WATER FILTRATION AND PURIFICATION
WATER FILTRATION AND PURIFICATION
The most efficient and cost-effective solution for water purity is to treat only the water you plan to consume. A point-of-use water treatment system eliminates the middleman costs associated with bottled water, and can provide purified water for pennies per gallon. Devices for point-of-use water treatment are available in a variety of sizes, designs, and capabilities. Some systems only improve your water's taste and odor. Other systems reduce the various contaminants of health concern. Different systems work most effectively against certain contaminants. A system that utilizes more than one technology will protect you against a broader spectrum of biological pathogens and chemical impurities.
When considering a treatment device, always pay careful attention to the independent documentation of the performance of the system for a broad range of contaminants. You should carefully read the data sheets provided by the manufacturer to verify its claims. Many companies are certified with the National Sanitation Foundation (NSF), whose circular logo appears on their data sheets.
The following is a brief analysis of the strengths and weaknesses of each option.
Mechanical Filtration
Mechanical filtration can be divided into two categories:
Activated Carbon Filtration
Carbon adsorption is the most widely sold method for home water treatment because of its ability to improve water by removing disagreeable tastes and odors, including objectionable chlorine. Activated carbon filters are a very important piece of the purification process, although they are only one piece. Activated carbon effectively removes many chemicals and gases, and in some cases it can be effective against microorganisms. However, generally it will not affect total dissolved solids, hardness, or heavy metals. Only a few carbon filter systems have been certified for the removal of lead, asbestos, VOCs, cysts, and coliform. There are two types of carbon filter systems, each with advantages and disadvantages: granular activated carbon, and solid block carbon.
Activated carbon is created from a variety of carbon-based materials in a high-temperature process that creates a matrix of millions of microscopic pores and crevices. One pound of activated carbon provides anywhere from 60 to 150 acres of surface area. The pores trap microscopic particles and large organic molecules, while the activated surface areas cling to, or adsorb, small organic molecules.
The ability of an activated carbon filter to remove certain microorganisms and certain organic chemicals, especially pesticides, THMs (the chlorine by-product), trichloroethylene (TCE), and PCBs, depends upon several factors:
Granular Activated Carbon
Any granular activated carbon filter has three inherent problems. First, it can provide a base for the growth of bacteria. When the carbon is fresh, practically all organic impurities (not organic chemicals) and even some bacteria are removed. Accumulated impurities, though, can become food for bacteria, enabling them to multiply within the filter. A high concentration of bacteria is considered by some people to be a health hazard.
Second, chemical recontamination of granular activated carbon filters can occur in a similar way. If the filter is used beyond the point at which it becomes saturated with the organic impurities it has adsorbed, the trapped organics can release from the surface and recontaminate the water, with even higher concentrations of impurities than in the untreated water. This saturation point is impossible to predict.
Third, granular carbon filters are susceptible to channeling. Because the carbon grains are held (relatively) loosely in a bed, open paths can result from the buildup of impurities in the filter and rapid water movement under pressure through the unit. In this situation contact time between the carbon and the water is reduced, and filtration is less effective.
To maximize the effectiveness of a granular activated carbon filter and avoid the possibility of biological or chemical recontamination, it must be kept scrupulously clean. That generally means routine replacement of the filter element at six- to twelve-month intervals, depending on usage.
Solid Block Carbon
These are created by compressing very fine pulverized activated carbon with a binding medium and fusing them into a solid block. The intricate maze developed within the block ensures complete contact with organic impurities and, therefore, effective removal. Solid block carbon filters avoid the problems just discussed with granular carbon filters.
Block filters can be fabricated to have such a fine porous structure that they filter out coliform and associated disease bacteria, pathogenic cysts such as giardia, and lighter-weight volatile organic compounds such as THMs. Block filters eliminate the problem of channeling. Also, they are so dense that they do not allow the growth of bacteria within the filter.
Compressed carbon filters have two primary disadvantages compared with granular carbon filters. They have smaller capacity for a given size, because some of the adsorption surface is taken up by the inert binding agent, and because they tend to plug up with particulate matter. Thus, block filters may need to be replaced more frequently. In addition, block filters are substantially more expensive than granular carbon filters.
Limitations of Carbon Filters
To summarize, a properly designed carbon filter is capable of removing many toxic organic contaminants, but it will fall short of providing protection against a wide spectrum of impurities.
Reverse Osmosis (Ultrafiltration)
Reverse osmosis (RO) is a water purification technology that utilizes normal household water pressure to force water through a selective semi-permeable membrane that separates contaminants from the water. Treated water emerges from the other side of the membrane, and the accumulated impurities left behind are washed away. Eventually, sediment builds up along the membrane and it needs to be replaced.
Reverse osmosis is highly effective in removing several impurities from water: total dissolved solids (TDS), turbidity, asbestos, lead and other heavy metals, radium, and many dissolved organics. RO is less effective against other substances. The process will remove some pesticides (chlorinated ones and organophosphates, but not others), and most heavier-weight VOCs. However, RO is not effective at removing lighter-weight VOCs, such as THMs (the chlorine by-product) and TCE (trichloroethylene), and certain pesticides. These compounds are either too small, too light, or of the wrong chemical structure to be screened out by an RO membrane.
Reverse osmosis and activated carbon filtration are complementary processes. Combining them results in the most effective treatment against the broadest range of water impurities and contaminants. Many RO systems incorporate both a pre-filter of some sort, and an activated carbon post-filter.
RO systems have two major drawbacks. First, they waste a large amount of water. They'll use anywhere from 3 to 9 gallons of water per gallon of purified water produced. This could be a problem in areas where conservation is a concern, and it may be slightly expensive if you're paying for municipal water. On the other hand, this wastewater can be recovered or redirected for purposes other than drinking, such as watering the garden, washing the car, etc. Second, reverse osmosis treats water slowly: it takes about 3 to 4 hours for a residential RO unit to produce one gallon of purified water. Treated water can be removed and stored for later use.
Other Treatment Processes
There are a couple of other water treatment processes that you should know about. Distillation is a process that creates clean water by evaporation and condensation. Distillation is effective against microorganisms, sediment, particulate matter, and heavy metals; it will not treat organic chemicals. Good distillers will have a carbon filter to remove organic chemicals. Ultraviolet (UV) systems use UV light to kill microorganisms. These systems can be highly effective against bacteria and other organisms; however, they may not be effective against giardia and other cysts, so any UV system you buy should also include a 0.5-micron filter. Other than some moderately-expensive solar-powered distillers on the market, any distiller or UV unit will require a power source.
A Final Word on Water
Our best advice for those of you considering a drinking water treatment system has three simple points.
These actions will enable you to purchase a system that will most effectively meet your particular needs for safe drinking water.
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