The Basics of an RODI Filter

Reef-junky

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An RO membrane forces molecules through a semipermeable membrane (certain substances can pass through but not others) to form a less concentrated solution then you started with. The inside of the membrane looks like so:

8B66DE89-563A-4C59-9510-16593F2827D9.jpeg DC516531-2E45-4BAA-966A-1E965D3E66BE.jpeg E71BFC63-C68B-456B-99B3-8B769E5E3F77.jpeg

Some of the things an RO membrane can filter out are:

Lead
Copper
Arsenic
Nitrates and Nitrites
Chromium
Selenium
Fluoride
Radium
Barium
Cadmium
TDS

The filters on an RO/DI filter are called stages. The first few stages consist of prefilters that help protect the RO membrane. The first prefilter is a sediment filter. It helps remove large sediment. The next stage is a carbon filter which takes out chlorine. Some filters run two sediment and or two carbon filters. You start with a course filter and then go to a tighter one. From there we go into the RO membrane. The last stage is running the water though a DI filter this takes out the last bit of contaminates that the RO can’t get. The DI filter will take out anything with an ionic charge. You can run two DI filters to get the maximum amount of life out of your DI resin. This is a digram of an RO/DI filter.


1D464E6A-1A2A-49BD-8EA4-361FDC9E6613.jpeg

An important thing to note about RO filters is that the membrane is flushed with water the whole time you are running the filter. The water that’s used to flush the membrane is waste water. The amount of waste water your filter will produce is giving in a ratio so for example a filter might say it has a 3:1 waste water ratio. For every one gallon of good water you make you will make 3 of waste water. Let’s say we want to make 5 gallons of water for our tank that means we will end up with 15 gallons of waster water and we will end up running 20 gallons of water through the filter. There are a few things we can do to lower the amount of waste water the filter will produce. We can run two RO membranes. We could also run a booster pump to feed the filter.

There are a few things we can do to get the maximum amount of life out of our filters. We can test the water for Cl after the last carbon filter to tell us when to change the carbon out. We can also put a pressure gauge before the RO membrane when we see the pressure drop this lets us know the prefilters are starting to plug and we should change them. We can also add a flush kit to the RO membrane. This allows water to bypass the flow restrictor and flush the membrane clean. The membrane should be flushed before and after each use. You can buy an automatic flush kit which will flush the membrane for you. This will help extend the life of the RO which is the most expensive to replace. The flow restrictors job is to keep the membrane under pressure. The last thing we can do is use a TDS meter. There are two types inline and hand held. If we test the TDS after the DI resin anything higher then 0 tells you it’s time to change the resin. We can find out how well our membrane is preforming by testing the TDS of our tap water and the TDS after the RO membrane. Let’s say we have tap water that’s 200 TDS and after the RO we get 4 TDS our rejection rate is:

4/200=0.02

100(0.02)=2

100-2=98%

Once the rejection rate starts dropping by 3 or 4 % it’s time to start thinking of replacing the membrane. The membrane should tell you the expected rejection rate you will get.

There are a few things that can cause an RO/DI filter not to work well. One is if your on well water and your water has CO2 in it. If that’s the case then we need to degass the water before we run it though the DI filter. This can be done by collecting the water after the RO and putting and air stone in it. Let it sit for a day then use a small pump to run it through the DI resin or something like this can be made to degass the water as your running.


Another potential problem is if your water is treated with Chloramines. If that’s the case then you need to run an additional prefilter.


An RO/DI filter looks like this. You can see the 2nd RO on top of the main one and you can see the flow restrictor in the 2nd picture.

BA629D5B-4DE9-4962-8CFE-56318273887E.jpeg 1D665E11-29D6-4584-8ECF-4CE545E23877.jpeg

An automatic RO/DI flush kit looks like this. FBEB2A82-7BDA-4044-8939-0CD453B047E0.jpeg

And a booster pump looks like this: A228A482-02C3-4BE6-A511-4BE57E8EFAA2.jpeg
 
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Daniel@R2R

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Good overview! Thanks for writing this up!
 

