Brita Water Filter lowers KH substantially but not TDS - The Planted Tank Forum
 
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post #1 of 2 (permalink) Old 01-22-2013, 04:53 AM Thread Starter
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Brita Water Filter lowers KH substantially but not TDS

Can someone explain this to me?

Out of the tap my water is about 160-170ppm TDS. I run it through a brita water filter and it goes to about 120ppm TDS. However, kH will drop from 4-5 to 0-1 after filtration. What is going on?

From what I understand it softens the water but it doesn't reduce hardness. What is chemically occurring?

edit:

Some searching has told me that it's a Cation exchange resin that sequesters Ca2+ ions. However why should the pH drop so dramatically? It goes from ~7.4-8 to below 6.0. Is there an acid being produced? Also could there be any substance being outted from the filter that could harm aquatic life.

edit 2:
Ion exchange resins exist in the form of cation exchange resins and anion exchange resins. The BRITA water filter cartridge is filled with cation exchangers. These replace positively charged ions such as calcium, magnesium, lead or copper with positively charged hydrogen ions. Anion exchange resins replace negatively charged ions from tap water (e.g. nitrate, sulphate and phosphate) with negatively charged chloride or hydroxide ions. These are primarily used as nitrate filters.

Looked on brita's site and it apparently turns CaCO3 into H2CO3....is this right? Could it really be carbonating the water?

Last edited by Jeffww; 01-22-2013 at 05:06 AM. Reason: more q's
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post #2 of 2 (permalink) Old 01-22-2013, 07:33 AM
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Originally Posted by Jeffww View Post
Out of the tap my water is about 160-170ppm TDS. I run it through a brita water filter and it goes to about 120ppm TDS. However, kH will drop from 4-5 to 0-1 after filtration. What is going on?

From what I understand it softens the water but it doesn't reduce hardness. What is chemically occurring?

Looked on brita's site and it apparently turns CaCO3 into H2CO3....is this right? Could it really be carbonating the water?
"Soft" water by human standards means water that is devoid of calcium and magnesium ions. A Brita filter removes calcium cations, and thus reduces hardness and softens the water.

If Brita filters use hydrogen in the exchange resin, then when calcium ions are removed, your TDS will go down slightly. The hydrogen ions most likely react with the carbonate species, forming carbonic acid, which then offgases to CO2 rapidly. This results in your kH being reduced (carbonates are the main species that contribute to carbonate hardness, as the name would imply).

Anthony

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