Laboratory Analysis, Sampling, and Process Control Calculations
Getting numbers you can defend
Sampling is a legal act. Know grab versus composite (composites for load-based parameters like BOD and TSS, grabs for chlorine, pH, temperature, dissolved oxygen, and bacteria), proper containers, preservation, holding times, and chain of custody. Chlorine residual is measured immediately by DPD; turbidity uses a nephelometer; BOD is a 5-day, 20 C test; TSS uses glass fiber filtration and drying at 103 to 105 C; coliform methods report presence or absence and counts converted to a geometric mean.
Quality control makes the result admissible: method blanks, duplicates, matrix spikes, calibration verification, and documented detection limits. Significant figures and units are graded too - mg/L versus ug/L, gallons versus cubic feet, day versus hour.
Process control math worth memorizing:
- Pounds formula: lb/day = mg/L x 8.34 x MGD.
- Feed rate: solution feed, gpd = (lb/day desired) / (8.34 x solution strength as decimal).
- Detention time = volume / flow, with matching units.
- Percent removal = (in - out) / in x 100.
- Surface loading = flow / area; hydraulic loading in gpd/ft2.
- MCRT = solids in aeration system / solids leaving per day.
Traps: using gallons when the formula expects MG, forgetting that 1 ft3 = 7.48 gal, mixing percent with decimal concentration, averaging pH values arithmetically (use the hydrogen ion), and treating a composite as valid when a grab is required.
Worked examples
Chlorine dose: 1.5 MGD at 2.0 mg/L = 2.0 x 8.34 x 1.5 = 25 lb/day. If the hypochlorite is 12.5 percent strength by weight, solution feed = 25 / (8.34 x 0.125) = 24 gpd. Percent removal: influent BOD 240 mg/L, effluent 18 mg/L gives (240 - 18) / 240 x 100 = 92.5 percent.