When looking at source water monitoring for drinking water treatment, the terms colour and natural organic matter (NOM) are often interchanged. This may be due to organic matter, such as humic acids, causing visible colour in surface water. However, its important to note that the absence of colour in surface water does not always translate into an absence of organic matter. As an example, pesticides in water will not show up with a colour instrument but will be detected by a UV254 or UV spectral analysis instrument. Therefore, organics are best measured with a UV254 or UV spectral absorbance instrument as direct colour measurements will not provide an accurate or reliable surrogate for organic matter.
UV254 AND COLOUR
Organic measurements, such as UV254, can however provide a good indication of colour if most of the colour comes from organic matter. If this is the case, a UV254 instrument can be used to provide an approximation of colour based on organics. The correlation will be dependent on the particular matrix of organics at that site but the two are often very linear. Real Tech’s UV254 analyzers provide a scaling factor function so that the measured level of organics can be displayed in units of Hazen.
SPECTROPHOTOMETERIC DETERMINATION OF COLOUR
Colour can be determined using spectrophotometry between 450 and 465 nm wavlengths. There are two types of colour measurement, true colour and apparent colour. True colour is the colour after turbidity has been removed (filtered through a 0.45 micrometer pore size) so that all the colour measured in due to dissolved material. Apparent colour is measuring colour of a water sample as is, that is, including both true colour and turbidity.
Both apparent colour and true colour can be monitored continuously using an online UV-VIS spectrophotometer. While apparent colour can be measured directly, the true colour measurement will be an approximation as filtering is not typically feasible and turbidity affects are approximated and compensated for using additional reference wavelengths.