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CSA Z320-11 (2016): Standard Method for Biochemical Oxygen Demand (BOD5) Testing in Water and Wastewater
Comprehensive guidelines for the laboratory determination of biodegradable organic matter in aqueous samples using the 5-day BOD test
The biochemical oxygen demand (BOD) test is a critical measure of the biodegradable organic content in water and wastewater. CSA Z320-11 (2016) establishes the definitive procedure for the 5-day BOD test (BOD5) across Canada. This standard ensures consistency, accuracy, and comparability of results across laboratories, supporting regulatory compliance and treatment process control. This article outlines the scope, key technical requirements, implementation recommendations, and compliance notes for this essential standard.
1. Scope and Applicability
CSA Z320-11 (2016) specifies the method for determining the 5-day biochemical oxygen demand (BOD5) of natural waters, municipal wastewater, industrial effluents, and treated aqueous samples. The standard applies to samples with BOD5 values ranging from < 1 mg/L to several thousand mg/L when appropriate dilutions are applied. It is intended for use in environmental monitoring, wastewater treatment plant operations, and regulatory compliance. The method relies on the measurement of dissolved oxygen depletion over a five-day incubation period at 20 ± 1 °C in the dark. The standard explicitly excludes alternative incubation periods (e.g., BOD7) and does not address non-biodegradable oxygen demand (COD).
Important: The standard is not applicable to samples exhibiting acute toxicity to the microbial seed without pretreatment. Toxicity screening is recommended prior to performing the test on industrial wastewaters.
2. Technical Requirements
2.1 Sample Collection, Preservation, and Storage
Samples must be collected in pre-cleaned glass or hard plastic bottles, filled completely to avoid headspace, and transported to the laboratory at ≤ 6 °C. If analysis cannot begin within 6 hours of collection, samples may be stored at 4 °C for up to 24 hours. Freezing is not permitted. Samples containing residual chlorine must be dechlorinated immediately after collection using sodium sulfite solution.
2.2 Dilution Water Preparation
Dilution water must be prepared from deionized or distilled water with low organic content. The standard specifies the addition of phosphate buffer (pH 7.2), magnesium sulfate, calcium chloride, and ferric chloride to support microbial growth. The water must be aerated to dissolve oxygen saturation at 20 °C and checked for residual chlorine and heavy metals prior to use.
2.3 Seeding Requirements
A microbial seed is required when the sample does not contain an adequate indigenous microbial population. The seed is obtained from clean surface waters, secondary effluent, or a commercial preparation. The standard requires that the seed be acclimated to the test conditions and that its performance be verified using a glucose-glutamic acid (GGA) standard. The seed correction value must be determined from a separate seed blank and subtracted from the sample BOD calculation.
2.4 Incubation and Dissolved Oxygen Measurement
Incubation is carried out at 20 ± 1 °C in the dark for exactly 5 days (± 6 hours). Dissolved oxygen (DO) is measured before and after incubation using either the azide modification of the Winkler titration method or a membrane electrode DO probe meeting the standard’s precision criteria. The DO depletion in each bottle must be at least 2.0 mg/L and the residual DO at least 0.5 mg/L for the result to be valid.
2.5 Calculation of BOD
The BOD5 is calculated using the formula:
BOD5 (mg/L) = [(D1 – D2) – (B1 – B2) × f] / P
where D1 and D2 are DO levels (mg/L) of the diluted sample before and after incubation, B1 and B2 are DO levels of the seed blank, f is the seed fraction, and P is the decimal dilution factor (e.g., 0.1 for a 1:10 dilution). Results are reported to three significant figures.
2.6 Quality Control
The standard mandates the following quality control measures:
- Analysis of a GGA standard (200 mg/L each of glucose and glutamic acid) with each batch to verify seed viability and operator technique. The acceptable range for BOD5 is 198–235 mg/L.
- Duplicate analyses on at least 10% of samples (relative percent difference ≤ 20%).
- A seed blank to correct for background oxygen demand.
