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What is the maximum amount of different bacteria in the drinking water in Europe?

What is the maximum amount of different bacteria in the drinking water in Europe?


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I am looking for a statistical amounts which are allowed. Some students say it is 0 for all bacteria, which I think is false. I found this USA source.

I found there

  • Total Coliforms (including fecal coliform and E. Coli) mg/l. No more than 5.0% samples total coliform-positive (TC-positive) in a month.
  • Giardia lamblia: 99.9% removal/inactivation.

What is the right place to look for such information?


No, your colleages are right: There should be no coliforms in drinking water in Europe following the Council Directive 80/777/EEC. This PDF from the Northern Ireland Environment Agency (which follows the directice mentioned above) shows this a bit nicer. It makes sense, since coliforms are a sign of mixing sewage and fresh water. Nothing you want to have happening.

This is the relevant table from the linked PDF:


NSF Standards for Water Treatment Systems

While no federal regulations exist for residential water treatment filters, purifiers and reverse osmosis systems, voluntary national standards and NSF International protocols have been developed that establish minimum requirements for the safety and performance of these products to treat drinking water. The standards and protocols are explained in detail below. The numbers in the names reflect the order in which the standard or protocol was developed and are not a ranking or rating system.

  • NSF/ANSI 42
    Filters are certified to reduce aesthetic impurities such as chlorine and taste/odor. These can be point-of-use (under the sink, water pitcher, etc.) or point-of-entry (whole house) treatment systems.
  • NSF/ANSI 53
    Filters are certified to reduce a contaminant with a health effect. Health effects are set in this standard as regulated by the U.S. Environmental Protection Agency (EPA) and Health Canada. Both standards 42 and 53 cover adsorption/filtration which is a process that occurs when liquid, gas or dissolved/suspended matter adheres to the surface of, or in the pores of, an adsorbent media. Carbon filters are an example of this type of product.
  • NSF/ANSI 44
    Water softeners use a cation exchange resin that is regenerated with sodium or potassium chloride. The softener reduces hardness caused by calcium and magnesium ions and replaces them with sodium or potassium ions.
  • NSF/ANSI 55
    Ultraviolet treatment systems use ultraviolet light to inactivate or kill bacteria, viruses and cysts in contaminated water (Class A systems) or to reduce the amount of non-disease causing bacteria in disinfected drinking water (Class B).
  • NSF/ANSI 58
    Reverse osmosis systems incorporate a process that uses reverse pressure to force water through a semi-permeable membrane. Most reverse osmosis systems incorporate one or more additional filters on either side of the membrane. These systems reduce contaminants that are regulated by Health Canada and EPA.
  • NSF/ANSI 62
    Distillation systems heat water to the boiling point, and then collect the water vapor as it condenses, leaving behind contaminants such as heavy metals. Some contaminants that convert readily into gases, such as volatile organic chemicals, can carry over with the water vapor.
  • NSF/ANSI 177
    Shower filters attach directly to the pipe just in front of the homeowner’s showerhead and are certified to only reduce free available chlorine.
  • NSF/ANSI 244
    The filters covered by this standard are intended for use only on public water supplies that have been treated or that are determined to be microbiologically safe. These filters are only intended for protection against intermittent microbiological contamination of otherwise safe drinking water. For example, prior to the issuance of a boil water advisory, you can be assured that your filtration system is protecting you from intermittent microbiological contamination. The standard also includes material safety and structural integrity, similar to other NSF/ANSI drinking water treatment unit standards. Manufacturers can claim bacteria, viruses and cysts reduction for their filtration system.
  • NSF/ANSI 401
    Treatment systems for emerging contaminants include both point-of-use and point-of-entry systems that have been verified to reduce one or more of 15 emerging contaminants from drinking water. These emerging contaminants can be pharmaceuticals or chemicals not yet regulated by the EPA or Health Canada.
  • NSF P477
    These point-of-use filters reduce microcystin (toxins produced by blue-green algae) below the health advisory set by the EPA.
  • NSF P473
    PFOA/PFOS water filters or systems are evaluated on their ability to reduce PFOA and PFOS in drinking water and to meet strict material safety and structural requirements as defined in NSF/ANSI 53.
  • NSF P231
    Microbiological water purifiers are certified for health and sanitation based on the recommendations of the EPA’s Task Force Report, Guide Standard and Protocol for Testing Microbiological Water Purifiers (1987) (Annex B).
  • NSF/JWPA P72
    Iodine radioisotope point-of-use treatment options are evaluated for reduction of all forms of iodine in drinking water. This protocol was developed in conjunction with the Japan Water Purifier Association (JWPA).

