The measurement principle is based on the absorption of UV light by unsaturated organic molecules at 254nm according to the Beer-Lambert law:

[C]= k log (Iin/Iout)

With: [C]: sample concentration
k: absorption coefficient (specific to each molecule)
Iin: light intensity at the input of the sample
Iout: light intensity at the output of the sample

Generally, the sample is a mixture of many different molecules giving an average absorption coefficient. That means that the instrument must be calibrated according to a COD laboratory measurement before use.

Also, the ratio of the different molecules must remain approximately constant to assume a good correlation with laboratory COD.

UV absorption can be considered as an alternative method for COD (Chemical Oxygen Demand) when fast, reliable and inexpensive measurements with very low maintenance are required.

The correlation for industrial waste water may need an experimentation to be validated while results are guaranteed on river water  or urban wastewater.

Turbidity, suspended solid or dirty on the flow cell is automatically compensated by a differential measurement with a second detector at a reference wavelength.

This method is in accordance with DIN38404-C3 standard and can be considerate as an alternative method referring to AFNOR XPT90-210 standard.

Once a day, a low-cost cleaning solution (5% sulphuric acid) automatically cleans the flow cell. An auto-zero is performed at the same time.

The autonomy is about 2 weeks with the built-in 2-litre tank.

An alarm is generated if the cleaning solution tank is empty.

Ranges and calibration

Several ranges are available according to the flow cell:

The instrument is pre-calibrated for river water on the low range and for urban wastewater on the high range.

For a specific effluent, the ratio COD/UV can be entered in the instrument according to laboratory COD measurements.

Operating cost

The operating cost is limited to the refilling of the 2-litre tank of cleaning solution every 2 weeks with 5% sulphuric acid on distilled water (for effluent measurements, tap water instead of distilled water can be tolerated after checking).

It can be estimated to about US$100 per year including labour.

Many studies have been conducted on river water to demonstrate the relation between UV absorption and COD such as:

American Public Health Association - “Standards methods for the determination of water and Wastewater – UV Absorbing organic constituents, p5-60”, 1995.

J.C. ROSTANG, C.MOUVET - “Contribution of UV spectrometry and dissolved organic matter to the studies of the functioning of freshwater ecosystems”. Science de l’Eau, 5(1986) 9-28.

V.LEMAUVIEL - “Utilisation de la spectrometrie U.V. dans l’analyse des eaux naturelles”. Rapport DIREN-SEMA Basse Normandie, 1994.

The applications concern mainly river monitoring for environmental studies and drinking water treatment plants.

Successful and cost-efficient applications are also reported on automatic coagulant dozing.

The graph bellow shows a typical relation between UV absorption and COD on river water.

The COD of the final effluent is a critical parameter for environmental requirements on many chemical and food industries. 

A fast and reliable on-line measuring system is needed.

If the correlation with COD is assumed, UV absorption appears to be the most adapted and cost-effective method (no filtering, no reagents, no consumable, no dilution).

The CX1000 can drive a sampler using the high alarm relay if a standard method is required to meet the regulations.

The samples are taken only above a critical value to limit the number of laboratory analysis.

Measuring time	< 10 seconds
Correlation with COD on river water	very good
Correlation with COD on industrial effluent	variable
Detection of unsaturated organic molecules	yes
Detection of alcohol and linear hydrocarbons	no
Reagents	no
Consumable, gas cartridge	no
Distilled water for dilution	no
Periodic change of electrode or lamp or parts	no
Maintenance	very low
Filtering	no need
Operating cost	low
Size	compact
Weight	15 kg approx.
Transportable by car or passenger plane	yes
Installation time	few minutes

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