Re: RO/DI decision time
<a href=showthread.php?s=&postid=11764404#post11764404 target=_blank>Originally posted</a> by five.five-six
membrane / media price
membrane / media availability
membrane / media longevity
TDS output
GPD output
rejection rate / water wasted
this stuff is co confusing I just do not know where to start
I answer some of your questions a little more specifically since it seems my last response was more of a ramble than anything.
membrane / media price In terms of membranes, price is a tiny factor. You really want to get a 70GPD dow filmtec membrane so just get the cheapest price possible (from a reliable source of course!). The only real other price concern regarding membranes, I think, is whether or not you want to go for a hand tested membrane by spectrapure. See, dow filmtec membranes are rated to remove 98% of dissolved solids (tds) before your DI resin. That is a great thing. However, this is only an AVERAGE. The membranes are not considered defective unless they get below 95% I believe. This is a big deal when it comes to di usage. Normal dow filmtec membranes can be had for about $40. Now if you want someone to hand test your membrane and guarantee you that you will get one of the ones that is ABOVE 98% rejection, then you can pay about $80 and get a tested membrane from spectrapure. The value of this is directly related to how much TDS is i nyour tap water. If you have high tds tap water, the spectrapure membrane will save you money in no time at all since you will use less DI resin. If you have great tap water, then it is certainly not a necessity. You will need to evaluate that on your own. For my high tds (500ppm), it was obviously worth it.
membrane / media availability Membranes are available everywhere and from every RO vendor. This is a cerrtain non issue with any standard RO system. The parts and filters are more or less interchangeable. if you get a system from the Filterguys you can put spectrapure filters in it. If you get a unit from melevsreef you can throw buckeyefield supply units in it also. the only exception might be specialty units like costco unit that I would stay away from for that reason alone (someone correct me if I am wrong). But in general, media availability is not a concern. Customer service is a better concern and all of the RC sponsors will have good service.
Media longevity Better go with media quality instead. Longevity isn't much of an issue with most media b/c you will want to replace the prefilters every 6 months anyways. As previously mentioned, just get high quality prefilters and they will do the job just fine. The longevity of the membrane will be determined by the quality of the prefilters. the longevity of the DI resin will be determined by the quality of the membrane (which is determined by the prefilters again. so we come full circle).
TDS output. All units should give 0 tds. Even the worst of the worst should give you 0 tds. This is because DI resin is very effective at stripping ions from water. The issue is how did we get to 0 tds. Bad units will rely more heavily on the DI resin and so it will need to be changed more often. Good units will rely on the membrane so the operating cost will be cheaper. Initial impressions of cheap units are almost universally good, but within a few years it becomes obvious that the units are more expensive to run because of the heavy DI dependence. I make tons of water, always have 0 tds out, and I do it on the super cheap b/c I use the best membrane possible to do all of the work for me. So the end tds result is almost always 0 tds as long as you are willing to throw enough DI resin at the problem (the goal is not to).
Rejection rate: I described this above. Stick with dow filmtec and you should get around 98% rejection. Avoid 100gpd membranes as they get an average of 96% rejection. Think of that as 2% vs 4% getting through. That's twice as much DI you will use with the 100gpd membrane vs the 70gpd membrane. Your only real other option is the hand tested membrane for a guarantee.
Waste water.
Most high quality units run at about 4 gallons waste to 1 gallon good water. This is pretty much a standard so not a lot you can do. Spectrapure has a new high efficiency unit that runs 1:1 ratio but be prepared to drop about a grand. Filterguys have a water miser system that they claim works well to reduce waste water but it is almost certainly a better idea to use it on low tds tap water rather than high tap tds. Last, if you will use this for drinking water (pressurized storage tank) then a permeate pump will save you a huge amount of water. otherwise, almost all units from the RC sponsors will run at about the same 4:1 ratio.
HTH,
FB
that sounds like what I wan't. I do not want to save $50 upfront just to have problems downstream. I am wondering if I can T the output and run one end to the refrigerator in my kitchen and my ice maker and the other end run to some sort of 
