BOD and COD: Connections and Differences of Two Key Organic Pollution Indicators in Wastewater

In wastewater treatment, achieving discharge compliance is the ultimate goal. Each treatment unit relies on reliable water quality monitoring data to dynamically adjust process parameters and ensure progressive pollutant removal. Among numerous indicators, BOD (Biochemical Oxygen Demand) and COD (Chemical Oxygen Demand) are representative parameters of organic pollution, present throughout the treatment process. They are used to assess influent load, determine biological treatment suitability, and guide operational decisions such as aeration and sludge recirculation.

Understanding the essential relationship and differences between BOD and COD helps engineers diagnose water quality more accurately and optimize processes. This article analyzes their definitions, measurement principles, and practical applications, while exploring the critical role of the BOD/COD ratio in assessing biodegradability. Nexisense, a brand specializing in water quality sensors, provides stable online monitoring equipment in multiple treatment projects, supporting precise management.

BOD (Biochemical Oxygen Demand): Organic Load from a Microbial Perspective

BOD represents the oxygen consumed by microorganisms while decomposing organic matter under aerobic conditions. It simulates natural pollutant degradation and directly measures the biodegradable fraction of organic matter.

In practice, BOD5 (five-day incubation period) is used for comparability. The sample is maintained at 20°C, and the difference between initial dissolved oxygen and residual oxygen on day five determines the BOD5 value (mg/L). Higher values indicate more easily degradable organic matter and greater potential oxygen demand. If BOD5 is high and oxygen replenishment is insufficient, hypoxia and odor/darkening of the water may occur.

BOD5 closely reflects biological load for designing aerobic processes such as activated sludge or biofilm systems. Its disadvantage is the long measurement period (5 days) and influence from microbial activity, temperature, and toxic substances, limiting rapid decision-making.

COD (Chemical Oxygen Demand): Comprehensive Chemical Oxidation of Organic Matter

COD uses a strong oxidant (e.g., potassium dichromate) under acidic conditions and heat to oxidize reducing substances in the sample (mainly organic matter, plus some inorganic reducers like sulfides or Fe²⁺). The oxygen equivalent consumed is expressed in mg/L O2, representing total reducing matter.

Compared to BOD, COD is fast (2–3 hours), reproducible, and less affected by toxins, making it ideal for online monitoring. Higher values indicate greater total organic and some inorganic pollutant content. According to GB3838-2002, COD classifies surface water quality: Class I ≤15 mg/L (close to drinking water), Class V ≤40 mg/L (high pollution).

COD oxidizes both biodegradable and recalcitrant/toxic organics, so values are generally higher than BOD5.

Intrinsic Connection between BOD and COD: Same Target, Different Scope

Wastewater contains various organic compounds. BOD and COD provide unified indicators by quantifying oxygen consumed in the oxidation of organics to CO2 and H2O, reflecting pollution load. The difference lies in:

• BOD reflects only the biodegradable fraction.

• COD covers nearly all organics and some inorganic reducers.

Therefore, COD is usually greater than BOD5. In domestic wastewater, COD/BOD5 ≈ 1.8–2.5 (predominantly biodegradable). In industrial wastewater, COD/BOD5 ≈ 3–5 or more, indicating an increase of recalcitrant compounds.

BOD/COD Ratio: Key Biodegradability Indicator

• B/C > 0.5 (COD/BOD < 2): high biodegradability, suitable for direct biological treatment.

• B/C 0.3–0.5: moderate biodegradability, may require longer retention time or nutrient addition.

• B/C < 0.3 (COD/BOD > 3.3): low biodegradability, biological treatment limited; physical-chemical or advanced oxidation pretreatment needed.

For example, food processing wastewater has high B/C and goes directly to biological treatment; textile or pharmaceutical wastewater has low B/C and requires pretreatment to improve biodegradability.

Monitoring B/C in influent, effluent, and treatment stages helps detect anomalies: sudden drop indicates toxins; rise in effluent indicates accumulation of recalcitrant compounds.

Practical Applications: Using BOD and COD to Guide Treatment

• Influent monitoring: COD quickly identifies load impact; BOD5 evaluates biological stage design.

• Process control: Real-time monitoring of DO and COD/BOD ratio optimizes aeration.

• Effluent control: COD as primary discharge indicator; BOD5 validates biological efficiency.

• Abnormality diagnosis: High COD but normal BOD → inorganic interference; high BOD but low COD → microbial activity issue.

Nexisense online water sensors enable continuous monitoring of COD and BOD-related parameters, with RS485 or 4–20 mA outputs, integrable into PLC or SCADA systems, supporting data-driven decisions.

FAQs

Which is more important, BOD or COD?
Complementary: COD is fast and comprehensive for monitoring and compliance; BOD reflects actual biological treatment, useful for design and biodegradability assessment.

Why is COD/BOD lower in domestic wastewater and higher in industrial wastewater?
Domestic wastewater contains easily degradable organics; industrial wastewater contains synthetic compounds, dyes, and complex metals that are harder to degrade.

How to improve biodegradability if B/C is low?
Common methods: acid hydrolysis, Fe/C micro-electrolysis, ozone/Fenton advanced oxidation to break macromolecules.

COD exceeds standard but BOD is normal, what to do?
Check for interference from sulfides, Fe²⁺, or other inorganic reducers, or use partial oxidation to separate organic/inorganic contributions.

Conclusion: Mastering BOD and COD for Intelligent Water Management

BOD and COD, although focusing differently, provide a complete picture of organic pollution. BOD shows biodegradable fraction; COD shows total load. The B/C ratio is essential for process design and performance evaluation. With stricter discharge standards and energy optimization, accurate real-time data is crucial. Nexisense online sensors enable precise monitoring, transforming data into operational improvements and supporting sustainable water protection.