Dead-End Chlorine Residual Loss and Musty Odors
A 6-in dead-end main serves 12 homes and a small strip mall. Chlorine is fed as free chlorine at the treatment plant; residual leaving the plant is 1.1 mg/L. At the dead end, residual is 0.05 mg/L, and a routine sample shows no coliform but a heterotrophic plate count of 800 CFU/mL. Customers complain of musty odors and occasional sediment. The main was installed in 1965 and has no blow-off. The utility flushes the dead end only when complaints occur. A new restaurant is about to open at the end of the line and will increase demand.
1. What is the most likely explanation for the low residual and high HPC, and what else would you check?
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The pattern points to stagnation and biofilm growth in an old dead-end main, not a single treatment failure. Water sits in the line, chlorine decays, sediment accumulates, and biofilm produces musty odors and high HPC counts. The absence of coliform is reassuring but does not prove the water is biologically stable. Check the actual flow direction and velocity, whether valves are closed or short-circuited, the main's interior condition, sediment at the dead end, chlorine residual at several points along the line, and whether the HPC sample was collected properly. Also confirm that the plant feed is steady and that no cross-connection or unintended bypass is affecting the area.
2. What immediate actions would you take before the restaurant opens?
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Flush the dead end at a controlled rate until the water is clear and residual is restored, then resample for chlorine residual, HPC, and coliform. Because there is no blow-off, use a hydrant or temporary flushing point and dechlorinate the flush water if required. Increase monitoring frequency in the area, inspect and clean the main if sediment persists, and consider a temporary booster chlorination point if residual cannot be maintained. Notify the restaurant owner of the situation and do not rely on a single clear flush as proof that the problem is solved.
3. What long-term improvements would you recommend?
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Loop the dead-end main or replace/line the old pipe to eliminate stagnation and sediment. Install a permanent blow-off or automatic flusher and set a routine flushing schedule based on residual monitoring, not complaints. Add chlorine residual monitoring at the dead end and track HPC trends. Coordinate with the restaurant's expected demand and consider whether added flow will improve or worsen circulation. A flushing program alone may control symptoms, but looping or rehabilitation addresses the cause.