Loss of Chlorine Residual on a Dead-End Main
A 6-inch unlined cast-iron dead-end main runs 2,800 feet from a booster station to a school and 14 homes. The booster discharge free chlorine is 1.9 mg/L, the first hydrant reads 1.2 mg/L, and the last hydrant at the school reads 0.05 mg/L. The system must maintain at least 0.2 mg/L free chlorine in the distribution system. Two customers have reported rusty water, and a main repair was completed three weeks ago about 600 feet from the dead end. The utility has no routine unidirectional flushing program, and the main includes a 4-inch section near the school. The state has asked for a plan before the next sanitary survey. The operator has one two-person crew, a 500-gallon flush tank, and a chlorine test kit.
1. What immediate actions and sampling would you take before flushing the main?
Read the worked answer
First confirm the chlorine result is real: check the test kit's reagents and expiration date, then resample at the booster, the first hydrant, and the school using the same method and holding time. Sample for bacteriological quality at the worst site, the school, and at a representative midpoint; do not flush first if flushing would hide the actual condition. Inspect the repair area for a closed valve, a bypass, or a temporary connection that could be causing stagnant flow. Check the school for cross-connection risks such as irrigation, boiler makeup, or fire-sprinkler connections.
Because 0.05 mg/L is below the 0.2 mg/L minimum, notify the supervisor and determine whether the state requires immediate reporting or a public notice. The utilities should not assume the dead end is the only problem; a closed valve can isolate a section and create a low-chlorine pocket. Until sampling is complete, increase booster output only if the downstream residual can be raised without exceeding the maximum residual disinfectant level. Document every reading and time.
2. Design a flushing strategy for this main using the available crew and equipment. Include flow direction, velocity, and how you would know when to stop.
Read the worked answer
Use unidirectional flushing from the booster toward the dead end. Isolate the 4-inch section if valves allow, and flush at the hydrant nearest the school. For a 6-inch main, a velocity of about 2.5 feet per second requires roughly 220 gallons per minute; for the 4-inch section, about 98 gallons per minute is enough for the same velocity. The 500-gallon flush tank is not a supply source for flushing; it is for hauling sample water or for dechlorination needs, so the crew should plan on using system water and dechlorinating the discharge if required.
Flush in stages, starting near the source and moving toward the dead end. At each stage, measure free chlorine and turbidity before and during flushing. Stop when chlorine residual has improved and stabilized and turbidity has cleared, not simply after a fixed number of minutes. If the residual does not improve after two passes, the problem may be biofilm, a closed valve, or a cross-connection, and flushing alone will not fix it. Resample bacteriology after flushing and before returning the main to normal service.
3. What long-term corrective actions would you recommend in the plan requested by the state?
Read the worked answer
The root cause is likely a combination of long detention time in a 2,800-foot dead end, old unlined cast iron, and no routine flushing. The plan should include a written flushing schedule with maps, valve numbers, target velocities, dechlorination procedures, and documentation. It should also evaluate looping the main or connecting it to another street to eliminate the dead end, which is the most reliable fix. If looping is not feasible, consider automatic flushing devices, a small recirculation or bleed line, or increased booster residual with careful monitoring.
Because a repair occurred three weeks ago and customers report rusty water, inspect the repair and the 4-inch section for internal corrosion and valve problems. Conduct a cross-connection survey at the school. Set a monitoring plan that samples the dead end at a frequency the state accepts, and track whether residuals stay above 0.2 mg/L between flushings. The plan should distinguish temporary flushing from permanent hydraulic improvements, because the state will want to see that the utility is addressing the cause, not just the symptom.