Suspicious TDS readings

[ccs]

Hi everyone, I tried to search for other threads but none of them really helped me understand what's going on.

I juts purchased a 5 stage RODI unit with brand new filters and the membrane is a Dow 100 gpd.

I have a 550 flow restrictor and a booster pump (I don't have a pressure gauge, but I think the pressure should be ok)

After the water goes through the pre filters and the membrane (pre DI) it reads 14-15 tds,

The incoming tds is only about 95-100 tds. So the rejection rate is about 85% which seems really poor...

The water is from a drill/well and we periodically send it for analysis.

The only red flag from the analysis, I think, is extremely low ph (about 5) possibly due to high levels of dissolved CO2.

I was burning through DI resin very fast (again probably due to high CO2) but I am sure that the 15 tds after membrane was also not helping.

I have checked everything and the membrane is correctly installed.

The flow restrictor makes a fairly high hissing sound, but I think that is normal.


Could the CO2 be so high that its messing the TDS reading? Anything else I might be missing?

 

[Biglew11]

Looks like brs recommends around 800ml.

with lower incoming tds you can probably go a little lower. To low like 500ml may force to much water to go through the membrane. I have a 600ml on mine using 100gpd membranes.

To me It sounds like you may have restricted the water flow around the membrane too much, forcing some tds to go through to the product side.

I'm no chemist but I dont think co2 would affect tds. It will deplete your di resin.
Some else with well water can expand better than me about how to off gas the co2.

 

[Mike konesky]

800ml is the correct restrictor for a 100gpd membrane as stated above. 3 options for co2.
1. Ignore it and spend tons on anion resin
2. Inline degassing chamber
3. Separate tank for degassing, then pump through di.
Number 3 is my preferred choice, but requires the space to store RO while it's being degassed.
I'd start with replacing the restrictor and see what your resin consumption is.

 

[ccs]

Thanks to both! I will try to had a 800 ml restrictor and see how it goes.

I have space for a degassing container so I will try that too.

 

[KStatefan]

800ml is the correct restrictor for a 100gpd membrane as stated above. 3 options for co2.
1. Ignore it and spend tons on anion resin
2. Inline degassing chamber
3. Separate tank for degassing, then pump through di.
Number 3 is my preferred choice, but requires the space to store RO while it's being degassed.
I'd start with replacing the restrictor and see what your resin consumption is.

I do a combination of 2 and 3 for my degassing. PVC pipe gegassing chamber then into a five gallon container that is pumped into the DI resin

 

[Mike konesky]

I do a combination of 2 and 3 for my degassing. PVC pipe gegassing chamber then into a five gallon container that is pumped into the DI resin

Nice. I pump RO to a 44 and 32 brute and use a skimmer pump with venturi to kick co2. Only takes about 45min to aerate the 32 gal and remove CO2. An hour for the 44.

 

[KStatefan]

Thanks to both! I will try to had a 800 ml restrictor and see how it goes.

I have space for a degassing container so I will try that too.

I would recommend measuring your waste to permeate ratio so you know you do not have a restrictor that is mismarked

 

[ccs]

I would recommend measuring your waste to permeate ratio so you know you do not have a restrictor that is mismarked

And what ratio should I be looking for?

 

[KStatefan]

And what ratio should I be looking for?

I target 3:1 on my system but I have much higher tds in my source water then you do you could probably get a way with a lower ratio depending on how hard your water is.

 

[Biglew11]

And what ratio should I be looking for?

4 to 1 is a good starting point. if the tds in the product water out of the membrane is acceptale you can decrease the ratio. when i set mine up i think i ended up at about 2.7 to 1 and get 3tds out of 2 membranes in series.

 

[Buckeye Hydro Verified Sponsor]

And what ratio should I be looking for?

That all depends upon your feedwater chemistry, and your pretreatment. For example - do you have high hardness? Iron? Manganese? Do you run your well water through a softener?

Feel free to email us your well water lab results and we'd be happy to take a look at it for you.

Russ
[email protected]