16.3 Disinfection of Mains, Storage Facilities & Repaired Components
Key Takeaways
- New and repaired mains must be disinfected and cleared bacteriologically before being returned to service, following AWWA C651 for mains and C652 for storage facilities.
- The continuous feed method holds 25 mg/L free chlorine for 24 hours, while the tablet method places calcium hypochlorite tablets in the pipe during assembly for a 24-hour contact period.
- Slug application moves a short column of 100 mg/L chlorine through the main so every surface receives at least 3 hours of contact.
- Highly chlorinated water must be neutralized before disposal, because discharging it to a wash or storm drain kills aquatic life.
- Two consecutive satisfactory bacteriological samples taken at least 24 hours apart are required before a new main is placed in service.
16.3 Disinfection of Mains, Storage Facilities & Repaired Components
Any time the distribution system is opened, contamination can enter. The Need-to-Know Criteria list "clean and disinfect the finished water storage facilities" and "disinfect components used during install/repairs" as scored operator tasks. The governing standards are AWWA C651 for mains and AWWA C652 for storage facilities.
Before Disinfection: Cleanliness and Flushing
Disinfection cannot compensate for a dirty pipe. Organic material and soil exert chlorine demand and physically shield organisms from contact.
- Keep pipe interiors clean during construction and plug open ends whenever work stops.
- Swab pipe interiors with a hypochlorite solution as they are laid where the specification requires.
- Flush the completed main at a velocity sufficient to carry debris out — commonly at least 2.5 to 3.0 ft/s.
For an 8-inch main, area is 0.785 × 0.667² = 0.349 ft², so 2.5 × 0.349 × 448.8 ≈ 392 gpm.
The Three AWWA C651 Methods
| Method | Chlorine concentration | Contact time | Application |
|---|---|---|---|
| Continuous feed | 25 mg/L free chlorine throughout the main | 24 hours, retaining at least 10 mg/L at the end | The standard method for most new mains |
| Tablet | Calcium hypochlorite tablets affixed inside the pipe during assembly | 24 hours | Short runs; pipe must be kept dry during construction |
| Slug | 100 mg/L free chlorine in a moving column | At least 3 hours of contact with every surface | Long transmission mains where filling the whole line at 25 mg/L is impractical |
Continuous Feed
Fill the main slowly from a potable source while injecting chlorine so the entire length reaches 25 mg/L free chlorine. Operate every valve and hydrant on the section so chlorinated water contacts all appurtenances. Hold 24 hours, then verify at least 10 mg/L free chlorine remains at the far end. A residual below 10 mg/L means demand exceeded expectation — indicating contamination or debris — and the procedure must be repeated.
Tablet Method
Calcium hypochlorite tablets are attached to the inside top of each pipe section with an approved adhesive as the pipe is laid. When the main is filled, the tablets dissolve and produce the disinfecting concentration. The critical constraint: the pipe must be kept dry during construction, because premature wetting dissolves the tablets before the main is complete. This method also cannot be used where the main cannot be kept clean.
Slug Method
A continuously moving slug of water dosed to 100 mg/L free chlorine is passed slowly through the main so that every interior surface receives at least 3 hours of contact. Chlorine is re-dosed as needed to keep the slug above 100 mg/L as it travels. Used on long transmission mains where holding the entire volume at 25 mg/L would require an impractical quantity of chemical.
Final Flushing and Bacteriological Clearance
After the contact period, flush the highly chlorinated water out until the residual matches that of the distribution system entering the main.
Bacteriological clearance then requires two consecutive sets of satisfactory samples taken at least 24 hours apart, collected from the new main after final flushing. Samples are analyzed for total coliform and, if total coliform is present, for E. coli. Any positive result requires re-disinfection and re-sampling.
[!IMPORTANT] The main does not go into service on the strength of the chlorine residual alone. Passing bacteriological results are the release condition, and they cannot be rushed — the 24-hour spacing between sample sets is a minimum, not a target. Sampling must use approved procedures with a sterile bottle containing sodium thiosulfate to neutralize residual chlorine so the sample reflects conditions at collection rather than continued disinfection in the bottle.
Disposal of Highly Chlorinated Water
[!WARNING] Water at 25 to 100 mg/L free chlorine is acutely toxic to aquatic life and must never be discharged to a wash, storm drain, canal, or any water body without neutralization. Arizona's ephemeral washes still support aquatic and riparian ecosystems, and a discharge can also violate AZPDES stormwater requirements.
Neutralization options include sodium bisulfite, sodium metabisulfite, sodium thiosulfate, sodium sulfite, and ascorbic acid, applied to reduce the residual to a level acceptable for discharge. Alternatives are discharge to the sanitary sewer with the treatment plant's permission, containment and hauling, or land application at a rate that will not run off. Dechlorination diffusers on flushing outlets are now common practice.
Erosion control is a companion requirement: the discharge must be directed so that it does not scour the receiving channel, which typically means a diffuser, splash pad, or energy dissipation device.
Storage Facility Disinfection (AWWA C652)
Storage tanks are disinfected after construction, after painting or major repair, and after any entry or cleaning.
| Method | Description |
|---|---|
| Method 1 | Fill the entire tank with water dosed to 10 mg/L free chlorine and hold 6 hours |
| Method 2 | Spray or brush all interior surfaces with a 200 mg/L solution, hold 30 minutes, then fill with potable water |
| Method 3 | Fill about 5 percent of tank volume with 50 mg/L chlorinated water, hold 6 hours, then fill the remainder with potable water and hold 24 hours |
Method 2 is common because it uses far less chemical and produces less waste to dispose of, and Method 3 is a middle path where full-volume dosing is impractical.
Tank cleaning before disinfection removes accumulated sediment, and confined space entry procedures apply to any tank entry — a point covered in the safety chapter but worth flagging here, since tank interiors are permit-required confined spaces.
Emergency Repairs
A main break under pressure is a different situation. AWWA C651 recognizes abbreviated procedures because the system cannot wait 48 hours for clearance.
Practical measures during an emergency repair:
- Keep the trench dewatered and keep the pipe ends out of standing water; contaminated water entering an open main is the principal risk.
- Swab or spray the interior of the repair components and the pipe ends with a hypochlorite solution — typically 1 to 5 percent — before assembly.
- Maintain positive pressure in the main where at all possible; if pressure is lost, treat it as a higher-risk event.
- Flush thoroughly after the repair until clear water and normal chlorine residual return.
- Collect bacteriological samples after the repair, even though the main returns to service before results are available.
- If the main was depressurized and contaminated, the utility must evaluate whether a precautionary boil-water advisory is warranted, in consultation with the primacy agency.
After a 24-hour continuous feed disinfection at 25 mg/L, the operator measures 6 mg/L free chlorine at the far end of the main. What does this indicate and what should be done?
A crew completes main disinfection and prepares to flush approximately 40,000 gallons of water containing 25 mg/L free chlorine into a nearby dry wash. What must be done first?
Under AWWA C652 Method 2 for disinfecting a storage tank, what concentration and contact time apply?