Laboratory Analysis and Process Control Testing
Core process control tests
BOD5 is a bioassay: sample incubated 5 days at 20 °C, seeded if needed, with a dissolved oxygen depletion of at least 2 mg/L and at least 1 mg/L residual DO remaining. A seed correction is applied when seeded dilution water is used. COD uses dichromate digestion and reflects nearly all oxidizable material. TSS is retained on a glass fiber filter and dried at 103–105 °C; VSS is the portion lost on ignition at 550 °C, and fixed solids are the ash remaining. Other routine tests include pH, alkalinity, DO by Winkler titration or membrane electrode, ammonia, nitrate, phosphorus, residual chlorine by DPD, settleability, SVI, and microscopic examination.
Quality control
Accuracy is checked with matrix spikes, laboratory control samples, and certified standards; precision is checked with duplicates. Method blanks, calibration blanks, calibration curves, control charts, and method detection limits round out the program. Preservation and holding times must be met: cool to 4 °C, acidify to pH below 2 with sulfuric acid for most nitrogen and metals samples, BOD within 48 hours, TSS within 7 days. Chain of custody documents every transfer.
Traps
Mixing the drying temperature (103–105 °C) with the ignition temperature (550 °C); reporting BOD without a seed correction; forgetting sodium thiosulfate in coliform bottles; and treating a duplicate as a measure of accuracy when it measures precision.
Rule of thumb
Percent removal = (influent − effluent) ÷ influent × 100. A BOD depletion below 2 mg/L is invalid - use a smaller sample volume or seed the dilution water.