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05-19-2011, 11:46 PM
Neither RO or DI is particularly effective at removing certain contaminants so it really takes both to do a good job.
RO is only marginally effective at all forms of ammonia including nitrites and nitrates, on the flip side weakly ionized substances like phosphates and silicates are tough for most DI resins.
If you had to choose one it should be a good RO.
I would think twice about the small "portable" RO and RO/DI units, they have major drawbacks. For one they use smaller non standard replacement sediment or prefilters, carbons and DI cartridges. These replacements are harder to find, especially in the low micron ranges reefkeepers prefer for our systems. They also do not last as long due to being smaller and most of all cost more to replace. Try to find a 1 or 0.5 micron prefilter and carbon block for a portable or a 20 oz DI cartridge
Along with that they really are not much if any smaller. They normally use the same top bracket as the full size systems so the footprint is the same, just a few inches shorter so no advantage really. Add to that they do not come with the necessary inline pressure gauge or a TDS meter and most do not have thingsmost of us find useful like an adjustable or capillary tube flow restrictor or a DI bypass valve so you can use RO only water for things like drinking or pet water.
What initially looks like a good deal ends up costing more to operate and often needs upgrades to function as intended. Add up the cost of more frequent replacements a TDS meter and pressure gauge and the cost exceeds the cost of a better full size reef quality system within the first 12 to 18 months. Most of us plan to keep our RO/DI systems for years so the long trerm operating cost is a big issue.
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