

Disinfection Options and Tradeoffs

Free chlorine gives a durable distribution residual and supports CT, but reacts with NOM to form regulated DBPs - trihalomethanes (TTHM MCL 0.080 mg/L) and haloacetic acids (HAA5 MCL 0.060 mg/L). Chloramines are weaker primary disinfectants but hold a long residual with far fewer THMs. Ozone and UV are strong primary disinfectants (UV is excellent for Cryptosporidium and Giardia) but leave no lasting residual, so a secondary disinfectant is still needed. The exam often wants the operational consequence, not just the strongest oxidant.

CT Logic and Workflow

CT equals disinfectant residual concentration C (mg/L) times effective contact time T (min); units are mg-min/L. The SWTR requires 3-log (99.9%) Giardia and 4-log (99.99%) virus removal/inactivation; filtration earns credit toward these logs and disinfection CT supplies the remainder. Required CT values come from EPA tables (40 CFR 141.74) indexed by disinfectant, target log, temperature, and pH - lower temperature and higher pH raise the required chlorine CT.

Do not use chemical dose as C unless the problem says residual equals dose; demand, decay, and short-circuiting reduce the effective value. Do not use theoretical detention blindly - multiply by a baffling factor (or use a measured T10, the time by which 10% of water has exited) to get effective contact time.

1. Identify target organism, required log, disinfectant, pH, and temperature.
2. Find required CT from the table.
3. Theoretical t = basin volume / flow.
4. Convert days/hours to minutes (x1,440 min/day).
5. Apply the baffling factor if T10 is not given.
6. Use the measured residual at the end of the contact segment as C.
7. Actual CT = C x T; compare to required CT.
8. If short, solve for residual, volume, flow reduction, or better baffling.

Exam Traps

CT problems are unit traps. Clearwell volume (MG) divided by flow (MGD) yields days - multiply by 1,440 before multiplying by C. Watch the operating flow: at maximum-day or peak-hour flow, contact time shrinks, so a basin that passes at average flow can fail at max day. For concept items, hold the multibarrier idea: filtration controls particles and turbidity, disinfection inactivates pathogens, and good operation runs both without creating avoidable DBP or residual problems.

Disinfectant Comparison and Log-Credit Splitting

Conventional filtration commonly earns about 2.5-log Giardia and 2.0-log virus removal credit, so disinfection must supply the remaining 0.5-log Giardia and 2.0-log virus to reach the 3-log and 4-log SWTR totals. A useful exam shortcut from the rule: a filtered system that holds final pH below 9 and achieves at least 0.5-log Giardia inactivation by chlorine is deemed to meet the 4-log virus requirement, because virus CT is far smaller than Giardia CT at the same conditions.

Backwash and Filter Run Economics

A filter run produces water minus the volume consumed by backwash. If runs are short, net production drops and backwash water demand climbs, straining the clearwell and waste-handling system. The exam may give clean-bed headloss, terminal headloss, and a headloss-buildup rate, then ask for run length: run time = (terminal headloss - clean headloss) / buildup rate. Always confirm whether the limiting event is headloss or turbidity breakthrough - whichever comes first ends the run.