Process Control, Monitoring, and Laboratory Analysis
Core Concepts
Process control ensures treatment meets effluent limits. Key concepts include monitoring parameters (pH, flow, turbidity, DO, ORP, conductivity, TOC, BOD, COD, TSS, nutrients, metals), control charts, and statistical process control (SPC). Laboratory analysis must follow approved methods (e.g., EPA methods, Standard Methods) with quality assurance/quality control (QA/QC) including blanks, duplicates, spikes, and calibration. Sampling must be representative; grab vs. composite sampling; chain of custody for legal defensibility. Instrument calibration and maintenance are critical. Traps include confusing accuracy vs. precision, misinterpreting control chart rules (e.g., Western Electric rules), and assuming that a single grab sample is representative of a variable discharge. Also, method detection limit (MDL) and practical quantitation limit (PQL) are often confused. pH calibration requires fresh buffers; DO meters need membrane and electrolyte maintenance. Turbidity is not a direct measure of TSS but correlates. Another trap: BOD seed control and nitrification inhibition in BOD tests.
Worked Example
A control chart for effluent pH shows 7 consecutive points above the mean. According to Western Electric rules, this indicates a shift and requires investigation. Rule of thumb: 7 points on one side of the mean = non-random pattern. This illustrates the trap of ignoring trends and reacting only to points outside control limits.