Coagulant Dose Creep After a Reservoir Turnover
A surface-water plant treating 6.2 MGD has run for three weeks on a stable alum dose of 28 mg/L, with settled turbidity near 1.8 NTU and finished water at 0.06 NTU. This week the raw water changed abruptly: turbidity dropped from 9 NTU to 3.5 NTU, but raw pH rose from 7.4 to 8.1 and alkalinity fell from 95 to 58 mg/L as CaCO3. Filter run times have shortened from 40 hours to 22 hours, and settled turbidity has climbed to 4.5 NTU. The operator's jar tests, run at the current coagulant dose, show pin floc that does not settle. Chlorine residual and disinfection CT remain in compliance. The plant has one alum feed pump with no spare, and the basin has a single rapid-mix unit with a fixed 30-second detention time.
1. Diagnose why settled turbidity worsened even though raw turbidity improved, and state what the operator should change first.
Read the worked answer
The raw turbidity drop is a red herring; the controlling changes are the pH rise and alkalinity drop. Alum coagulation depends on pH and on the hydrolysis products that form. At pH 8.1 with lower alkalinity, the operator is likely outside the optimum pH band for the alum dose in use, producing pin floc that will not enmesh or settle. The first move is to re-run jar tests across a range of coagulant doses and pH values, not just the current dose, to locate the new optimum. If the optimum requires a lower pH, evaluate acid addition or a coagulant aid; if the dose is simply insufficient for the new particle character, increase it incrementally while watching settled turbidity and filter run length. Do not chase the raw turbidity number downward; it is not the limiting variable.
2. Filter run times have halved. What does that indicate about the treatment train, and how should the operator respond before changing filter operations?
Read the worked answer
Shortened runs with elevated settled turbidity point upstream: poorly conditioned floc is reaching the filters and loading them faster than the media can store it. The correct sequence is to fix coagulation and sedimentation first, then reassess filter performance. Changing filter operations (for example, raising backwash frequency or altering flow) would mask the symptom and consume treated water. The operator should confirm settled turbidity returns toward the prior baseline before concluding the filters have a problem. If runs remain short after settled turbidity recovers, then investigate media condition, backwash effectiveness, and filter loading rate.
3. The single alum feed pump and fixed rapid-mix detention time are constraints. How should the operator manage risk during the correction?
Read the worked answer
A single feed pump is a single point of failure for the entire plant's coagulation; the operator should verify spare parts, a manual backup feed method, and a documented contingency. Rapid-mix at a fixed 30 seconds may be inadequate if the new chemistry requires a different mixing intensity or time; the operator can test whether changing mixing energy (if adjustable) improves floc, but should not assume detention time must change. During the transition, increase monitoring frequency for settled and finished turbidity, and confirm disinfection CT remains valid since higher settled turbidity can shield pathogens. Any chemical change should be recorded and communicated to the next shift.