Water Quality 5-Day Biochemical Oxygen Demand (BOD5) Testing

The 5-day biochemical oxygen demand (BOD5) test is one of the most widely applied parameters for measuring the concentration of biodegradable organic matter in water and wastewater. This article provides a comprehensive technical overview of BOD5 testing, covering its principle, applicable sample types, reference standards, dilution and seeding methodology, calculation, quality control, and typical regulatory applications. It serves as a practical reference for environmental engineers, laboratory personnel, and compliance managers.

Technical Overview

Five-day biochemical oxygen demand (BOD5) is defined as the mass of dissolved oxygen consumed by aerobic microorganisms while decomposing the biodegradable organic matter present in a water sample during five days of incubation at 20 ± 1 °C in complete darkness. Because microbial degradation of organic pollutants directly consumes oxygen from the receiving water body, BOD5 is a key indicator of the pollution load exerted on rivers, lakes, and coastal waters: the higher the BOD5 value, the greater the risk of oxygen depletion and damage to aquatic ecosystems.

The test is performed by diluting the sample with oxygen-saturated, nutrient-fortified dilution water, filling airtight incubation bottles, measuring the initial dissolved oxygen (DO), incubating the bottles for five days, and measuring the final DO. The difference between the two measurements, corrected for seed and blank contributions and divided by the dilution factor, yields the BOD5 result in mg/L. The method is internationally harmonized through ISO 5815-1, EN 1899-1, GB 7488-87, and HJ 505-2009, and is routinely carried out by accredited third-party laboratories operating under ISO/IEC 17025 quality management systems.

• Parameter: BOD5 (and carbonaceous BOD5, CBOD5, when nitrification is inhibited)
• Incubation condition: 20 ± 1 °C, dark, 5 days (120 ± 4 hours)
• Determination range: approximately 2 to 6000 mg/L, depending on the dilution factors selected
• Typical method detection limit: on the order of 0.5 to 2 mg/L

Testing Samples

Municipal Wastewater and Treatment Plant Samples

Influent (raw sewage), primary effluent, secondary biological effluent, and final discharges from municipal wastewater treatment plants are the most frequently tested sample types. Influent samples typically require high dilution factors (often 20- to 100-fold), whereas biologically treated effluents with BOD5 values below 20 mg/L require very low dilutions or, where permitted by the applicable standard, direct incubation of undiluted samples in accordance with ISO 5815-2.

Industrial Wastewater

Effluents from food and beverage processing, fermentation, brewing, pulp and paper making, textile dyeing, pharmaceutical production, petrochemical refining, and slaughterhouse operations contain high concentrations of biodegradable organics and often exhibit BOD5 values from several hundred to several thousand mg/L. Industrial samples may contain residual disinfectants, heavy metals, extreme pH, or biologically foreign compounds, all of which require specific pretreatment and, in many cases, inoculation with acclimated (adapted) microbial seed to obtain valid results.

Surface Water, Groundwater, and Receiving Waters

Rivers, lakes, reservoirs, groundwater, and coastal waters are tested to classify water quality, verify the impact of discharges, and support environmental monitoring programs. Clean surface water samples usually contain sufficient dissolved oxygen and indigenous bacteria to be analyzed with minimal or no dilution, provided the oxygen depletion over five days remains within the valid measurement window.

Other Sample Types

Landfill leachate, reclaimed water and reuse systems, recirculating aquaculture water, and stormwater runoff are also analyzed for BOD5. Leachate in particular combines very high organic strength with potential toxicity, so sequential dilutions and adapted seeding are essential for reliable quantification.

Testing Items

The core deliverable of the test is the BOD5 concentration of the sample, expressed in mg/L. Depending on the purpose of the analysis and the applicable standard, the following related items are determined or controlled within the same analytical sequence:

• BOD5 of the sample, calculated from the oxygen depletion of valid dilutions after seed and blank correction
• Carbonaceous BOD5 (CBOD5), obtained by adding allylthiourea (ATU) to suppress nitrifying bacteria, in accordance with ISO 5815-1
• Dilution water blank, used to verify that the dilution water itself consumes no more than 0.2 mg/L of oxygen over five days
• Seed control bottles, used to quantify and subtract the oxygen demand contributed by the inoculum
• Glucose-glutamic acid check standard (150 mg/L each), used to verify overall method performance; the accepted reference result is 198 ± 30.5 mg/L or the laboratory's established control limits
• Parallel duplicate samples at a prescribed frequency to demonstrate analytical precision

Testing Methods

Reference Standards

• ISO 5815-1:2019, Water quality — Determination of biochemical oxygen demand after n days (BODn) — Part 1: Dilution and seeding method with allylthiourea addition
• ISO 5815-2: Dilution-free method for samples with low oxygen demand
• EN 1899-1, European adoption of the dilution and seeding method for BOD5
• GB 7488-87 and HJ 505-2009, Chinese national and environmental standards for the dilution and seeding determination of BOD5
• Standard Methods for the Examination of Water and Wastewater, Method 5210 B, widely referenced internationally

Sample Collection and Preservation

Samples are collected in clean glass bottles with ground-glass or screw caps and filled completely without headspace or air bubbles. Plastic containers should be avoided because plasticizers can leach and bias the oxygen demand. Samples are transported and stored in the dark at 0–4 °C and should be analyzed as soon as possible, ideally within 6 hours and in no case later than 24 hours after collection, since microbial activity continues even at low temperature and will depress the measured value.

