Instrumentation Failure and Process Control Decisions
A 10 MGD surface water treatment plant relies on a SCADA system for process control. The plant has online turbidimeters on each filter, a streaming current detector (SCD) for coagulant dose control, and a pH analyzer at the filter influent. The plant uses alum and polymer. One morning, the operator notices that the SCD reading has been stable at 0.0 for the past 2 hours, but the raw water turbidity has increased from 5 to 20 NTU. The filter effluent turbidities are rising: Filter 1 is at 0.5 NTU, Filter 2 at 0.4 NTU, and others are at 0.2 NTU. The pH analyzer reads 7.0, which is normal. The operator checks the SCD and finds that the probe is fouled with algae. The plant has a backup SCD but it is not calibrated. The operator must decide how to control coagulant dose until the SCD is back online. The plant has a jar test kit and can perform tests, but it takes 30 minutes. The plant also has a manual backup for alum dose based on flow and turbidity, but the last jar test was a week ago at lower turbidity. The operator must act quickly to prevent filter breakthrough.
1. What immediate actions should the operator take to maintain coagulation control and prevent filter breakthrough?
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Immediate actions: 1) Switch coagulant dose control from SCD to manual or flow-paced mode. Use the last known good dose from the jar test a week ago (e.g., 20 mg/L) as a starting point, but adjust upward because raw turbidity has increased from 5 to 20 NTU. A rule of thumb: increase alum dose by 1-2 mg/L per NTU increase, but better to perform a jar test. 2) Perform a jar test immediately with the current raw water to determine the optimal dose. While waiting for results, increase the alum dose by 25-50% (e.g., from 20 to 25-30 mg/L) to account for higher turbidity. 3) Monitor filter effluent turbidities closely; if any filter exceeds 0.5 NTU, consider taking it out of service or reducing flow. 4) Check and clean the SCD probe; if possible, calibrate the backup SCD quickly or use a portable SCD. 5) Verify pH; if pH is stable at 7.0, that's good, but ensure alkalinity is sufficient. 6) Increase polymer dose if needed to strengthen floc; but be careful not to overdose. 7) Notify the supervisor and document the event. 8) After the jar test, adjust the dose accordingly and bring the backup SCD online. 9) Consider using the streaming current value from a portable unit if available. 10) If turbidity breakthrough continues, reduce plant flow to increase detention time and improve settling.
2. How should the operator handle the fouled SCD and the uncalibrated backup, and what long-term improvements should be considered?
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Handling the fouled SCD: 1) Clean the probe according to manufacturer instructions (e.g., using a soft brush and mild detergent, then rinse). 2) After cleaning, recalibrate the SCD using a standard or by comparing to a jar test. 3) If the SCD is damaged, replace it. For the backup SCD: 1) Calibrate it using a two-point calibration (or according to manufacturer) and verify with a jar test. 2) Once calibrated, put it into service and repair the primary. Long-term improvements: 1) Implement a preventive maintenance schedule for the SCD, including weekly cleaning and monthly calibration. 2) Install a second SCD in parallel for redundancy, or use a different coagulation control strategy (e.g., automatic dose control based on turbidity and flow with feedback from filter effluent). 3) Train operators on manual coagulant control and jar testing. 4) Maintain a log of jar test results to establish a dose-turbidity relationship. 5) Consider installing a particle counter for better filter performance monitoring. 6) Review SCADA alarms for instrument failure; set alarms for out-of-range or stagnant readings. 7) Stock spare probes and calibration supplies. 8) Develop a standard operating procedure for instrument failure. 9) Consider using a portable SCD as a backup. 10) Evaluate the need for an online TOC analyzer if TOC varies. The key is to have redundant, reliable instrumentation and trained staff to respond to failures.