6.2 AWWA C651 Disinfection of New Mains
Key Takeaways
- AWWA C651-23 lists four acceptable disinfection methods: Tablet/Granule, Continuous Feed, Slug, and Spray — each with a specific chlorine dose, contact time, and required residual.
- Tablet method: 25 mg/L initial chlorine, ≥24 hr contact (48 hr if water <41°F/5°C), ≥0.2 mg/L free residual after contact; cold water extends the hold only — the Table 2 tablet count is unchanged.
- Bacteriological sampling uses Option A (two sets ≥16 hr apart) or Option B (sit ≥16 hr, then two sets 15 min apart, no flush); collect every 1,200 ft, at the end, and at each branch >1 pipe length; all samples must be coliform-free.
- After chlorination, flush heavily chlorinated water until chlorine matches distribution levels and dechlorinate discharge per AWWA C655 before environmental release.
- Pressure test (C600) is always completed BEFORE disinfection (C651); potable water is used throughout.
Purpose and Standard
Every new, repaired, or reactivated water main must be disinfected before it is put into service. AWWA C651-23, Disinfecting Water Mains, is the controlling standard and is heavily tested. The goal is to destroy contamination introduced during construction (soil, groundwater, debris) and to verify the main is bacteriologically safe through sampling.
The Four Acceptable Methods
C651 allows four disinfection methods. Each has a specific initial chlorine dose, minimum contact time, and required free chlorine residual at the end of contact. Know all three numbers for each method.
| Method | Initial chlorine dose | Min. contact time | Required free residual after contact | Typical use |
|---|---|---|---|---|
| Tablet / Granule | 25 mg/L | ≥24 hr (48 hr if water <41°F / 5°C) | ≥0.2 mg/L | New small/medium mains; tablets placed in each section |
| Continuous Feed | 25 mg/L | ≥24 hr | ≥10 mg/L | Larger mains; feed chlorine solution until a stable residual is measured at the end |
| Slug | 100 mg/L | ≥3 hr | Maintain ≥50 mg/L throughout the slug | Long mains; a moving plug of high-chlorine water |
| Spray | 200 mg/L | ≥30 min | Surface wetted for 30 min | Large mains (too big to fill); sprayed on interior surfaces |
Tablet Method Detail
For the tablet/granule method, calcium hypochlorite tablets are placed in each pipe section as the main is laid. AWWA C651 Table 2 gives the number of 5-g tablets per section for a 25 mg/L dose. For example, a 12-in pipe in 18-ft sections needs 4 tablets per section to reach 25 mg/L. The C651 formula behind that table is 0.0012 × d² × L, rounded up to the next whole tablet, where d is the pipe diameter in inches and L is the section length in feet.
Cold water changes the hold time, not the dose. If the water temperature is below 41°F (5°C), C651 requires the chlorinated water to remain in the main for at least 48 hours instead of 24. The tablet count stays exactly what Table 2 prescribes, and the target dose stays 25 mg/L — a common exam trap invites you to double the tablets, and that is wrong. Either way, a detectable free chlorine residual of ≥0.2 mg/L must be present at each sampling point at the end of the 24- or 48-hour period.
Memory hook: Tablet = 25 / 0.2 / 24 hr. Continuous = 25 / 10 / 24 hr. Slug = 100 / 50 / 3 hr. Spray = 200 / — / 30 min. The pattern is dose / residual / time.
Continuous Feed Detail
Chlorine solution is fed continuously at one end while water is drawn off at the other, until water at the far end shows a stable residual. The method works well when you can measure residual in real time and when tablet placement is impractical.
Slug Detail
A concentrated plug of chlorinated water (100 mg/L) is moved slowly through the main so every foot of pipe wall sees at least a 3-hour contact at ≥50 mg/L. If the residual drops below 50 mg/L at the tail of the slug, re-dose to 100 mg/L to keep the slug effective.
Spray Detail
For very large mains where filling is impractical, the interior surfaces are sprayed with a 200 mg/L chlorine solution and kept wet for at least 30 minutes.
Flushing and Dechlorination (C655)
After the required contact time, the heavily chlorinated water must be flushed out until the chlorine residual in the main matches the normal distribution residual. The discharge from this flush is toxic to aquatic life and cannot be dumped directly to a storm drain, creek, or ditch. AWWA C655, Disinfecting Wastewater Discharges to the Environment (dechlorination), governs treatment of the discharge — typically by passing the water through a dechlorination tablet feeder or using a reducing agent such as vitamin C (ascorbic acid) or sodium sulfite/sulfate before release.
The standard sequence is: chlorinate → hold contact time → flush to distribution residual → dechlorinate discharge (C655) → sample → return to service.
Bacteriological Sampling
Disinfection is not complete until bacteriological samples are collected and confirmed coliform-free. C651 offers two sampling options:
- Option A — Collect two sets of samples at least 16 hours apart. The main is held under pressure (with a low residual) during the wait.
- Option B — Let the main sit for at least 16 hours without flushing, then collect two sets of samples 15 minutes apart (no flush between them). The 15-minute gap, without flushing, verifies the water in the main itself is clean rather than freshly flushed water.
Where to Sample
Collect samples:
- Every 1,200 ft along the new main
- At the end of the line (dead end)
- At each branch greater than one pipe length
Use sterile bottles (with sodium thiosulfate to neutralize chlorine), label the location, and ship chilled to the lab within the hold time.
If a Sample Is Positive
Any coliform-positive result requires:
- Repeat disinfection (typically the continuous-feed or tablet method)
- Re-flushing
- A new set of bacteriological samples
Do not return the main to service until two consecutive sets of samples (taken per Option A or B) are coliform-free.
Pressure Test Before Disinfection
Repeat this rule to yourself: pressure test (C600) before disinfection (C651). A leaking main found after chlorination wastes the chlorinated water, restarts the sampling clock, and risks a chlorine discharge. The correct sequence is install → pressure test → disinfect → flush → dechlorinate discharge → sample → return to service. Potable (drinkable) water must be used for filling, flushing, and chlorination throughout — never raw surface water or recycled water.
Common Exam Traps
- Mixing the four methods' numbers — the dose/residual/time pattern (25/0.2/24, 25/10/24, 100/50/3, 200/—/30) is the fastest recall.
- Forgetting the cold-water extension (48 hr at <41°F / 5°C).
- Forgetting to dechlorinate the discharge per C655.
- Confusing Option A (16 hr apart) with Option B (16 hr sit + two 15-min-apart samples).
A new 12-inch main is being disinfected with the tablet method and the water temperature is below 41°F (5°C). Per AWWA C651, what initial dose, contact time, and post-contact residual are required, and how many tablets per 18-ft section are used?
Which AWWA C651 sampling option is correctly described?
After disinfection, the heavily chlorinated water must be flushed from the main. Which standard governs treatment of that discharge before it reaches the environment?