How online COD, BOD and TSS analysers work
What the analyser measures, how it compares with the laboratory, and what affects accuracy
An online analyser does not run the laboratory test at the outlet. It measures something physical that follows the laboratory value - how the effluent absorbs and scatters light, or the voltage of a glass electrode - and does so every few minutes, all day. Knowing how that works explains most of the questions plants and boards ask about online readings.
Laboratory and online, side by side
| Parameter | In the laboratory | Online | What affects accuracy |
|---|---|---|---|
| COD | Digestion with dichromate, about two hours | UV–visible absorption of the effluent | A fouled window; an effluent different from the one it was set against |
| BOD | Incubation for three days at 27 °C | Derived from the spectrum with a BOD:COD correlation | A change in the effluent's make-up, which changes the BOD:COD ratio |
| TSS | Filtration, drying and weighing | Optical absorption and scatter | Air bubbles, fouling |
| pH | Glass electrode | Glass electrode with temperature compensation | An ageing electrode, a missed buffer calibration |
| TOC | Oxidation and measurement of the carbon dioxide | UV–visible absorption | As for COD |
| Turbidity | Nephelometer | Light scattered at 90° | Air bubbles, fouling |
Why the correlation matters
- A spectrum, not a number
- The optical probe records how strongly the effluent absorbs light across the ultraviolet and visible range. COD, BOD, TSS and TOC are calculated from that spectrum.
- Set for your outlet
- Every effluent is a little different, so the calculation is set at commissioning against laboratory results for samples from your own outlet.
- Checked over time
- A grab sample tested in the laboratory is compared with the online value for the same time; a lasting difference means the setting is checked again.
- No reagents
- Because light does the measuring, the optical parameters need no chemicals and produce no waste.
What keeps the readings right
- Clean optics
- Fouling of the measuring window is the most common cause of drift; it is cleaned on a schedule.
- Steady flow, no bubbles
- The probe sits where the effluent flows steadily and bubbles do not pass the window.
- A calibrated electrode
- pH is calibrated with standard buffers and the electrode replaced when it ages.
- A record of every check
- Each calibration and laboratory comparison is recorded, with its date and values.
Questions
Is an online reading the same as a laboratory result?
It follows the laboratory value closely when the analyser is set and kept correctly, but it is a continuous measurement, not the same test. The two are compared regularly.
Why might the board's laboratory result differ from our online reading?
A grab sample is one moment; the online value may be an average over minutes. A small difference is normal. A lasting one is a reason to clean the probe and recheck its setting against the laboratory.
The analyser and each parameter
- Online water quality analyser (8 parameters)Our main product: eight parameters as standard, made in India
- COD analyserChemical oxygen demand measured continuously at the outlet, without reagents
- BOD analyserContinuous BOD at the outlet, when the laboratory test takes three days
- TSS analyserTotal suspended solids measured continuously, by optical absorption and scatter
- pH analyserpH at the outlet on its own port, with automatic temperature compensation
- Calibration and upkeepClean, check, calibrate and record
See it working
Want to see the analyser's readings next to your laboratory's? Ask for a demonstration.