Food
Water filters
Introduction
Most people who treat their water do so to get rid of the taste of chlorine.
Many devices will do the job - but you need to watch for dubious claims.
In this report, we tell you what chemicals and organisms may be present in water and what to consider when you buy a water filter.
Note: this report does not contain test results or brand-specific recommendations.
Safe water
Supplying safe drinking water is the most important task for water authorities. Water catchment areas are increasingly under threat from animals and recreational use, which can contaminate the water, and from logging which results in increasing runoff and sediment in the water. Large-scale conversions to dairying in Southland and Canterbury have raised concerns about pollution of local water supplies.
To comply with the Drinking Water Standards for New Zealand, water supply authorities must monitor for bacteria and demonstrate that treatment can remove protozoan cysts.
Bacteria
Water authorities monitor for the bacterium E. coli, which is found in animal gut. While it's not necessarily harmful, it indicates the potential presence of other more dangerous organisms. Cholera and typhoid are just two of the diseases which can be spread by contaminated water. In New Zealand water supplies, campylobacter is the most common cause of illness.
Chlorination at the treatment plant is the most common means of killing bacteria in source water. But bacteria in the water mains are another threat, so water suppliers try to ensure there's a residue of chlorine to protect the water on its way from the treatment plant to your home.
Because deep underground water sources (such as in Christchurch) are usually protected from animal and human contamination, their water is usually not chlorinated.
Protozoan cysts
Giardia and cryptosporidium belong to a group of microscopic single-celled organisms called protozoa, which live in the intestines of humans and animals. Their cysts are passed out in faeces. If they get into the water source they can cause severe diarrhoea or worse. Cryptosporidiosis may be fatal in people with lowered immunity, and in small children and the elderly.
It's very difficult to check for these organisms - they're not distributed evenly, but exist in the water in clumps. They may settle to the bottom in still water, to reappear in the water flow when disturbed, perhaps by heavy rain in the catchment area.
To comply with the drinking water standards, there must be a level of treatment which can remove the cysts, or the source must be secure from contamination by animals or humans. Chemical coagulation causes particles, including giardia and cryptosporidium cysts, to form a sludge which can be filtered out.
Despite these measures, giardia and cryptosporidium may pop up in water supplies. In 2003 a cryptosporidium scare forced Masterton residents to boil their water.
Local water supplies are required to issue a "boil water" warning if there is a risk of your drinking water becoming contaminated.
Chemicals in your water
At the treatment plant chemicals are added to the water to kill or help remove bugs, or to improve its look, smell or taste. Traces of these chemicals may remain in the water, and some have aroused concerns about possible harmful side-effects. However, the health benefits of these chemicals are regarded as outweighing the possible risk.
To comply with the drinking water standards, authorities must monitor the levels of these chemicals to ensure they remain within World Health Organisation (WHO) guidelines.
THMs
Chlorine can react with organic matter such as decayed vegetation in water to form potentially harmful by-products called trihalomethanes (THMs). The most common THM is chloroform. Some studies have suggested a possible link between chlorinated tap water and a slightly higher risk of some forms of cancer.
Aluminium
The aluminium in drinking water is just a small proportion of our dietary intake. It occurs naturally in river and lake water and is often used in chemical coagulation to remove suspended particles from water, making it clearer. While most of the aluminium can be filtered out of water, small amounts may pass through.
Fears that aluminium in water supplies could cause Alzheimer's disease have not been backed by research.
Fluoride
Many water authorities add fluoride to water, as it has a proven record of reducing tooth decay. It's also an ingredient in many toothpastes. Critics say fluoridated water is unnecessary and that it leads to dental fluorosis (mottled teeth) in people who ingest too much. Some claim that at high levels it can cause cancer, but the WHO has not been able to establish a link. Negatively charged ion exchange resins will remove fluoride.
Natural contaminants
Some chemical contaminants occur naturally, for example, heavy metals in some areas, or nitrates in farming districts. In areas where there is likely to be an unacceptable level of chemical contaminants in the water supply, water authorities are required to carry out regular checks.