Keep in mind that certification to an NSF/ANSI standard or protocol does not mean that a filter, purifier or treatment system will reduce all possible contaminants. It’s important to verify that the filter, purifier or treatment system is certified to the applicable standard for the reduction of the contaminants of most concern to you or your family.


Colony Counts: The Microbiology of Drinking Water

In this issue of the Water Industry Journal, we gain an insight into how colony counts are used to ensure we have wholesome drinking water on tap. Shaun Jones, Public Health Manager at Wessex Water and Chair of Standing Committee of Analysts for Microbiology, talks about drinking water colony counts.

“The provision of safe and wholesome drinking water is a principal objective of the water industry, such is its importance to the health and wellbeing of our society. Water companies invest massive sums of money and work tirelessly to achieve this reliably.

“With global statistics often attributing the cause of over half a million deaths each year to the consumption of water contaminated with diarrhoeal disease-causing pathogens like, cholera, dysentery and typhoid [1] , perhaps the safety of drinking water supplies is more synonymous with microbiological quality than any other possible character.

“In England and Wales, we enjoy and almost take for granted our share of the 16,000 million litres of drinking water that somewhat underwhelmingly appears when we turn on the tap at home.

“Unlike elsewhere across the world, our drinking water is unlikely to make us sick and reaches an enviable regulatory standard that is greater than 99.9% compliant with the European Union (EU) Drinking Water Directive [2].

“Despite this, during 2017 over one million microbiological tests were carried out on public water supplies across England and Wales to verify its safety and compliance with regulations – approximately 2,900 tests every day. [2]

“These tests include for Escherichia coli, other coliform bacteria, enterococci, Clostridium perfringens. In addition to these tests, drinking water microbiologists also determine colony count – the subject of this very article.

“Out of all the drinking water microbiology tests I carried out as a bench analyst, the colony count tests were perhaps the most interesting.

“Also known as ‘total viable count’, ‘standard plate count’ or ‘heterotrophic plate count’, I much prefer the name ‘colony count’ – not only is it the regulatory name for the test it

also describes precisely what microbiologists currently spend much of their time doing counting colonies!

“Bacteria are single cell microorganisms that divide to grow in number one divides to make two, then numbers accumulate rapidly 2-4-8-16-32 etc. Given enough time the sheer number of cells is such that the collective mass appears as a visible colony that can be observed and counted.

Escherichia coli can divide every 20 minutes under laboratory conditions. This means that a single cell will produce a phenomenal 47,000,000,000,000,000,000,000 cells after just 24 hours. Whilst you can’t ordinarily see bacteria, when grown in a clump like this it’s no wonder they can be visualised in the form of a colony.

“The completely weird thing about a colony, which I find forever fascinating, is that its shape, size and colour is different for different bacteria. So much so, microbiologists use the morphology of colonies as a diagnostic tool to identify the bacteria present. Why this happens is as much as mystery as their shape and form at the microscopic level which again is different for different bacteria – how tremendously helpful!

“The role of the microbiologist is to provide the right nutrients, atmosphere and temperature to allow the bacteria of interest to grow as fast as possible, often in preference to the other bacteria present in the sample.

“Agar is a generic term for the solid nutrient growth media used to grow bacteria which usually all takes place within a petri dish – transparent circular dishes used to culture of microorganisms that you might otherwise remember from school. The variety of agar available corresponds to each of the different bacteria we can grow.

“Drinking water microbiologists continue to use agar and petri dishes to grow bacteria in order to determine their concentration in the sample examined. However, technology seeks to change this century old approach.

“Quantifying the number of microbes in a water sample is not straightforward, even for the most skilled microbiologist. There are two principal concepts which highlight these limitations:

The first is the concept of viability, which, in the context of this article, is the ability of a bacterium to grow (multiply) significantly under laboratory conditions so that a visible colony appears following incubation.

Secondly, that the colony may have been formed from the growth of a single bacterium or a cluster of bacteria.

