How to pick a good RO/DI unit

One of the most recommended pieces of equipment to reefers is an RO/DI (Reverse Osmosis/Deionization) unit. RO/DI units are used to clean and purify tap water. Tap water, whether it comes from your local municipality or from your own well can contain all types of compounds that may or may not be okay to drink and may or may not be fine to use in your reef tank. The only way to know for sure whats in your water is to have a complete work up done. If you were to pay for these tests yourself it would be more expensive then buying a RO unit in the first place, so i would suggest skipping the tests and investing your money in a decent RO unit.

So what makes a decent unit? Well, its not the name on the outside, its not the number of stages and its not the gallons per day(GPD). Its the components on the inside that matter. So whether you are buying a new unit or just replacing some of your filters there are a few things you should know and look for to make sure you are getting your moneys worth.

This is a typical 4 stage RO/DI unit



If you purchase a standard unit like this with 10" canisters (the three clear housings on the bottom) you have lots of options for components. This type of system is probably the most widely used and many manufactures make components for these and most all are interchangeable, so you don't have to keep going back to the original manufacture whenever you need replacements. That's a good thing because a lot of the companies overcharge for an inferior product.

So lets get started on the components.



The first component you want to look at is the particle filter(go's in the first clear canister on the left in the above photo). The purpose of a particle filter is to well, filter out particles These filters can be made a few ways and come in different filtering levels. The will be rated on the filter size they filter down to. The unit of measure the manufactures use is microns. You want to use the lowest micron filter you can get. I typically use a 1micron or lower filter. Some units will have two canisters for particle filters and recommend you use a 10 or 5micron filter first and then a 2 or 1micron filter second but i don't see the point in this because a 1micron or lower filter will still trap all those larger particles. Using just one particle filter, the filter may be used up a little faster then using two but my one filter lasts me 3 or 4 months and i go through a lot of water.

Carbon filter.



The main purpose of the carbon filter is to remove chlorine and chloramines(go's in the center clear canister in the above photo). Chlorine's will destroy your RO membrane so carbon filters are a key part of a good unit and you shouldn't skimp when buying a carbon filter. Carbon filters will also remove some chemicals and compounds and can easy the work load on the RO membrane. I recommend a carbon block filter, which is basically carbon dust that has been compressed and glued together into a solid block. Carbon blocks have a micron rating also and i would suggest a micron rating as low as you can get (typically .5microns). Some manufactures will recommend using a particle filter that has a larger micron rating then the carbon block filter and allowing the carbon block to do some particulate filtering, but i think this is a bad idea. I want my carbon block to filter chemical compounds like chlorine, not particles. After all that's why i have a particle filter

There is another rating for carbon filters that is much more important then the micron rating. That is the Gallon rating. Typical carbon filters have a gallon rating of anywhere between 6000 to 8000 gallons but for a few more $ you can get a carbon filter that has a higher affinity for chlorine's and has a gallon rating of up to 20,000 gallons. Remember, your carbon filter is there to protect your RO membrane so spend the extra $2 or $3 and get the best you can find.

The RO membrane. (top horizontal canister in above photo)



The RO membrane is the workhorse of the system and membrane choice is critical. Unfortunately this is where people often make a mistake. RO membranes are rated in Gallons Per Day (GPD) and people often think that buying a membrane or unit with a high GPD is better, this is not always the case. What you need to look at is the rejection rate of the membrane. The rejection rate tells you what percentage of Total Dissolved Solids (TDS) will be removed. A true RO membrane will have a rejection rate of 98% or better. So if your tap water has a TDS of 100ppm, the product water after the membrane will have a TDS of 2ppm. If you surf around the web and look at different membranes with different GPD ratings you will see that most membranes above 75GPD do not have a rejection rate of 98% or better. Typically these 100GPD and higher membranes have a rejection rate anywhere between 90% and 95%. You may say whats the big deal between 98% and 95%. Well depending on your source water TDS it may or may not be a big deal. It's not uncommon for people to have source water with a TDS of 500ppm or even 800ppm. Lets say your tap water is 800ppm, a membrane with 98% rejection will give you product water with a TDS of 16ppm. That same 800ppm tap water at a 95% rejection would be 40ppm, and with a rejection rate of 90% it would be 80ppm. That's leaving a whole lot of TDS behind for your Deionization (DI) to filter out and DI beads are not cheep.