- Comparison of initial and final DO in all bottles to confirm adequate depletion.
| Expected BOD5 (mg/L) | Sample Volume (mL) | Final Volume (mL) | Dilution Factor D |
|---|---|---|---|
| 2–6 | 400 | 1000 | 2.5 |
| 6–20 | 200 | 1000 | 5 |
| 20–60 | 100 | 1000 | 10 |
| 60–200 | 50 | 1000 | 20 |
| 200–600 | 25 | 1000 | 40 |
| 600–2000 | 10 | 1000 | 100 |
3. Implementation Highlights
Successful implementation of CSA Z320-11 (2016) requires careful attention to sample handling and laboratory technique. Use only glass incubation bottles with ground-glass stoppers to prevent atmospheric oxygen intrusion. Avoid exposure to sunlight or UV light during incubation, as algae may produce oxygen and skew results. For high-strength samples, prepare a series of dilutions to ensure at least one bottle meets the DO depletion criteria. When using a DO probe, calibrate daily and stir gently to avoid oxygen absorption.
Best Practice: Inoculate dilution water at a consistent seed concentration for all samples within a batch. Record the seed volume used for reproducibility.
Quality Check: The GGA standard should be analyzed with every batch. If the result falls outside 198–235 mg/L, investigate potential causes such as toxic dilution water, expired seed, or faulty reagents.
Critical: Never freeze BOD samples. Even partial freezing alters the microbial population and organic matter integrity, rendering the test invalid.
4. Compliance and Quality Assurance
Laboratories performing BOD5 testing under CSA Z320-11 (2016) are expected to operate under a recognized quality management system, such as ISO 17025. Regular participation in inter-laboratory proficiency testing is recommended to verify method performance. Documentation of all sample preparation, data calculations, and QC results must be maintained for audit purposes. The standard is reaffirmed as the authoritative BOD method in Canadian environmental regulation; any deviation from the specified procedure must be documented and justified.
Frequently Asked Questions
Q: What is the difference between BOD5 and BOD7?
A: BOD5 uses a 5-day incubation at 20 °C, while BOD7 uses 7-day incubation. CSA Z320-11 (2016) only describes the 5-day test. BOD7 is sometimes preferred for regulatory purposes in certain countries but is not included in this standard.
A: BOD5 uses a 5-day incubation at 20 °C, while BOD7 uses 7-day incubation. CSA Z320-11 (2016) only describes the 5-day test. BOD7 is sometimes preferred for regulatory purposes in certain countries but is not included in this standard.
Q: Can I use CSA Z320-11 (2016) for marine or saline water samples?
A: The standard is primarily designed for freshwater samples. If applied to saline waters, the dilution water must be adjusted to match the salinity of the sample to maintain osmotic balance for the microbial seed. Seeding with a marine-adapted microbe source is necessary. The standard does not provide specific guidelines for saline water, so validation is recommended.
A: The standard is primarily designed for freshwater samples. If applied to saline waters, the dilution water must be adjusted to match the salinity of the sample to maintain osmotic balance for the microbial seed. Seeding with a marine-adapted microbe source is necessary. The standard does not provide specific guidelines for saline water, so validation is recommended.
Q: What are the main limitations of the BOD5 test?
A: BOD5 measures only biodegradable organic matter under standard conditions and does not account for refractory compounds, microbial toxicity, or longer-term degradation. It also requires careful seeding, incubation temperature control, and dilution estimation. Results cannot be directly related to organic carbon or chemical oxygen demand (COD).
A: BOD5 measures only biodegradable organic matter under standard conditions and does not account for refractory compounds, microbial toxicity, or longer-term degradation. It also requires careful seeding, incubation temperature control, and dilution estimation. Results cannot be directly related to organic carbon or chemical oxygen demand (COD).
Q: How often must the glucose-glutamic acid (GGA) check be performed?
A: CSA Z320-11 (2016) requires that a GGA standard be included with each analytical batch. A batch is defined as a set of samples incubated together. At least one GGA per batch is mandatory. If multiple batches are run per day, each must have a GGA.
A: CSA Z320-11 (2016) requires that a GGA standard be included with each analytical batch. A batch is defined as a set of samples incubated together. At least one GGA per batch is mandatory. If multiple batches are run per day, each must have a GGA.
© 2026 Technical Article – CSA Z320-11 (2016) Overview for educational and reference purposes. Always consult the official standard for full requirements.