Biochemical oxygen demand (BOD) is one of the most important indicators for assessing biodegradable organic pollution in surface water bodies such as rivers, lakes, and reservoirs. This article provides a professional technical overview of BOD determination in surface water, covering sampling requirements, internationally recognized test methods such as the dilution and seeding method, relevant standards including ISO 5815, HJ 505-2009, and GB 3838-2002, as well as quality control criteria and typical applications in environmental monitoring.
Technical Overview
Biochemical oxygen demand (BOD) is defined as the mass concentration of dissolved oxygen consumed by aerobic microorganisms while decomposing biodegradable organic matter in a water sample under specified incubation conditions. The most widely used parameter is BOD5, which is determined by incubating a sample in the dark at 20 ± 1 °C for five days and measuring the decrease in dissolved oxygen (DO) before and after incubation. A high BOD5 value indicates a large amount of biodegradable organic pollution; when such water enters a river or lake, microbial degradation can deplete dissolved oxygen, threaten fish and other aquatic organisms, and accelerate eutrophication-related deterioration.
Because surface water is a critical resource for drinking water abstraction, irrigation, aquaculture, and ecological maintenance, BOD5 serves as a core classification and compliance parameter in surface water quality standards. Under the Chinese national standard GB 3838-2002 (Environmental Quality Standards for Surface Water), BOD5 limits are 3 mg/L for Class I and Class II waters, 4 mg/L for Class III, 6 mg/L for Class IV, and 10 mg/L for Class V. Comparable assessment frameworks exist internationally, making harmonized BOD methodology essential for reliable and comparable data.
The principal method standards applied by accredited third-party laboratories and government monitoring agencies include:
- ISO 5815-1:2019 — Determination of biochemical oxygen demand after n days (BODn), dilution and seeding method with allylthiourea addition
- ISO 5815-2:2019 — Dilution and seeding method for undiluted samples, particularly suited to relatively clean surface waters
- EN 1899-1 and EN 1899-2 — European adoption of the ISO 5815 methodology
- HJ 505-2009 — Determination of biochemical oxygen demand after 5 days (BOD5), dilution and seeding method (successor to GB 7488-87)
- HJ/T 86-2002 — Microbial sensor rapid test method for BOD in water
- US EPA Method 405.1 and APHA/AWWA/WEF Standard Methods 5210 B (dilution method) and 5210 D (respirometric method)
- GB 7489-87 / ISO 5813 (iodometric titration) and HJ 506-2009 / ISO 5814 / ASTM D888 (electrochemical and optical dissolved oxygen measurement)
Testing Samples
Applicable Sample Types
BOD determination is applicable to a broad range of aqueous matrices. For surface water monitoring programs, typical sample types include:
- Rivers, streams, and canals (including transboundary and urban reaches)
- Lakes, reservoirs, and ponded water bodies
- Estuarine and brackish surface water (with salinity correction considerations)
- Wastewater treatment plant influent and effluent, and industrial discharges for comparison with receiving-water objectives
- Groundwater and rainwater references in comprehensive watershed surveys
Sampling and Preservation Requirements
Representative sampling is essential because BOD reflects the biodegradable fraction of organic matter, which can change rapidly during storage. Key requirements include:
- Collect grab samples at designated monitoring cross-sections, typically 0.5 m below the water surface, avoiding floating debris and sediment disturbance
- Use clean glass or chemically resistant plastic bottles, fill completely with no headspace or air bubbles, and cap tightly
- Transport and store samples in the dark at 0–4 °C; analysis should begin as soon as practicable, ideally within 6 hours and no later than 24 hours after collection
- Recommended sample volume is at least 1 L (preferably 2 L) to allow multiple dilution levels and replicate determinations
- Pre-treat as required: neutralize residual chlorine with sodium sulfite, adjust pH to approximately 7 if the sample falls outside the range of about 6–8, and remove gross solids or oils that would interfere with homogeneity
Testing Items
The core deliverable of the test is the BOD5 value, expressed in mg/L, supported by several related determinations that ensure data validity and interpretive value. Common testing items include:
- BOD5 (5-day biochemical oxygen demand at 20 °C) — the primary reporting item
- Carbonaceous BOD (CBOD5) — BOD5 determined with a nitrification inhibitor such as allylthiourea or TCMP to isolate the carbonaceous oxygen demand
- Ultimate BOD (BODu or BODn for extended incubation periods) — optional, for oxygen-balance modeling and self-purification studies
- Dissolved oxygen at day 0 and day 5 — the fundamental measurement underlying all BOD calculations
- Supporting parameters for interpretation: COD (GB 11914 or HJ 828), TOC, ammonia nitrogen, and pH, used to check the BOD/COD ratio and sample biodegradability
The dilution and seeding method is suitable for BOD5 concentrations from approximately 2 mg/L upward, with higher values accommodated through dilution to several thousand mg/L. The undiluted-sample variant (ISO 5815-2) is optimized for clean waters, and the microbial sensor rapid method typically covers a range of about 2 to 500 mg/L.
