Water Main Disinfection Calculator (AWWA C651)
Disinfecting a new or repaired water main per AWWA C651? Enter the pipe size and length, pick the disinfection method, and get the pipe volume, chlorine chemical required, and the flushing flow needed to scour the main clean.
How the calculation works
The calculator first finds the volume of water in the main, then converts a target chlorine concentration into pounds of chlorine (as Cl2), and finally converts that into the amount of the chemical you selected.
Pipe volume:V (gal) = 0.785 × (D ÷ 12)² × L × 7.48052, where D is diameter in inches and L is length in feet.
Chlorine required:Cl₂ (lb) = V (gal) × 3.78541 × C (mg/L) ÷ 453,592. C = 25 mg/L for the tablet and continuous-feed methods, 100 mg/L for the slug method.
Slug volume (slug method): the volume swept by a slug moving at 3 ft/s for 3 hours, capped at the pipe volume: V_slug (gal) = min(V, 0.785 × (D ÷ 12)² × 32,400 × 7.48052).
Tablet count: tablets = ceil(Cl₂ (lb) × 453.592 ÷ (5 × 0.65)) for 5-gram, 65% tablets.
Flushing flow at 2.5 ft/s:Q (gpm) = 0.785 × (D ÷ 12)² × 2.5 × 448.831. Flush time shown is for 3 pipe volumes: t (min) = 3 × V (gal) ÷ Q (gpm).
Notes & limitations
- Dose targets (25 mg/L tablet/continuous-feed, 100 mg/L slug) and the 2.5 ft/s flushing velocity follow AWWA C651. Verify against the current AWWA C651 edition and your local specifications — tablet placement details and acceptance criteria vary.
- Retention and verification: the tablet method requires a 24-hour hold with at least 25 mg/L free chlorine residual at the end; otherwise flush and rechlorinate. The slug must remain in contact at least 3 hours.
- Highly chlorinated water must be dechlorinated before discharge to the environment.
- Satisfactory bacteriological testing — typically 2 consecutive sample sets collected 24 hours apart — is required before placing the main in service.
Reference: AWWA C651, Disinfecting Water Mains (current edition).
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