Steve_Skullsplitter0

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Buckets suck.
Rodi makes it easy peezy.
Just flush RO membrane before and after use. Change sediment and carbon blocks every other DI resin change. If your city water uses chlorine or chlorimines use prime in your storage tub.
That's what I do. It works for me.
I run two 75g membranes and 3 inline DI canisters. I can easily make 30g in 8 hours with 62 degree water at 70psi.
 
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Reef-junky

Reef-junky

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Buckets suck.
Rodi makes it easy peezy.
Just flush RO membrane before and after use. Change sediment and carbon blocks every other DI resin change. If your city water uses chlorine or chlorimines use prime in your storage tub.
That's what I do. It works for me.
I run two 75g membranes and 3 inline DI canisters. I can easily make 30g in 8 hours with 62 degree water at 70psi.

The biggest shock to me is the amount of waste water produced.
 

Ardeus

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I went through 3 different rodi systems until I finally found one that manages to give 0 tds from 250 tds tap water.

The 2nd one is a 175 GPD system and it didn't perform well. The pressure was 55 and I guessed it wasn't enough to make it work as it should. I tried to change the membrane to a 75 GPD, the resine, flow restrictor and the best I could get was 11 tds.

 
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Reef-junky

Reef-junky

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I went through 3 different rodi systems until I finally found one that manages to give 0 tds from 250 tds tap water.

The 2nd one is a 175 GPD system and it didn't perform well. The pressure was 55 and I guessed it wasn't enough to make it work as it should. I tried to change the membrane to a 75 GPD, the resine, flow restrictor and the best I could get was 11 tds.



So you got 11 TDS after the DI on 3 different filter systems?
 
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Reef-junky

Reef-junky

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No, I had 11 tds on the 2nd.

The first got me to 5 or 6 tds.

The last one gets to 0.

That doesn’t make sense. Are you on city or well water? I use to run an RO/Di buddie with higher TDS tap water then that and got 0 TDS after the DI every time and that’s a really cheap filter. If the other two filters had DI then something was interfering with the RO or set up wrong would be my guess.

45EE30BD-205E-4A12-B04E-41C43A42EDF9.jpeg
 
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Ardeus

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I have city water, the pressure is 55.

There was something really wrong with that second unit (I still have it).

The original resin leached something that made the water taste metallic. The TDS increased after the resin. I got a new resin and it reduced the TDS but only to 11.

I changed the membrane to a 75 gpd and the flow restrictor and it got worse.
 
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Reef-junky

Reef-junky

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There was something really wrong with that second unit (I still have it).

The original resin leached something that made thewater taste metallic. The TDS increased after the resin. I got a new resin and it reduced the TDS but only to 11.

Do you have a picture of it?
 
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Reef-junky

Reef-junky

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Looks like the small filter that looks kind a like the one in the picture doesn’t have DI.

My guess on the 2nd filter is the resin is dead.
 

Opus

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Flushing the membrane in our consumer systems really doesn't do anything. Unless you are making a lot of water you should get 4+ years out of a good membrane.
 

Ardeus

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The first one didn't have di and produced 5 tds for 3 years.

The 2nd had new resin that I bought from another brand.
 

Ardeus

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Flushing the membrane in our consumer systems really doesn't do anything. Unless you are making a lot of water you should get 4+ years out of a good membrane.

When I spoke to the customer support of the 2nd unit they asked me to flush it. Why doesn't it help? The Aquamedic unit does it automatically.
 

Opus

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When I spoke to the customer support of the 2nd unit they asked me to flush it. Why doesn't it help? The Aquamedic unit does it automatically.
I'm not sure of the "science" behind it. I've just been told by water guru's that I trust that it doesn't work on our little consumer units.
 

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