Water Quality Biochemical Oxygen Demand (BOD) Testing – Principles, Methods, and Applications

Biochemical Oxygen Demand (BOD) testing is one of the most widely applied methods for evaluating the concentration of biodegradable organic matter in water and wastewater. Accredited third-party laboratories conduct BOD5 analysis in accordance with recognized standards such as ISO 5815-1, EN 1899-1, and HJ 505-2009 to support discharge compliance, treatment process control, and environmental quality monitoring.

Technical Overview

Biochemical Oxygen Demand (BOD) is a measure of the quantity of dissolved oxygen consumed by aerobic microorganisms while decomposing biodegradable organic matter in a water sample under controlled conditions. The most commonly reported parameter is BOD5, defined as the oxygen demand measured after incubating a sample in the dark at 20 ± 1 °C for five days, expressed in milligrams per liter (mg/L). A higher BOD value indicates a greater concentration of biodegradable organics and, consequently, a higher potential for oxygen depletion in the receiving water body.

The analytical principle is straightforward: the dissolved oxygen (DO) of the sample is measured before and after the five-day incubation period, and the difference, corrected for dilution and seed contributions, represents the BOD5. The test is prescribed by numerous national and international standards, including ISO 5815-1 (dilution and seeding method), ISO 5815-2 (undiluted samples), EN 1899-1 and EN 1899-2, HJ 505-2009, GB 7488-87, and Standard Methods 5210B. BOD results are frequently evaluated alongside COD, TOC, and ammonia nitrogen to characterize overall organic loading, and serve as a core indicator in surface water quality classification (e.g., GB 3838-2002) and wastewater discharge regulations (e.g., GB 8978-1996).

Testing Samples

Surface Water

River, lake, reservoir, and estuarine water samples are tested to assess ambient water quality and compliance with surface water quality standards. Because natural surface waters usually contain low-to-moderate organic loads, direct incubation or low-dilution series are often applied, and results support water function zone classification and watershed management decisions.

Municipal Wastewater

Influent, aeration tank mixed liquor, secondary effluent, and final discharges from municipal wastewater treatment plants are routinely tested. BOD5 data are essential for verifying that final effluent meets permitted discharge limits, calculating pollutant removal efficiency, and adjusting aeration and sludge retention strategies.

Industrial Wastewater

Effluents from industries such as food and beverage processing, fermentation, pulp and paper, petrochemicals, textiles and dyeing, pharmaceuticals, and chemical manufacturing frequently contain high organic loads. These samples typically require multi-level dilution series and, where necessary, seeded incubation using acclimatized microorganisms to obtain valid BOD5 values.

Groundwater and Source Water

Groundwater and drinking water sources are tested to detect organic contamination from leachate, seepage, or agricultural runoff. Low BOD5 values confirm limited biodegradable pollution, while anomalous elevations trigger source investigation and protection measures.

Reclaimed Water and Treated Effluent

Reclaimed water intended for reuse in irrigation, cooling, or landscape applications is tested to verify stabilization of residual organics and to demonstrate conformity with reuse water quality requirements before distribution.

Testing Items

Core parameters and related determinations offered within a BOD testing program include:

• Five-day Biochemical Oxygen Demand (BOD5) – the standard regulatory parameter
• Carbonaceous BOD (CBOD5) – BOD5 determined with a nitrification inhibitor (e.g., allylthiourea, ATU) to suppress nitrogenous oxygen demand
• Ultimate BOD (BODu or BOD20) – long-term incubation to estimate total biodegradable oxygen demand
• Dilution water and seed controls – verification of blank stability and seed activity
• Supporting parameters – dissolved oxygen (DO), COD, TOC, ammonia nitrogen, and pH for data interpretation

Testing Methods

Dilution and Seeding Method (Standard Method)

Following ISO 5815-1 and HJ 505-2009, the sample is diluted with aerated, nutrient-fortified dilution water to at least two dilution levels so that residual DO after incubation remains above the method-specified minimum. Where the sample contains few indigenous microorganisms, a seed inoculum is added. Dissolved oxygen is determined before incubation and after five days at 20 °C in the dark, typically by iodometric titration or electrochemical/optical DO probes. The applicable determination range is approximately 2–6000 mg/L, with higher concentrations accommodated by additional dilution.

