Turbidity

Water Quality Standards

1. General description

Identity

Turbidity is caused by the presence in water of particulate matter, such as clay, silt, colloidal particles, and plankton and other microscopic organisms, and is a measure of the water's ability to scatter and absorb light. This depends on a number of factors, such as the number, size, shape, and refractive index of the particles and the wavelength of the incident light (1).

Sources

The particles that cause turbidity in water vary in size between 1 nm and 1 mm (2). They can be divided into three classes: clay particles, which have an upper particle size limit of about 0.002 mm diameter; organic particles produced by the decomposition of plant and animal debris; and fibrous particles, e.g. those of minerals such as asbestos.

Soil particles, produced by the erosion of the land surface, have been found to constitute the major part of the suspended material in most natural waters. The coarser sand and silt fractions are wholly or partially coated with organic material. Phyllosilicate clay particles, as well as non-clay material, such as iron and aluminium oxides and hydroxides, quartz, amorphous silica, carbonates, and feldspar, constitute the clay fraction. Clay and organic material are often associated as a "clayBorganic" complex. Humic substances have a much higher ionic exchange capacity than inorganic clays, and their effects often predominate (3).

The accumulation of large numbers of microorganisms has been reported to produce turbid water. Examples include summer blooms of algae in surface water, algal debris, and the detritus from iron bacteria in distribution systems (3).

2. Analytical methods

A number of methods may be used to measure water turbidity, but nephelometry and turbidimetry form the basis of present standard methods (4–6).

3. Environmental levels and human exposure

All natural waters are turbid, surface waters generally more than groundwater. Raw water turbidity has been reported to range from less than 1 to more than 1000 NTU (nephelometric turbidity units) (3). As processes such as simple filtration or coagulation, sedimentation, and filtration are effective in removing turbidity, drinking-water concentrations are usually less than 1 NTU. If turbidity is present in higher concentrations, it may be as a consequence of inadequate treatment or the resuspension of sediment in the distribution system (7). It may also be due to the presence of inorganic particulate matter in some groundwaters.

4. Relationship with water quality parameters

The turbidity of water is related to or affects many indicators of drinking-water quality. For example, there is a relationship between high turbidity and the appearance, colour, taste, and odour of both raw and filtered waters. It is reported that 50% of colour in water is due to a "colloidal fraction" of humic substances. This, however, is not true colour, which is the colour remaining after removal of turbidity (6).

Turbidity can have a significant effect on the microbiological quality of drinking-water. Its presence can interfere with the detection of bacteria and viruses in drinking-water (8); more importantly, turbid water has been shown to stimulate bacterial growth (2) since nutrients are adsorbed on to particulate surfaces, thereby enabling the attached bacteria to grow more rapidly than those in free suspension.

The major problem associated with turbidity is its effect on disinfection, because high levels have been shown to protect microorganisms from the action of disinfectants (8–10) and to increase the chlorine and oxygen demand (11). Coliform bacteria have been found in waters of turbidity between 4 and 84 NTU, free chlorine residuals between 0.1 and 0.5 mg/litre, and a minimum contact time of 30 min (3). In turbid water, Escherichia coli has been shown to be protected in the presence of chlorine levels of 0.35 mg/litre or greater (3).

The adsorptive capacity of some suspended particulates can lead to the presence of undesirable inorganic and organic compounds in drinking-water. Most important in this respect is the organic or humic component of turbidity. For example, herbicides such as 2,4-D, paraquat, and diquat can be adsorbed on to clay–humic particulates, the adsorption being greatly influenced by metal cations present in the humic material. In addition, the strength of the bonds in some metal–humate complexes in the turbidity fraction may complicate the measurement of trace metals in natural waters, resulting in an underestimation of the metal concentrations.

5. Effects on humans

The consumption of highly turbid water may constitute a health risk, because, as mentioned above, excessive turbidity can protect pathogenic microorganisms from the effects of disinfectants, stimulate the growth of bacteria in distribution systems, and increase the chlorine demand. In addition, the adsorptive capacity of some particulates may lead to the presence of harmful inorganic and organic compounds in drinking-water.

6. Conclusions

The appearance of water with a turbidity of less than 5 NTU is usually acceptable (see Acceptability aspects) to consumers, although this may vary with local circumstances. However, because of its microbiological effects, it is recommended that turbidity be kept as low as possible. No health-based guideline value for turbidity is proposed.