water systems: MATERIALS ENCYLOPEDIA
Water filters INTRO:
The treatment of water to potable quality is extremely important, and qualified professionals should design filtration and treatment systems if there is any concern that the water source may have harmful contaminants.
Where municipal water supply is available to residents, most jurisdictions require that the service be used. Special permission may be required to use a private water collection and treatment system.
Local municipalities or health authorities regulate privately owned water systems in most jurisdictions. There may be minimal requirements specifying only that a suitable water system be installed, or prescriptive regulations and requirements for testing of water to meet established standards.
A homeowner’s selection of filters and/or treatment systems will vary greatly depending on the quality of the source water. It is important to remember that standards for “safe” drinking water change over time, and that only a small number of thousands of potential contaminants are covered by regulations and detected in testing. Whether a municipal or private water source is being used, filters and treatments may be needed or desired to address issues ranging from taste to concern about particular minerals or contaminants.
All water systems should be subjected to regular testing, whether mandated by law or not. It is in the homeowner’s interest to ensure that their water supply is as clean as it can be. When sending water for testing, request the most comprehensive test available. Basic testing may only be detecting bacteria (coliform and E. coli), and not addressing minerals and heavy metals, VOCs, pesticides and herbicides, PCBs and organic and inorganic chemicals of many types, including those that may be derived from components of the water system itself.
Once thorough testing has been completed, the proper types of filters and/or treatment can be installed.
Filters are rated by pore dimension, measured in microns, or thousandths of a millimeter. The “micron size” of a filter is determined by one of two testing standards that can give different results for filters with the same nominal micron size.
Nominal Micron Rating (NMR) indicates the filter can capture a given percentage of particles of the stated size. For example, a filter might be said to have a nominal rating of 90 percent at 5 micron.
Absolute Micron Rating (AMR) results are obtained by making a single pass of fluid containing particles through a flat sheet of filter material and capturing and measuring the particles in the passed fluid.
Cryptosporidium and Giardia are two common water-borne microorganisms that can reliably be removed by a 1 absolute micron filter. When using filters to remove microorganisms, be sure to get “absolute micron” filters, as the nominal rating means that some percentage will be getting through.
It is not uncommon to use multiple particle filters of descending micron size, to stop large particles from clogging up the finer filters. Screen filtration typically covers sizes 5 microns and larger; micro filtration covers the 0.1 to 1 micron range and ultra filtration from 0.01 to 0.1 micron. Nano filtration is used for anything finer than 0.01 micron, and is rarely used in household water filtration.
Whole house filtration
uses one set of filters through which all water used at every fixture in the home will pass.
Filters require regular inspection and replacement. It can require a good deal of pressure to force water through very fine filters, and some drop in the quantity and pressure of water may be experienced. Clogged filters can seriously restrict water flow.
The material that is caught in a water filter may be biologically active, and may colonize in the filter. If a water system is dormant for a long time or has a high degree of biological activity, change the filters frequently.
Backwash capability in a filter system allows a reverse flow of water to unclog surfaces and flush the freed particulate out of the system. These systems will be more expensive to install but will reduce the cost of filter replacement over the lifespan of the system.
Types of Water filters
Particle or Sediment Filters
The most basic type of filtration removes particulate from the water. These filters may be screen, mesh, woven fabric, string or filaments of various materials. They are most commonly used as the first filters in a series, as they are less expensive and will remove large particles, extending the lifespan of the finer, more expensive filters downstream.
In cases where water is free of microorganisms and other health issues, a sediment filter may be all that is required to remove particulate and suspended solids.
Activated Carbon filters
Carbon filters are a common component of many home water filtration systems. The filter contains “activated” carbon (typically derived from coconut husk), which has been given a positive electrical charge and is good at removing sediment, chlorine and VOCs. The carbon, in either block or granular form, has a large amount of surface area (as much as 80 hectares in 1 kg), and is able to adsorb a great deal of material. These types of filters are not effective at removing salts or minerals.
Carbon filters may be sold at particular micron ratings, and can take the place of sediment filters in some systems. They are more expensive than sediment filters, so if there is a high volume of particulate it would be beneficial to use a less expensive sediment filter in advance of the carbon filter.
Some carbon filters are impregnated with activated silver to stop bacteria from growing in them.
Ceramic filters use a core of diatomaceous earth, a matrix of microscopic shells. This type of filter can remove particles as small as 0.2 micron, providing very effective filtration that can be considered bacteriologically safe.
Ceramic filters have less surface area than carbon filters, and will typically reduce flow rates more than other filter types. They are cleanable and can be washed and reused many times, reducing waste and saving money.
Some ceramic filters are impregnated with activated silver to prevent bacterial growth within the filter.
These filters have been used to clean water for hundreds of years, but have only recently been the focus of a great deal of study. They can offer an inexpensive and accessible form of water filtration that is highly effective.
A biosand filter is a container filled with clean drainage gravel topped with clean sand. A diffuser is used above the sand to prevent incoming water from agitating the sand.
The small pore sizes between the grains of sand work to mechanically trap particles and pathogens as gravity carries the water through the deep sand bed. In a relatively short period of time, an active biosand layer develops in the top 5–10 centimeters (2–4 inches) of the filter. Microorganisms colonize the sand in the upper layers and consume pathogens as they pass through. Pathogens that are not consumed will often complete their life cycle within the sand bed, where they die because of lack of food or oxygen. By the time water reaches the gravel layer and exits the filter to a storage container, it has been significantly cleaned.
Biosand filters are widely used in the developing world, where they provide a low-cost option that can be made with locally available materials to provide filtration on a household or village scale. A growing number of peer-reviewed field and laboratory tests attest to the potential of this type of filtration.* These filters do not remove most salts or metals.
Biosand filters cannot process pressurized water on demand. Water must be allowed to move through the filter at a slow rate, and there must be pause time between dousing. Filtered water trickles into a separate storage container for later use. Some commercial systems mechanize the dousing process to ensure maximum output, while others require the owner to fill the filter as required.
The biosand layer requires some maintenance. If it gets too thick, flow rates will be drastically reduced. Agitating and skimming the surface will restore flow.
There are commercial manufacturers in North America that offer biosand filters in a range of different sizes and flow rates. To date, authorities do not approve these filters as a potable water solution, but they may be used as highly effective “pre-filters” to help extend the lifespan of other filters.