Laboratory Analysis and Process Control Calculations
Core tests
BOD5 measures oxygen consumed over 5 days at 20 °C; CBOD suppresses nitrification with an inhibitor. TSS is retained on a glass fiber filter dried at 103–105 °C, and VSS is the portion burned off at 550 °C. pH, alkalinity, ammonia, nitrate, and phosphorus round out nutrient control. Dissolved oxygen is measured by membrane electrode (calibrated in water-saturated air) or Winkler titration. Chlorine residual uses DPD or amperometric titration. Coliforms use membrane filtration or MPN.
Sampling and quality control
Grab samples are point-in-time; composites (time- or flow-proportional) represent an average. Cool samples to 0–6 °C and respect holding times. Each analytical batch should carry a method blank, duplicate, and known standard; calibration curves, control charts, and correct significant figures are part of the record.
Conversions to memorize
1 ft³ = 7.48 gal; 1 gallon of water weighs 8.34 lb; lb/day = mg/L × MGD × 8.34; ppm is approximately equal to mg/L in wastewater.
Traps
Applying a dilution factor when the whole bottle was used; forgetting seed correction; reporting an arithmetic mean where the permit specifies a geometric mean.
Worked example
30 mL of sample is diluted to 300 mL. Initial DO is 8.0 mg/L and final DO is 3.2 mg/L. BOD = (8.0 − 3.2) × (300/30) = 4.8 × 10 = 48 mg/L. Had the full 300-mL bottle been used, the same depletion would mean 4.8 mg/L - a tenfold error if the dilution factor is misapplied.