I'm trying to price out both purchasing costs and operating costs of various RO/DI units and I have a few questions.

I'm partial to the MaxCap 90 GPD unit at this point in my research.

Before I ask my questions, this info might help...
My home water supply ranges from 180-210 TDS based on the past 4 years of municipal water quality reports. Chlorine is used here as opposed to chloramine (if that matters). Unfortunately there is no mention of the silica levels in any of the reports. Also, my home water pressure is great and the water temp of my tap water is in the mid to high sixties F during the summer and barely 40F during the coldest part of the winter (I know that might be significant).

My questions:

What's the difference between the 10" and 20" cartridges?

How long can I expect the cartridges to last, in gallons produced, (prefilter, carbon, RO, DI-1, DI-2)? Just trying to get an idea of how often I'll have to replace each. I couldn't find it anywhere on their website but I may have missed it.

Do I really need to spend the money on a MaxCap or would a premium unit from Buckeye Field Supply suffice? Also, if I get a Buckeye unit, once the cartridges need replacing can I replace them with the better-quality MaxCap filters if I choose? I know with the Buckeye unit I lose the convenience of Dual in-line TDS meters and the extra DI silicate removal resin. I'm just wondering if the price of the MaxCap is worth it in the end.


Thanks for your help

The larger cartridges will be able to handle much higher water volumes compared to the 10". As far as how long any given set of cartridges will last, that is extremely variable. Sediment blocks are changed (or in some cases, cleaned) when the flow pressure drops. Carbon blocks are also variable, but usually have volume ratings in usual chlorine levels--change them every 6 months or sooner if chlorine is measurable after the cartridge(s) or if it reduces pressure. Membrane and DI life is dependent on TDS. For example, in my area, TDS is around 20-40 and cartridges last the full 6 months without clogging. Membranes last for well over 5 years. DI lasts about as long or longer. Don't expect the same with your water, though. For resins, at least, every 2% increase in TDS decreases life by 50%. It is all very dependent. That being said, the MaxCap is a top-notch system and Spectrapure is the absolute best in the business, as far as RO/DI water purification is concerned. Other units are potentially just as good, but you don't have the guaranteed 98% rejection on the membranes, as well as their proprietary resin mixes. The bottom line is that the resins are higher quality and the membrane is guaranteed. You'll also get comparatively more water out of the MaxCap before things must be changed out. All that being said, if you know where to look, you can replicate most of those results with cheaper units.

Edit: I should point out that the unit itself doesn't matter so much. What does matter is the filters you use, which is what I meant about replicating results. Semiconductor grade resins, along with silicate resins, can be found at thefilterguys.biz, whereas the best pre-filtration cartridges can be found at buckeye. Of course, you can also purchase all of these from Spectrapure for a relatively small amount more (say $1-2).

We have had nothing but semi-conductor grade resin for about the last year or so. The difference between semiconductor grade and nuclear grade is negligible for purposes of this hobby, so we've not bothered to change the text on our webpage.

We also have special di cartridges including the silica buster cartridges.

Russ

Thanks for all of the advice. Maybe Buckeye Field Supply can answer some specifics about their unit or point me to another resource/thread where I'm sure they've already answered these questions...

How long can I expect your resins, cartridges and membranes to last given my tap water parameters?

A good rule of thumb is to replace your sediment filter and carbon block after six months. A more precise way to maximize the useable life of these two filters is to use a pressure gauge to identify when pressure reaching the membrane starts to decline. This is your indication one or both of the filters is beginning to clog.

Also be cognizant of the chlorine capacity of the carbon block. A good 0.5 micron carbon block for example will remove 99% of chlorine from 20,000 gallons of tap water presented at 1 gpm. Some original equipment suppliers commonly provide carbon cartridges rated at 2,000 to 6,000 gallons.

Regarding your RO membrane and DI resin, use your TDS meter to measure, record, and track the TDS (expressed in parts per million) in three places:
1. Tap water
2. After the RO but before the DI
3. After the DI.

The TDS in your tap water will likely range from about 50 ppm to upwards of 1000 parts per million (ppm). Common readings are 100 to 400 ppm. So for sake of discussion, let's say your tap water reads 400 ppm. That means that for every million parts of water, you have 400 parts of dissolved solids. How do we go about getting that TDS reading down to somewhere near zero?