Pretreatment

• Adjust samples with extreme pH to approximately 6.5–7.5 using dilute acid or base solutions
• Neutralize residual chlorine or other oxidants with sodium sulfite or sodium thiosulfate before analysis
• Bring samples to 20 ± 1 °C before dilution; supersaturated samples, for example those containing algae, are shaken or aerated briefly to reduce DO to saturation
• Allow samples with high settleable solids to settle for 1 to 2 hours and use the supernatant, where the applicable standard permits
• Inoculate disinfected or biologically inhibited samples with suitable seed, such as settled domestic wastewater, biological treatment effluent, commercial seed cultures, or microorganisms acclimated to the specific industrial matrix

Dilution and Seeding Procedure

Nutrient-fortified dilution water is prepared from aerated, oxygen-saturated water dosed with phosphate buffer, magnesium sulfate, calcium chloride, and ferric chloride. At least two dilutions are prepared for each sample so that at least one bottle falls within the valid measurement window. BOD incubation bottles of 250 to 300 mL are filled carefully to prevent air bubbles, water-sealed, and the initial DO of a representative bottle is measured with an iodometric (Winkler) titration or a calibrated electrochemical or optical dissolved oxygen probe. The remaining bottles are incubated in a thermostatically controlled incubator at 20 ± 1 °C in darkness for five days, after which the final DO of each bottle is measured.

Calculation

The result is calculated as: BOD5 (mg/L) = [(D1 − D2) − (B1 − B2) × f] / P, where D1 and D2 are the dissolved oxygen concentrations (mg/L) of the diluted sample before and after incubation, B1 and B2 are the corresponding values for the seed control, f is the fraction of seed present in the sample bottle relative to the seed control, and P is the volumetric fraction of sample in the diluted mixture. For unseeded dilutions, the seed correction term is omitted.

Quality Control and Acceptance Criteria

• Oxygen depletion of the dilution water blank must not exceed 0.2 mg/L over five days
• Valid bottles must show an oxygen consumption of at least 2 mg/L and a residual DO of at least 1 mg/L after incubation
• The glucose-glutamic acid check must fall within the accepted range, and results are tracked on laboratory control charts
• The incubator temperature is maintained at 20 ± 1 °C and verified with calibrated thermometers
• Dissolved oxygen measuring devices are calibrated daily and periodically cross-checked against iodometric titration
• Reported values are the average of results from all valid dilutions, and results from invalid bottles are excluded with documented justification

Applications

BOD5 data underpin a broad range of environmental, regulatory, and engineering decisions:

• Compliance monitoring of municipal and industrial discharges against permit limits, including the Class 1A (BOD5 ≤ 10 mg/L), Class 1B (≤ 20 mg/L), and Class 2 (≤ 30 mg/L) requirements of GB 18918-2002
• Surface water quality classification under GB 3838-2002, where BOD5 limits range from 3 mg/L for Class I and II waters up to 10 mg/L for Class V waters
• Process control at wastewater treatment plants, where influent and effluent BOD5 quantify organic loading and removal efficiency
• Evaluation of biological treatability using the BOD5-to-COD ratio, with ratios above approximately 0.3 generally indicating good biodegradability
• Environmental impact assessment, construction completion acceptance monitoring, and regulatory enforcement investigations
• Verification of reclaimed water quality for reuse in irrigation, landscaping, and industrial cooling
• Research on biodegradation kinetics, receiving-water assimilative capacity, and ecological risk of organic pollution

FAQ

Why is the incubation period fixed at five days at 20 °C?

The five-day period at 20 °C was historically established as a reproducible reference condition; it corresponds roughly to two-thirds of the ultimate carbonaceous oxygen demand for typical domestic wastewater and matches the natural self-purification timescale of many receiving waters. Fixing both temperature and duration makes results directly comparable between laboratories and regulatory jurisdictions worldwide.

What is the difference between BOD5 and COD?

BOD5 measures only the portion of organic matter that microorganisms can biodegrade within five days, whereas chemical oxygen demand (COD) measures nearly all chemically oxidizable matter, including biologically resistant compounds. COD results are available within hours, while BOD5 requires at least five days; the ratio between the two parameters is widely used to judge biodegradability and the suitability of biological treatment.

How long does a BOD5 test take from sample receipt to report?

Because the incubation alone lasts 120 hours, a typical turnaround is five to seven working days after sample receipt, allowing time for pretreatment, dilution setup, incubation, final DO measurement, calculation, and quality control review. Samples received with insufficient remaining holding time or improper preservation may be rejected.

How should samples containing residual chlorine, disinfectants, or toxic substances be handled?

Oxidizing residues are chemically neutralized before dilution. Samples containing toxic metals or biocidal compounds are diluted sufficiently to bring the toxicant below its inhibitory concentration and are inoculated with seed microorganisms acclimated to the wastewater matrix; otherwise the measured oxygen demand will be artificially low and the sample will show obvious signs of inhibition in the dilution series.

When should carbonaceous BOD5 (CBOD5) be reported instead of BOD5?

CBOD5 is determined by adding allylthiourea to suppress nitrification, isolating the oxygen demand of carbon oxidation from that of nitrogen oxidation. It is appropriate for nitrifying biological effluents and wherever the applicable permit or standard explicitly specifies CBOD5, because nitrification can otherwise inflate the apparent carbonaceous oxygen demand.

Which industries and facilities most commonly require BOD5 testing?

Municipal treatment plants plus food and beverage, fermentation, brewing, starch and sugar, pulp and paper, textile and dyeing, pharmaceutical, petrochemical, and slaughterhouse facilities routinely monitor BOD5, both for discharge compliance and for controlling the biological treatment processes on which their effluent quality depends.