Chlorine Residual Collapse in a 180-Person Mobile Home Park
You operate a 180-resident mobile home park served by a single 6-inch well supply. The well pumps 45 gpm through a 1,000-gallon atmospheric contact tank where you feed 12.5% sodium hypochlorite with a metering pump at 0.4 gallons per day. The distribution system is roughly 4,200 feet of 6-inch main with low flow, especially during winter when irrigation stops. For three weeks your daily DPD free chlorine reading at the farthest sample station (Station 7, at the end of a dead-end loop) has been 0.9, 0.7, 0.5, 0.3, and today 0.1 mg/L. Your entry-point residual is steady at 1.1 mg/L. Total coliform samples collected yesterday at Station 7 and two other sites came back: Station 7 positive, the other two absent. Your state requires a detectable residual throughout and 5 routine samples per month for your population.
1. Diagnose why the entry-point residual is stable while the far-end residual has collapsed, and rank the plausible causes before you change any chemical feed setting.
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The entry point is fine, so the problem is downstream of the contact tank, not the dose. Work through causes in order of likelihood and cost to test:
- Chlorine demand in the mains. Long residence time in low-flow 6-inch pipe plus biofilm and sediment on the walls consumes free chlorine. A 4,200-ft, 6-inch main holds about 0.785 x (0.5 ft)^2 x 4,200 = ~825 gallons, roughly 18 minutes of run time at 45 gpm - but in a dead-end loop with near-zero winter demand, actual travel time can be many hours to days. That alone explains a decaying residual.
- Dead-end stagnation. Station 7 sits on a loop with no through-flow; chlorine decays toward zero with age.
- Nitrification or a chloramine problem - only if you were feeding ammonia, which you are not. Rule it out by confirming you feed hypochlorite only.
- Metering pump drift or hypochlorite degradation. Check the pump stroke and verify the bleach is fresh (12.5% degrades fast in heat); but since the entry point is steady, this is a secondary check, not the primary cause.
Do not simply crank up the feed pump first - that raises entry-point residual and trihalomethanes without fixing the stagnation that is actually causing the loss.
2. Given the positive coliform at Station 7, what immediate actions and follow-up sampling does this trigger, and how do you confirm the problem is fixed?
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A single routine positive is a routine trigger, not automatically an acute violation - but it requires action and repeat samples.
Immediate actions:
- Collect the required repeat samples (typically 3: the original site, one up- and one downstream) within the state's window (commonly 24 hours).
- Check whether any repeat is also positive and whether E. coli is present - that changes the classification to acute.
- Flush the dead-end loop at Station 7 to restore turnover, and increase flushing frequency until residual recovers.
- Inspect for cross-connections or a broken main near the positive site.
Confirming the fix:
- Restore and hold a detectable free chlorine residual at Station 7 across multiple consecutive days, not one reading.
- Get all repeat samples absent.
- If repeats are also positive, you are in a repeat/assessment-monitoring path and may need a Level 1 assessment and corrective action.
Document everything - dates, readings, flush volumes, sample results - because the regulator will ask for the corrective-action record.
3. What operational change would most durably keep residual at Station 7 without over-chlorinating the rest of the system?
Read the worked answer
The durable fix targets water age at the dead end, not dose.
Options and trade-offs:
- Automatic or scheduled flushing at the dead-end hydrant to force turnover. This is the cheapest, fastest control and directly attacks stagnation. Size the flush so you cycle the ~825-gallon loop volume regularly (e.g., a timed flush a few times a week, adjusted seasonally).
- Loop the dead end into the rest of the distribution so water flows through instead of sitting - capital cost, but permanent.
- Blend the dose so the entry point carries enough residual to survive travel time without pushing DBPs up. Since your entry residual is already 1.1 mg/L, raising it further is a blunt tool; flushing is better.
- Seasonal plan: winter low demand is the root driver, so increase flush frequency when irrigation stops and reduce it in summer.
Recommendation: install a timed flush at Station 7 as the primary control, verify residual daily for two weeks, then move to a monitoring schedule. Loop the dead end when budget allows.