Page 5: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Colour

Relationships with Other Water Quality Parameters

Colour is directly related to almost every other drinking water quality parameter. In general, these relationships are due to colour itself, which can interfere with the colorimetric analysis of some constituents of potable water; the formation of complexes or adsorption of some water constituents by the dissolved humic substances that colour comprises; reactions between humic substances and chemicals added during treatment; and the provision of nutrients to some micro-organisms by humic substances and their metal complexes. In addition, combinations of some of the above mechanisms are involved in the corrosion and incrustation of drinking water distribution lines.

Physical Characteristics

Although some of the older water quality literature claims that dissolved humic substances cause taste in water,Footnote 40 no recent research appears to have been done on this question. Highly coloured, polluted water will frequently have an associated objectionable taste, but the degree to which this association is causative is unknown.

In order for any substance to exhibit an odour, it must have an appreciable vapour pressure, and it is implausible that this could be the case for the polyelectrolytic, high molecular weight, dissolved humic substances. It is known, however, that the organic colouring material of water stimulates the growth of many aquatic micro-organisms,Footnote 25,Footnote 26,Footnote 41 some of which are directly responsible for the production of odour in water.

As humic acid and certain metal complexes of humic substances are poorly soluble at the pH of potable water, they will be responsible for a portion of the turbidity in a water sample. Another constituent of the suspended solids fraction of water is known to be a complex of clay particles with humic substances. Furthermore, as "dissolved" humic substances in water exist predominantly as colloidal dispersions, and as optical measurements of turbidity are influenced by particles in the colloidal size range,Footnote 2 the presence of colour in water will have an effect on the measurement of turbidity. It is also well known that the presence of moderate amounts of colour in certain types of raw water has an adverse effect upon the removal of turbidity by coagulation and sedimentation.Footnote 13 Laboratory studies have shown that the presence of fulvic acid lowers the optimum pH for turbidity removal and increases the amount of coagulant required to treat dilute clay suspensionsFootnote 42,Footnote 43; the coagulant dose and optimum pH for colour removal, however, are independent of the presence of clay.Footnote 42

Microbiological Characteristics

Humic substances are generally regarded to be very resistant to the action of bacteria and higher micro-organisms in the environment,Footnote 28,Footnote 29 and laboratory studies have confirmed this stability in water samples that were stored for months under conditions favourable to the growth of bacteria.Footnote 1,Footnote 44 One recent preliminary study, however, has presented evidence for two different aquatic microbiological transformations of soluble humic substances; one of these processes appears to increase the colour of the water, whereas the other decreases it.Footnote 45 Several authors have reported that certain bacteria, fungi and higher micro-organisms are capable of utilizing humic substances as a source of energy.Footnote 44,Footnote 46-48 Studies of other specific micro-organisms have shown that they are unable to utilize humic substances as a food source.49 The most common colour problem of microbiological origin is the production of "red water." This phenomenon occurs because many genera of bacteria are capable of oxidizing iron (II) to iron (III), which precipitates from solution as the hydroxide and imparts a characteristic reddish colour to tap water. In severe cases, distribution lines have been blocked, or nearly blocked, by the action of these "iron bacteria." Similarly, a black discoloration may be imparted to drinking water by the action of bacteria that are capable of oxidizing dissolved manganese to its insoluble oxides. This colour problem occurs more frequently in groundwater than in surface water supplies.

A few investigators have reported the formation of coloured, humic-like substances by aquatic micro-organisms.Footnote 25,Footnote 26 The proportion of colour in water that originates in this way is thought to be very small, however.Footnote 13

The presence of organic colour in treated water that is disinfected by chlorination is one factor that can cause difficulty in maintaining a free available chlorine residual in distribution systems. Although this has been known since at least 1949,Footnote 40 it was not until the discoveryFootnote 50,Footnote 51 in 1974 of relatively large amounts of chloroform and other trihalomethanes in chlorinated water that the reaction of chlorine with dissolved humic substances was subjected to careful study. Since 1974, it has been established that the trihalomethanes are reaction products of chlorine (and adventitiously present bromine and iodine) with humic substances, and that conventional water treatment can remove most of the organic precursors from the raw water.Footnote 52,Footnote 53

Chemical Characteristics

It is well known that colour can interfere with the chemical analysis of many of the other constituents of water. It has been noted in Standard Methods for the Examination of Water and Wastewater, for example, that colour must be compensated for or removed in colorimetric analytical methods,Footnote 4 and the use of cupric acetate precipitation has been shown to be effective in removing the interference due to colour in standard colorimetric analyses for chloride, fluoride, nitrate (and nitrite), phosphate and sulphate.Footnote 54

Non-colorimetric methods of analysis can be interfered with by colour because of the metal-complexing properties of humic substances. It is for this reason, for example, that samples for the determination of total hardness by the titrimetric method involving ethylenediaminetetraacetic acid (EDTA) must be ashed prior to analysis in order to completely oxidize organic material.Footnote 4 Specific-ion electrode methods for the analysis of metals in water, such as calcium and cadmium, give low results as a result of complex formation if humic substances are present.Footnote 4 Humic substances can also interfere with trace metal analysis when the sample is concentrated by extraction with an organic solvent that contains a complexing agent.Footnote 55,Footnote 56

Some of the other relationships between colour and chemical parameters include the following observations: that the bicarbonate alkalinity of water can be destroyed by humic substancesFootnote 57; that total dissolved solids and organics may include humic substances; that humic substances can interfere with the analysis of methylene blue active substancesFootnote 4; and that nitrilotriacetic acid (NTA) can form stable mixed complexes with several divalent metals and fulvic acids.Footnote 58,Footnote 59 and one recent reviewer has suggested that all phosphate in water is complexed with humic substances.Footnote 60

The relationship between corrosion and incrustation and the humic content of water is a complex and important one. Small amounts of humic substances (1 to 2 mg/L) aid in the deposition of a protective layer of calcium carbonate in distribution systems for mildly incrustive watersFootnote 14; however, larger amounts may be responsible for the deposition of flow-restrictive "humus mud" in distribution systems where lime is added as a post-treatment step for corrosive waters.Footnote 61 The presence of humic substances in water has differing effects, in kind and in degree, on the corrosion of iron, copper, aluminum and lead.Footnote 62,Footnote 63 Water containing very little dissolved humic material can be more corrosive than water containing larger amounts. Distilled water at pH 7, for example, is more corrosive to lead than other types of water,Footnote 64 and activated carbon treatment, which removes humic substances from drinking water, has been related to increased corrosion problems in Germany in instances in which all of the humic substances have been removed.Footnote 64

Severe chemical corrosion in distribution systems can also lead to the production of colour (and turbidity) at the consumer's tap. Red colours tend to be associated with iron, black colours with manganese and faint blue colours with copper corrosion.

Report a problem or mistake on this page
Please select all that apply:

Thank you for your help!

You will not receive a reply. For enquiries, contact us.

Date modified: