Monitoring, Sampling, and Laboratory Analysis
What you must know
Monitoring ensures compliance and process control. Sampling must be representative: composite vs grab, flow-proportional vs time-proportional, and proper preservation (cooling, acidification, etc.). Chain of custody and documentation matter. Key lab analyses: BOD5 (5-day, 20°C, seeded, ATU for NBOD suppression), COD (dichromate, digestion), TSS (filterable residue), VSS, ammonia (Nessler or electrode), nitrate, phosphorus, pH, alkalinity, and DO (Winkler or meter). Quality control includes blanks, duplicates, spikes, and calibration.
Process monitoring includes MLSS, SVI, DO, microscopy, and settleability. Field measurements (pH, DO, temperature) require calibration and proper probe maintenance. Data validation and trend analysis support process decisions.
Traps in questions
- Confusing grab vs composite sampling requirements.
- Forgetting BOD seeding and ATU for nitrification suppression.
- Ignoring sample preservation and holding times.
- Assuming a single sample represents a variable discharge.
Worked example / rule of thumb
A flow-proportional composite is preferred when flow varies; grab samples suit pH and DO. Rule of thumb: BOD holding time is generally 48 hours if preserved at ≤6°C; COD holding time is typically 28 days if preserved. Always verify against the permit and method.