Funky tds measurement on ro

[lowsingle]

So I added a tds measurement between my input water and after my 2 carbon blocks and found that the tds went up slightly after the blocks.  I attributed this to the carbon blocks needing to be better rinsed, but after 1 hour and 20 minutes of washing the tds was still higher than the input....any idea what is going on?

 

thanks,

Darren

 

[Jon Lazar]

Remove the sediment filter and carbon block from inside their housings and re-run your checks.  If TDS is still higher, then it's an instrumentation error.  If TDS #2 drops so that it's the same at both locations, then the filter or carbon block is releasing tiny amounts of dust.  You can run additional checks with different combinations to figure out which is causing the increase.

 

Either way, I wouldn't worry about a slight rise in measured TDS _before_ the RO membrane.  The sediment filter and carbon block aren't intended to reduce TDS anyway.  The RO membrane will take care of nearly all the TDS, and the DI will remove whatever gets by the RO.

[DFR]

I wouldn’t worry too much about the raise following the carbon block (But out of curiosity what were the values before and after?) providing the readings following the RO membrane is near 0, and 0 after the DI resin.

[lowsingle]

I was getting 105 tds in from my plumbing line and after the blocks and carbon filters I was getting 109 tds to the ro membrane.  After the ro membrane I get ~ 15 and my di resin takes care of the rest.  I thought I would see some drop as I was hoping the carbon would take out stuff, not add it. 

 

Darren

 

[Origami]

First, remember that TDS meters actually measure conductivity - that is, the ability of your water to conduct a tiny amount of current. To conduct current, you need free ions (charged particles) in the water.

 

Now, many jurisdictions use chloramine as a disinfecting agent. Monochloramine has the formula NH2Cl. When passed through catalytically active carbon, two monochloramine molecules (i.e. 2 NH2Cl) break down to form nitrogen gas (N2), two positively charged hydrogen atoms (H+) and two negatively charged chlorine atoms (Cl-). These charged ions make the water slightly more conductive, thereby giving you a tiny boost in TDS. Ultimately, your RO membrane and DI resin stages will remove these larger and ionic components.

[lowsingle]

Then why have the carbon blocks if it is left to di resin?  is it to protect the ro membrane?  I thought the carbon block captured the chloramine, but you are saying that it only breaks it down....

 

Thanks for the insights though....it sounds like a slight bump in tds is expected which was new to me.....

 

thanks,

 

Darren

 

[WheresTheReef]

Carbon removes some chemicals such as chlorine. There are special carbon filters you can buy to remove chloramine. That’s why you should look at the water quality report from your water company to see if they use it during their filtration process. Yes, sediment and carbon filters protect the much more expensive RO filter.

[Origami]

2 hours ago, lowsingle said:

Then why have the carbon blocks if it is left to di resin?  is it to protect the ro membrane?  I thought the carbon block captured the chloramine, but you are saying that it only breaks it down....

 

Thanks for the insights though....it sounds like a slight bump in tds is expected which was new to me.....

 

DI resin removes ions - charged atoms and molecules. Di resin does not remove dissolved molecules that are not charged. Certain pollutants in water, for example various organic compounds, may be present in the water but are not dissolved in the salt-sense - that is, they're not in ionic form. Granular Activated Carbon (GAC) is useful in removing a lot of these non-polar pollutants.

 

Carbon blocks work great at removing chlorine from water. Please keep in mind that chlorine is not the same as the chloride ion (Cl-). Instead, the aqueous chlorine that you're used to is formed by mixing chlorine gas with water to create hypochlorous acid (HOCl) and hypochlorite ions (OCl). (Sometimes sodium hypochlorite or calcium hypochlorite is used, but the dissolution of both still produce HOCl and OCl as byproducts.) Note the presence of oxygen (O) in both of these. Removing that oxygen effectively dechlorinates the water. But, it would be incorrect to think that dechlorination removes the chlorine ion (that is, chloride). Instead, dechlorination is a form of chemical neutering.

 

When passed over GAC, hypochlorous acid and hypochlorite oxidize the surface of the GAC, tearing away the oxygen and leaving H+ and Cl- ions to pass through.

 

The catalytic carbon modules that you buy for removing chloramine does so by 1) breaking the chloramine down to chloride (Cl-), a proton (e.g. ionic hydrogen (H+)), and dissolved nitrogen gas (N2). Again, the Chloride (Cl-) passes through and is removed by the DI stage.

 

There's a reason all that prefiltering is there before your RO membrane and the DI stage. One big reason is to remove as much as possible so that your DI resin lasts longer and is more cost effective. You can certainly buy two or three stage DI filters today - basically sediment, followed by carbon, followed by resin. For small quantities of water, this may work. But, in the end, the DI resin exhausts so quickly that you have to replace that cartridge often and end up spending a lot of money that you don't have to.

[DFR]

@Origami nice write-up, and very informative!