There are true RO membranes out there that filter more then 75GPD but they are typically much more expensive then the 75GPD units. I use a 75GPD system and it ea sly makes enough water for my reef which is about 450 gallon combined, and I also have multiple clown fish tank and rotifer cultures that get water changes every day so unless you have a really large system i doubt you need anything larger then a 75GPD unit.

Deionization (clear canister on the bottom right of above photo)

Deionization or DI is the final step in RO/DI filtration. The DI canister holds a cartridge containing cation and anion beads that remove any minerals the RO membrane could not. You want to buy a DI canister that sits vertical so the water flows up through the beads. The horizontal units generally don't hold much media, and in time the beads shrink and settle You can buy prepackaged cartridges or the beads by them selves and pack your own cartridge. The beads also come in color changing and non-color changing forms and the only difference i know between these two is that one changes color . The color change is meant as a way for you to tell when the beads are exhausted but a better way to tell is when you notice the product water TDS increase above 0ppm.

Extra components and add on's.

Three stage, four stage, five stage, whats the difference? It all depends on what the extra stages are. If some of these extra stages consist of particle filters rated at different sizes then i don't think they are worth the extra money. IMO get a small size rated particle filter and you are all set.

Two carbon blocks? This might not be a bad idea. If you are slack about changing your filters when they need it having two carbon blocks can help save your membrane.

Two DI canisters? This also might not be a bad idea but it also depends on how bad your water is to start with. I only use one DI and i can go many months between changes but I'm luck to have source water with a TDS of around 125ppm.

Pressure gages? It's a good idea to know what your water pressure is. RO membranes give their GPD ratings based on a certain water pressure so if your unit is under producing its rated GPD the first thing you should check is the water pressure.

TDS meters? I feel these are a must have. How else are you going to know your unit is working correctly unless you have some way of testing the product water? I like the in-line units because they are connected right to the unit. You cant drop them in the water or misplace them so its always right where you need it.

RO/DI video overview

That's a great post Chris - and should be something people can refer to over time.

A few tweaks:

There actually is quite a bit of difference in the hardware used in systems available on the retail market. We've had opportunity over the years to try lots of different housings, fittings, and brackets for instance. We've seen lots of parts we won't use because of quality issues that would be invisible to retail users until the parts failed. For example: Clear RO membrane housings that are brittle and frequently crack, thin-walled RO membrane housings that develop internal cracks, clear filter housings that are thin-walled and crack, poor quality fittings, brackets that are inexpensive but require "messy" routing of tubing, plastic RO membrane brackets that are brittle and crack. Aluminum vs steel brackets (which will rust over time). The list goes on, but you get the idea. None of these would be apparent to a buyer until they had problems.

Use of multiple sediment filters is sometimes appropriate with feedwater heavy in sediment. It is a way to lengthen the period of time between filter changes.

Re DI resin - it is the DI bed that "shrinks" (settles) over time, rather than the beads themselves.

Under normal circumstances I agree with your recommendation re a 4 stage being appropriate. For most people this is the way to go. "More stages" does not necessarily equate to a better system. There are 6 and 7 stage systems on ebay that people buying their first system are sometimes enamored with, but I can't tell you the number of these systems we've reconfigured for people once they understand what they bought.

Again - great post!

Russ

great chris you should put it sooner maybe i didn't go trough all that problems thanks great work thanks

Originally Posted by marvik

great chris you should put it sooner maybe i didn't go trough all that problems thanks great work thanks

...well hopefully you can still benefit from it!

Well I have a spectrapure model and watts model, When the Spectrapure was running it got my water here to 0 TDS, Also rid all my chloramines, the watts gets it down 20 TDS but does not have a DI resin in it. I am goin gtog to switch them back to the spectrapure being primary. They are top Builder here in AZ because our water sucks so bad, they were built for this water specific.

yep as always barb i am a nubie here cause lot of nohow in fresh water but here ??????????? thanks

Originally Posted by Barbara

...well hopefully you can still benefit from it!

Great! just keep comin' back!

Added two videos to the first post