6.7 Main Disinfection, Breaks & Repair
Key Takeaways
- New and repaired mains must be disinfected under AWWA C651 before being returned to service.
- The continuous feed method holds at least 25 mg/L free chlorine for 24 hours, retaining at least 10 mg/L at the end.
- Two consecutive satisfactory bacteriological samples collected 24 hours apart are required before a new main is placed in service.
- Water mains must be separated from sanitary sewers by at least 10 feet horizontally and 18 inches vertically with the water main above.
- Highly chlorinated water from main disinfection or flushing must be dechlorinated before discharge to a storm drain or watercourse.
6.7 Main Disinfection, Breaks & Repair
A main break is the moment a distribution system stops being a sealed barrier. Soil, groundwater, and whatever is in them sit directly against the opening while the pipe is depressurized. Everything in this section exists to close that window safely.
1. Separation From Sewers
The cheapest protection is distance. The standard requirements:
| Separation | Requirement |
|---|---|
| Horizontal (parallel) | At least 10 feet edge to edge between a water main and a sanitary sewer |
| Vertical (crossing) | At least 18 inches, with the water main above the sewer |
| When separation cannot be met | The sewer must be constructed to water main standards and pressure tested, and joints located as far as possible from the crossing |
Commit the 10 feet / 18 inches / water on top rule to memory — it appears on virtually every distribution exam.
2. Disinfecting New Mains — AWWA C651
Every new main, and any main opened for repair, must be disinfected before service.
Step sequence
- Keep it clean during construction. Cap or plug open ends whenever work stops; keep the trench dewatered; never let the pipe interior contact trench water. If the interior is flooded, swab or heavily chlorinate it.
- Flush the main at a velocity sufficient to remove construction debris — typically at least 2.5 to 3 ft/s.
- Disinfect by one of the three accepted methods below.
- Hold for the required retention time.
- Flush out the highly chlorinated water until the residual matches the rest of the system, dechlorinating the discharge.
- Sample bacteriologically — two consecutive satisfactory samples collected at least 24 hours apart.
- Pressure and leakage test, then place in service.
The three disinfection methods
| Method | Procedure |
|---|---|
| Continuous feed | Fill the main with water dosed so free chlorine is at least 25 mg/L throughout; hold at least 24 hours; at the end, residual must be at least 10 mg/L |
| Slug | Move a slug of water at at least 100 mg/L free chlorine slowly through the main so all interior surfaces contact it for at least 3 hours |
| Tablet | Calcium hypochlorite tablets adhered inside the pipe during assembly, dissolving as the main is slowly filled; 24 hour retention. Only for mains not flooded during construction, and not permitted where solvent-weld joints are used |
If the bacteriological samples fail, the main is re-disinfected, re-flushed, and re-sampled.
3. Main Breaks and Repairs
Response sequence
- Locate and isolate — use the system map and valve records, closing the fewest valves that will isolate the break. This is where the valve exercising program from Section 6.4 either saves you or fails you.
- Notify affected customers, and coordinate with fire authorities since fire flow is compromised in the isolated area.
- Assess pressure impact. If pressure fell below 20 psi in occupied portions of the system, plan for a boil water advisory.
- Excavate safely — see Section 13.3 for excavation, shoring, and utility locating requirements. Call before you dig.
- Dewater the trench and keep it dewatered throughout the repair so contaminated water cannot enter the pipe.
- Repair with a repair clamp, sleeve, or pipe section, keeping all materials and the pipe interior clean and swabbed with 1% hypochlorite solution.
- Flush thoroughly in the direction that carries material out of the system, not deeper into it.
- Restore pressure, verify disinfectant residual returns to normal, and sample bacteriologically.
- Rescind any advisory only after satisfactory samples and restored pressure and residual.
Keeping pressure on
Where it is feasible, repairing under pressure or maintaining a small positive flow out of the break prevents contaminated water from entering. Positive pressure is the barrier; losing it is what turns a repair into a contamination event.
Classifying the break
| Break type | Typical cause |
|---|---|
| Circumferential (beam) break | Ground movement, bedding failure, differential settlement — pipe acts as a beam and snaps |
| Longitudinal split | Internal pressure surge, external crushing load, or water hammer |
| Blowout hole | Localized corrosion pitting from aggressive soil |
| Joint / bell failure | Thrust, gasket deterioration, movement |
Recurring breaks in one area indicate soil corrosivity, poor bedding, or excessive surge — the fix is investigation, not repeated patching.
Thrust restraint
At bends, tees, reducers, valves, and dead ends, internal pressure produces an unbalanced thrust force that will push the fitting out of the joint. This is resisted with thrust blocks poured against undisturbed soil or with mechanical joint restraint. A break at a bend immediately after a new installation almost always means inadequate thrust restraint.
4. Dechlorination of Discharge Water
Both main disinfection water and routine flushing water can be acutely toxic to aquatic life. Before discharging to a storm drain, ditch, or watercourse, the water must be dechlorinated using:
- Sodium bisulfite, sodium sulfite, or sodium thiosulfate solution fed into the discharge
- Ascorbic acid or vitamin C tablets in diffuser devices, which are non-toxic and increasingly common for field flushing
- Discharge to the sanitary sewer with the receiving treatment plant's approval — never assume this is acceptable without asking, since a large chlorinated slug can damage the biological process
Operators should also control erosion and sediment from the discharge, using diffusers, splash pads, or dechlorination bags at the hydrant.
Under the AWWA C651 continuous feed method, what free chlorine concentration and retention time are required for a new water main?
What separation is required where a water main crosses a sanitary sewer?
A repair crew must discharge highly chlorinated water from a newly disinfected main into a roadside ditch that drains to a creek. What must be done first?
A newly installed main fails at a 90-degree bend shortly after being pressurized. What is the most likely cause?