Testing Methods
Dilution and Seeding Method (HJ 505-2009, ISO 5815-1, EN 1899-1, EPA 405.1)
This is the reference method for regulatory-grade BOD5 determination. The sample, or a diluted portion of it, is mixed with aerated, nutrient-buffered dilution water containing phosphate buffer (pH ≈ 7.2), magnesium sulfate, calcium chloride, and ferric chloride. Where the microbial population is insufficient, a seed inoculum (such as settled domestic wastewater supernatant or a commercial seed culture) is added. Duplicate bottles are prepared: dissolved oxygen is measured immediately in one set, while the remaining bottles are water-sealed, incubated in the dark at 20 ± 1 °C for five days, and then measured for residual DO.
For each valid bottle, the BOD5 is calculated as: BOD5 = [(c1 − c2) − (c3 − c4) × f1] / f2, where c1 and c2 are the DO concentrations in the diluted sample before and after incubation, c3 and c4 are the DO concentrations in the seed control bottle before and after incubation, f1 is the fraction of seed in the diluted sample relative to the seed control, and f2 is the dilution fraction (sample volume divided by total volume). Dilution levels are selected so that measured results satisfy the validity criteria below; running at least two or three dilutions in parallel is standard practice for unknown samples such as surface water.
Undiluted Sample Method (ISO 5815-2, EN 1899-2)
For clean surface waters with low BOD5, direct incubation without dilution avoids the uncertainty introduced by the oxygen demand of dilution water. Because oxygen depletion is small in such samples, stringent control of blanks and highly accurate DO measurement (iodometric titration or calibrated probes) are required, and the method is generally applicable to BOD5 values up to approximately 6 mg/L.
Manometric (Respirometric) Method
In the respirometric technique, the sample is incubated in a sealed bottle above which carbon dioxide released by microbial respiration is absorbed by an alkali trap (for example, sodium hydroxide or lithium hydroxide pellets). The pressure drop in the closed system, measured electronically or manometrically, is proportional to the oxygen consumed and is converted directly into BOD. The approach, consistent with Standard Methods 5210 D, provides continuous or daily readings, requires no titration, and is convenient for laboratory screening and treatment-process monitoring, although classical dilution-method results remain the benchmark for compliance reporting in many jurisdictions.
Microbial Sensor Rapid Method (HJ/T 86-2002)
The microbial sensor method couples a membrane bearing immobilized aerobic microorganisms with a dissolved oxygen probe. When the sample flows past the membrane, the microorganisms consume oxygen and DO at the probe decreases; the steady-state signal difference relative to a standard solution is correlated to BOD. Results are obtained within approximately 20 to 30 minutes, making the technique valuable for rapid screening, on-site assessment, and high-frequency monitoring, with results generally reported alongside periodic confirmation by the standard dilution method.
Dissolved Oxygen Measurement Techniques
Because BOD accuracy ultimately depends on DO measurement, the following techniques are used: the Winkler iodometric titration (GB 7489-87, ISO 5813) as the classical high-accuracy reference; electrochemical DO probes (HJ 506-2009, ISO 5814); and optical luminescent DO sensors (covered by ASTM D888-type probe methods), which offer stable, flow-independent readings well suited to multi-bottle BOD workflows.
