BOD (Biochemical Oxygen Demand) and COD (Chemical Oxygen Demand) are two fundamental parameters in water and wastewater analysis that quantify the organic load of a water body. Both are expressed in milligrams of oxygen per litre (mg O₂/L) and are critical for assessing water quality, wastewater treatment efficiency, and environmental impact.
- BOD (Biochemical Oxygen Demand): The amount of dissolved oxygen consumed by biological organisms when decomposing organic matter in a given water sample at a specified temperature over a set time period (typically 5 days — BOD₅). BOD reflects the biologically degradable organic content and is an indicator of the ecological burden a discharge places on a water body.
- COD (Chemical Oxygen Demand): The total amount of oxygen required to chemically oxidise all organic matter in the sample using a strong oxidising agent (potassium dichromate). COD measures total oxidisable organic content, including compounds resistant to biological degradation. COD is always ≥ BOD for the same sample.
- Both parameters are key to Ovalab’s water quality testing service for industrial wastewater, surface water, and effluent monitoring.
Analytical Methods
BOD and COD are determined using standardised ISO methods:
- BOD — ISO 5815-1:2019 (Water quality — Determination of biochemical oxygen demand after n days — Part 1: Dilution and seeding method): The sample is diluted with oxygen-saturated water, seeded with appropriate microorganisms if needed, and incubated in the dark at 20 ± 1 °C for 5 days (BOD₅). Dissolved oxygen is measured at day 0 and day 5; BOD is calculated from the difference. The method uses allylthiourea (ATU) to inhibit nitrification for carbonaceous BOD determination.
- COD — ISO 6060:1989 (Water quality — Determination of the chemical oxygen demand — Open reflux method): The classic method using potassium dichromate in sulphuric acid at reflux temperature (148 °C for 2 hours). Remaining dichromate is titrated; COD is calculated. Applicable for COD values 30–700 mg/L.
- COD — ISO 15705:2002 (Water quality — Determination of the chemical oxygen demand index — Small-scale sealed-tube method): A miniaturised version using sealed cuvettes (dichromate reagent in sealed tubes), with photometric measurement of the colour change. Results are determined using UV-Vis spectrophotometry. This is the most commonly used routine method.
Environmental and Regulatory Significance
BOD and COD are central to wastewater discharge permitting and surface water quality assessment:
- EU Urban Wastewater Treatment Directive (91/271/EEC): Sets BOD₅ limits for effluent discharged from urban wastewater treatment plants — e.g., BOD₅ ≤25 mg/L and COD ≤125 mg/L for discharges from populations above 2,000 person equivalents (p.e.) receiving sensitive area treatment. These are the established reference limits; a recast Directive is under development.
- EU Water Framework Directive (2000/60/EC): Requires Member States to achieve ‘good ecological status’ and ‘good chemical status’ for all water bodies. BOD/COD monitoring of rivers, lakes, and transitional waters is a key tool for tracking organic pollution pressure.
- Industrial wastewater permits: Food processing, brewery, dairy, paper, chemical, and pharmaceutical industries must comply with BOD/COD discharge limits specified in their environmental permits (integrated pollution prevention and control — IPPC).
- Wastewater treatment monitoring: Regular BOD and COD testing of influent and effluent at treatment plants measures removal efficiency and compliance with operating permits.
Frequently Asked Questions
What is the difference between BOD and COD?
BOD₅ refers to the Biochemical Oxygen Demand measured after 5 days of incubation at 20 °C. The 5-day period was historically standardised because it represents the time for a typical river’s water to flow from a pollution source to the sea in the UK (where the test was originally developed). In practice, 5-day incubation captures the bulk of the carbonaceous (organic carbon) oxygen demand while remaining practical for routine monitoring. For full ultimate BOD (BODu), longer incubation periods (20–28 days) are sometimes used in research applications.
Why are BOD and COD important for water quality?
COD is always higher than or equal to BOD because COD measures the total oxidisable organic matter (using a strong chemical oxidant — potassium dichromate), including compounds that are not biodegradable by microorganisms. BOD measures only the fraction that bacteria can biologically degrade within 5 days. The difference between COD and BOD provides insight into the proportion of non-biodegradable organic load — important information for wastewater treatment design, as non-biodegradable compounds cannot be removed by conventional biological treatment processes.
What are typical BOD and COD limits?
The COD/BOD ratio is used to assess the biodegradability of a wastewater: a ratio close to 1 indicates high biodegradability (most organic matter can be treated biologically), while a high ratio (e.g., >3) suggests significant non-biodegradable content, indicating the need for physico-chemical pre-treatment or advanced oxidation before or instead of biological treatment. Typical domestic wastewater has a COD/BOD ratio around 1.5–2.0; industrial effluents can vary widely depending on the industrial process.
How are BOD and COD measured?
The most widely used routine method is the sealed-tube (cuvette) method per ISO 15705:2002: a measured volume of sample is added to a pre-dosed reagent cuvette containing potassium dichromate in sulphuric acid, digested at 148 °C for 2 hours, then measured photometrically using UV-Vis spectrophotometry at 600 nm (for COD ranges 15–150 mg/L) or 420 nm (for lower ranges). Results are read directly against a calibration curve. The method is faster and uses less hazardous waste than the traditional open reflux method (ISO 6060:1989).
What does a high COD/BOD ratio indicate?
BOD and COD monitoring is mandatory for industries with significant organic effluent discharges, including: food and beverage processing (dairy, meat, brewery, sugar production), paper and pulp manufacturing, chemical and petrochemical plants, pharmaceutical manufacturing, textile dyeing, and municipal wastewater treatment plants. Discharge limits are specified in individual environmental permits (water consents) issued under national water law transposing the EU Urban Wastewater Treatment Directive (91/271/EEC) and Water Framework Directive (2000/60/EC). Ovalab’s water quality testing service provides accredited BOD and COD analysis for industrial clients.