“These limitations mean that only the viable fraction of the population of bacteria as a whole appear as colonies. The absence of colonies may not therefore necessarily translate to the absence of bacteria in the sample – the reason why test results are often reported as ‘not detected’ and why so many tests are carried out each year.

“Therefore, it is impossible to know the precise number of bacteria in a sample and collectively gives rise to the use of the unit of measure for such tests – ‘colony forming unit’ usually expressed as per unit volume examined.

“Incidentally, this is part of the reason why drinking water safety planning risk assessment is promoted as the best means to ensure supplies are reliably wholesome and safe.

“Anyway, returning to the colony count test and why I find them a little quirky.

“Although the 1998 Drinking Water Directive required colony counts to be determined at 22°C, the Water Supply (Water Quality) Regulations required water companies to carry out two colony count tests one at 22°C with incubation for three days and the other 37°C with incubation for two days [3] .

“Since 2016 and the amendments to the Water Supply (Water Quality) Regulations, water companies are now only required to test for colony counts at 22°C.

“The standard that water companies must attain is ‘no abnormal change’ and, to my knowledge, no water company has ever contravened this standard. Having said that, as a former DWI Inspector, I know with certainty colony count results are used in the consideration of enforcement action and in this regard water companies use them to monitor and identify changes to long term trends.

“To better identify water quality trends and avoid the limitations of the traditional culture method, water companies have been exploring using state of the art technology [4] . The technology isn’t new but has rather re-materialised in a more accessible format due to the vastly improved capital cost and footprint.

“Flowcytometry, is not reliant on the viability of microbes or the visualisation of colonies but is a direct measurement of the number of bacteria present. The clever part about flow cytometry is that the cells in a sample are streamed past a laser beam in single file scattering the light in a characteristic way. The scatter is measured by a detection and computer system and the cells characterised on a chart.

“This technique has been further enhanced by the use of fluorescent dyes such SYBR Green and Propidium Iodide. When used in combination, water companies are able to count the entire bacterial population and determine the number of viable and damaged bacterial cells.

“This state of the art method provides a much more realistic picture of water quality than the traditional culture method and only takes less than one hour compared to the 24 hours of growth needed for visible colonies to form.

“There is a lot of good work being carried out by the water industry to refine the flow cytometry technology further, including to create better fluorescent markers that would allow us to also identify and count the number of different types of bacteria present.

“Some of the work to create a standardised flow cytometry method is being carried out through the Standing Committee of Analysts [5] and may ultimately lead to a new regulatory standard, like it has been elsewhere around the world.

“Back at home, for now, water companies will continue to utilise the technique to further improve the quality of our drinking water supplies alongside traditional microbiology.”

[2] DWI – Chief Inspector’s Report 2017

[3] The Microbiology of Drinking Water (2002) – Part 1 – Water Quality and Public Health

[4] Water Res. 2014 Nov 1565:224-34. Assessing microbiological water quality in drinking water distribution systems with disinfectant residual using flow cytometry.


Water Disinfection with Chlorine and Chloramine

Water comes from a variety of sources, such as lakes and wells, which can be contaminated with germs that may make people sick. Germs can also contaminate water as it travels through miles of piping to get to a community. To prevent contamination with germs, water companies add a disinfectant&mdashusually either chlorine or chloramine&mdashthat kills disease-causing germs such as Salmonella, Campylobacter, and norovirus.

Chlorine and chloramine are the major disinfectants used in public water systems.

You can find out whether there is a disinfectant in your water, what kind of disinfectant is used, and how well your utility has followed the rules about disinfection by obtaining a copy of your utility&rsquos consumer confidence report .

Most communities use either chlorine or chloramines. Some communities switch back and forth between chlorine and chloramines at different times of the year or for other operational reasons. Less commonly, utilities use other disinfectants, such as chlorine dioxide. Some water systems that use water from a groundwater source (like community wells) do not have to add a disinfectant at all.

What is chlorination?

Chlorination is the process of adding chlorine to drinking water to kill parasites, bacteria, and viruses. Different processes can be used to achieve safe levels of chlorine in drinking water. Using or drinking water with small amounts of chlorine does not cause harmful health effects and provides protection against waterborne disease outbreaks.

Are there any health issues associated with chlorine?

Your water company monitors water quality regularly to provide you with safe drinking water. Some people are more sensitive than others to chemicals and changes in their environment. Individuals who have health concerns should seek medical advice from their healthcare provider before contacting their local health department external icon .

