Activated Charcoal Filters. These systems use replaceable filter cartridges containing granular or powdered block carbon. Water needing treatment passes through the cartridge, coming into contact with the activated charcoal on its way to the faucet. These filters have a limited ability to adsorb contaminants and eventually need to be replaced. For best results, these filters need to have sufficient contact time with the water, so the depth of the filter and the flow rate of water can be important. In addition, activated charcoal becomes a growth medium for bacteria when the filter is saturated with organic contaminants, which supply the food source for the bacteria and when the filter has not been used for a long period of time.
Activated carbon filters remove organic contaminates, such as pesticides, industrial solvents, components of gasoline, radon, and many other organic compounds that cause the water to taste and smell bad.
Reverse Osmosis. This unit is usually installed underneath the kitchen sink. Water passes through a thin membrane with very tiny pores. The system works by applying a high water pressure on one side of the filter, which forces pure water through the membrane. The contaminants accumulate on the other side of the membrane. The pure water is collected for drinking and cooking use. These systems remove all major types of contaminants - nitrates, metals, and organics.
Check into the cost of installation, filter exchanges and the amount of waste water that is generated. Typically, these units use several gallons of water to produce 1 gallon of treated water. Pre- and post-treatment of the water is often needed.
Distillation. This unit treats water by heating it until it forms steam. When water becomes steam, any contaminants in the water are left behind. The steam then cools and condenses to form pure water in a separate storage container. The contaminants are periodically flushed from the unit. Distillers remove inorganic compounds, such as lead and nitrates, and some organic compounds. They also disinfect the water through the boiling process. These units may be found on the countertop, attached to the wall or on a cart.
Disadvantages to this system are high energy use and cost, the heat produced during its use, the loss of beneficial minerals from the water and possibly flat tasting water. Also, care must be taken to avoid bacterial contamination of the storage container.
Ion Exchange. Water softeners are the best known example of ion exchange systems. These systems work by exchanging a compound in the water for a chemical on the filter resin. Calcium and magnesium, which make the water hard, are exchanged for sodium ions attached to the resin surface. Once all the sodium ions have been replaced by the calcium and magnesium ions, the system must be recharged by flushing the system with high amounts of sodium. This creates a great deal of waste water that is discharged to the sewer or septic system. Ion exchange systems have been developed to remove nonhardness chemicals, however recharging the resins is more difficult.