Riverside Drive WTP: Filter Upset and a CT Shortfall After a Storm
Riverside Drive Water Treatment Plant is a 1.6 MGD conventional plant (alum coagulation, dual-media filters, free chlorination) serving about 9,000 people from a river source. In 18 hours the watershed receives 3.2 inches of rain. Raw turbidity climbs from 5 to 78 NTU, raw pH falls to 6.8, alkalinity to 22 mg/L, and the water temperature is 6 °C. Over the next shift the combined filter effluent averages 0.28 NTU with a peak of 0.7 NTU. One filter runs at 0.48 NTU for most of the day and spikes to 1.4 NTU on two consecutive 15-minute readings after backwash. Clearwell free chlorine is 1.9 mg/L, theoretical detention time is 55 minutes, the baffle factor is 0.55, and pH is 7.4. Your approved CT tables require 230 mg·min/L for 3-log Giardia at this pH and temperature. On Maple Street, a dead-end main, the free chlorine residual is 0.15 mg/L; it is 0.6 to 0.8 mg/L elsewhere in the system.
1. Evaluate the filter effluent readings. Is this a violation, and what do you do on this shift?
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Separate the two standards - they are not the same limit.
- Combined filter effluent (CFE): the rule is 0.3 NTU in at least 95% of monthly measurements and never above 1.0 NTU. Average of 0.28 with a peak of 0.7 NTU is inside both, so no CFE violation yet - but the margin is thin and one more spike could erase the 5% allowance.
- Individual filter effluent (IFE): 0.48 NTU for most of the day will almost certainly fail the 95% IFE criterion for that filter by the end of the month, and two consecutive 15-minute readings at 1.4 NTU is the trigger for a filter self-assessment under the surface water treatment rules as NH DES applies them. Read your rule text for the exact deadline before you commit to a date.
Immediate actions, in order of payoff:
- Verify the turbidimeters - grab a sample and check instrument calibration and the sample line for air bubbles before you chase a chemistry ghost.
- Jar-test the coagulant. Alkalinity dropped to 22 mg/L, which starves alum floc; consider a coagulant change or caustic/alkalinity addition rather than simply pushing more alum into low-alkalinity water.
- Reduce filter loading (lower plant rate, put the bad filter on filter-to-waste, or take it out of service) and check backwash efficiency, media loss, and headless profiles.
- Log everything with times and readings - the self-assessment and any compliance determination depend on your records, not memory.
2. Compute actual CT. If the plant cannot meet the required CT today, what are your options and obligations?
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The arithmetic: CT = residual × time × baffle factor = 1.9 mg/L × 55 min × 0.55 = 57.5 mg·min/L against a required 230 mg·min/L. That is about 25% of the required credit - a large shortfall, not a rounding error.
Why raising the dose alone will not fix it: to hit 230 with 55 minutes and a 0.55 baffle factor you would need 230 ÷ (55 × 0.55) = 7.6 mg/L free chlorine. That is not a defensible operating residual: DBP formation, taste and odor complaints, and chlorine demand in a raw water at 78 NTU would eat much of it anyway. Even with perfect plug flow (baffle factor 1.0) you would need 4.2 mg/L.
The real constraint is contact volume, and it gets worse with flow. The 55 minutes is a design figure; at higher plant flow during the storm, actual detention is shorter and CT falls further.
Options:
- Put every available clearwell in service, verify baffle and valve positions, and stop short-circuiting.
- Engineer improvements: baffling to raise the factor toward 0.9, additional contact volume, or an alternative primary barrier (UV or ozone).
- Confirm with NH DES whether the deficiency is a treatment technique violation. If it is, a Tier 2 public notice is generally required on the rule's schedule, and you should not wait for the engineer's report to notify the state.
3. What do you do about the 0.15 mg/L free chlorine residual on Maple Street?
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Treat it as a real finding first, then verify it. The state minimum residual in the distribution system is 0.2 mg/L free chlorine under NH's rules - 0.15 is below it, but a single handheld reading can be off. Re-check the meter with a fresh DPD reagent, take a second sample, and confirm the number before you act on it.
If it holds, work the cause, not the symptom:
- Check for a recent main break or repair, new construction, or a valve change that could have introduced stagnant or sediment-laden water.
- Look for a cross-connection or backflow path at the dead end - dead ends are where unprotected connections show up first.
- Flush the main until the residual recovers, then re-sample after the system settles. Flushing alone does not count as a fix if the residual falls back within a day.
- Consider a booster chlorination point or dead-end blowoff with scheduled flushing if this is chronic.
Pair the residual sample with bacteriological sampling. Low residual plus a coliform hit is a completely different conversation than low residual alone. If the residual cannot be restored, talk to NH DES about notification before the next compliance sample is due rather than after it comes back positive.