Does chlorine affect patients during dialysis?

During dialysis, large amounts of water are used to clean waste products out of a patient&rsquos blood. Dialysis centers must treat the water to remove all chemical disinfectants, including chlorine and chloramine, before the water can be used for dialysis. Home dialysis users should consult the machine manufacturer for instructions on how to properly treat their water before use.

What are safe levels of chlorine in drinking water?

Chlorine levels up to 4 milligrams per liter (mg/L or 4 parts per million (ppm)) are considered safe in drinking water external icon . At this level, harmful health effects are unlikely to occur.

Will chlorine affect my water&rsquos taste or smell?

Chlorinated water can taste and smell different than untreated water. Some people like the taste and smell of chlorinated water, and others do not. Taste and smell problems may arise depending upon the water quality and amount of chlorine in the water.

Will chlorine affect my pets?

Chlorine and chloramine are toxic to fish, other aquatic animals, reptiles, and amphibians. Unlike humans and other household pets, these types of animals absorb water directly into the blood stream. Don&rsquot keep these animals in water that contains these disinfectants. Chlorine can be removed from water by letting it sit out for a few days or by buying a product at your local pet store that removes the chlorine. Ask your local pet store about methods of removing disinfectants from water for these pets.

The small amount of chlorine added to water will not affect other pets (such as mammals and birds) and can be used regularly for watering and bathing animals.

Why is my water provider temporarily switching from chloramine to chlorine disinfection?

The U.S. Environmental Protection Agency (EPA) allows drinking water treatment plants to use chloramine and chlorine to disinfect drinking water. Water system pipes develop a layer of biofilm (slime) that makes killing germs more difficult. Water providers may temporarily switch from chloramine to chlorine disinfection to help remove this slime layer.

Is chlorine treatment new?

Chlorine was first used in the United States as a major disinfectant in 1908 in Jersey City, New Jersey. Chlorine use became more and more common in the following decades, and by 1995 about 64% of all community water systems in the United States used chlorine to disinfect their water.

What is chloramination?

Chloramination is the process of adding chloramine to drinking water to disinfect it and kill germs. It is sometimes used as an alternative to chlorination. Chloramines are a group of chemical compounds that contain chlorine and ammonia. The particular type of chloramine used in drinking water disinfection is called monochloramine which is mixed into water at levels that kill germs but are still safe to drink.

Are there any health issues associated with chloramine in water?

Studies indicate that using or drinking water with small amounts of chloramine does not cause harmful health effects and provides protection against waterborne disease outbreaks. These studies reported no observed health effects from drinking water with chloramine levels of less than 50 milligrams per liter (mg/L) in drinking water. A normal level for drinking water disinfection can range from 1.0 to 4.0 mg/L.

Your water company monitors water quality regularly to provide you with safe drinking water. Some people are more sensitive than others to chemicals and changes in their environment. Individuals who have health concerns should seek medical advice from their healthcare provider before contacting their local health department. Contact your local health department for more information external icon .

What are safe levels of chloramine in water?

Chloramine levels up to 4 milligrams per liter (mg/L) or 4 parts per million (ppm) are considered safe in drinking water. At these levels, harmful health effects are unlikely to occur.

Does chloramine affect patients during dialysis?

During dialysis, large amounts of water are used to clean waste products out of a patient&rsquos blood. Dialysis centers must treat the water to remove all chemical disinfectants, including chlorine and chloramine, before the water can be used for dialysis. Home dialysis users should consult the machine manufacturer for instructions on how to properly treat their water before use.

Why is my water provider switching from chlorine to chloramine disinfection?

The U.S. Environmental Protection Agency (EPA) allows drinking water treatment plants to use chloramine and chlorine to disinfect drinking water. Research shows that chloramine and chlorine both have benefits and drawbacks.
Chlorine is a highly effective method of disinfection. However, while in the pipes it produces small amounts of chemicals (called &ldquodisinfection by-products&rdquo) if the source water has higher levels of dirt or germs that may react with chlorine.

Chlorine is also used up quickly in water systems. Sometimes there is not enough chlorine left to kill germs in the water by the time it reaches the end of the pipes. Chloramine can last longer in the water pipes and produces fewer disinfection by-products. To meet EPA standards intended to reduce disinfection by-products, some water utilities are switching to chloramine.

