High PPM from my RO unit

[Galabar]

Looking at various diagrams, it seems that the large, threaded end of the RO membrane compartment is (usually?) the input for water:

However, you are saying that you are using that as the waste water output:

On 11/3/2023 at 9:44 AM, Ruud said:

Here it is. White tube on the left filter is my RO out tube. Top tube on the right is the waste tube, lower tube on the right (into the sediment) is the tap water IN tube that runs from the pump. 

First stage sediment, then pressed carbon, then RO membrane, then granulated carbon

Try attaching things as in the diagram above. Once you've done that test both the waste and good water outputs (to make sure you know which is which).

 

Edited November 5, 2023 by Galabar

[tolstoy21]

I haven't read the whole thread, but I would guess that you've already discussed the idea that the stages are incorrectly connected to one another so something is bypassing the RO membrane, and the TDS is being lowered a tad by one of the other stages (sediment, carbion, DI resin, etc.)

What goes first in the chain of stages depends on what your different stages do. A sediment filter and/or carbon filter should go before the RO membrane, and the DI resin should go after the membrane.

The other thing I can think of is that if you are using DI resin, when it becomes exhausted, it can start dumping some of the ions it stripped from the water in favor of other heavier ions.  This would lead to a sudden, measurable surge of TDS when this happens. However, this should not continue indefinitely. 

But honestly it sounds to me like water is bypassing the RO stage.

You'd have to see what your RO membrane is rated for, but typically it should have a 98% rejection rate, so if you have 400TDS water going in, the TDS coming out, before hitting any other stages should be like 8 TDS.  A failing RO membrane should not produce water of 200+ TDS, unless there is a big hole in it. 

So it's either seated wrong, or the tubing is connected incorrectly from one stage to the next.

Edited November 5, 2023 by tolstoy21

[tolstoy21]

On 11/3/2023 at 12:01 PM, TOtrees said:

In terms of target/expectation, even without a DI resin, you should be able to get under 10ppm with a new membrane.

Agreed, if there is a DI stage after the RO membrane, the TDS should be zero or close to it. If the RO membrane is getting to end of life or the input pressure is off and not enough TDS is getting rejected by the membrane, then the DI resin should clean that all up. But, you'd also be burning through DI resin as very fast rate, since there is more TDS to clear from the water.

With an old membrane you'd maybe see a TDS of 40 tops post RO stage (and post DI if the resin is exhausted).

Edited November 5, 2023 by tolstoy21

[tolstoy21]

On 11/3/2023 at 12:01 PM, TOtrees said:

You can also try flushing your membrane, and see if that makes a difference. Here's how to install a valve to do it regularly https://www.youtube.com/watch?v=T5CFco_Ceog, but you can also do it by manually removing the flow restrictor. 

Good one. Had not thought about that. There is a thing known as TDS creep, where, when your unit is sitting idle, the TDS of the source water will slowly make its way through the membrane. So that initial minute or so of water will have a high TDS if you have no DI stage after (or exhausted DI resin). 

A lot of people do what TOtrees said, which is flush their membrane on startup (mine does this automatically via a doo-hickey I installed), and/or allow the first minute of water from the RO unit to go down the drain (You'd need a three-way valve to act as a bypass).

My TDS is about 330, and even with flushing, the initial minute of water has a TDS of close to 200. But this quickly drops, and I have a bypass value open to that goes down the drain. Within a minute everything is working well, and I turn the valve on the bypass to let the water git my DI stages.

The reason a lot of folks do this is so they aren't prematurely exhausting their DI resin.

 

Edited November 5, 2023 by tolstoy21

[TOtrees]

The schematic from @Galabar is helpful, it confirms that it is possible for a post-membrane carbon stage to exist (something I never knew). A quick check with google suggests this is something that drinking water systems add, to improve taste. It wouldn't be necessary for an aquarium system. 

@tolstoy21 makes a great point, that if you're measuring the first water coming out the system, that could be why you're seeing such high numbers. I do the same thing tolstoy describes, but manually - ie let the system run for a few minutes until the output tds drops and settles at the range/numbers I expect. 

[tolstoy21]

On 11/5/2023 at 5:07 PM, TOtrees said:

but manually - ie let the system run for a few minutes until the output tds drops and settles at the range/numbers I expect. 

I do half of the process manually. The automatic part is the initial flush of sediment from the membrane. I have an auto flush kit something like this --> https://www.bulkreefsupply.com/aquatec-auto-flush-flow-restrictor.html.  The flushing is meant to extend the life of the membrane and make it operate at its best.

But I also have a three-way bypass valve installed that I switch over manually to let the first minute of water go down the drain. --> https://www.bulkreefsupply.com/3-way-ball-valve-with-john-guest-1-4-push-connect.html?queryID=e8c62bbb246712798164f41471d5b67c&objectID=982&indexName=brs_prod_m2_default_products

I keep a dual inline TDS monitor on my system to measure the water post membrane, and post DI. Watching it post-membrane allows me to see when the TDS has dropped to a number I like before switching the 3-way bypass into operation. It also let me know when the membrane needs to be replaced. The probe post DI allows me to know the TDS of the water going into my bucket, which should read zero if everything is running correctly. If it's not zero, typically my DI resin needs to be changed.

Edited November 5, 2023 by tolstoy21

[Ruud]

Hello all, and thanks for the input. A few answers: 

- its not the water pressure that's the problem: I have tried anywhere between 3 and 8 bar and at no point I get close to acceptable PPM

- Yes I am collecting the RO water and not the waste water. The waste water measures much higher PPM still

 

In the end I gave up on the unit ever working as it should be, whether its a plumbing issue or not. Yesterday I forked over 154 euro for a very basic house model RO system, connected it and it runs 10ppm water consistently as it should, after bypassing the re-mineralization phase.