Carbon filters are rated by volume of water treated, since they can hold only a limited amount of sediment. They should be changed regularly to avoid dumping more bacteria and chemicals back into the drinking water and because the filtration slows down when they near the end of their effectiveness. The carbon filter may clean roughly 400–1,000 gallons, and each unit may vary depending on the amount of sediment in the incoming water. A unit should probably be changed at about 75 percent of its maximum capacity for best results. Figure your average daily usage and mark the time for change on your calendar. Activated carbon filters/purifiers, though more expensive than tap water, are usually less expensive than distillers or units that use reverse osmosis.

Reverse Osmosis (RO) is thought by some authorities to be the best way to purify water. Under pressure, usually from the tap, water flows through special membranes with microporous holes the size of the water molecule. These pores allow the water molecule to pass through while rejecting the larger inorganic and organic materials.

Reverse osmosis units usually have two or three filtering mechanisms. First is a sedimentation filter, which merely allows particulate matter to settle. Then comes the RO filter. It is followed by a carbon filter, which removes most any contaminants that may have passed through the RO membrane. With this system, virtually 100 percent of the organic material is removed, along with almost all the minerals.

Reverse osmosis units range from small home units to those of industrial size. Home units can make from three to ten gallons per day. They are energy efficient, as they require only tap water pressure, yet are not water efficient. Until recently, they were very expensive, but now there are good units available at competitive prices. Since the life of the RO filter is usually about five years, the price per gallon of water is approximately 20–30 cents. The carbon filter (and possibly the RO membrane) in this unit should be replaced every year or so, and this is relatively inexpensive. Disadvantages of RO units include their bulky size, the limitation of water production determined by the size of the holding tank (usually one to two gallons), and the time involved to prepare the water for drinking (often three to six hours per gallon). The units produce many gallons of "waste water" per gallon of drinking water because only 10–25 percent of the incoming water goes through the unit; waste can run between 2–30 gallons daily depending on the unit’s efficiency. This is not ideal in droughts, though this waste water can be collected for other uses. RO units may not clear all bacteria and chemicals, though the addition of carbon filtration/purification makes them very efficient. Furthermore, RO units remove almost all minerals (high-calcium waters may clog their filters), which many authorities feel are an important component in our water. Concern over the same hazard of leaching body minerals from drinking distilled water exclusively is not yet well founded scientifically, though people drinking only these waters while fasting run the risk of depleting themselves more rapidly. Deionized water, though, different from RO or distilled, should not be used for drinking as it can deplete body minerals more readily.

Overall, reverse osmosis may be our key filtration system now and in the future, especially with more efficient and economical systems available. Reverse osmosis is best for removing dissolved solids, organic chemicals, and lead and other heavy metals.