Will chloramine affect my water&rsquos taste or smell?

If you notice any change in the taste or smell of your water, it may be that the water treated with chloramine has less of a &ldquochlorine&rdquo taste and smell than water treated with chlorine.

Will chloramine increase the amount of lead or copper in my drinking water?

Chloramine can change the chemical properties of the water, which can affect lead and copper pipes. Lead and copper levels are strictly regulated in drinking water by the EPA Lead and Copper Rule external icon . EPA provides guidance for local water authorities switching to chloramine on how to minimize lead and copper levels.

If you are concerned about lead or copper levels in your household water, call EPA&rsquos Safe Drinking Water Hotline at 800-426-4791 for testing information.

Will chloramine affect my pets or plants?

Chlorine and chloramine are toxic to fish, other aquatic animals, reptiles and amphibians. Unlike humans and other household pets, these types of animals absorb water directly into the blood stream. Don&rsquot keep these animals in water that contains these disinfectants. Unlike chlorine, chloramine cannot be removed by letting water sit out for a few days. However, products are available at aquarium supply stores that can remove chloramine. Ask your local pet store about methods of removing disinfectants from water for these pets.

The small amount of chloramine added to water will not affect other pets (such as mammals and birds) and can be used regularly for watering and bathing animals.


Rainwater Collection

Collecting and using rainwater can be a great way to conserve resources. Some people use rainwater for watering plants, cleaning, bathing, or drinking. However, it is important that the rainwater system is maintained properly and the water quality is appropriate for the intended use.

Germs and other contaminants are found in rainwater.

While useful for many things, rainwater is not as pure as you might think, so you cannot assume it is safe to drink. Rain can wash different types of contaminants into the water you collect (for example, bird poop on your roof could end up in your water barrel or tank). Rainwater can carry bacteria, parasites, viruses, and chemicals that could make you sick, and it has been linked to disease outbreaks.

The risk of getting sick from rainwater may be different depending on your location, how frequently it rains, the season, and how you collect and store the rainwater. Dust, smoke, and particles from the air can contaminate rainwater before it lands on your roof. Roofing materials, gutters, piping, and storage materials can introduce harmful substances such as asbestos, lead, and copper into the water. Dirt and germs can be washed into collected rainwater from the roof, especially when rain follows several days of dry weather.

Prevent illness

To lower your risk of getting sick, consider using rainwater only for uses such as watering plants that you don&rsquot eat or washing items that are not used for cooking or eating. Avoid using rainwater for drinking, cooking, brushing your teeth, or rinsing or watering plants that you intend to eat. Instead, use municipal tap water if it is available, or purchase bottled water for these purposes.

If you have a weakened immune system, you should be especially careful when choosing your drinking water source. Discuss this with your healthcare provider.

Rainwater might not be safe for household use without additional treatment.

Before using collected rainwater for drinking, bathing, or cooking, consider whether treatment is needed to make it safe. Testing the water can determine if there are harmful germs, chemicals, or toxins in it. Water treatment options include filtration, chemical disinfection, or boiling. Filtration can remove some germs and chemicals. Treating water with chlorine or iodine kills some germs but does not remove chemicals or toxins. Boiling the water will kill germs but will not remove chemicals. Using a simple device called a &ldquofirst flush diverter&rdquo to remove the first water that comes into the system may help avoid some of these contaminants. The amount of water that should be removed by a first flush diverter depends on the size of the roof feeding into the collection system.

Consider adding a screen to the water inlet or emptying the rain barrel at least every 10 days to prevent mosquitoes from using the rain barrel as a breeding site.

Some people add purchased, treated water to the rainwater they collect in their cistern. This may make the treated water less safe.

Regularly test your collected rainwater and maintain your rainwater system.

If you collect and store rainwater for drinking, you have an individual water system and are responsible for ensuring that your water is safe. You should have your water and your system tested regularly and maintain the system external icon properly. When rainwater is used as a supplemental water source, homeowners should ensure that rainwater cannot enter pipes containing safe drinking water. Contact your state or local health department for more information.

Check local regulations and guidance.

Your local health authorities might provide additional guidance on safely collecting rainwater. Rainwater collection is not allowed in all places. Some states consider rainwater the property of the state and regulate its collection, so you should consult with your local government (for example, your environmental quality department external icon or health department) before proceeding.


