15.1 Water Main Disinfection (AWWA C651)
Key Takeaways
- Water Main Disinfection is a named sub-topic in the Disinfection category of the SWRCB distribution exam, which carries 15 to 20 questions at D1 through D3.
- 22 CCR 64580 incorporates AWWA Standard C651-05 by reference and requires samples from new mains to be coliform-negative before the main is placed into service.
- The three C651 methods are the tablet method, the continuous feed method at 25 mg/L held 24 hours, and the slug method at 100 mg/L moved along the main.
- After disinfection the main is flushed until the residual is comparable to the distribution system, then cleared by two consecutive sets of coliform-negative samples.
- Heavily chlorinated water may not be discharged without neutralization, and the discharge must be controlled to protect receiving waters and aquatic life.
The Regulatory Requirement
22 CCR 64580: Prior to use, newly installed water mains, or water mains that have been taken out of service for maintenance or repair, shall be disinfected and sampled for bacteriological quality in accordance with American Water Works Association Standard C651-05. And: Samples from new mains shall be negative for coliform bacteria prior to the new main(s) being placed into service.
That single section makes AWWA C651-05 enforceable California law. The standard itself becomes the exam content.
Preventive Practice During Construction
C651 emphasizes keeping the pipe clean in the first place, because disinfection cannot reliably overcome gross contamination:
- Store pipe with caps or plugs in place. Keep the interior dry and closed.
- Do not allow pipe to lie in a flooded trench. If it does, the interior must be cleaned and, at the engineer's direction, may require the higher-effort procedures reserved for contaminated pipe.
- Plug the open end whenever work stops - lunch, end of shift, weekends. An open pipe end overnight is an invitation for rodents, debris, and runoff.
- Swab joints and gaskets with a 1 to 5 percent hypochlorite solution as they are assembled.
- Keep the trench dewatered so groundwater cannot enter the pipe.
The Three C651 Disinfection Methods
1. Tablet Method
Calcium hypochlorite tablets are adhered to the inside top of each pipe section with an approved adhesive before the pipe is lowered into the trench, then the main is slowly filled with potable water to dissolve them.
- Simple, no chlorine feed equipment, low volume of chlorinated water
- Requires the pipe interior to be dry and clean and the pipe to be installed dry
- Not permitted when the pipe has been wet or when solvent-weld joints are used in a way that would be compromised
- Fill velocity should not exceed about 1 ft/s so the tablets dissolve rather than being washed to the end
- Contact time is typically 24 hours at a target of at least 25 mg/L average
2. Continuous Feed Method
The main is filled with potable water, flushed to remove air and debris, then refilled while chlorine solution is injected at a constant rate so the water entering the main carries at least 25 mg/L free chlorine.
- The chlorinated water is held in the main for at least 24 hours
- At the end of the retention period the residual must be at least 10 mg/L free chlorine throughout the main, measured at the far end and at all branches
- Valves are operated during the retention period so the chlorinated water contacts all appurtenances - operate every valve and hydrant while the chlorinated water is in the main, or those parts never get disinfected
Dose calculation. To dose a 12-inch, 1,800-foot main to 25 mg/L using 12.5 percent sodium hypochlorite weighing 10.0 lb/gal:
Volume per foot of 12-inch pipe = 0.785 x (1.0 ft)² x 7.48 = 5.87 gal/ft Main volume = 5.87 x 1,800 = 10,566 gallons = 0.01057 MG Chlorine required = 0.01057 x 25 x 8.34 = 2.20 lb Hypochlorite solution = 2.20 / (10.0 x 0.125) = 2.20 / 1.25 = 1.76 gallons
3. Slug Method
Used for large-diameter transmission mains where filling the whole main at 25 mg/L would be wasteful. A slug of water dosed to at least 100 mg/L free chlorine is moved slowly through the main so that every part of the pipe is exposed to the slug for at least 3 hours.
