The importance of good drinking water in maintaining human health was recognised early in human history, with water storage and treatment mentioned in historical records dating back to at least several hundred years BC.
In more recent times, the studies of Dr John Snow on the transmission of cholera by London's drinking water in the mid-1800s contributed to the recognition that specific microorganisms cause specific human diseases.
By the early 1900s, better protection of water supplies from sewage pollution and simple but effective methods of water treatment (chlorination, sand filtration) greatly reduced rates of waterborne disease in developed nations. However, waterborne diseases continue to be a major cause of illness and death in the developing world, especially in children.
Since most people drink water every day, contamination of a public drinking water supply has the potential to expose nearly all members of a community to harmful chemicals or microorganisms in a very short period of time. For this reason, it is important that the protection of public health is the first consideration in managing any water supply.
Microorganisms that are capable of causing disease are called pathogens. The pathogens of concern in water supplies are mainly those that are found in the excrement (faeces) of humans or animals. If these microorganisms are present in water, and are not removed by water treatment or disinfection, then consumers may suffer infections.
Many types of pathogenic bacteria, viruses, protozoa and helminths may be transmitted by contaminated water supplies. These pathogens can also be transmitted directly from human to human, from animal to human, from swimming in contaminated water, by contaminated food, or indirectly through contact with contaminated objects. The fact that contaminated water causes an outbreak of a particular disease does not mean that the disease is only or mainly transmitted by water under normal circumstances.
Generally, faecal contamination from human sources is regarded as the greatest risk to water supplies, as some diseases such as cholera, typhoid, and gastroenteritis viruses are found only in humans. However, some pathogens from animals including mammals and birds can also cause illness in humans.
|
Zoonotic diseases Zoonotic diseases or zoonoses are those diseases that are naturally transmitted between vertebrate animals and humans. Mammals in catchments and birds in service reservoirs are the most common cause of waterborne zoonotic disease. For example, Salmonella bacteria entering the water supply from such sources can result in waterborne zoonotic disease. Zoonoses can be spread from pets, such as dogs and cats, and agricultural animals (cattle, sheep and pigs), or from native or feral animals.
|
The lack of a safe water supply and waste disposal system causes waterborne disease to spread easily in poorer countries. For example, several serious diseases are spread by freshwater snails associated with irrigation canals in certain areas, as well as by bathing, swimming, wading and washing clothes in such waters.
Water also plays a role in the transmission of other types of diseases. For instance, insects that breed in water, such as mosquitoes, may also spread disease to humans by sucking blood. Such diseases include typhus, dengue fever, malaria and yellow fever.
The amount of illness in a community is affected by the quantity of water that is available, as well as its microbiological quality. If there is not enough water for people to bathe themselves, or to wash cooking utensils or clothing, high rates of gastroenteritis are common. In this situation, increasing the amount of water for people to use will generally produce a health benefit even if the quality of the water is not changed.
More on water-related diseases
The 2004 edition of the Australian Drinking Water Guidelines incorporates a Framework for Management of Drinking Water Quality that provides guidance on assessing and managing risks to drinking water supplies throughout the water supply system. This Framework emphasises a preventive approach so that potential problems can be identified and managed to avoid impacts on the quality of water supplied to consumers. Ensuring the microbiological safety of a water supply entails a wide-ranging program of protection, treatment and monitoring, with barriers to the entry and transmission of pathogens throughout the system.
The barriers should include most of the following:
In addition, the Guidelines discuss monitoring for microbiological quality as a check that the barriers to contamination are working.
|
In summary, Australian Drinking Water Guidelines highlights the concept of multiple barriers to prevent pathogens or other contaminants reaching consumers. Such barriers may include:
|
Protection of public health depends on having multiple barriers in place to keep bugs (pathogenic organisms) out of the water supply. This includes trying to keep them out of the catchment or water source in the first place. Other barriers to these organisms are water treatment technologies, disinfection and a closed distribution system.
Measurement of pathogens in the water may appear to be the best method to determine whether the water supply is safe. However, this is an immensely difficult task, requiring expensive and sophisticated technology and taking considerable time.
It is also a very complex task due to the diversity of pathogens that exist and the different test methods required. A further problem is the inability of existing technology to continually monitor for pathogens. Because of these factors, indirect methods are used to measure microbiological water quality.
One such method is to measure the concentration of disinfectant at various points throughout the distribution system. If chlorine is used as the disinfectant, what is referred to as the chlorine residual is measured. According to the Australian Drinking Water Guidelines, a chlorine residual of 0.2 milligrams per litre (mg/L) to 0.5mg/L is generally adequate. Chlorine at this level kills the target organisms.
The second method of measuring microbiological water quality is to monitor for organisms that might indicate that the water is contaminated with faecal material or that disinfection is inadequate. These organisms are referred to as indicator organisms. They are not harmful to health but their presence indicates that other faecal organisms (including harmful pathogens) may also be present in water.
Members of the coliform group of bacteria are used as indicators of water quality. This group contains many species of bacteria that grow in the environment, but a sub-group of coliform bacteria, called thermotolerant coliforms (coliforms preferring warmer temperatures), are found predominantly in the intestine and faeces of humans and other warm-blooded animals.
One member of the thermotolerant coliform group, Escherichia coli (often referred to as E. coli ) is recognised as the most specific indicator of recent faecal contamination in water supplies. This organism is now the preferred indicator for assessing the microbiological quality and safety of drinking water.
In some instances, a more general test for thermotolerant coliforms may be used instead of a specific test for E. coli , however thermotolerant coliforms are a less specific indicator of faecal contamination. Some non-faecal environmental coliform bacteria ( Klebsiella, Citrobacter and Enterobacter ) are also thermotolerant, and in certain circumstances these may produce a positive result on the test even if faecal contamination is absent.
