It may be true that colour in water is harmless when the water is utilised for human consumption, however, it is certainly unappealing.
Colour in our water does matter when it comes to drinking it, in other home and industrial uses, and in some aquatic and recreational environments.
Colour, taste and odour, along with the generally monitored chemical parameters that include pH, hardness and alkalinity, are basic World Health Organisation drinking water quality specifications.
What exactly is the colour of water, the impact of colour on water quality, and how is colour removed from water?
What colour is water?
Artists generally depict water as blue or blue-green. Ideally, the water from your tap should not be blue, blue-green, milky or any other colour – it should appear colourless and clear.
While relatively small quantities of water may appear colourless, pure water has a slight turquoise colour that becomes deeper as the thickness of the observed sample increases.
The turquoise colour is caused by the weak absorption of light in the red part of the visible spectrum. The hue of water is an intrinsic property that is affected by the dissolved and suspended impurities in the water.
There are many surface-water colours depending on the dissolved and suspended impurities that may be organic or inorganic in nature.
Colour can be tested through a quick and easy test, which reflects the amount of colour imparted by dissolved organic material as well as inorganic components, such as iron and manganese, in the water.
Colour variations are measured with reference to a standard colour scale. An example of such a scale is the Platinum-Cobalt scale, where discolouration is expressed in Hazen units (HU). Water rated as having a colour of 5 units means that the colour of the water is equal in intensity to the colour of distilled water containing 5 milligrams of platinum (as potassium chloroplatinate) per litre.
SANS241:2015, aligned with U.S. EPA Secondary Drinking Water Regulations, recommend that potable water possess a colour of fewer than 15 units.
The colours of water
Water colour can reveal physical, chemical and bacteriological conditions in the water.
Pure water from the natural environment is very rare, as water contains dissolved minerals and organic material, as well as colloidal and non-colloidal suspended solids that vary depending on the environment through which the water has flowed, which will impact the colour of the water.
The plus side is that many of these dissolved minerals are beneficial for human health and the overall flavour of the water.
The dissolved minerals remove the “flat” taste associated with pure water which is devoid of all minerals, such as deionised water.
While most people prefer water with dissolved minerals, they still want it to be clear.
Examples of colours that may form in water
Light to Dark Brown:
Dissolved organic material from vegetation, including humus and tannins, may cause the water to discolour to various shades of brown. The presence of dissolved and colloidal iron also causes a brown hue in water. Iron contamination may be a result of corrosion in the water reticulation system, or it may be naturally present in the raw water.
Red can be a sign of rust from iron pipes or airborne bacterial contamination when observed in surface water bodies such as lakes.
Green colouration may be caused by unicellular, planktonic algae floating in the water. This is particularly prevalent in water supplies served by open reservoirs or rivers. Algae is generally not a health threat, and reservoirs can be managed and monitored to prevent algae from growing to the point where they discolour water.
Dissolved copper, whether naturally present or resulting from the corrosion of copper components in the water reticulation system, may also cause water to be blue or blue-green. Blue and blue-green discolouration may be caused by certain copper and tin-based biocidal chemicals.
Purple to black:
The presence of manganese may cause the water to be dark purple or black. Manganese is usually present in natural (raw) water and becomes a problem when it is inadequately removed in the raw water treatment processes. Black can also indicate the growth of sulphur-reducing bacteria.
Excess calcium and magnesium carbonate, naturally occurring minerals found in varying concentrations in most waters, may cause water to be milky. Excessive bacterial growth may turn clear water milky. Milky-white water from taps is also often caused by harmless air bubbles in the water. The milky-ness usually clears from the bottom of a glass within minutes when this is the case.
The impact of colour on water quality
Dissolved metals, such as iron and copper, will cause the water to have a metallic taste and odour. Iron, copper and manganese are known to cause discolouration of laundry and fixtures in domestic applications.
Excessive calcium and magnesium carbonate may cause scaling and blockage of screens and filters of domestic appliances and industrial infrastructure. Scaling negatively impacts the operational efficiency of the affected equipment.
On the other hand, insufficient calcium and magnesium carbonate will cause the water to be corrosive, also negatively impacting the integrity of the equipment.
Bacterial growth, specifically sulphur-reducing bacterial growth, can cause an unpleasant sulphur-like taste and odour in the water.
This taste and odour are caused by the formation of hydrogen sulphide gas which is formed during bacterial metabolism.
The bacteria that typically produce this type of gas are not a health risk. Excess bacterial numbers contribute to the suspended solid loading of the water, negatively impacting the turbidity of the water, giving it a milky appearance.
Water contaminated with high bacterial numbers may pose a high health risk if consumed.
Coloured water has a negative effect on the natural water ecosystem. Light is critical for the growth of aquatic plants, and coloured water limits the penetration of light, negatively impacting the metabolism and growth of plants and algae present in the water body.
A highly coloured body of water will not sustain a balanced and healthy aquatic ecosystem. Very high algae growth will prevent light penetration, and use up dissolved oxygen in the water body, causing a eutrophic condition that can drastically reduce all life in the water body. This could lead to the long-term impairment of the ecosystem.
How is colour removed from water?
Colour from dissolved metals such as iron, copper and manganese may be removed by raising the pH, causing the soluble metal to precipitate from the solution. The precipitate can then be filtered from the water using conventional filtration techniques.
The metals may also be oxidised by aeration, or by the addition of a chemical oxidant such as ozone, chlorine, peroxide or potassium permanganate. The precipitated solid may similarly be removed by filtration. Aeration is also effective for the removal of the odorous hydrogen sulphide formed by sulphur-reducing micro-organisms.
Coagulation on its own or with settling, followed by conventional filtration, may be effective in reducing or removing colour from water.
Activated carbon is also widely used for the removal of colour, taste and odour from bulk water supplies. Activated carbon is generally applied after sedimentation and filtration.
The clarified overflow water is passed through filters containing granular activated carbon, a medium with excellent properties of attracting gases, fine suspended solids and organic impurities, such as phenols.
Nanofiltration is another technology that may be applied to remove colour, particularly if demineralised water is desired at the end of the water treatment process.
Nanofiltration is membrane-based technology, ideal for waters containing low levels of total dissolved solids and organic material.
Case Study: The removal of iron and manganese from borehole water for potable consumption in a residential eco estate
The eco estate’s design philosophy is based on its commitment to environmental conservation, guided by the principle of “Return to Nature”. The ideal is to go “off-grid” and be self-sufficient in terms of both water and electricity.
The developers chose to access borehole water as the main source of reticulated water. The untreated borehole water contains iron and manganese, minerals that cause the water to discolour (even at very low concentrations), creating an array of problems for the residents, including staining laundry.
Watercare Innovations was approached to provide a water solution. Watercare Innovations designed, built and commissioned a turnkey solution to treat the borehole water for the removal of the colour, and to produce SANS241:2015 quality potable water. The treatment process consists of the following unit operations:
- Upfront coagulation, pH correction and ozone injection for the oxidation of unwanted minerals dissolved in the water
- Filtration Step 1 to remove the insoluble minerals
- Filtration Step 2 to remove the remaining suspended solids
- Disinfection compliant with SANS 241:2015
- Final Product Monitoring – real-time, remote monitoring of the final product water for pH, conductivity and ORP (oxidation-reduction potential, as a disinfection control measure)
The plant has been operated and maintained by Watercare Innovations for six years since commissioning.
Understanding the water impurity indication colour spectrum makes identifying and solving aesthetic, microbiological and chemical water quality problems easier to resolve.