Colour is an often misunderstood water quality parameter. Colour is present in most surface and ground water sources. While colour can be the result of dissolved or suspended contaminants, colour in surface water is generally caused by dissolved organic matter. In managing quality of drinking water supplies, management of colour is a key measure to ensure control of disinfection by-products (DBPs).
Colour can be measured as either true colour or apparent colour. Apparent colour is the colour of a water sample containing both dissolved and suspended contaminants and is appropriate for aesthetic purposes. For treatment applications, analysis of true colour is more appropriate, as apparent colour is greatly affected by turbidity. The use of true colour to inform treatment decisions also ensures that suspended contaminants removed by filtration processes do not influence the result.
Colour measurements are expressed in Hazen Units (HU), or Platinum-Cobalt Units (PCU/PtCo). Both measures are equivalent and do not require conversion. Drinking water supplied should maintain colour below 15 HU to limit the formation of DBPs during post-filtration chlorination. Colour is generally perceptible in a glass of water at 15 HU. For comparison, freshly brewed tea is cited in the ADWG at 2500 HU.
Measurement of colour can be achieved using either spectrophotometry or using a visual comparator. The spectrophotometric method uses absorption of a specific wavelength of light in the visible range (generally 436nm). Visual comparators offer a lower cost solution to spectrophotometry, although rely on the judgement of operators to determine the colour level. Additionally, this method has a low resolution, with colour in the 0-20 HU range generally only determined to an accuracy ±2.5 HU or higher. Regardless of the method used, suspended content in samples are typically filtered using a 0.45µm filter prior to measurement to obtain a true colour reading.
Colour is generally controlled using coagulation and filtration processes. Removal of colour is improved by the implementation of enhanced coagulation, where coagulation is optimised for natural organic matter removal. While colour can be 'bleached' using chlorination processes, this can result in an increase in DBPs, and requires close monitoring to ensure that levels of DBPs remain within ADWG limits.
For assistance in managing colour and DBPs in your water supply, don't hesitate to contact Viridis.