Installing two RO membrane in line

Sipadan

New member
Ok, so haven't been on the board in ages, so I'm glad to be back. I live on a small island in Indonesia and have recently setup a small 100 gpd RO system for filtering my well water. Because I'm quite close to the beach, it's got a very high TDS (4000+ ppm in dry season) when I first installed it the RO unit got the TDS down to 200-300. It started to taste a bit brackish after about 8 months (TDS still around 300) but I figured it would be a good idea to change the membrane early because the water is so bad (I'm under no illusion it will last the full 2 years you guys can get out of yours) so after changing it the TDS shot up to 1100 ppm, and after tinkering with it a bit and running it for a day that has yet to change [emoji848]. So I changed back to the original membrane and now it's hovering around 700 ppm. So I'm a bit confused and was wondering if:

1: I have done something wrong in the instillation?

2: since the membranes I can get here seem to be of lower quality or filtration abilities than those you can get in the states, would it be a good idea (or is it even possible) to run two in line to put the water through two membranes before using?

I'll include a photo of the system below as it does not rely on water pressure from the main and has its own booster pump, which may make a difference. As well as a sketch of the proposed system I am envisioning as I see some people DO run two membranes, but mostly for re-filtering the waste water for better efficiency or conversely running them side by side to boost output. Ok thanks guys, I'm counting on you here, because the local filter system sellers have a limited imagination and or knowledge of their own products [emoji23]. Thanks in advance and happy reefin'!

Nick
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I don't think that is going to accomplish much but double your waste water. Things that will get past the first membrane will be more likely to get past the 2nd membrane. You will still want to use a DI stage.

If you are trying to save on DI costs, use 2 DI stages and when the 2nd DI stage starts passing TDS rotate it into the first position to protect the 2nd DI stage
 
I don't think that is going to accomplish much but double your waste water. Things that will get past the first membrane will be more likely to get past the 2nd membrane. You will still want to use a DI stage.



If you are trying to save on DI costs, use 2 DI stages and when the 2nd DI stage starts passing TDS rotate it into the first position to protect the 2nd DI stage



Perfect, exactly what I was looking for. All the systems here are just RO for some reason, but found a place to get DI filters online. Thanks man.


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Look at bulk reef supply or other vendors who offer refillable DI cartages and bulk DI media to save more money. just be sure to pack the cartages tight when you refill them. It's so easy that even I can do it!
 
I missed the part about 200-300 TDS output. How bad is the water you are starting with? with a 97% rejection rate membrane, you would basically have to start with mud to get 300PPM output. I think something is very wrong with your setup
 
I missed the part about 200-300 TDS output. How bad is the water you are starting with? with a 97% rejection rate membrane, you would basically have to start with mud to get 300PPM output. I think something is very wrong with your setup



Haha, basically yes it's not so much mud as just brackish water and sand. it's currently at 4000+ ppm TDS straight out of the well. We are just getting into rainy season now so it should drop significantly... but even then never less than 2000 ppm.


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Haha, basically yes it's not so much mud as just brackish water and sand. it's currently at 4000+ ppm TDS straight out of the well. We are just getting into rainy season now so it should drop significantly... but even then never less than 2000 ppm.


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Even starting a 2000 parts per million, and 97% rejection rate would leave you at 60 parts per million. At 60 or even 300 parts per million you're going to run through DIY cartridges really fast. Your second Ro membrane starts to make sense however you will need a booster pump to get a proper pressure to the membrane and relieve pressure from the first membrane


I would first store water from the first membrane in some sort of reservoir and set up a float switch to activate a pump to run the second membrane. The bigger your intermediate Reservoir the better because membranes don't like to start and stop.

I would also probably protect the first membrane with more carbon blocks and mechanical filters.
 
With your TDS that high to start I wonder if pumping in to a container and letting it settle out may help. Than pumping it through your system. I would defiantly us additional sediment and carbon filters before the membranes. Us a large micron sediment filter first then a smaller one. You can do the same with the carbon blocks.
Adding DI will defiantly reduce your TDS but your going to use resin very quickly. For that reason I would not use mixed resins. Using he 2 separate resins makes it a little easier to rejuvenate them. The mixed resins can be seperated in the rejunivation process.
There must be a better filtration method for your application. I wonder if distilling would be more efficient. How about collecting and storing rain water?
 
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