Storm-Driven Coagulation Upset at a 12 MGD Conventional Plant
A 12 MGD conventional surface water plant treating a river source is running at 9.5 MGD when a 36-hour storm raises raw turbidity from 4 to 22 NTU, color to 45 CU, and drops raw pH from 7.6 to 6.9 and alkalinity from 70 to 45 mg/L as CaCO3. The plant feeds 28 mg/L alum and 0.3 mg/L cationic polymer; the lime feeder has been idle for two years. Settled turbidity climbed from 1.8 to 4.5 NTU, filter runs fell from 60 to 22 hours, and combined filter effluent is running 0.35 to 0.85 NTU against a permit limit of 0.30 NTU (95th percentile) and an internal goal of 0.10 NTU. Jar tests show 45 mg/L alum gives the best settled turbidity (1.2 NTU) but leaves the jar pH at 6.2. Filter-to-waste piping and valves exist but have not been operated since 2019, and no current operator has used them. The primacy agency requires notification within 24 hours if combined filter effluent exceeds 1.0 NTU. Chlorine residual and bacteriological results remain normal.
1. What should the operator do in the next 8 hours? Rank the actions and justify each one with the data given.
Read the worked answer
Treat this as a loading problem first, a dose problem second.
- Cut hydraulic loading now. Drop plant output toward 6-7 MGD and take the worst filter out of service. Settled turbidity of 4.5 NTU is roughly 2.5 times the pre-storm value; that solids load is what collapsed runs from 60 to 22 hours, and lower rate buys settling time.
- Raise the alum dose toward the jar-test optimum, but only with lime online first. 45 mg/L without pH correction will push coagulation pH below the effective alum range and can carry over soluble aluminum.
- Start filter-to-waste on any filter returning to service or currently above 0.30 NTU. Placing 0.5 NTU water on the clearwell is what is putting the plant near the 1.0 NTU notification trigger.
- Add polymer as a filter aid, not as a substitute for coagulation, if settled turbidity stays above 2 NTU.
- Notify the primacy agency if any combined effluent reading exceeds 1.0 NTU, and document the operational changes in the log.
Restoring lime feed is the gating step; the other actions depend on it.
2. Quantify the alkalinity and pH trade-off of moving from 28 to 45 mg/L alum, and explain how you would dose hydrated lime to stay in the effective coagulation range.
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Alum consumes about 0.45 mg/L of alkalinity as CaCO3 per 1 mg/L of alum.
- At 45 mg/L: 45 x 0.45 = about 20 mg/L as CaCO3 consumed. Raw alkalinity is 45 mg/L, so roughly 25 mg/L remains, which is consistent with the jar pH of 6.2 and is too low for reliable alum coagulation (target roughly 6.8-7.2 and 40-50 mg/L residual alkalinity).
- The 17 mg/L increase over the current dose accounts for about 7.7 mg/L of that consumption, so most of the pH drop is caused by the higher dose, not by the raw water alone.
Hydrated lime adds about 1.35 mg/L of alkalinity as CaCO3 per 1 mg/L of Ca(OH)2, so roughly 15 mg/L of hydrated lime restores about 20 mg/L of alkalinity and lifts pH back near 7.0. Feed it ahead of rapid mix with good slaking, control on both pH and alkalinity, and watch for overfeed, which drives pH above 7.5, wastes alum, and scales the lime system.
3. The plant manager wants to put every filter back in service as soon as possible. What criteria would you set for ending filter-to-waste and returning a filter to normal service?
Read the worked answer
Return-to-service should be performance-based, not time-based.
A filter should stay on filter-to-waste until its individual effluent turbidity is at or below 0.10 NTU on at least three consecutive 15-minute readings, with no upward trend, and headloss and flow are stable. If particle counting is available, require counts consistent with the plant's clean-filter baseline.
Before that, verify the filter was properly backwashed, the media surface is level with no mudballs or cracks, and there is no air binding. Check that the filter-to-waste valve actually moves and seats fully, since it has been idle since 2019, and exercise it before the first use rather than during a storm.
Once in service, hold the filter on 15-minute individual effluent turbidity monitoring, and keep the combined filter effluent under 0.30 NTU at all times with a target of 0.10 NTU. Filters that were not taken out of service but are producing above 0.30 NTU should also be pulled and returned through filter-to-waste. Document every decision, because the primacy agency will look at the records after the event.