BOD and COD appear on every ETP monitoring report, every SPCB compliance submission, and every effluent test certificate. Yet the distinction between them — and what the ratio of one to the other actually means for your treatment system — is frequently misunderstood. This guide explains both parameters from first principles, with the practical implications for ETP design and operation.
What Is BOD and How Is It Measured?
Biochemical Oxygen Demand (BOD) is the amount of dissolved oxygen (in mg/L) consumed by microorganisms as they biodegrade organic matter in a wastewater sample over a standard period — typically 5 days at 20°C, reported as BOD₅. It is a measure of the biologically degradable organic pollution in the water.
The BOD test works by taking two samples of the wastewater: one is measured for initial dissolved oxygen (DO), and the other is incubated in the dark at 20°C for 5 days. The final DO of the incubated sample is then measured. BOD₅ = (Initial DO − Final DO) × dilution factor. Because microorganisms consume oxygen as they break down organic matter, higher BOD means more biodegradable organic pollution.
The 5-day test is the standard in India and most regulatory frameworks. An "ultimate BOD" test runs for 20+ days and captures complete biodegradation, but is impractical for routine monitoring. The ratio of BOD₅ to ultimate BOD is typically 0.6–0.8 for most industrial wastewaters.
What Is COD and How Is It Measured?
Chemical Oxygen Demand (COD) is the amount of oxygen (in mg/L) required to chemically oxidise all oxidisable organic and inorganic matter in a sample, using a strong chemical oxidant (typically potassium dichromate at reflux temperature in acidic conditions). COD measures all oxidisable pollution — both biodegradable and non-biodegradable organic compounds.
COD can be measured in 2–3 hours using the closed reflux method, making it far more practical than BOD₅ for day-to-day ETP monitoring. In most ETP control rooms, COD is the primary metric tracked in real time — daily inlet, post-biological, and final effluent COD measurements give the operator a rapid picture of biological treatment performance.
Since COD includes non-biodegradable compounds, COD is always equal to or greater than BOD for any wastewater sample. The difference between COD and BOD represents the non-biodegradable fraction of organic matter.
The BOD:COD Ratio: What It Tells You
The BOD:COD ratio is one of the most useful single numbers for evaluating wastewater treatability:
| BOD:COD Ratio | Interpretation | Treatment Implication |
|---|---|---|
| ≥ 0.5 | Highly biodegradable | Standard biological treatment effective (MBBR, ASP, SBR) |
| 0.3–0.5 | Moderately biodegradable | Biological treatment works with adequate HRT; some residual COD expected |
| 0.1–0.3 | Poorly biodegradable | Advanced oxidation (AOP) before or after biological treatment needed |
| < 0.1 | Predominantly refractory | Physico-chemical or AOP primary treatment; biological of limited use alone |
Most food and beverage wastewaters have BOD:COD ratios of 0.5–0.7, making them excellent candidates for conventional biological treatment. Textile dye wastewater typically shows ratios of 0.1–0.3 due to non-biodegradable synthetic dyes — requiring advanced oxidation in addition to biological treatment. Pharmaceutical and chemical wastewaters can drop below 0.1 when significant solvent or API content is present.
Typical BOD and COD Values by Industry
| Industry | BOD (mg/L) | COD (mg/L) | BOD:COD |
|---|---|---|---|
| Dairy processing | 500–2,000 | 1,000–5,000 | 0.5–0.6 |
| Biscuit / bakery | 800–3,000 | 2,000–8,000 | 0.5–0.6 |
| Sugar mill | 1,000–3,000 | 2,500–7,000 | 0.5–0.6 |
| Distillery (spent wash) | 30,000–70,000 | 80,000–150,000 | 0.4–0.5 |
| Textile dyeing | 200–600 | 800–3,000 | 0.15–0.3 |
| Pharmaceutical | 500–2,000 | 2,000–10,000 | 0.1–0.3 |
| Tannery | 800–2,500 | 2,000–6,000 | 0.3–0.5 |
Discharge Standards for BOD and COD in India
The CPCB sets General Standards under the Environment (Protection) Rules, 1986:
- Inland surface water: BOD ≤30 mg/L, COD ≤250 mg/L
- Public sewer: BOD ≤350 mg/L, COD ≤600 mg/L
- Land irrigation: BOD ≤100 mg/L, COD not typically specified
- Marine coastal areas: BOD ≤100 mg/L
State PCBs frequently impose stricter limits under individual Consent to Operate conditions, particularly for sensitive receiving water bodies, ZLD-required sectors (textile in Gujarat, distillery in Maharashtra), and industries in ecologically sensitive zones. Always verify your specific CTO conditions rather than assuming General Standards apply.
How to Reduce BOD and COD in Wastewater
For high BOD:COD wastewater (≥0.5), a well-designed MBBR or activated sludge system running at appropriate F/M ratio and with adequate HRT will achieve 90–98% BOD removal and 85–95% COD removal. The most common causes of failure to achieve target BOD/COD are: insufficient HRT during production peaks, high F/M ratio from underloaded or poorly controlled biological tanks, and inadequate pre-treatment (DAF bypass leading to FOG fouling of biological media).
For low BOD:COD wastewater (below 0.3), advanced oxidation processes — ozonation, Fenton reaction (H₂O₂ + Fe²⁺), or UV/H₂O₂ — are used to break down refractory compounds before biological treatment. This increases the BOD:COD ratio of the pre-treated effluent, making it more amenable to biological degradation. Activated carbon adsorption can also remove refractory COD polishing after biological treatment.
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