Water quality standards
As mentioned above, customer complaints may be about water quantity and breaks in supply, but they may also be about water quality. This could be an undesirable colour or taste in the water that consumers have detected. More seriously, it could be a problem related to ill health that has been linked to the water supply. To check that the water supplied is both potable and palatable requires standards to be established. Drinking water standards are a set of limits for a wide range of substances, organisms and properties of water, with the protection of public health as the focus.
As you have read, water can contain many different types of biological contaminants. Water is considered a good and universal solvent, which means all kinds of dissolved substances can be found in it as well. As a result, there is a need to have precise assessment methods that are used to test for the presence and concentrations of these substances to find out if they are within the water quality standards. The World Health Organization publishes international guidelines for drinking water quality (WHO, 2011) and most national governments also produce their own country standards.When the standards are breached, it gives a signal to the water supplier to investigate and take remedial action. Although standards are set in most low-income countries, quality assurance is not always present.
In addition to measuring health-related parameters set in international and national drinking water quality standards, water suppliers need to carry out a wide range of analyses important to the operation and maintenance of water treatment and distribution systems. These analyses (Figure 9.5) should include acceptability, which is a critical parameter. (Parameter means a measurable factor.) If the water supplied to the community is not acceptable (for example, because it has a tinge of colour in it due to natural compounds present, which may not be harmful), consumers may choose a more palatable, but possibly unsafe, water source. Acceptability may be assessed by physical observation (such as taste, colour, odour, visible turbidity), without laboratory determinations. Water utilities will monitor the water quality to check that the standards are adhered to.
Drinking water can be contaminated with human or animal faeces, which are a source of pathogenic bacteria, viruses, protozoa and helminths. Measurement of microbial parameters involves the microbiological tests for faecal contamination that you read about in Study Session 2.
What are the tests widely used to assess faecal contamination?
The usual tests are for total coliform bacteria, faecal coliform bacteria and E. coli.
Table 9.1 shows appropriate bacteriological standards for drinking water. It includes total coliforms and E. coli, and also mentions total visible colonies of organisms (Figure 9.6) and faecal streptococci. These are additional ways of measuring the microbial contamination of water. For all four parameters shown here the standards state that none should be present in drinking water.
Table 9.1 Bacteriological quality standards for drinking water. (ESA, 2013)
|Substance||Maximum permissible level|
|Total visible organisms, colonies per 1 ml||Must not be detectable|
|Faecal streptococci per 100 ml||Must not be detectable|
|E. coli, number per 100 ml||Must not be detectable|
|Total coliform organisms, number per 100 ml||Must not be detectable|
To conduct a visible colony test, a known volume of water is spread on a nutrient layer in a Petri dish. After incubation, colonies of bacteria grow and can be counted. If there are too many colonies to count (as in B) then a diluted sample of the same water should be used (A).
Do you experience a taste while drinking potable water? Water in its natural state is tasteless, odourless and colourless. Although changes in these parameters may have no direct health effects, a slight change will result in consumer dissatisfaction. Important physical characteristics of drinking water quality are listed in Table 9.2.
Table 9.2 Physical parameters of drinking water.
|Parameter||Maximum permissible level||Adverse effect|
|Colour||15 platinum-cobalt units (PCU)*||Unpleasing appearance|
|Odour||Not objectionable||Unappealing to drink|
|Taste||Not objectionable||Unappealing to drink|
High pH imparts taste and soapy feel, low pH causes corrosion.
For effective disinfection pH is preferably <8.0.
|Turbidity||5 nephelometric turbidity units (NTU)**||Visibly cloudy|
* In platinum-cobalt units the colour is measured by visual comparison of the sample with platinum-cobalt standards. One unit of colour is that produced by 1 mg of platinum per litre of solution (1 mg/l or 1 mg l–1), in the form of the chloroplatinate ion.
**The nephelometric turbidity unit is a measure of the turbidity of the water.
Natural water has different elements and compounds in it due to the ability of water to dissolve many substances. Unlike microbial contaminants, most chemical constituents in drinking water only cause adverse health effects after a prolonged period of exposure. A huge concern is if a massive accidental discharge of chemicals occurs in the drinking water system. Experience has shown, however, that in many such incidents, the water will exhibit changes in appearance, taste and odour that prompt consumers to cease using it. For example, if the residual chlorine level exceeds a certain level, consumers notice the taste and immediately reject the water. An example of chemical quality standards for the usual parameters in drinking water are presented in Table 9.3. So, taking aluminium for example, its quantity in 1 litre of water must not exceed 0.2 mg. There are other components that also have limits, such as pesticides, but these will only be analysed for if a case of contamination by these types of substances is suspected.
Table 9.3 Chemical quality standards of drinking water. (ESA, 2013)
|Substance||Maximum permissible level (mg l–1)||Health and other risks associated with high intake|
|Aluminium||0.2||Deposition of aluminium hydroxide flocs in water pipes and exacerbation of discoloration of water by iron|
|Ammonia||1.5||Objectionable (pungent) odour|
|Arsenic||0.01||High incidence of skin and possibly other cancers|
|Cadmium||0.003||The kidney is the main target organ of toxicity|
|Chloride||250||Undesirable (salty) taste|
|Chromium||0.05||Carcinogenicity suspected of chromium (VI) compounds|
|Copper||2||Acute toxicity is high; effects on thyroid and particularly the nervous system if long-term exposure occurs|
|Cyanide||0.07||Effects on thyroid and particularly the nervous system if long-term exposure occurs|
|Fluoride||1.5||At low concentration, prevents dental caries; at high (7 mg l–1) concentration, increased risk of dental fluorosis; much higher concentration leads to skeletal fluorosis.|
|Iron||0.3||Causes reddish-brown colour, promotes iron-bacteria growth and stains laundry and plumbing fixtures|
|Lead||0.01||Toxic to both the central and peripheral nervous systems, causing neurological effects|
|Manganese||0.5||Neurotoxicity and other toxic effects|
|Mercury (total)||0.001||The kidney is the main target for inorganic mercury, whereas methyl-mercury mainly affects the central nervous system|
|Nitrate (as NO3)||50||Causes methaemoglobinaemia in infants and suspected risk of certain forms of cancer|
|Nitrite (as NO2)||3||Causes methaemoglobinaemia in infants and suspected risk of certain forms of cancer|
|Sulphate||250||Causes noticeable taste and corrosion of pipes|
|Total dissolved solids||1000||Undesirable taste|
|Zinc||5||Imparts astringent taste and opalescence; develops a greasy film on boiling|