Drinking-Water Disinfection, US EPA SWTR & Clearwell Hydraulics
Disinfection CT Inactivation, Baffling Factor & Log Formula
Calculate Clearwell Theoretical Hydraulic Residence Time HRT (min), Tracer T10 Effective Contact Time (min), Effective Disinfection CT (mg·min/L), and Pathogen Log-Inactivation Credit.
Never use theoretical V / Q for regulatory CT—hydraulic short-circuiting mandates T10 = HRT * BF!
Under the US EPA Surface Water Treatment Rule (SWTR) and Chick-Watson disinfection kinetics, pathogen kill depends on CT = Disinfectant_Residual_mg_L * T10_min! Using mean Hydraulic Residence Time HRT = Clearwell_Volume / Peak_Hourly_Flow fatally overstates disinfection because jetted inlet water short-circuits straight to the outlet while dead zones stagnate! Regulatory credit uses T10—the time it takes the fastest 10% of a fluoride/lithium tracer pulse to exit—computed via the Baffling Factor BF = T10 / HRT (0.10 for unbaffled tanks, 0.50 for serpentine baffled clearwells, 0.70 for superior perforated baffles, 1.00 for plug-flow pipelines)!
Verify on a Serpentine Baffled Municipal Clearwell—Clearwell_Volume = 1,500 m³ (B2), Peak_Hourly_Flow = 25.0 m³/min (B3 -> Theoretical HRT = 1,500 / 25 = 60.00 min in E2), Serpentine_Baffling_Factor = 0.50 (B4 -> Effective T10 = 60 * 0.50 = 30.00 min in E3), Free_Chlorine_Residual = 1.60 mg/L (B5 -> Effective CT = 1.60 * 30 = 48.00 mg·min/L in E4), and Required_CT_Per_Log_Giardia = 32.0 mg·min/L (B6): Achieved Giardia Log-Inactivation Ratio equals 48.00 / 32.00 = 1.50 logs (E5, clearing a 1.0-log post-filtration target with a 50% margin)!
| EPA SWTR Clearwell CT KPI (1,500 m³, 25 m³/min, BF = 0.50, 1.6 mg/L Cl2) | Formula | Verified Output |
|---|---|---|
| Theoretical Detention HRT (min) | Clearwell_Volume_m3 / Peak_Flow_m3_min | 60.00 min |
| Effective Tracer Contact T10 (min) | Theoretical_HRT_min * Baffling_Factor | 30.00 min (6.00 min Unbaffled) |
| Effective Disinfection CT (mg·min/L) | Free_Chlorine_mg_L * Effective_T10_min | 48.00 mg·min/L (9.60 Unbaffled) |
| Giardia Log-Inactivation Credit | Effective_CT / Required_CT_Per_Log | 1.50 logs (0.30 logs Unbaffled!) |
Baffling-Adjusted CT & Pathogen Log-Inactivation Formula
Validate Volume > 0, Peak_Flow > 0, Baffling_Factor in (0, 1.0], Chlorine_Residual > 0, and Required_CT_Per_Log > 0!
=LET(vl_c, B2, q_pk, B3, bf_t, B4, cl_r, B5, ct_q, B6, IF(OR(vl_c <= 0, q_pk <= 0, bf_t <= 0, bf_t > 1, cl_r <= 0, ct_q <= 0), "Invalid Clearwell CT Input", LET(t_10, (vl_c / q_pk) * bf_t, ct_e, cl_r * t_10, ROUND(ct_e / ct_q, 2))))Unbaffled Tank Short-Circuiting Compliance Failure Check (Set BF = 0.10)
Remove internal baffle curtains so Baffling_Factor B4 drops from 0.50 to 0.10 (unbaffled circular reservoir): Effective T10 E3 plunges from 30.00 min to 6.00 min, Effective CT E4 drops to 9.60 mg·min/L, and Giardia Inactivation Credit E5 collapses to 0.30 logs—failing the 1.0-log drinking-water permit!
=IF(ISTEXT(E5), E5, ROUND((B2 / B3) * B4 * B5, 2))Try the example
Use the original values or change them to test your understanding.