How to Choose a BOD Analyzer for Wastewater Monitoring
BOD5, manometric & respirometric methods
Biochemical Oxygen Demand (BOD) measures the oxygen microbes consume while breaking down organic matter in your water — the classic indicator of organic pollution and a permit parameter in nearly every discharge standard. Choosing between the 5-day dilution, manometric, and respirometric methods decides your throughput, lab workload, and whether you can react in hours or wait five days.
1. What BOD actually tells you
BOD is the amount of dissolved oxygen (mg/L) consumed by microorganisms during aerobic decomposition of the organic matter in a sample, standardized as BOD5 at 20°C over five days. Because it reflects what biology can actually oxidize, regulators use it alongside COD. The COD/BOD ratio (roughly 2–3 for typical sewage) is a quick indicator of biodegradability — a very high ratio means a lot of the COD is refractory and will not be removed by conventional biological treatment.
2. The three measurement methods
- Dilution method (classical BOD5). Seed-inoculated sample, dissolved-oxygen measured at day 0 and day 5 (Winkler titration or DO probe). Low capital cost and the regulatory reference, but labor-intensive and a five-day wait.
- Manometric. Sample in a sealed bottle; the pressure drop from oxygen consumption is measured automatically. Removes the titration step and logs the curve, but it is still a ~5-day test.
- Respirometric / biosensor. Real-time measurement of oxygen uptake (often hours, not days). Higher capital cost, but ideal for process control, toxicity screening, and shortcut BOD for aeration basins.
3. Throughput vs. labor
For a small number of monthly compliance samples, manual dilution BOD5 is cheapest. For routine high-throughput compliance, manometric instruments automate the bottle count. For a treatment plant that wants to optimize aeration or catch toxic shocks, respirometric online probes pay back through energy and downtime savings.
4. Sample and matrix realities
Residual chlorine must be neutralized (it kills the seed), and heavy metals, extremes of pH, or high salinity can inhibit the inoculum — additions of a healthy seed and a check blank are routine. Temperature must be held at 20°C and the dilution water must be calibrated (blank < 0.2 mg/L). These controls decide whether your number is defensible.
5. Online vs. laboratory BOD
Online BOD sensors (respirometric probes) exist and are excellent for early warning and process control, but their range and matrix tolerance are limited. The laboratory BOD5 (or manometric) result remains the compliance reference. Most plants run both: online for control, lab for the report.
6. Relating BOD to your COD permit
Many discharge permits set both COD and BOD. COD is faster and chemical; BOD is biologically meaningful. Our COD analyzer guide and the QCVN 40:2025 calculator explain how the limit is derived from your permitted flow — size the BOD method to the same discharge column.
7. Buying checklist
- Method fit: dilution (low volume), manometric (routine), respirometric (control/toxicity)
- Sample throughput and bottle/seed automation
- Measurement range vs. your expected BOD
- Data export (USB / LIMS) and GLP traceability
Related buying guides
Frequently asked questions
BOD5 vs COD — which should I report?
COD is a fast chemical oxidation test; BOD5 is the slower, biologically relevant test. Many discharge standards require both. The COD/BOD ratio (about 2–3 for municipal sewage) tells you how biodegradable the load is — a very high ratio means much of the COD will not be removed by conventional biology.
How long does a BOD test take?
The classical dilution BOD5 and most manometric instruments take five days at 20°C. Respirometric / biosensor methods give a result in hours, which is why they are used for process control rather than statutory reporting.
Can one instrument measure both BOD and COD?
No. They are different principles — BOD is biological oxygen consumption, COD is chemical oxidation with dichromate. They are usually separate instruments; laboratories often run them side by side to characterize a stream.