Quality Control and Acceptance Criteria
Robust quality assurance is indispensable for defensible BOD data. Accredited third-party laboratories typically implement the following controls:
- Validity criteria for each bottle: DO consumption of the diluted sample of at least 2 mg/L and residual DO of at least 1 mg/L after incubation
- Dilution water blank with negligible 5-day oxygen demand, commonly not exceeding 0.2 mg/L
- Glucose–glutamic acid check standard, with the widely applied acceptance window of 198 ± 30.5 mg/L (approximately ±15%)
- Seed correction applied whenever inoculated dilution water is used
- Incubator controlled at 20 ± 1 °C, bottles protected from light and sealed with water traps to prevent atmospheric oxygen ingress
- DO probes calibrated before use (zero with sodium sulfite solution; span with water-saturated air) and titration endpoints verified against standardized reagents
- Duplicate determinations and control charts to monitor precision and bias over time
Applications
BOD determination data from surface water testing support a wide range of regulatory, engineering, and scientific purposes:
- Surface water quality assessment and functional zone classification against GB 3838-2002, including routine national and local monitoring network programs
- Compliance verification for wastewater discharges, including municipal treatment plant effluents under GB 18918-2002 (for example, BOD5 ≤ 10 mg/L for the highest discharge tier), evaluated together with the assimilative capacity of the receiving surface water
- Environmental impact assessments, pollutant discharge permit applications, and water pollutant total-load control programs
- Water pollution incident investigations, pollution source tracing, and enforcement actions
- River and reservoir self-purification studies, dissolved-oxygen balance modeling, and ecological flow assessments
- Research applications such as biodegradability evaluation (BOD/COD ratio), treatment process optimization, and long-term trend analysis of watershed health
FAQ
What is the difference between BOD and COD?
COD measures the total chemically oxidizable organic and some inorganic matter using a strong oxidant, whereas BOD measures only the fraction that aerobic microorganisms can biologically degrade. BOD therefore better reflects the ecological impact of organic pollution on receiving waters. For most wastewaters, BOD5 is roughly a fraction of COD; the BOD/COD ratio is commonly used to judge biodegradability and the suitability of biological treatment.
Why does the standard BOD test take five days?
The five-day incubation at 20 °C corresponds to the oxygen demand of the rapidly biodegradable carbonaceous fraction and historically approximates the time for river water in northwestern Europe to reach the sea. Five days also limits interference from slower nitrifying bacteria. The five-day convention (BOD5) is embedded in ISO 5815, HJ 505-2009, and most national regulations.
How are dilution factors selected for surface water samples?
Dilutions are chosen so that the expected oxygen depletion after five days remains within the measurable window: consumption of at least 2 mg/L and residual DO of at least 1 mg/L. Because surface water BOD5 is usually low (often below 6–10 mg/L), high dilution fractions close to the undiluted sample, or the undiluted-sample method, are typically used. The COD value can provide an initial estimate for selecting two or three parallel dilution levels.
What interferences can affect BOD results?
Common interferences include residual chlorine or other disinfectants (removed with sodium sulfite), extreme pH (adjusted toward neutral), toxic substances such as heavy metals or pesticides that inhibit microbial activity, excessive ammonia causing nitrification oxygen demand (suppressed with allylthiourea or TCMP when CBOD is required), and high salinity. Poor sample handling, such as warm storage or long holding times, can also bias results significantly.
How quickly are results available?
The classical dilution and seeding method requires five days of incubation, so validated results are typically reported within one week of sample receipt. The microbial sensor rapid method delivers results within about 30 minutes, and respirometric systems provide interim readings throughout the incubation period, both of which are useful for screening purposes.
Which BOD5 values correspond to each surface water quality class in China?
Under GB 3838-2002, BOD5 must not exceed 3 mg/L for Class I and Class II waters (source water and drinking water protection zones), 4 mg/L for Class III (secondary drinking water abstraction and aquaculture), 6 mg/L for Class IV (industrial and recreational non-contact use), and 10 mg/L for Class V (agricultural and landscape use). Results above Class V limits classify the water body as inferior Class V, indicating severe organic pollution.
Can BOD be measured on-site?
Portable dissolved oxygen instruments and rapid microbial sensor analyzers allow approximate on-site assessment of oxygen conditions and screening-level BOD estimation. However, regulatory-grade BOD5 determinations are performed in controlled laboratory conditions with calibrated incubators, validated dilution water, and full quality control, which is why accredited third-party laboratories remain the standard route for compliance testing.