Undiluted Sample Method

For clean surface waters with sufficient initial dissolved oxygen, ISO 5815-2 and EN 1899-2 permit direct incubation of the undiluted sample in completely filled bottles, simplifying handling for low-pollution matrices.

Manometric (Respirometric) Method

Pressure-based respirometric systems measure the pressure drop in a sealed bottle as microorganisms consume oxygen and release carbon dioxide into an absorbent. Continuous readings generate a BOD curve, enabling rapid process monitoring and estimation of BOD kinetics, while reducing mercury-free titration work and reagent consumption.

Quality Assurance and Control

Valid BOD determination relies on strict quality control, including:

• Blank dilution water controls with five-day oxygen consumption within the acceptance limit
• Glucose–glutamic acid reference checks to verify overall method and seed performance
• At least two dilutions per sample producing valid residual DO values
• Duplicate determinations and incubation at 20 ± 1 °C in darkness to prevent algal oxygen production
• Sample collection in glass bottles, completely filled without headspace, and analysis as soon as possible, or storage at approximately 4 °C within the standard-specified holding time (typically no more than 24 hours)

Applications

BOD testing results are used across a broad range of regulatory, engineering, and scientific contexts:

• Regulatory compliance – demonstrating conformity of wastewater discharge with national and local discharge standards and pollutant emission permits
• Wastewater treatment plant operation – monitoring influent load, biodegradability (BOD/COD ratio), aeration demand, and biological treatment efficiency
• Environmental impact assessment and acceptance – providing baseline data for new projects and verification data for completed wastewater facilities
• Surface water quality management – classifying water bodies and tracking organic pollution trends in rivers, lakes, and reservoirs
• Water reuse evaluation – confirming the suitability of reclaimed water for irrigation, industrial cooling, and ecological replenishment
• Research and product development – assessing biodegradability of formulations, chemicals, and treated effluents

FAQ

What is the difference between BOD and COD? BOD measures only the organic fraction that microorganisms can biologically degrade over five days, whereas COD measures the total chemically oxidizable organic and inorganic reducing matter through a chemical oxidant. BOD is slower to obtain but reflects the actual biological oxygen burden on a receiving water body; the BOD/COD ratio is widely used to judge the biological treatability of wastewater.

Why is BOD measured over five days at 20 °C? The five-day, 20 °C incubation is a historically standardized convention that captures the readily biodegradable fraction of oxygen demand while remaining practical for routine laboratory operations. It also provides a compromise between domestic wastewater characterization and the lag phase of microbial degradation.

What is the difference between BOD5 and CBOD? Nitrifying bacteria can consume oxygen while oxidizing ammonia, inflating the apparent BOD. Adding a nitrification inhibitor such as allylthiourea (ATU) yields the carbonaceous BOD (CBOD), which isolates the oxygen demand attributable to organic carbon oxidation; many discharge permits specify CBOD for treated effluents.

How should samples be preserved and how quickly must they be tested? Samples should be collected in clean glass bottles, filled completely without air headspace, and kept at low temperature (approximately 0–4 °C) in the dark. Analysis should begin as soon as possible, generally within six hours of sampling and no later than 24 hours, to avoid changes in the microbial population and organic content.

What is the reporting timeframe for BOD testing? Because the standard method requires a mandatory five-day incubation plus sample preparation and verification, final BOD5 results are typically reported within seven to ten working days after sample receipt. Clients with time-critical monitoring needs may supplement BOD with rapid COD or online BOD measurements for interim control.