Other groups of bacteria may be used by water suppliers as operational indicators to assess whether water supply systems are operating normally as expected. These organisms (total coliforms and heterotrophic plate count bacteria) have no significance for assessing health risks but unusual variations in their numbers may signal a change from normal operating conditions that requires investigation.
|
Indicator organisms can be used to:
|
The indicators most commonly used to measure the microbiological quality of water are E. coli , and thermotolerant (or faecal) coliforms.
A wide variety of chemicals may enter a body of water used for water supply purposes via stormwater runoff. Such chemicals can be natural or manufactured substances.
Inorganic chemicals, such as mineral salts, can be leached from the natural environment. Manufactured chemicals such as pesticides, herbicides, insecticides, pharmaceuticals and industrial waste products can also be picked up from the land in the catchment or discharged into a waterway from a specific source.
Every chemical has an effect on living organisms exposed to it. The study of the negative or harmful effects of chemicals on living organisms is known as toxicology.
Living organisms respond in different ways when exposed to chemicals. Some effects in organisms are immediate; that is, they show up within 24 to 48 hours. Other effects may be delayed and not show up for 10 or 20 years or more; for example, cancer in humans.
The response of a living organism exposed to a chemical depends upon the chemical dose or the exposure level. Generally, the higher the dose the more significant the effect. Simply knowing that the compound is carcinogenic is not sufficient to assess the risk to human health – it is necessary to know the harmful dose as well.
Ingestion of low levels of some chemical contaminants in drinking water over long periods of time has been associated with negative health effects, but these associations are not fully understood.
The Australian Drinking Water Guidelines provide guidance to water authorities on safe levels of chemicals in drinking water based on the best scientific information available.
Inorganic chemicals may be present naturally in raw water, be derived from contamination of source water or obtained from contact with piping and plumbing materials used to transport water.
Generally a naturally occurring phenomenon, hardness is a measure of the calcium and magnesium salts dissolved in the water. Hardness levels of less than 200 milligrams per litre (expressed as a concentration of calcium carbonate) are described as good quality water in Australian Drinking Water Guidelines.
On the basis of taste, a concentration of total dissolved solids of less than 500mg/L is also described as good quality water in Australian Drinking Water Guidelines.
Sometimes, domestic plumbing can be a source of elevated levels of copper or iron measured at the tap. Copper in drinking water can have health effects. Plumbing can also be a source of lead in drinking water.
Several elements are essential to human nutrition at low doses, yet can have negative effects at high doses. These include arsenic, selenium, chromium, copper, molybdenum, nickel, zinc and sodium. The elements lead, arsenic and cadmium are suspected carcinogens.
A summary of the scientific evidence that the guideline values for these and other chemicals are based upon can be found in Australian Drinking Water Guidelines.
Organic chemicals in water derive from:
The breakdown of naturally occurring organic materials is the predominant source of organic chemicals in water. These chemicals are derived from vegetation, soil humus, and microbiological activity. Water scientists refer to this material as natural organic matter (or NOM). These organics are typically benign, although they can be responsible for such aesthetic problems as colour, taste and odour.
Excessive algal growth in source waters can lead to the tainting of drinking water supplies with complex and unpleasantly scented organic components such as geosmin and methylisoborneol.
The toxins produced by some blue-green algae – or cyanobacteria – are an exception to the usually benign character of NOM. These toxins are harmful to human health.
A wide range of organic substances can enter the water source from human activities in the catchment. These sources include agriculture, runoff from urban settlements, wastewater discharge and leachate from contaminated soils. Most organics in water supplies that have harmful health effects are part of this group. They include pesticides and solvents.
Organic contaminants formed during water treatment include disinfection byproducts formed, for example, when chlorine reacts with natural organic matter.
Disinfection of water, using treatment methods such as chlorination, has removed the threat of waterborne epidemics and reduced infant mortality rates to very low levels in Australia. Without disinfection, Australians would still be at risk from diseases such as cholera.
However, there is a downside to disinfection; the use of oxidants for disinfection, taste, odour and colour removal can produce undesirable organic byproducts.
During chlorination of water supplies, the chlorine reacts not only with the microorganisms but also with most of the other organic material present in the water, either dissolved or in suspension. This produces a range of organic compounds known as disinfection byproducts. The presence of these compounds has been detected only as more and more sensitive scientific equipment has been developed.
These disinfection byproducts contain halogens, a group of elements with similar chemical properties. These halogens are fluorine, chlorine, bromine and iodine. While a lot remains to be known about many of these disinfection byproducts, they include a group of chemicals called trihalomethanes (THMs), mainly chloroform (trichloromethane), plus a broad range of other compounds including haloacetic acids, halonitriles, haloaldehydes and chlorophenols.
In order to keep the level of disinfection byproducts low in the water supply, treatment of raw waters is carried out to remove as much NOM as possible before disinfection. Less NOM means there is less material to form disinfection byproducts and also less chlorine is required to achieve adequate disinfection of the water supply.
Several epidemiological studies have indicated a possible association between chlorinated drinking water and increased risks from a variety of cancers, mainly to do with the bladder, colon and rectum. However, other studies have not found such associations. Therefore, because of the limitations of the data, no definite conclusions can be based on these studies.
Alternative disinfectants - chloramines, chlorine dioxide and ozone – can also react with organic matter in source water to produce disinfection byproducts. The byproducts from these reactions are also not widely understood.
The Australian Drinking Water Guidelines suggest guideline values for a range of disinfection byproducts.
Consumer's Guide to Drinking Water - May 2006