Chloramine Residual Loss in a Dead-End Distribution Zone
A town of 18,000 switched from free chlorine to chloramines six weeks ago to lower TTHM. One pressure zone is fed by a 12-inch main and ends in 2,400 ft of 8-inch PVC dead-end serving two hydrants and a 300,000-gal elevated tank. Plant effluent total chlorine is 4.0 mg/L and free chlorine is 3.8 mg/L. At the dead end, the operator finds free chlorine 0.05 mg/L, total chlorine 0.35 mg/L, free ammonia 0.12 mg/L as N, nitrite 0.09 mg/L, pH 7.9, water temperature 24°C, and HPC 280 cfu/mL. Flow in the dead-end main is near zero except during hydrant use. Customers report earthy taste and odor. Coliform results are absent, but the state requires a written response for HPC above 200 cfu/mL. The utility must keep DBP results below Stage 2 limits and cannot afford a new main this year.
1. What does the dead-end chemistry indicate, and how would you distinguish nitrification from ordinary chloramine decay?
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The pattern points to nitrification in the stagnant dead-end main, not just chloramine decay. Ordinary chloramine decay lowers total chlorine but does not produce free ammonia and nitrite; here free ammonia is 0.12 mg/L as N and nitrite is 0.09 mg/L, with HPC 280 cfu/mL and 24°C water. Those are classic nitrification indicators: nitrifying bacteria oxidize free ammonia and consume chloramine residual, and the dead-end main has near-zero flow. I would confirm by checking ammonia and nitrite at the plant and at several points along the main, reviewing total/free chlorine ratios, and repeating samples at the same time of day. A chloramine residual that is mostly combined (free 0.05, total 0.35) with free ammonia present is not simple decay; decay alone would leave little free ammonia. Also verify the ammonia feed pump is not overfeeding and that the chlorine-to-ammonia-N ratio is in the intended 4:1–5:1 range.
2. List immediate operational actions for the next 72 hours, including flushing and chlorination, and state a risk of each.
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Immediate actions: (1) Flush the 2,400-ft dead end at a controlled rate, e.g., 200–300 gpm from the end hydrant, until free chlorine recovers and nitrite/ammonia drop; calculate volume (8-in main holds about 6,300 gal) and flush at least 2–3 main volumes, then confirm with field tests. (2) Increase circulation by exercising valves to loop the dead end if possible and by cycling the elevated tank; avoid stranding chloraminated water. (3) Temporarily raise chloramine dose or shift to a free-chlorine burn in that zone to 1–2 mg/L free chlorine, but check DBP formation and notify the state if the burn changes treatment. Risk: flushing wastes water and may stir sediment; a free burn can raise TTHM. (4) Increase monitoring for free/total chlorine, free ammonia, nitrite, and HPC at the dead end daily until stable. Do not rely on total chlorine alone.
3. What long-term controls and monitoring would you recommend before next summer?
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Long-term: eliminate or reduce the dead end by looping the 8-inch main; install an automatic flusher sized to turn over the main and operate it on a schedule or on chlorine/nitrite setpoints; improve tank turnover and monitor tank chlorine; optimize ammonia feed with a ratio controller and continuous ammonia analyzer; keep the chlorine-to-ammonia-N ratio high enough to avoid excess free ammonia but low enough to limit nitrification; monitor nitrite and HPC monthly in warm months; and flush before summer. Also review the HPC response and DBP results. If nitrification recurs, a seasonal free-chlorine burn may be needed, but it must be planned with DBP monitoring and state notification.