Chlorine Feed Failure and Disinfection CT
A groundwater system chlorinates with sodium hypochlorite (12.5%) fed by a metering pump into the main before a 4,000-gallon contact tank. Design flow is 400 gpm; the tank provides roughly 10 minutes of theoretical contact time at design flow. Target free chlorine leaving the tank is 0.8 mg/L; the state requires a CT sufficient for 4-log virus inactivation, and the system's approved CT table assumes a pH of 7.2 and a temperature of 10°C. At 14:00 the analyzer at the tank outlet reads 0.05 mg/L and the low-residual alarm sounds. The metering pump is running but the suction line from the day tank is empty; the day tank was filled at 06:00. The distribution system is a single pressure zone with about 1,200 connections and a 300,000-gallon elevated tank at 70% level.
1. What is the immediate sequence of actions, and what makes this urgent beyond a low chlorine reading?
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The urgency is not the 0.05 mg/L itself - it is that an empty day tank means no disinfectant has been fed for some unknown period, possibly hours, and water with no residual has been moving into the distribution and the elevated tank the whole time.
Immediate sequence:
- Restore feed. Refill or swap the day tank and confirm the pump is actually moving chemical (stroke, suction, discharge pressure). Verify with a grab sample, not just the analyzer.
- Find out how long it was empty. The tank was filled at 06:00 and is empty by 14:00. Calculate expected usage from dose and flow; if the numbers don't match, there is a leak or a feed-rate error. That tells you how many hours of under-chlorinated water were produced.
- Protect the distribution. Check residuals at representative points and in the elevated tank. If residual is gone in the tank, you have a large volume of unprotected water.
- Notify the supervisor and the state per the reporting rule for a treatment failure or loss of disinfection.
- Consider a boil-water notice if CT was not met and residual was lost in the distribution - that is a judgement call made with the state, leaning toward notice for an acute risk.
Do not simply refill and walk away. The question is what already left the plant.
2. How do you determine whether the CT requirement was actually met during the outage, and what does the empty tank tell you?
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CT is concentration times contact time. The requirement was calculated for 0.8 mg/L and about 10 minutes at design flow. If concentration went to zero, CT went to zero regardless of contact time - the tank was full of water with no disinfectant in it. So the CT requirement was not met for the duration of the outage.
To bound the duration:
- Back-calculate from the day tank: 12.5% hypochlorite at the dose needed to hit 0.8 mg/L at 400 gpm gives a known daily volume. If the day tank holds, say, a day's worth and it emptied in 8 hours, either the dose was set high, the flow was higher than design, or there is a leak. Each has a different meaning, and you need to know which.
- Check the analyzer trend and any grab samples to find when residual actually started falling.
- Check the elevated tank: 300,000 gallons at 70% is about 210,000 gallons of stored water. If it filled during the outage, that volume is unprotected and will blend into the system as it drains.
Also check the CT assumptions themselves: pH 7.2 and 10°C. If the actual pH or temperature has drifted, the approved CT table no longer applies and the required contact time changes. The empty tank means zero concentration, so the answer to 'was CT met' is no - the work is establishing for how long and what volume was affected.
3. The residual is restored and distribution samples are clean. What changes to prevent recurrence, and what would you tell the state?
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Prevention, in order of effectiveness:
- Level alarm and auto-shutoff on the day tank. A tank that can run dry silently is the root cause. Add a low-level alarm at minimum, and ideally an interlock that alarms before empty and logs the event.
- Daily verification of chemical usage. Compare actual hypochlorite consumed against expected dose × flow. A mismatch is an early warning that catches a leak or feed error before the tank empties.
- Redundant feed or automatic switchover to a second day tank or drum so a single empty tank cannot stop disinfection.
- Analyzer alarm response procedure - the low-residual alarm worked; make sure the response is written down and practiced, including who calls the state and when.
- Verify the CT basis - confirm pH and temperature are monitored and that the approved table still applies.
What to tell the state: the timeline (when feed likely stopped, when discovered, when restored), the volume and area affected, residual and bacteriological results, the notification and any public notice issued, and the corrective actions with dates. Be factual and complete. The state's concern is whether you understood the failure and fixed the cause, not whether you had a bad day.