Dual carbons are a hold over from decades ago when carbon development and technology was not what it is today. In the old days you used two GAC or granular activated carbon filters to protect the RO mmembrane from chlorine because its all we had and GAC has much less capacity than a carbon block. GAC can be exhausted in as little as 300 total gallons of normally chlorinated water, thats 60 RO/DI gallons and 240 waste gallons at the industry standard 4:1 waste ratio so they used two carbons to make it last longer.
Today the technology is head and shoulders above back then, they can blend multiple types of carbon together when forming steam extruded carbon blocks and get micron ratings in the sub micron range like 0.5 or 0.6 microns. A single 0.5 or 0.6 carbon block can last up to 20,000 gallons, 4,000 treated and 16,000 waste, of normally chlorinated OR chloraminated water without the need for multople cartridges.
The key is keeping the carbon clean so the billions of tiny microscopic pores which adsorb the chlorine and VOC's do not become fouled or plugged renedring it useless. In my experience, a properly designed RO/DI system will have a single, low micron sediment filter of the same or smaller micron size as the single solid carbon block. In my personal RO/DI I run a 0.2 micron absolute rated pleated sediment filter which has 10 the surface area and capacity as a normal spun poly sediment filter so lasts longer and filters better and beleiev it or not has lower headloss than a normal 1 micron sediment filter. The carbon block is a 0.5 micron, usually a Matrikx+1 variety then the RO membrane followed by dual DI with TDS measuring points and ball valves to draw samples after the carbon block, after the RO membrane, after the first DI and on the final RO/DI.
Many vendors still insist on a coarse sediment filter followed by a sacrificial carbon to make up for the poorly performing sediment filter followed by a second usually lower micron carbon which is doing all the work. The problem is the sediment filter is coarse and usually nominal rated so silt and colloidal materials pass right through to the first, often 5 or 10 micron micron carbon which traps some of it but in the process becomes fouled and useless and lets particles pass on to the last line of defense which is working its tail off but not getting much help from the first two filters so is pooped out quickly leading to shorter membrane life and shorter DI life, all adding up to a higher cost of ownership and operating cost over time.
Design it correctly, as a system, start to finish since every piece depends on what is in front of it, and it will last longer, perform better and cost you less in the long run. Everything you remove up front never gets to the membrane or DI so they last longer and perform better. Membranes on many systems last 18-36 months when they can easily last 10 years if kept in good working order. Add up the cost of a membrane every 2 years and the resulting DI replacements due to its lower performance and you can spend a fortune.
You also have additional head loss with each canister, cartridge, tubing and fittings you add so the efficiency or rejection rate of the RO membrane goes down, the higher the pressure the better it performs. Then you have the added cost of all the canisters, filters and fittings plus a larger bracket to hold them all. Another thing I like to mention too is you can see 40 microns with the unaided human eye, so many of the 10 micron or whatever filters are about as effective as a screen door in reality. Which would you rather having protecting your system, a 0.5 micron absolute rated sediment filter and carbon block at $20-$25 replacement cost or a 10.0 or 5.0 micron nominal rated (research the difference between absolute and nominal, it is huge), 5.0 or whatever micron carbon block and 1.0 or 0.6 micron tired as heck carbon block at about the same replacement cost? How many levels of protection does the dual 0.5 micron asbolute version offer versus the other? Once that overworked carbon fails its all over as it is foing the work of three filters plus some since the upstream micron size is so large.
Just something to think about.
This also applies to granular catalytic carbons or so called chloramine carbons which really are not needed, its the DI that is the most important piece not the carbon. A good, clean 1 micron or less carbon block is perfectly fine for the chlorine portion of chloramines, its the ammonia portion that is the biggest threat and where good DI resin and sufficient contact time are critical, not carbon.
