Wastewater Characterization and Regulatory Requirements
Core Concepts
Industrial wastewater is characterized by physical, chemical, and biological parameters. Key physical parameters include pH, temperature, color, turbidity, and solids (total, suspended, dissolved, volatile). Chemical parameters include BOD, COD, TOC, oil and grease, nutrients (N, P), heavy metals, and toxic organics. BOD measures biodegradable organic matter via oxygen consumption over 5 days; COD measures chemically oxidizable matter and is always ≥ BOD. The BOD/COD ratio indicates biodegradability. Regulatory frameworks include the Clean Water Act (CWA) and its National Pollutant Discharge Elimination System (NPDES) permits, which set effluent limitations based on technology-based standards (BAT, BCT) and water quality standards. Pretreatment standards (40 CFR 403) apply to industrial dischargers to publicly owned treatment works (POTWs), including categorical standards and local limits. Traps include confusing BOD5 with ultimate BOD, misinterpreting TSS vs. total solids, and assuming that a pollutant without a numeric limit is unregulated-narrative standards may apply. Also, whole effluent toxicity (WET) testing is often required even when specific chemicals are below limits. Another trap: grab vs. composite sampling requirements differ by permit; composite samples are flow-weighted or time-weighted.
Worked Example
An industrial user discharges wastewater with BOD5 = 250 mg/L and COD = 500 mg/L. The BOD/COD ratio is 0.5, indicating moderately biodegradable waste. If the local limit for BOD5 is 200 mg/L, the user must pretreat. Rule of thumb: BOD/COD > 0.5 suggests readily biodegradable; < 0.3 suggests poorly biodegradable or toxic inhibition. This helps in selecting treatment (biological vs. chemical).