Dead-End Chlorine Residual Versus TTHM Headroom
A 9,000-connection distribution system serves a desert community north of Las Vegas. The system uses free chlorine and has a 0.8 MG welded-steel elevated tank on a 12-inch transmission main. In late July, the farthest hydrant on a 2,000-foot 6-inch dead-end main reads 0.08 mg/L free chlorine at 2 p.m.; the entry point is 0.85 mg/L. Water temperature is 91°F. The utility flushes the dead end for five minutes weekly. Recent distribution samples show TTHM at 72 µg/L and HAA5 at 51 µg/L against LRAA limits of 80/60 µg/L. The utility has had two main breaks this month on the same line, and one fire hydrant was recently replaced. Customers have not reported taste or odor. The utility wants to raise residual without pushing DBPs over the limit before the next quarterly sample.
1. Identify the most likely causes of the low residual and rank them. What field checks would you make before changing chemical feed?
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Rank causes by likelihood and check them in the field. The dead end has low turnover, 91°F water, and only a 5-minute weekly flush, so chlorine decay is expected; the two breaks and hydrant replacement introduce sediment and biofilm that exert demand. First measure free and total chlorine at the entry, at the tank outlet, at the break sites, and at the dead-end hydrant, then calculate loss per 1,000 feet. Check for a closed or throttled valve that isolates the dead end from the transmission main, because a 0.77 mg/L drop over 2,000 feet is too large for bulk decay alone. Open a hydrant upstream and downstream to verify flow direction. Inspect the replaced hydrant for a swivel or gate left partially closed. Only after confirming flow and demand should you change feed; raising chlorine at the plant will increase TTHM in the trunk main, where residence time is already long.
2. Propose operational changes that would restore residual at the dead end while limiting DBP formation. Explain the trade-offs.
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Restore turnover before raising dose. Fully open and exercise valves around the new hydrant and the two break repairs; if a valve is throttled, that alone may restore residual. Increase flushing at the dead end from 5 minutes weekly to a controlled 10–15 minutes twice weekly, and flush after any repair, directed to a dechlorination setup if discharge reaches storm drains. If residual remains below 0.2 mg/L, boost chlorine at the tank outlet rather than at the plant, so the higher dose does not age in the transmission main. Keep the entry-point free chlorine below the level that drives TTHM formation; at 72 µg/L TTHM, the system has little headroom. Consider looping the dead end or adding a small circulation pump as a capital fix. Do not simply raise the plant dose; it increases DBP precursor contact time without fixing the local stagnation.
3. What monitoring plan would confirm the fix and demonstrate compliance?
Read the worked answer
Sample at the dead-end hydrant and at the tank outlet for free and total chlorine, TTHM/HAA5, and iron. Collect a bacteriological sample at the dead end and at the mid-zone, because sediment and biofilm can mask regrowth. Track daily free chlorine at three points until the dead end holds at least 0.2 mg/L for five consecutive days, then return to the routine schedule. For DBP compliance, collect the required quarterly LRAA samples at the established sites; do not substitute the dead-end sample for a compliance site unless it is already in the monitoring plan. If flushing is increased, record volume and dechlorination. If the valve fix restores residual, document before-and-after flow and chlorine so the utility can justify the capital loop or pump. Recheck after the next main break, because repairs disturb biofilm and can reproduce the problem.