Laboratory Analysis and Process Control
Measuring to control
Lab questions test whether you can collect a valid sample, run or interpret a test, and use the result to adjust the process. Understand grab vs. composite sampling, chain of custody, and quality control: blanks, duplicates, spikes, and calibration. Know the core tests: turbidity (nephelometric), pH, chlorine residual (DPD or amperometric), alkalinity, hardness, TSS/VSS, BOD, and coliform testing (presence/absence and membrane filtration).
Concepts to master
- Calibration and standard curves; why instruments drift.
- Detection limits and reporting; holding times and preservation.
- Process control math: dose calculations (mg/L × MGD × 8.34 = lb/day), percent removal, and flow conversions.
- Recordkeeping and reporting to regulators.
- Interpreting trends rather than single points.
Traps
A frequent trap: using a grab sample where a composite is required, or vice versa. Another: reporting a result below the detection limit as a hard number. Watch for unit errors in dose math (MGD vs. gpm, lb vs. mg/L).
Worked example
A plant treats 2 MGD with a 10 mg/L dose: 10 × 2 × 8.34 = 166.8 lb/day. If the chemical is 65% active, divide by 0.65 to get product weight. Always check units before computing.