Definition
UV absorption & transmissivity measurement is used in the testing of water and waste-water, to predict water quality and the amount of dissolved organic carbon. The UV254nm optical measurement can be supplied for either transmission (UVT) or absorption (UVA) measurement. Correlation may also be made with TOC/BOD/COD equivalent for some applications.
UV254nm transmissivity measurement is used to control UV disinfection processes, to account for absorbance by dissolved organics and suspended solids. To ensure efficient sterilisation, the lamp intensity may be regulated by an instrument measuring % UVT.
UV254nm absorption can also be used as a general indicator of organic loading and is also often referred to as SAC (Spectral Absorption Coefficient). However, since each chemical species absorbs the UV spectrum differently, the test must be applied carefully. Where the sample matrix is highly variable or contains organic compounds which do not absorb at the 254nm frequency, the alternative analyser UV- persulfate methodology may be more accurate, reporting Total Organic Carbon (TOC).
Methodology
Flow through or direct in-situ probe measurement is available. Different path lengths may be needed to accommodate different application / measuring range requirements thus providing various levels of sensitivity. Automatic cleaning (either chemical or air blast) can be used to maintain reliable measurement especially in waste-water applications.
Limitations Of Test
Only organic chemicals containing a double bond within their structure strongly absorb UV254nm, single bonded chemicals do not. Every organic chemical absorbs different amounts of UV254nm. The technique is not typically suitable for applications where the contaminant is unknown or highly variable in it's chemical form. The technique can be applied to a known process where the contaminant is known to strongly absorb at this frequency. Absorption across the UV spectrum may be used to evaluate a more complex sample matrix but care still needs to be taken to establish the suitability of the optical technique.
Alternative On-line Methodology
When analysing Total Organic Carbon (TOC), the alternative UV-persulphate technique may be beneficial for some applications with variable sample matrix. The methodology is not compound sensitive. A correlation may also be set up with other test such as COD and BOD. Peak organic loads may therefore be determined in real-time (response time from 3 minutes) allowing for improved measurement and control.
More Information
TOC
On line continuous measurement of Total Organic Carbon for water and waste-water applications.
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