TDS

[Mark Bradley]

I have an RO unit that is only a couple of months old. I measured the water it produced, this morning, and it showed 6ppm. I assumed it would be closer to zero. I only have it producing about 30 litres every fortnight as only use it for top up (buy my saltwater ready mixed).
What do I need to do to reduce this measurement? surely the filters can’t need replacing after such a short time? Is 6ppm unacceptable for my reef?

 

[homer1475]

Are you flushing the system before use?

In other words, running low pressure water through the membrane, and putting it all to waste before producing water?

Your short run times will greatly increase TDS(we call this TDS creep) inside the membrane and needs to be "cleaned", or "flushed" out before actually producing water.

I do not use anything above 1TDS.

 

[Mark Bradley]

Are you flushing the system before use?

In other words, running low pressure water through the membrane, and putting it all to waste before producing water?

Your short run times will greatly increase TDS(we call this TDS creep) inside the membrane and needs to be "cleaned", or "flushed" out before actually producing water.

I do not use anything above 1TDS.

Um - the answer to that would be “no”! I did when new but assumed it was a one off.
How long should I run it at low pressure?

 

[homer1475]

I use my TDS meter(3 inline meter) to determine when to stop flushing.

My incoming TAP water measures 20PPM, I flush my unit(startup TDS can be as high as 45TDS) until the TDS is around 5, then turn the inline valve to create pressure, and open the valve to the DI resin.

While flushing, it's a great practice to install another valve before the DI resin so your not pushing that high TDS through your DI which would deplete it rather fast.

This old video by @SpectraPureInc can explain it, and how to install one better then I can.

I will say I can produce way more 0TDS water after implementing a flush valve and DI bypass, before my filters, and DI resin are depleted.

 

[piranhaman00]

Are you using just an RO without the DI? 6 TDS after RO is good, I am about 9 TDS after RO, it’s the DI that then takes it to 0tds

 

[homer1475]

Good point!

I didn't consider just RO as most units we buy for the hobby include DI.

 

[Mark Bradley]

I have the DI unit too:

 

[homer1475]

That large white tube with the valve on it is the flush valve. You should measure your incoming TDS. Then with the valve open(flushing), measure your TDS when you first turn the unit on(before the Di stage), then wait until that number drops significantly, all the while bypassing your DI stage. Then turn that valve to create pressure, causing the Ro membranes to actually work.

In my case my incoming TDS is right around 20. When I first turn my unit on, the TDS can go as high as 35(TDS creep). When I see the number drop to around 5, I turn the flush valve to make water, and turn the valve to send the made water through the DI to get 0TDS water.

 

[Mark Bradley]

That large white tube with the valve on it is the flush valve. You should measure your incoming TDS. Then with the valve open(flushing), measure your TDS when you first turn the unit on(before the Di stage), then wait until that number drops significantly, all the while bypassing your DI stage. Then turn that valve to create pressure, causing the Ro membranes to actually work.

In my case my incoming TDS is right around 20. When I first turn my unit on, the TDS can go as high as 35(TDS creep). When I see the number drop to around 5, I turn the flush valve to make water, and turn the valve to send the made water through the DI to get 0TDS water.

Thanks - I’ll give that a try

 

[Mark Bradley]

Flushed the unit but it hasn’t made a difference. I wonder whether my measurer needs calibrating? Ordered some calibration fluid so will try that.

 

[Randy Holmes-Farley Verified Community Expert]

Flushed the unit but it hasn’t made a difference. I wonder whether my measurer needs calibrating? Ordered some calibration fluid so will try that.

Very unlikely. TDS meters do not move off the zero, so zero is always zero. Whether 5 ppm TDS is really 10 ppm TDS is relatively unimportant for determining if the TDS is 0 ppm.

 

[Randy Holmes-Farley Verified Community Expert]

DO you use well water? High CO2 water rapidly depletes the DI part of an RO/DI.

 

[Graffiti Spot]

That tube on the top is your di resin? That won’t last long with 6 tds coming off your filters. I would get a normal sized di chamber.

 

[Mark Bradley]

DO you use well water? High CO2 water rapidly depletes the DI part of an RO/DI.

No - from the mains

 

[Mark Bradley]

That tube on the top is your di resin? That won’t last long with 6 tds coming off your filters. I would get a normal sized di chamber.

That is what came with the unit. I’m only doing an average of 15 litres a week!

 

[Mark Bradley]

So got the calibration fluid and the meter was a little out but nothing major. As a ‘test’ I thought I would see what my LFS RO water measured, for comparison, and was surprised that it was 20ppm!

 

[Graffiti Spot]

Most local fish stores don’t keep up with their rodi filters very well. Very common which is why I tell people not to use their water unless they guarantee it to be 0 tds.