Process Control, Sampling, and Laboratory Analysis
Sampling
Choose grab versus composite deliberately: grab samples suit pH, DO, chlorine, and bacteria, while flow-proportioned composites suit pollutant averages and loading calculations. Follow preservation and holding times, document chain of custody, and neutralize chlorine in effluent bacteria samples with a reducing agent.
Core laboratory tests
BOD5 incubates at 20 °C for 5 days in a seeded, diluted sample; CBOD uses a nitrification inhibitor. TSS is retained on a glass fiber filter and dried at 103–105 °C; VSS is the portion lost on ignition at about 550 °C. Ammonia, nitrate, and phosphorus are run by colorimetric, electrode, or chromatographic methods. DO comes from a meter or Winkler titration, and fecal coliform by membrane filtration (0.45 µm, incubated near 44.5 °C).
Process control tests
Settleability (SV30), SVI, MLSS and MLVSS, DO profiles across the basin, microscopic examination for filaments and protozoa, and the VFA-to-alkalinity ratio give early warning long before permit limits are affected.
QA/QC
Duplicates, blanks, spikes, standards, control charts, and calibration records support defensible data.
Traps
Never report a raw settleometer reading as an SVI. Ignoring seed correction inflates BOD. Sampling chlorinated effluent without neutralizer kills coliforms and produces false low counts. Results below the detection limit are not zero for averaging.
Worked example
SVI = SV30 (mL/L) × 1,000 ÷ MLSS (mg/L). If sludge settles to 250 mL/L in 30 minutes with MLSS of 2,500 mg/L, SVI = 250 × 1,000 ÷ 2,500 = 100 mL/g, indicating good settling.