Low Chlorine Residual at the End of a Long Distribution Line
A 1.8 MGD groundwater system chlorinates at the wellhead with sodium hypochlorite (12.5%, stored in an outdoor bulk tank). Free chlorine at the plant is 1.1 mg/L. At a sampling station 5.5 miles out on a 12-inch line, the free residual has fallen to 0.05 mg/L on three consecutive weekly samples; total chlorine is 0.4 mg/L. The state requires a detectable residual throughout the distribution system and a minimum of 0.2 mg/L free (or an approved alternative) at representative points. The area beyond the station is a growing residential subdivision. A recent customer complaint mentioned a slight chlorine odor at a tap near the plant. The bulk hypochlorite tank was last filled 7 weeks ago and sits in direct summer sun.
1. Diagnose why residual is low at the far station and why total chlorine exceeds free chlorine there.
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Two clues point in different directions, and both matter.
1. Total > free at the far station. When total chlorine (0.4) is much higher than free (0.05), most of the chlorine has been consumed reacting with something - ammonia, organics, or pipe biofilm - and converted to combined chlorine (chloramines). That is a demand problem in the distribution system, not a dosing problem at the plant. Possible sources: ammonia in the source water, nitrification in a chloraminated system, or a long residence time in the 5.5-mile main.
2. The plant-side odor and the 7-week-old, sun-exposed hypochlorite. Sodium hypochlorite degrades - faster with heat, sunlight, and time. A 12.5% solution stored outdoors in summer can lose a large fraction of its strength in weeks, and degradation also raises chlorate. So the 1.1 mg/L at the plant may be delivered by a dose that is larger than you think, and the effective strength is uncertain. The odor complaint near the plant is consistent with a high dose of degraded product.
So: the residual decays over 5.5 miles because of demand (biofilm, ammonia, long detention), and the feed strength is unreliable because of storage. Both must be addressed; raising the dose on degraded product alone may just create taste/odor and chlorate without fixing the far end.
2. What corrective actions do you take, in order, and how do you verify they worked?
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Immediate (this week):
- Test the hypochlorite strength (or have it tested) and check the tank for age/temperature. If strength is low, draw down and refill; move or shade the tank, and set a maximum storage age (commonly 30–60 days) and a rotation practice. Verify the feed pump calibration against the actual strength.
- Sample along the line (plant, midpoint, far station) for free and total chlorine, ammonia, and heterotrophic plate count to locate where decay happens. If ammonia is present, the system may be inadvertently chloraminating; decide whether to switch to a deliberate chloramine strategy or remove the ammonia.
- Flush the far main to turn over stagnant water and re-check residual; if biofilm is the demand, flushing plus a temporary residual boost at the plant may restore the far end.
Short term: 4. Consider a booster chlorination station partway out on the 12-inch line, or looping the dead-end subdivision to reduce detention time. Size it from the decay curve, not by guess. 5. If the subdivision is growing, model or estimate future demand and detention; a 12-inch line serving low flow has long residence time, which is the root cause.
Verification:
- Re-sample the far station weekly until residual is stably ≥0.2 mg/L free for at least 4 consecutive weeks.
- Track free/total ratio and ammonia at the far end.
- Confirm no taste/odor complaints and that chlorate (if tested) is within limits.
- Document the hypochlorite strength log and tank temperature going forward.