Let's say you have a 75 gpd membrane. During winter, you could go with a 50 gpd capillary restrictor, and trim it shorter if needed to get to a 4 to 1. Because nearly all of the restriction occurs in about 1/4 of the motion of a ball valve, they aren't a good choice. An adjustable restrictor/needle valve is an option:
Russ
This might work for folks with soft water sources, but where I am a DI cartridge on its own might only last 30-50 gallons because our water has 200-300ppm calcium out of the tap. Hardness is like around 17-20 dGH in my parts due to limestone bedrock... I used a strictly DI based filter for a while (AP Tap Water Purifier) and it makes about 30 gallons of water from the tap for $30 cartridge. That is completely out of the question and FAR more expensive than an RO/DI system. Water is dirt cheap(edit:relatively cheap compared to dry regions of NA) where I am; Ontario has more freshwater than most of the world, so its pretty inexpensive.
I'm sure it would work just fine in Northern Ontario, where bedrock is granite and the water is soft as a baby's bottom. Moot point though; the water is still pure enough to use straight from the tap in those parts.I'm sure there are certain places where this method would be similar in cost to a RO/DI system.
What I'm trying to say here is that the effectiveness of a system like this is highly dependant on the qualities of your source water.
Also, if you are using large amounts of DI resin - deinfately check out recharging them, its not difficult and can be done many many times to the same resin.
http://reefkeeping.com/issues/2008-09/nftt/index.php
I don’t understand why people waste there time, water, and money with RO filters.
Green maybe, if fresh water is a concern where you live (it isn't everywhere). Anywhere the water goes back into a freshwater system (like the Mississippi or one of the Great Lakes, avoiding water use by using extra chemicals (those needed to make and recharge the resins, for instance) isn't necessarily green.
Cost? Don't agree. You'd be more convincing if you actually had cost numbers to compare, rather than stating something as fact when many of us disagree with it.
The use of an RO membrane cuts down the DI resin costs by a factor of 10-50. In my system, and especially in larger systems, the initial cost of an RO membrane is low compared to the cost of all the resins needed over time. Yes, you may be willing to recharge resins yourself, but you need to figure all the time involved. My time is worth a lot more than I can make back recharging resins myself.
Time? I would instantly assume the opposite if you mean the time an aquarist spends on something (which is the primary time factor, IMO), especially if you recharge the resins yourself. Time needed to make a batch of water is immaterial, if you turn it on and come back the next day to a filled reservoir.
That all said, I think DI plus a carbon block and sediment filter is a fine way to go for many folks. Particularly those with low water needs, or with low TDS water.
Such a system is not as effective at removing everything, but it is adequate for most reefers.
