Distribution Residual Collapse in a Well-Based System
A 3,400-person community draws from two 350-gpm wells and chlorinates with chlorine gas at the wellhouse. Finished water enters a 0.40-MG elevated tank; average demand is 0.45 MGD, so stored water often sits for nearly a day. In late July, plant free chlorine is 1.0 mg/L, but a dead-end sample 1.8 miles out shows free chlorine 0.15 mg/L and total chlorine 0.55 mg/L. Water temperature is 27 C, ammonia is 0.06 mg/L as N at the plant, and coliform samples remain absent, but taste complaints are rising. HPC has not been run. The system has no booster station and no breakpoint chlorination setpoint beyond a target of 1.0 mg/L free at the plant. The operator must decide whether to flush, increase chlorine, or move toward chloramination before the August demand peak.
1. What is the most likely explanation for the free-to-total chlorine gap and the dead-end residual, and what immediate actions would you take? Include monitoring.
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The free-to-total gap is the key clue. A free residual of 0.15 mg/L with total chlorine of 0.55 mg/L means about 0.40 mg/L is combined chlorine. In a 27 C system with 0.40 MG storage and 0.45 MGD demand, theoretical storage time is 0.40/0.45 = 0.89 day, and dead-end travel time adds more; that is enough for ammonia released by chloramine decay or nitrification to tie up free chlorine. The plant ammonia of 0.06 mg/L as N could consume roughly 0.3 mg/L chlorine at breakpoint, which is consistent with the gap. Immediate steps: confirm with free/total chlorine at several points, ammonia, nitrite, HPC, and coliform; flush the dead-end main and exercise valves; increase plant free residual in small steps only if DBP formation and tank turnover are checked; reduce storage age by cycling the tank. Do not simply dose to a higher number without finding the demand source.
2. Would chloramination be a sound fix here? What data and approvals would you need before changing disinfection?
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Chloramination is not an automatic fix and can make nitrification worse if ammonia is already present. It would require Oklahoma DEQ approval, an ammonia feed and monitoring plan, free and total chlorine and nitrite monitoring, and a plan for breakpoint/chloramination transitions. The data needed first are a 24-hour chlorine profile at the tank and dead ends, ammonia and nitrite trends, HPC, and a tank turnover calculation. A better first step is to improve turnover, flush the dead ends, and possibly add a booster rather than switch chemistry. If chloramination is chosen, keep free chlorine during the transition, establish a nitrification action level, for example rising nitrite or falling total chlorine, and plan to revert to free chlorine if nitrite rises. The recommendation should be conditional, not a blanket switch.