If you do some experimenting with your TDS meter, you'll note that your sediment filter and carbon block filter (collectively called “prefilters”) do very little to remove dissolved solids. So with your tap water at 400 ppm, you can measure the water at the “in” port on your RO housing and you'll see it is still approximately 400 ppm.

The RO membrane is really the workhorse of the system. It removes most of the TDS, some membranes to a greater extent than others. For instance, 100 gpd Filmtec membranes have a rejection rate of 90% (i.e., they reject 90% of the dissolved solids in feed water). So the purified water coming from your 100 gpd membrane would be about 40 ppm (a 90% reduction). Filmtec 75 gpd (and below) membranes produce less purified water (aka “permeate”), but have a higher rejection rate (96 to 98%). The life span of a RO membrane is dependant upon how much water you run through it, and how dirty the water is. Membranes can function well for a year, two years, or more. To test the membrane, measure the total dissolved solids (TDS) in the water coming in to the membrane, and in the purified water (permeate) produced by the membrane. Compare that to the membrane’s advertised rejection rate, and to the same reading you recorded when the membrane was new. Membranes also commonly produce less water as their function declines.

After the RO membrane, water will flow to your DI housing. DI resin in good condition will reduce the 40 ppm water down to 0 or 1 ppm. When the DI output starts creeping up from 0 or 1 ppm, you know that your resin needs to be replaced. Sometimes people complain that their DI resin didn't last very long. Often the culprit is a malfunctioning RO membrane sending the DI resin “dirty” water. This will exhaust the resin quicker than would otherwise have been the case. Sometimes the problem is poor quality resin – remember that all resins are not created equal!

Russ

That's good to know, Russ. I hadn't really looked at the resin you guys sell in a while, so I don't doubt that it would have changed. You guys may be my one stop place for filters now. Thanks.

With Spectrapure and Buckeye you are looking at the only two vendors I would recommend.
The MaxCap has some advantages over other systems such as a individually hand tested and guaranteed in writing 98+% rejection rate RO membrane and their proprietary SilicaBuster and MaxCap DI resins. Along with those the use an absolute rated 0.5 microm prefilter compared to most others which use a 1 micron nominal rated prefilter or larger. I believe Russ and Buckeye uses a 1 micron standard but he does carry the lower micron rated filters too,
Both use a similar if not identical carbon block and the housings and fittings are similar if not the same. Basically what you are paying for is the labor to test and certify the RO membrane and their research time and costs to mix the DI resins. Personally I find it is worth the difference but your water conditions are not as bad as mine so the payback period would be longer, maybe 18 months to 2 years depending on usage.

Russ uses the handheld TDS meters which are my preference but the inlines serve a purpose too in monitoring the condition of each step in the process. I actually use both the inlines and handheld so bought a seperate handheld to go along with the inlines.

You will be happy with either vendor or unit, as I said they are the only two I depend on myself and I run large treatment plants and train operators for a living.

What Buckeye system is comparable to the MaxCap 90 gpd? I've been looking at buying an RO/DI unit too, specifically the Spectrapure MaxCap system. I know nothing about RO/DI units, but I like the idea of inline TDS meters - what's the benefit of a handheld meter? What is the benefit of the tested RO membrane?

I need to check my tap water parameters, but I know we have (had) hard water and now have a water softener with some type of "big blue" (carbon?) filter.

Buckeye does not have anything comparable to the MaxCap, no one does.

Their Premium system is comparable to the ProPlus though.

The benefits of a handheld meter are it is portable so can be used to test tap water TDS, RO only, RO/DI, storage containers, the LFS, the grocery store, your buddy system etc. The inlines are dedicated so only test two spots per meter. The handheld is also more accurate since it is temperature compensated and reads water temperature not air temperature which are hardly ever the same. They both have their uses though and I own both.

The benefits of the hand tested membranes in the MaxCap is it is bench tested to ensure better than 98% rejection and guaranteed in writing. Off the shelf membranes, even Dows, are shown to be right at 97% in most cases. That 2% or more increase in efficiency doubles the life of your DI resin so cuts the operating costs significantly.

RO membranes love softened water, it does much or the work for them and they last longer.

Those are the two systems from Spectrapure I've been considering - ProPlus and MaxCap. Right now, leaning towards the ProPure based upon cost. Thinking the water softener might allow me to get away with cheaper system. Testing just ensures you get what you pay for - makes sense!

Handheld is much more versatile, but I'm mainly concerned about the RO/DI water and can buy a TDS meter later, if needed.

Thanks for the info.