Problem supplies
People in rural areas, or with private supplies, may face problems of contamination by agricultural runoff, pesticides and herbicides. In particular, nitrates from fertiliser runoff or sewage may contaminate groundwater supplies.
While nitrate is harmless in itself, it can be converted into nitrite which is hazardous. It mainly poses a problem to babies and young children as it can decrease the blood's ability to transport oxygen, but there have also been claims it is linked to cancer.
Systems which collect water from the roof are vulnerable to contamination by birds and animals. Bore water, particularly shallow bore water, may contain nitrates, pesticides and harmful bacteria.
If you're concerned about the quality of your water, you can check it online. Public libraries also hold copies of the Register of Community Drinking-Water Supplies in New Zealand, published each year by the Ministry of Health.
If the grading suggests there is a problem, or your supply has not been graded, talk to your local water supply authority or to the health protection officer at your local hospital. They can tell you what contaminants are most likely to be present in your supply, and advise you on how to treat them. If you wish to have your water supply tested, they'll recommend a laboratory accredited to carry out the required testing.
For more information on rural water supplies, see Water Collection Tanks and Safe Household Water, a resource from the Ministry of Health.
Carbon filters
For problems such as bad taste or smell, an inexpensive carbon filter may be all you need.
These filters contain a disposable cartridge of activated carbon. Carbon is an extremely porous material: the tiny particles provide an immense surface area to attract and hold impurities. A single gram may have a surface area of more than 10 square metres!
Some grades of carbon will remove impurities such as chlorine by-products (trihalomethanes, or THMs), herbicides, pesticides and other organic chemical compounds. But most just make water taste and smell better.
Some cartridges will work for longer than others, or remove more chlorine from the water. Several factors affect their performance. A poorly designed cartridge may allow water to stream through too quickly. The longer water is in contact with the surface of the filter material, the more chance there is for chemicals to stick to it.
The carbon particle size also matters: the smaller the particle, the more outside surface area is available to attract impurities. That's why it's important to check that the filter you're buying complies with an accepted standard, such as the Australia/New Zealand performance standards for water filters AS/NZS 4348 or AS 3497:1998, or the relevant US National Sanitation Foundation (NSF) standard. See Our advice for more information on these standards.
Bacterial growth
Activated carbon can't remove disease-causing organisms (pathogens). They should be used only on water which has already been treated to make it safe.
If the cartridge is not changed regularly, you may end up making the water even more contaminated. This is because each time the filter is used some water remains in the carbon. Any bacteria in the water will also remain, along with organic substances that get trapped in the filter. Because there is no chlorine left to kill the bacteria, they can grow and multiply, feeding off the trapped organic material. Eventually they emerge in the water from your filter.
Some carbon filters claim to be "bacteriostatic". This does not mean they remove or kill bacteria; they simply contain a chemical, usually silver, to limit bacterial growth. The cartridges need to be replaced every six months or so. It's a good idea to write the installation date or expiry date on the cartridge before installing it. Ideally, there should be a device to warn you that the cartridge is no longer working: for example, an automatic shut-off or a reduction in flow.
For filters which don't provide such a warning, the standards require that the cartridge removes contaminants for twice the claimed capacity.
The performance of the cartridge alone may tell you when it's time for a new one. As impurities build up in the filter material, the filter slows or even stops. As the filter loses its ability to absorb any more chlorine, tastes and smells will probably reappear.
When replacing the cartridge look for one that is specified for the unit - ill-fitting cheaper substitutes may cause leaks.
Other filter options
If you want to buy a filter to remove any chemical contaminants, look for one which complies with NSF standard 53 (Health Effects) or the Australia/New Zealand water filter standards AS/NZS 4348 or AS 3497:1998. The label should list clearly what it will remove.
Filters can be mixed and matched to increase the number of contaminants removed. Activated carbon may be combined with an ion exchange unit to remove some metals and improve taste. Pre-filters are often used to remove sediment so that the filter works better and doesn't clog quickly.
