RO/DI TDS....add a 5th stage?

[nivenethan]

I have a Spectrapure MPDI60 4 stage (Sediment, Carbon, RO, DI). Incoming TDS is about 180. Effluent TDS ranges from 0 to 8 from time to time.

Can I add a 5th stage to bring TDS to 0?
What would that stage be?

Any other recommendations?

Thanks,

[iamwrasseman]

a second Di cartridge will certainly remove the rest of the tds in your situation IMO.

[HighlandReefer]

I agree with Dave. One thing to keep in mind is that your effluent TDS reading should read zero and once it rises to 0001 then you need to change your DI filter imediately to prevent a sudden release of loosely held ions like silica & ammonia. Many TDS meters can not be calibrated and may read with a little higher reading, pehaps up to 0002. In that situation, perhaps testing distilled water to see what it reads for a test of determining if your TDS meter is off a bit.

Randy provides more details in this article:

Reverse Osmosis/Deionization Systems to Purify Tap Water for Reef Aquaria
http://reefkeeping.com/issues/2005-05/rhf/index.htm

[bertoni]

Does the TDS rise gradually, or is it jumping around? It should just rise slowly. I would replace the cartridge as soon as the TDS rises measurably. You can use two DI cartridges, which can be useful. When the first one in line starts to fail, you can use the second cartridge in its place and add a new cartridge in the last cartridge. That will keep the water quality very high.

[sjm817]

The TDS should not be going from 0 - 8 after the DI. An extra stage is not needed. What is the TDS after the RO? The DI should be able to bring whatever that is down to 0 even if the TDS coming into it is high. It would just get exhausted quickly. Basically, the flow is High tapwater TDS -> low RO TDS (single digits) -> 0 DI TDS.

[PurdueWaterGuy]

Does the TDS rise gradually, or is it jumping around? It should just rise slowly. I would replace the cartridge as soon as the TDS rises measurably. You can use two DI cartridges, which can be useful. When the first one in line starts to fail, you can use the second cartridge in its place and add a new cartridge in the last cartridge. That will keep the water quality very high.

+1 That's the way the big boys do it! We have two 30 cubic foot mixed beds in series and rotate in a new one when the lead bed starts leaking boron.

[sjm817]

To expand on the use of a dual DI stage. A 5th stage (2nd DI) is not necessary for 0 TDS. A single DI can do that just fine. It is to get the maximum life from your DI cartridges and to take some of the guesswork and monitoring out of when to change out the DI stage. You always have a fresh one as stage 5 with the dual setup.

On the 0 - 8 TDS issue, if the RO stage is producing a TDS of 8, and the TDS is still 8 at the DI stage, the DI has long been exhausted.

I was using the dual stage mixed bed DI as described in the above posts, but just yesterday put in the 2 stage Spectrapure Max-Cap DI cartridges which work differently. The first stage is a "roughing" stage and the 2nd is the final. This is supposed to give 4x the life to the final stage and have better final product as well.