Laboratory Analysis, Sampling, and Monitoring
Core concepts
Sampling integrity is the foundation: grab vs. composite (time- or flow-proportioned), representative sampling points, clean containers, preservation (cooling to 4°C, acidification for metals, no headspace for BOD/DO), and holding times (BOD 48 hours, TSS 7 days, pH analyze immediately). Chain of custody protects legal defensibility.
Key tests: BOD5 (5-day, 20°C, seeded, dilution water, DO depletion), CBOD vs. BOD, TSS (glass fiber filter, 103–105°C drying), VSS (ignition at 550°C), pH (two-point calibration), DO (Winkler or membrane electrode), ammonia (Nessler or salicylate), phosphorus, chlorine residual (DPD), and coliform (membrane filtration or multiple tube).
Quality control: blanks, duplicates, spikes, standards, control charts, and method detection limits. Calibration is not the same as verification.
Traps
Questions confuse BOD with CBOD (nitrification inhibitor), or ask why a BOD result is invalid - common causes are insufficient seed, low DO depletion (<2 mg/L), or excessive depletion (<1 mg/L remaining). Watch for preservation errors: samples not cooled, or pH not measured in the field. Don't confuse TSS with settleable solids (Imhoff cone).
Worked example / rule of thumb
BOD: initial DO 8.0, final DO 3.0, sample volume 300 mL in a 300-mL bottle (no dilution). Depletion = 5.0 mg/L. If a dilution were used, BOD = depletion × (bottle volume ÷ sample volume), corrected for seed. Rule of thumb: valid depletion is at least 2 mg/L and at least 1 mg/L remains; otherwise rerun.