Understanding Water and Total Dissolved Solids (TDS)

We get questions everyday about whether our filters remove TDS (Total Dissolved Solids). The questions go one of two ways:

1. Do your filters remove TDS?

2. Why is my filtered water getting a high TDS meter reading?

These are very good questions because they get to the heart of how we can tell whether we are drinking clean water. But, before we answer them let's try to understand TDS a little better. 

What is TDS in Water? 

Total Dissolved Solids (or TDS) is a unit measuring the amount of particles within a solution of water. TDS is usually measured in parts per million, or how many particles other than water are there in 1 million water particles. 

These particles can consist of any combination of ionically charged chemicals and contaminants but also beneficial minerals and nutrients. Though TDS is often equated with contaminated water, that is not always the case. Ions and ionic compounds making up TDS usually include carbonate, bicarbonate, chloride, fluoride, sulfate, phosphate, nitrate, calcium, magnesium, sodium, and potassium. But any ion that is present will contribute to the total. As you can see, some of these are minerals and nutrients that are beneficial to the human body. The organic ions include pollutants, herbicides, and hydrocarbons. In addition, soil organic matter compounds such as humic/fulvic acids are also included in TDS. So just because water has a high TDS reading, it doesn't necessarily mean that your water is filled with toxins or chemicals, as it is likely a combination of the toxins and minerals, and once the water is filtered (and depending on what type of filter you use), you would be left with a better understanding of the types of TDS remaining in the water. 

Now this is where it gets interesting. In order to measure TDS, one typically uses a TDS meter. A TDS meter does not initially measure TDS, which is where much of the confusion arises. TDS meters, also known as TDS water teste readings or indicators, are digital or analog meters that measure the electrical conductivity of water. Based on that conductivity, the meters estimate what the true TDS level might be. 


What does a TDS meter measure?

In reality, a TDS meter measures the electrical conductivity of water or, in other words, the total amount of mobile charged ions found in water. If an element is dissolved in water and can conduct electricity, it is called an electrolyte. Salt, for example, is an electrolyte. This conductivity measurement is then put through a statistical regression equation to convert it to a concentration measurement such as Parts Per Million (ppm). But some materials (sugar, for instance) is not an electrolyte. Therefore it will not register whatsoever on a conductivity or TDS water meter. Check out the video below to see what we mean. This is one reason why a TDS meter reading is only an estimate of the true TDS.

A TDS meter is simply not capable of indicating the types of dissolved solids that might be present in the water or at which level those specific contaminants (or minerals) are present.

Although many essential elements may contribute to TDS, the measurement technique does not, itself, differentiate essential from toxic elements.

For example, if your water's reading was a TDS of 150 (which remember isn't a true calculation but more a gross estimation) you have no way of determining what exactly that reading is made up of. If it were Lead, you'd be talking about an extremely toxic (possibly deadly) amount of contamination, while if it were Potassium, it wouldn't be harmful at all, it would actually be healthy!

There are two main measurements when it comes to the electrical properties of a substance: Resistance, which is typically measured in Ohms (Ω) and Conductivity which is typically measured in Siemens (S).

Resistance is the measure of a material’s opposition to the flow of electric current.

Conductivity is the measure of the ease with which electric currents passes.

These two scalar measurements are the fundamental basis of understanding how the TDS meter works but what if we want to do more than measure the pure magnitude of these to measurements? What if we wanted to measure the rate at which electric current flows through a material with a given resistance/conductivity? For that we will need to convert these scalar quantities to vectors:

-Resistivity (⍴ / Ω·m )

-Conductivity (σ / S·m)

The only difference is that now we are measuring the Resistance or Conductivity multiplied by a measure of distance (in this case a meter).

By using these vector measurements we can assess the magnitude and rate at which an electric current passes through a given substance like a copper wire, an electrical resistor, or even water!

All substances conduct electricity but some do it better than others.

Here are two articles that suggest that using TDS as an indicator of clean water gives people a false sense of security and can actually lead them down a harmful and toxic path:


Now that you have a good understanding of the basic electrical measurements that relate to a TDS meter you're ready to learn how they work!


How does a TDS meter work?

As stated earlier a TDS Meter is really just a conductivity meter that relies on a statistical regression equation to convert conductivity into a concentration measurement.

How a TDS meter works

The theory behind the equation goes something like this: all substances conduct some amount electricity, but some do it better than others, so if we were to start with pure water we would get very consistent conductivity readings at around .018 µS, but if we added some table salt, which has been measured to have a conductivity of over 20µS, the conductivity would increase quite noticeably.

Typically when establishing a linear regression for a conversion from conductivity to parts per million a researcher will start by acquiring the purest water reasonably attainable (usually distilled water but some places have access to ultra-pure water and thus use that). Next, a baseline reading will be taken of the distilled water after which a know amount of a known solute (let us say 1 mg of table salt) will be added to the water and then one will measure and record the subsequent conductivity, and the researcher will continue adding and recording until the solution is either saturated or the meter is no longer within range. This entire procedure will be repeated several times until the researcher is satisfied that they have an accurate set of measurements.


Does Clearly Filtered remove TDS?

This is a tougher question to answer because the answer is both YES and NO. It really depends on the type of Total Dissolved Solids that you are looking to remove and/or are found in your water. 

