De-ionization (DI) units come in two basic varieties: mixed bed and separate bed. Two chambers are used in separate bed units, one for anion resins (to filter negatively charged ions), the other for cation resins (to filter positively charged ions). Mixed bed units use a single chamber with a mix of anion and cation resins. DI units are 100% water efficient with no waste water, and are typically rated in terms of grains of capacity (a grain is 0.065 grams). Once the capacity of the unit is reached it either needs to be replaced or recharged (using strong acids and bases). Recharging is normally only an option for separate bed units. Water production rates for DI units varies, but is typically around 10-15 gallons/hour.
Reverse osmosis (RO) units are normally based upon one of two membrane technologies: cellulose triacetate (CTA) and thin film composite (TFC). CTA-based systems are typically cheaper and do not filter as well, while TFC-based systems cost more but have higher pollutant rejection rates (95%-98%). RO filters work by forcing water under pressure against the membrane, which allows the small water molecules to pass through while rejecting most of the larger contaminants. RO units waste a lot of water. The membrane usually has 4-6 times as much water passing by it as it allows though. Unfortunately, the more water wasted, the better the membrane usually is at rejecting pollutants. Also, higher waste water flows are usually associated with longer membrane life. In practice, 300 gallons of total water may be required to produce 50 gallons of purified water.
Like any filter, RO membranes will eventually clog and need replacement. Prefilters are often placed in front of the membrane to help lengthen the lifetime. These filters usually consist of a micron sediment filter and a carbon block filter. The micron filter removes large particles and the carbon filter removes chlorine, large organic molecules and some heavy metals. Of course, the use of prefilters makes the initial unit more expensive but should pay for themselves in longer membrane life. RO units are rated in terms of gallons per day of output with 10-50 gallon/day typically available.
The ultimate in home water purification comes from combining the two technologies and processing the water from an RO unit though a DI unit. With a high grade DI unit, water equivalent to triple distillation purification levels can be achieved. Since the water entering the DI unit can be 50 times purer than tapwater, the DI unit can process 50 times as much before the resins are exhausted, reducing the replacement or recharging cost of the DI unit. Using two DI units in tandem, moving the 2nd in as a replacement for an exhausted 1st unit, and replacing the 2nd unit with a new unit will insure that no undesirable elements "break through" the exhausted 1st unit and enter your supply.
If you can only afford one filter, and waste water is not a concern, then we recommend purchasing a TFC RO unit with pre-filters. If waste water is a concern, or if only a small quantity of make-up water will be required (say, for a single 20 gallon tank), then a DI unit would be the preferred choice.
