Don't get hung up on stages. A dual DI is very nice but some vendors call the final taste and odor drinking water carbon filter a "stage" and others use two carbons and count them as two stages. In reality all it takes is 4 "stages" if they contain the correct micron rated filters. You need a good prefilter in the 0.2 to 1.0 micron range prefferable one with an absolute rating so it really does what it claims to do. You need a carbon block in the 0.5 to 1 micron range to adsorb chlorine and VOCs for up to 20,000 gallons of normally chlorinated water. You need a good RO membrane in the 98% +/- rejection range, the higher the rejection rate the better as this is what makes your DI resin last longer and work better. This is where the Spectrapure systems with their hand tested and guaranteed membranes shine, mine is averaging 99.25% rejection. Finally you need a full sized 24 oz DI canister and cartridge.
If you want to use RO for drinking water then the carbon taste and odor filter is used, do not use it for reef DI water though as it adds TDS back in. Two carbons are unneeded with a good 1 micron or less carbon block and that includes if you have chloramines, catalytic carbon is nice but really not necessary and if you do use it you want to follow it up with a 0.5 micron carbon blcok or sediment filter to trap the fines as it erodes.
Dual DI will extend the life of resin if you are using normal mixed bed resins as you can rotate cartridges into the lead position as the first one is exhausted and replace the second one. In the case of the MaxCap system the resins are a proprietary blend and they work best with the Max Cap in first and the Silica Buster in last. The Max Cap should go somewhere in the 600+ gallon range or more and the Silica Buster should last through at least 3 MaxCaps before requiring a change so a minimum of 1800 gallons. These numbers are with Phoenix water with a tap TDS of around 800 so most people will see lifespans much greater than this since the average nationwide TDS is only 250 to start with.
kfowler, any RO/DI should be capable of 0 TDS water after DI. The question is how well does the RO membrane work since this is what determines how long the DI lasts. I always tell everyone, test Tap Water TDS, RO only TDS and RO/DI TDS to determine your total packages efficiency not just the DI. If your membrane is only doing 90 or 95% DI will not last, although I think Marcs membranes are capable of at least 96% if not 98%. An important piece of information to remember is, For every 2% you increase the RO membranes efficiency, you DOUBLE the life of the DI resin! This is why I am glad I have a 99.25% efficient membrane with my 800 TDS tap water. My latest numbers are 795 TDS tap, 6.4 TDS RO only and RO/DI way down in the 18+ megaohm range which is much less than an indicated 0 on most TDS meters.
