C = disinfectant residual (mg/L) at the outlet; T = theoretical detention time (min). For chlorine at pH 7 and 15°C, CT = 60 achieves 3-log Giardia inactivation.

Effective CT (Baffling Factor)

min

T₁₀ = BF × HRT

Example

HRT = 60 min, BF = 0.5 → T₁₀ = 30 min

BF = baffling factor (0.1–1.0 per regulatory tables). T₁₀ is the time for 10% of tracer to pass through. Use T₁₀ for CT credit calculations.

Log Inactivation (Chick-Watson)

dimensionless

log(N₀/N) = k × C^n × T

Example

k = 0.02, C = 1 mg/L, n = 1, T = 30 min → log inactivation = 0.6

k = rate constant (varies by organism and disinfectant); n = dilution coefficient (typically 1 for chlorine). Used to calculate inactivation credit for regulatory compliance.

Ozone CT for Giardia (3-log)

mg/L · min

CT_O₃ = 0.63 mg/L·min (at 15°C, pH 7)

Example

Ozone residual = 0.3 mg/L, T₁₀ = 3 min → CT = 0.9 > 0.63 ✓

Ozone is far more effective than chlorine. At 15°C, only 0.63 mg/L·min achieves 3-log Giardia inactivation vs. 60 mg/L·min for chlorine at pH 7.

UV Dose for Giardia (3-log)

mJ/cm²

UV Dose = E × t (mJ/cm²)

Example

3-log Giardia requires ≥5.8 mJ/cm²; 4-log Cryptosporidium requires ≥22 mJ/cm²

UV dose = irradiance (mW/cm²) × exposure time (s). UV provides no residual — must be combined with chemical disinfection for distribution system protection.

Disinfection Byproduct (DBP) Formation

μg/L

THM_f = k × [NOM] × [Cl₂] × t^n × pH^m × T^p

Example

Higher NOM, higher Cl₂ dose, longer contact time, higher pH → higher THM formation

Simplified empirical relationship. Key levers: reduce NOM before chlorination (enhanced coagulation, GAC), minimize Cl₂ dose, lower pH, reduce contact time before distribution.

⚡ Quick Reference — Class IV Key Values

Parameter	Typical Value / Limit	Regulatory Reference	Notes
Giardia CT (Cl₂, 15°C, pH 7)	60 mg/L·min (3-log)	GUDI / Surface Water Regs	Use T₁₀ with baffling factor
Ozone CT (Giardia, 3-log)	0.63 mg/L·min	GUDI	Far more efficient than Cl₂
UV Dose (Giardia, 3-log)	5.8 mJ/cm²	UV Disinfection Guidance	No residual — combine with Cl₂
UV Dose (Crypto, 4-log)	22 mJ/cm²	UV Disinfection Guidance	Crypto resistant to Cl₂
Filtered turbidity (95th %ile)	≤0.3 NTU	O. Reg. 170/03 / Prov. Regs	No single reading >1.0 NTU
THM MAC (RAA)	100 μg/L	Health Canada GCDWQ	Running annual average
HAA MAC (RAA)	80 μg/L	Health Canada GCDWQ	Running annual average
Lead MAC	10 μg/L (90th %ile)	Health Canada GCDWQ	First-draw samples
Fluoride optimal	0.7 mg/L	Health Canada	MAC = 1.5 mg/L
Chlorine residual (distribution)	≥0.05 mg/L free Cl₂	Provincial regulations	Minimum at all points
Notification of adverse result	Within 24 hours	O. Reg. 170/03	To MOH and Director
Baffling factor (poor)	0.1	CT Guidance	Unbaffled tanks
Baffling factor (excellent)	0.7–1.0	CT Guidance	Plug flow / serpentine
Membrane flux (UF typical)	20–80 LMH	Manufacturer guidance	Monitor for fouling
VFD power savings (20% speed reduction)	~49% power reduction	Affinity laws	P ∝ N³

📝 Class IV Practice Quiz 🎯 Class IV Mock Exam ← Class III Formulas 🌊 WPI Hub