Process Control, Monitoring, and Laboratory Analysis
Process control means measuring, comparing to a target, and adjusting deliberately. Know the instruments: turbidimeters (NTU; nephelometric), pH meters (calibrate with two buffers bracketing the sample), conductivity, chlorine analyzers (DPD colorimetric and amperometric), flow meters, level sensors, and online vs grab sampling. Understand jar testing as the coagulation control tool and filter effluent turbidity as the filtration control tool.
Laboratory quality: know precision vs accuracy, calibration vs standardization, blanks, duplicates, spikes, and control charts. Recognize that DPD measures free and total chlorine and that high combined chlorine or interfering oxidants can bias results. Traps: calibrating a pH meter with only one buffer; reporting a result without checking it against a known standard; confusing NTU (optical) with mg/L; and assuming an online analyzer is correct without verification against a grab sample. Also watch holding times and sample preservation-improper preservation invalidates a compliance sample.
Rule of thumb: every critical online reading should be verified against a grab sample and a standard; if the two disagree, find out why before acting.
Worked example: An online turbidimeter reads 0.08 NTU but a grab sample reads 0.15 NTU. That gap matters against a 0.3 NTU limit. Recheck calibration and standard, then investigate the sample line-do not simply trust the lower number.