- Requires careful flow control so the slug is not diluted below 100 mg/L
- Residual is measured continuously as the slug advances; if it falls below 100 mg/L, stop and re-dose
- Valves and branches are operated as the slug passes
| Method | Target dose | Contact time | Best for |
|---|---|---|---|
| Tablet | ~25 mg/L | 24 hours | Small mains installed dry |
| Continuous feed | 25 mg/L, ≥10 mg/L remaining at 24 hours | 24 hours | The general-purpose method |
| Slug | 100 mg/L | 3 hours contact for every part | Large transmission mains |
Flushing and Neutralization
After the retention period the main is flushed until the chlorine residual is no higher than that prevailing in the distribution system, and until water quality (turbidity, color) is comparable.
[!WARNING] Heavily chlorinated water may not be discharged without neutralization. Water at 25 to 100 mg/L free chlorine is acutely lethal to fish and aquatic invertebrates, and discharging it to a storm drain, creek, wetland, or the ocean can be an enforcement action. Neutralize with a reducing agent - sulfur dioxide, sodium bisulfite, sodium sulfite, sodium thiosulfate, or ascorbic acid - dosed to the measured residual, and verify with a field test that the discharge shows no detectable chlorine. Control erosion, protect traffic, and account for the volume in the water audit.
Approximate neutralizing doses per mg/L of chlorine residual: sulfur dioxide 1.0, sodium bisulfite ~1.5, sodium sulfite ~1.8, sodium thiosulfate ~1.5-2.0. Ascorbic acid and its sodium salt are increasingly used because they are non-toxic and add no oxygen demand.
Bacteriological Clearance
C651 requires two consecutive sets of acceptable bacteriological samples, separated by a standing interval specified in the standard, collected from the new main after final flushing and before it is placed in service. Check the edition your specification cites - California incorporates C651-05, and the required interval between the two sample sets was changed in later editions (the 2005 edition uses a 24-hour separation; more recent editions allow a shorter separation after a specified standing period). When a project specification, a State Water Board condition, and an AWWA edition disagree, follow the most stringent applicable requirement and confirm with the Division of Drinking Water.
- Samples are collected from every 1,200 feet of new main, plus one from the end of the line and one from each branch
- Samples must show no coliform and, in California under 22 CCR 64580, samples from new mains shall be negative for coliform bacteria prior to being placed into service
- Sample bottles must contain sodium thiosulfate to quench any residual chlorine at the moment of collection, or the residual keeps disinfecting in transit and the result is meaningless
- Samples must be collected from a properly disinfected, non-threaded sampling tap, not from a hydrant with a hose bib screwed on
- Analysis must be at an ELAP-certified laboratory within the method hold time
If a sample is positive: re-flush, re-disinfect if warranted, and resample. Repeated failures point to a real defect - a contaminated section, an unflushed branch, a cross-connection with a temporary supply, or a sampling technique problem.
Repairs and Emergencies
C651 scales the procedure to the severity of the contamination.
| Situation | Procedure |
|---|---|
| Clean, dewatered repair - the trench stayed dry, the pipe interior stayed clean | Spray or swab all pipe and fitting interiors with 1 to 5 percent hypochlorite solution, thoroughly flush after the repair, and sample |
| Trench flooded or pipe interior contaminated | Slug or continuous-feed chlorination of the repaired section, plus thorough flushing and sampling |
| Emergency repair where service must be restored immediately | Restore service after flushing and disinfecting to the extent practical, then collect samples and continue monitoring, and evaluate whether a precautionary boil water notice is warranted in consultation with the Division of Drinking Water |
[!IMPORTANT] Restoring service before clearance is sometimes necessary, and it is a decision with consequences. If the segment dropped below 20 psi and the trench was flooded, the correct posture is to treat the water as potentially contaminated: flush, maintain an elevated residual, sample, and consult DDW about a precautionary boil water notice. The notice is lifted on coliform-negative results plus restored pressure and residual, not on the repair being finished.
Under the AWWA C651 continuous feed method, what free chlorine residual must remain in the main at the end of the retention period?
A 12-inch main 1,500 feet long must be dosed to 25 mg/L. Approximately how many gallons does the main hold?
After the retention period, a crew prepares to flush 10,000 gallons of water containing 45 mg/L free chlorine from a new main into a street gutter that drains to a creek. What is required?