Storm Turbidity and a Lime Outage at a Conventional Plant
A 12-MGD conventional surface-water plant treats a river source. At 06:00 Monday, raw turbidity is 42 NTU (up from 3 NTU), raw pH 7.0, alkalinity 38 mg/L as CaCO3, and color 35 CU after a 2-inch rain. Alum feed is 22 mg/L, polymer 0.4 mg/L, and pre-lime is off because the lime slaker is down for bearing replacement until Wednesday. Filtered turbidity has risen from 0.07 to 0.38 NTU at the combined effluent; one of four filters shows 0.9 NTU, and run times have fallen from 55 to 18 hours. Clearwell chlorine demand is up 1.2 mg/L, and finished-water pH is 6.6. The lab is staffed only until 15:00, and the next shift has one operator. The state requires notification if a filter exceeds 1.0 NTU or if CFE exceeds 0.3 NTU in more than 5% of monthly readings.
1. What three checks should you make before changing the alum dose, and what would each tell you?
Read the worked answer
Before touching the alum dose, verify the data and the cause.
- Jar-test the current raw water. Run a series at 22 mg/L and at 14, 18, 26, and 30 mg/L, with and without polymer. At pH 6.6 and 38 mg/L alkalinity, alum at 22 mg/L consumes roughly 10–11 mg/L of alkalinity, so low pH may be the real problem, not an insufficient dose. Overdosing alum would depress pH further and increase finished-water corrosivity.
- Check the coagulant feed system. Confirm pump stroke, calibration, solution strength, and that the feed point is not plugging. A 42-NTU spike with a stable dose can look like coagulation failure when it is actually a feed failure.
- Check each filter. Review backwash records, media condition, air scour, and turbidimeter calibration. The 0.9-NTU filter may have a media boil or a bad meter; one bad meter can make the whole plant look worse.
Also confirm raw pH and alkalinity with a second meter, because the lime outage makes the plant sensitive to small chemistry errors.
2. With no lime until Wednesday, outline a 24-hour treatment strategy to keep the combined filter effluent below 0.3 NTU.
Read the worked answer
Keep the plant within its turbidity limits by fixing coagulation chemistry and reducing loading, not by raising alum blindly.
- Restore pH/alkalinity. If caustic soda or soda ash is available, feed it to raise filtered-water pH toward 7.0–7.3 and maintain positive alkalinity. This may be more effective than more alum. Do not wait for lime.
- Jar-test an alternative coagulant. A polyaluminum chloride or alum/polymer combination may work better at low alkalinity and cold water. Increase polymer only within the filterable range; excess polymer shortens runs and blinds media.
- Reduce hydraulic loading. Cut plant output, if necessary, by 15–25% and purchase or borrow finished water. At 12 MGD, a short-term reduction is cheaper than a turbidity violation.
- Protect the filters. Backwash on headloss or turbidity, not on a fixed clock; use filter-to-waste after backwash; consider taking the worst filter out of service.
Document each change and the jar-test result. If individual filter effluent exceeds 1.0 NTU for two consecutive 15-minute readings, the filter is out of compliance and the state must be notified.
3. What monitoring and notification actions are required if one filter reaches 1.2 NTU for two consecutive 15-minute readings?
Read the worked answer
Two consecutive 15-minute readings at 1.2 NTU exceed the 1.0-NTU individual filter limit. Treat it as a compliance event.
- Confirm the reading. Check the turbidimeter calibration and grab-sample the filter effluent. If the meter is wrong, document the calibration and repair.
- Notify the state primacy agency as required, and start a filter assessment. Record the time, readings, backwash history, and what changed.
- Take the filter out of service or filter-to-waste if the reading remains high, and increase monitoring frequency.
- Watch the combined filter effluent. The CFE must stay at or below 0.3 NTU in at least 95% of monthly readings and never exceed 1.0 NTU. One bad filter can push the combined value over.
- Record everything in the operating log and prepare a corrective-action note. If the cause is the lime outage or coagulant upset, say so and show the fix.
Do not average the two readings or wait for the next shift to notify. The notification clock starts when you know.