The wastewater from an RO
filter isn't really a big deal. In my area water rates are about $4 per 1000 gallons. I doubt it's that much more expensive where you live. If you are still concerned about it, you can use the waste water for all sorts of things. Watering plants and animals are all great uses of the waste water. Plumbing the wastewater back into the system is possible, but you'd need a pump to re-pressurize it.
As for the difference between RO
: Reverse Osmosis and Deionization are complementary. RO
essentially removes things from the water that are significantly bigger than water. Deionization removes ions(these have a charge) from the water. RO
can remove large organic molecules like NO3
, but has difficulty with smaller ones like NH4. Luckily, NH4 is charged and gets removed by the DI
stage. Here is a great article on the subject.