There is a very high likelihood that all tap water throughout the world contains and mix of Total Dissolved Solids that are contaminants (both naturally occurring and added chemicals) and beneficial minerals and nutrients (like Calcium and Magnesium). Clearly Filtered water filters are designed to remove the Total Dissolved Solids that are contaminants but leave the Total Dissolved Solids that are beneficial minerals. Most filters are not smart enough to be able to identify this discrepancy and therefore either remove everything from your water (ie. ZeroWater) or leave in many contaminants (ie. PUR, SOMA). 

On top of that, because the Affinity Filtration Technology used by Clearly Filtered filters is an ionic absorption process, there is a chance that the water molecules are various minerals that pass through the filter are actually electrically charged differently once they pass through the filter. In the rare case, we have seen TDS readings that are higher once the water actually passes through the filter, further showcasing the idea that using TDS is not an accurate means of determining water cleanliness at all.


Is measuring for TDS an effective way of seeing if your water is clean?

The answer for tap water is ABSOLUTELY NOT! TDS meters are a gimmicky way of trying to show you that your water is either clean or dirty by giving you a number, therefore convincing you that the number is either good or bad. Without knowing exactly what is in your tap water, and what concentration those dissolved solids are present, it is impossible to determine the cleanliness of your water using a TDS meter.

The long answer is that this theory backing TDS meters is quite sound for controlled laboratory/manufacturing scenarios where the parameters affecting the conductivity of a solution are known. In this case, TDS will be a convenient and accurate means of assessing an unknown concentration of a known solute, but outside of these controlled situations such methodology is insufficient. The linear regression for TDS must be calibrated for both a solution’s solvent as well as its mixture of solutes, but if one does not know all these factors? It is at this point that TDS meters become ineffective means to assess one’s water quality.

For example: Calcium, a mineral shown to be beneficial to human health, typically carries an ionic charge of +2. This means that when a molecule of a calcium salt is dissolved into water it “frees up” 2 electrons which in turn are now available to complete the circuit between the two probes on the TDS; the transit time goes down and thus the TDS reading goes up.

If we were to add Bromine, a chemical known to be highly poisonous to humans, this salt typically carries an ionic charge of -1. So if we add a molecule of a brominated salt, only one electron would be “freed up” to complete the circuit between the two probes, and again the electrical transit time would decrease and the TDS reading would increase, but in this specific instance the reading would only be half as much as when we added the Calcium (because it only had half the amount of ionic charge).

Essentially, the TDS would go up twice as much for the healthy mineral as it would for the highly poisonous one. In both instances the TDS will increase, but only in one instance is the water unsafe to drink and, furthermore, the solution with a lower TDS is much more poisonous while the solution with a higher TDS is perfectly safe. In this instance the only thing a TDS meter is really good for is determining the absolute purity of the water, not its overall quality, and this distinction is important as pure water is different from contaminant free water.

... this distinction is important as pure water is different from contaminant free water.

There is an additional concern regarding common TDS meters available for consumer use and that is their sensitivity. As stated before, nearly all TDS meters convert a conductivity reading into a measurement of concentration, part per million (ppm), but the most concerning water contaminants have negative health consequences when present at at concentrations that are far less than even 1 ppm.

For example the the EPA’s federal action level for lead is 15 Parts Per BILLION (ppb). This means that for a standard TDS meter purchased via Amazon (or one that comes with a ZeroWater pitcher) would not register any lead until it contained nearly 70 times (66.667 times to be more accurate) the legal limit and even then it would only read 1 ppm, assuming the solution only contained pure water and pure lead!


What About Lead, Arsenic and Chromium 6?

Though each of these contaminants carry an ionic charge, and a therefore deemed Total Dissolved Solids, all of these contaminants are toxic at levels much lower than a TDS meter can realistically measure. 

As stated above, Lead would have to be nearly 70 times the legal limit before registering on a TDS meter.

Arsenic would need to be even more than this as the EPA's federal action limit is 10 ppb. This means that in order to for a TDS meter to pick up a reading of Arsenic, the levels of that toxin would be 100 times greater than the allowable legal limit.

And Chromium 6 is where it gets even scarier. Though Chromium 6, a known carcinogen made famous by Erin Brockovich, is not currently regulated by the EPA, the public health goal for this contaminant is 0.02ppb. That means that in order for a TDS to finally get a reading of Chromium 6 the toxin would need to be present at 50,000 times the public health goal limit. 

It fairly easy to see why when it comes some of the most dangerous contaminants found in our tap water, that a TDS meter is not an accurate way of determining the cleanliness of the water either pre or post filtration. Even a 0.00 reading is not giving you the complete picture when truly understanding what chemicals might still be present in your water.


What is the best way to determine if your water is safe to drink?

Unfortunately, because tap water varies so drastically between countries, states, cities and even homes, it is impossible to determine the exact chemicals that are found in the water once it leaves the tap.

Many water filters do not share their test data, which should be the first factor in determining whether a filter is capable of removing the various contaminants that can be found in tap water. Clearly Filtered prides itself on being extremely transparent with our testing data. Each filter system is taken off the shelf and given to a 3rd party EPA-approved testing facility to manage the testing and provide the final data. Every time a filter is re-formulated, all of the tests are run again to ensure we have the most up-to-date test results and verifiable data to back up our claims. 

The most appropriate way of determining the effectiveness of a filter is to send your water pre and post filtration to a lab that has the ability to test the water. This process is very expensive and not ideal, but would give you the most accurate results.

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