Laboratory Sampling, Analysis, and Process Control Calculations
Sampling that stands up, and the math that follows
A result is only as good as the sample. Know container and preservation rules: bacteriological samples go in sterile 100-mL bottles containing sodium thiosulfate to neutralize chlorine, collected aseptically, chilled, and analyzed within holding time. Chlorine residual, pH, temperature, and turbidity are measured in the field because they change in transit. TTHM and HAA5 samples require proper quenching and headspace control; metals are preserved with acid. Chain of custody, labeling, and the use of blanks, duplicates, spikes, and calibration standards make the data defensible.
Recognize the methods: DPD for chlorine residual, nephelometric turbidity, pH electrode, alkalinity by titration to the carbonate and bicarbonate endpoints, fluoride by ion-selective electrode or colorimetry, nitrate by colorimetry, and jar tests for treatability. Coliform results are presence/absence, and compliance is judged on a geometric mean, not an arithmetic average.
Practice these process-control calculations: percent removal = (in − out) ÷ in × 100; dose (lb/day = mg/L × MGD × 8.34); filter rate and overflow rate; detention time (volume ÷ flow); and conversions - 1 grain/gallon = 17.12 mg/L, 1 ft³ = 7.48 gal, 1 mg/L = 8.34 lb/MG.
Worked example - geometric mean: routine coliform results of 1, 5, 2, and 10 give a product of 100; the fourth root of 100 is 3.2, not the arithmetic average of 4.5.
Worked example - percent removal: influent 8 NTU, effluent 0.4 NTU → (8 − 0.4) ÷ 8 × 100 = 95% removal.
Traps: arithmetic averaging of coliform data; expired buffers or reagents; reporting a chlorine or turbidity value from a sample that sat in a cooler.