Ion exchange
These filters contain resins which attract either positively charged or negatively charged chemicals. Most models contain a mixture of both types.
Depending on the resins, they can remove positively charged chemicals such as metals or negatively charged metals such as fluoride or nitrate. They will also soften hard water.
On their own they won't remove chlorine or pesticides, and they don't work well when the water contains a lot of sediment, for example in an unfiltered supply. The cartridges require frequent changing.
Reverse osmosis
These filters remove most contaminants. Water is passed under high pressure through a thin membrane. Most contaminants are physically blocked and washed away. Reverse osmosis is slow and wastes a lot of water - for every litre of filtered water four or five litres go down the drain. They're also expensive to buy and run - the fragile membrane needs to be replaced regularly as it can become clogged or damaged and allow bacteria through.
Reverse osmosis filters won't remove THMs and some are damaged by chlorine.
Distillation
A distiller boils the water then condenses the steam, leaving impurities behind. Distillation removes most contaminants, including bacteria and protozoan cysts. It won't remove volatile organic compounds such as THMs unless it is combined with a carbon filter.
A distillation system is expensive to buy and run as it uses quite a bit of electricity. It's also slow and takes up a lot of room on your bench.
Home water treatment options
Brita Marella Cool filter jug
The Brita Marella Cool water filter jug promises "if you can't taste the improvement - your money back".
We put it to the test by carrying out a "blind" taste comparison with Wellington tap water, and three staff members also tried it out at home (each using a different water supply).
How does it work?
The working part of any filter is the cartridge. The jug's Maxtra 4-stage cartridge has an initial mesh filter to remove particles from water, an ion-exchange filter to remove heavy metals such as lead and copper, an activated carbon filter to reduce off-tastes such as chlorine and chlorine by-products, and another mesh filter.
Brita has shown us test evidence that the cartridge will do what it claims.
You need to change the cartridge regularly as the filter's effectiveness declines with use. In the lid of the jug there's an exchange indicator to remind you to get a new cartridge every eight weeks.
The jug won't remove fluoride, nor does it claim to. And it doesn't remove disease-causing organisms such as bacteria, giardia and cryptosporidium.
Better tasting water?
Three staff members compared the jug's filtered water with their local water supplies in Featherston, Otaki and Waikanae. All thought the filtered water tasted better, and two said they'd be likely to buy the jug; the third thought Featherston tap water was OK anyway.
The 13 staff members who compared the water with Wellington tap water all noticed a difference and all except two preferred the filtered water.
Our verdict
The Brita Marella Cool jug improved the taste of our tap water - and it's a lot cheaper than buying bottled water. The jug costs $35 to $40 and replacement Maxtra filters cost $14. That's a running cost of $1.75 per week.
Also ... a warning
Remember the Brita Marella Cool jug's filter doesn't remove disease-causing organisms. So only use the jug if your tap water has already been treated to make it safe.
Our advice
Check the labels on a water filter or any other water treatment device carefully. They should state:
- What the filter is designed for.
- When it should be changed.
- Whether it has been properly tested or certified.
The National Sanitation Foundation (NSF), a statutory body in the United States, tests water filters and certifies those which meet its performance standards. The label will list the contaminants which the cartridge will remove or reduce. The joint Australia/New Zealand standard (AS/NZS 4348:1995) specifies performance requirements for household water filters. However, you're more likely to find NSF standards quoted.
Only a complete assembled filter can be certified by NSF. This is because the interface between the cartridge and housing is critical to filter performance. Keep a record of the cartridge model number when you buy a filter unit, so that you can replace it with the same model.
The effective, really low-cost option
If you want to improve the taste of your water, you don't need to buy it in bottles and you certainly don't need to buy an expensive filter. Just fill a jug from the tap and leave it in the fridge. Some of the chlorine will dissipate and the chilling is often enough to mask any flavour that remains.