Chromium Reduction Dosing Drift
Your batch chemical treatment train reduces hexavalent chromium with sodium bisulfite at pH 2.5, then precipitates with lime to pH 8.5. The ORP controller is set to hold 250 mV during reduction. This morning the effluent chromium is 0.9 mg/L against a 0.5 mg/L limit, and the batch passed ORP control. The pH probe in the reduction tank reads 2.6. The bisulfite metering pump stroke was recalibrated last week by a contractor. The ORP probe was last cleaned and calibrated 9 days ago; the calibration record shows a 40 mV span error that was noted but not corrected. The lime feed is running normally and the clarifier is producing clear supernatant. Lab confirms the 0.9 mg/L is hexavalent, not total.
1. Diagnose the most probable cause and explain why ORP control alone did not catch it.
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The 0.9 mg/L is hexavalent chromium, so reduction - not precipitation - failed. ORP control is a proxy for the bisulfite/Cr(VI) redox couple, and a probe with a 40 mV span error means the controller was holding the true potential well below the 250 mV setpoint. At pH 2.6 the reduction is thermodynamically favorable, but the controller was satisfied by a false reading, so it under-dosed bisulfite. The recalibrated pump stroke is a secondary suspect: if the stroke now delivers more per count, the controller would cut run time and still read 'in control' on a bad probe. The lesson: ORP is only as good as its calibration, and a noted-but-uncorrected span error is a known defect that should have triggered a work order, not a batch release.
2. What immediate batch disposition and corrective actions do you take before the next batch?
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Immediate: hold and re-treat the failed batch - re-acidify to pH 2.5, dose bisulfite to a verified ORP of at least 250 mV using a freshly calibrated probe, confirm with a spot test or lab Cr(VI) before neutralizing and precipitating. Do not release on ORP alone. Corrective: replace/calibrate the ORP probe and verify against quinhydrone or a standard redox solution; verify the metering pump stroke against a graduated cylinder; add a secondary verification (diphenylcarbazide spot test or periodic lab Cr(VI)) at the end of reduction. Then investigate why a noted span error was not corrected - that is a calibration-record and work-order control gap.
3. How would you change the control strategy so a single failed probe cannot cause a permit exceedance?
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Move from single-loop ORP to a redundant, cross-checked scheme: (1) two ORP probes with alarm on divergence; (2) a pH interlock so reduction cannot proceed outside the 2.0–3.0 window; (3) a daily or per-batch Cr(VI) verification that gates release; (4) a bisulfite dose calculated from the batch's Cr(VI) load as a feed-forward setpoint, with ORP as feedback trim; and (5) a calibration due-date alarm on the controller. The principle is defense in depth: the chemistry (pH, stoichiometry, verification) should be able to catch a bad sensor, and the sensor should catch a chemistry upset.