What is the problem with swimming in water that doesn’t meet water quality standards?

Primary contact with water containing high levels of faecal bacteria and other pollutants can lead to disease, infection, and rashes. Swallowing contaminated water can lead to gastrointestinal infection such as nausea, diarrhoea and vomiting. More serious diseases and illnesses may also be contracted in heavily polluted waters including typhoid fever, hepatitis, gastroenteritis, and dysentery.

Even when you are on or near but not “under” the water, there is still a risk to your health from touching and inhaling water contaminated with bacteria and other pollutants. Skin contact with contaminated water can cause rashes and other skin problems. Bacteria can enter the body through openings, such as ears, nose, eyes, and through cuts and scrapes in the skin. Furthermore, inhaling contaminated water can lead to respiratory illnesses, and infections in the eyes, ears, and nose.

Anyone can be affected from exposure to contaminated water, but small children, the elderly, and people with compromised immune systems are the most vulnerable.


Different water disinfectants produce different types of disinfection byproducts. In addition, the presence and amount of different inorganic and organic matter, the water temperature and pH, and the dose of disinfectant all contribute to the formation and variation of the DBPs. The table below lists the major DBPs produced by different disinfectants in drinking water treatment.

Water disinfectantsDisinfection byproducts
ChlorineChlorate, Trihalomethane (THM), Haloacetic Acids
ChloramineChlorate, Trihalomethane (THM), N-nitrosodimythlamine (NDMA)
Chlorine dioxideChlorite, Chlorate
Ozone*Bromate

* In water containing bromide and organic bromine species


Fragmentation of Regulations

In order to implement the original intentions of the EAS, France, Netherlands, the United Kingdom and Germany agreed in 2007 to pursue a common approach for testing and assessing products in contact with drinking water on a voluntary basis. This program is generally known as the Four Member States (4MS) Initiative. Other countries have also become involved to one degree or another.

In 2011, the 4MS signed a formal declaration of intent. Up to now, several harmonized (among the 4MS) approval systems for materials and products in contact with drinking water have been developed, although these have not been systematically implemented at the national level.

Furthermore, in 2014, Denmark, Finland, Norway and Sweden began the MAID project—an initiative focused on material and product innovation through knowledge-based standardization in the drinking water sector. The first report, Nordic Drinking Water Quality, and the second report, Regulations and Approval Systems in the Nordic Countries, were published in November 2017.

During the past few years, there have been reports of other member states interacting with the above groups, plus some evaluating their own standards. The result has been a chaotic situation with little or no progress toward the standardization of materials throughout Europe.


6 - Chemistry, microbiology and biology of water

This chapter discusses some of the physical and chemical characteristics of water and their significance. The WHO guideline values are of worldwide application. The EC Directive applies only to member states of the European Community. The Directive sets numerical standards for health related chemical parameters, along with standards for a number of indicator parameters. The unit of measurement and the notation for a particular parameter can vary between the above standards. The WHO Guidelines and the European standards use a mixture of mg/l and μg/l, while the USEPA standards are mainly in mg/1. The levels of monitoring and analysis required for chemical parameters are described. It examines the microbiology of water and the most common waterborne diseases the requirements for the bacteriological quality of drinking water and the testing of water for pathogenic organisms. It is of paramount importance that correct procedures are followed when taking water samples to ensure that the samples are representative. Whenever possible, samples should be taken by trained and experienced personnel using dedicated sampling bottles and equipment. Water biology is described in terms of the significance of macro organisms on water quality and new areas of concern in respect of drinking water quality.


US EPA

The Safe Drinking Water Act defines the term "contaminant" as meaning any physical, chemical, biological, or radiological substance or matter in water. Therefore, the law defines "contaminant" very broadly as being anything other than water molecules. Drinking water may reasonably be expected to contain at least small amounts of some contaminants. Some drinking water contaminants may be harmful if consumed at certain levels in drinking water while others may be harmless. The presence of contaminants does not necessarily indicate that the water poses a health risk.

Only a small number of the universe of contaminants as defined above are listed on the Contaminant Candidate List (CCL). The CCL serves as the first level of evaluation for unregulated drinking water contaminants that may need further investigation of potential health effects and the levels at which they are found in drinking water.



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