What is the TDS of drinking water? (Total Dissolved Solid)

Many of us, who are looking for a water filter, have come across the word TDS much more often than not. 

But, we can’t always make the meanings out of it. Visiting the Wikipedia pages doesn’t always become fruitful.

In most cases, it is full of technical terms that most of us can’t comprehend properly.

That’s why, in this article, we’ll be discussing this important issue which might help you choose the perfect water filter for your home.

Also, there are common misconceptions around what the TDS of drinking water should be. 

Many think it is the indicator that shows the amount of pollution in the water. 

We’ll try our best to cover all the necessary issues that most of us get stuck when it comes to TDS of water.

So, let’s start- 

What is TDS?

The word “TDS” stands for total dissolved solids. This is a feature that we can use to find out how many particles are in the water.

We know that water that we get from water treatment plants have some substances other than water. If you’d like to know about how water treatment works, you should visit our post here.

There are many organic and inorganic materials in the tap water. Not all of these can dissolve in the water.

For example, if you mix sand with water in a glass, they don’t mix at all. And, therefore, the sand molecules will accumulate at the bottom of the glass.

The picture is different when you take salt instead of sand. If you add a bit of salt into the water and stir, it will dissolve. And, there is no way of identifying pure water and water that has salt in it.

So, you see, we can visually identify if the water is containing any clay, sand, or debris as they don’t dissolve into the water.

But, the scenario changes when it comes to salt. There are thousands of molecules other than salt that can perfectly blend with the water. 

And, as they dissolve, we can’t visually identify if the water is pure or not.

That’s why we need to measure the TDS of the water. With this, we can find out how much of these molecules are in the tap water. 

So, in short, TDS measures the amount of organic or inorganic compounds that the water is holding.

What inorganic substances can be in the tap water?

Let’s first see what are the inorganic substances in the water. Many of these are mineral salts that come from the soil.

If you’re familiar with chemistry, you’ve probably come across ionic compounds. You can find many ionic compounds in the water as well.

These compounds break into ions when they dissolve into water. Look at the well-known example of edible salt, i.e., Sodium Chloride (NaCl).

As soon as it comes in contact with water, it decomposes into sodium (Na+) and Chloride (Cl-) ions. And, after that, you can’t take it apart from water.

There are numerous other mineral salts like that. For instance, you will find Calcium Carbonate (CaC0₃), Potassium Sulfate (K₂SO₄), or Magnesium Nitrate (MgNO₃). 

The list goes on and on and on.

All these are ionic compounds, and as a result, they easily dissolve into the water. Owing to these, we can’t filter them out fast.

Apart from these, there are other materials like lead, Chlorine (not Chloride ions), Arsenic, and others. 

Lead can find its way in from leaky and old pipes. Water treatment plants use chlorine to disinfect. And, arsenic can be in larger quantities when you get water from an underground well.

That’s not all. Also, in rural areas, the water might contain pollutants like particles from fertilizers or insecticides. These all can dissolve into the water.

Also, in some cases, you can trace out pharmaceutical chemicals from tap water.

All these materials belong to the list of inorganic compounds that we come across in tap water.

What organic substances can be in the water?

Now, let’s talk about the organic compounds that find their way in our tap water.

We can divide them into two categories-

• Organic Chemicals
• Micro-organisms

Again, if you took elementary chemistry, you probably have some ideas about what organic chemicals are.

Organic compounds or organic chemicals fall into the category of compounds that hold chains of Carbon bonds inside them. Apart from carbon, there will be other molecules like Hydrogen, Oxygen, Nitrogen, and so on.

The most common example is Methane (CH₄). It is the primary component of natural gas. There are other organic compounds like Benzene (C₆H₆), Ethane (C₂H₆), propane (C₃H₈), and many more.

If you test your water, you will find some existence of these materials.

In addition to those, there are polymer chemicals like polyethylene (-C₂H₂-), PVC, or clusters of Teflon that you might find remaining to some extent in the tap water.

These set up the list of organic chemicals in the tap water.

Now, let’s shed some light on the micro-organisms.

If we test river water and tap water side by side, we’ll find many different types of microorganisms in both. Surely, the river water will contain those in larger quantities.

So, what are these microorganisms? There are life-forms like algae, moss, virus, bacteria, and protozoa that come floating with the water.

They can all dissolve into the water pretty fine. For example, a bacteria named E. Coli is the primary reason causing diseases like Cholera. 

Municipal water treatment plants clinically wipe out most of these life-forms. But, still, some of these can remain in the tap water.

How? They can find a way due to the leaky pipes from water distribution.

So, these are the primary constituents of dissolved organic compounds in the water. And, as they dissolve, we can’t visually identify them out of the water.

What is the unit of TDS?

So, as we’ve discussed earlier, TDS means how much dissolved solid is in the water.

Now, let’s try to define it quantitatively so that we can quickly measure it and get an idea about the quality of the water.

The quantitative definition of TDS should be, how many milligrams (mg) of dissolved solid is in per liter (L) of water.

So, you can easily guess the unit of TDS is mg/L. For example, suppose you have two liters of tap water. After testing, you find out that there is 500mg of solids dissolved in it.

That means the TDS of that water would be = (500/2) = 250 mg/L. Very straightforward, right?

Apart from this, there is another unit for measuring TDS. That is ppm or parts per million. So, what does it mean?

Let’s take a glass of water. Most of it is water. But, there are other molecules here as well. So, let’s assume, there are 10000 molecules of water. Besides, there are 20 molecules of other chemical species.

So, let’s use the unitary method.

Among 10000 molecules there are 20 different molecules.

Therefore, in 1 million or 1000000 molecules there would be = (20 X 1000000/10000) = 2000 molecules.

As a result, this sample of water has a TDS of 2000 ppm. 

So, you see, these two are the conventional units of TDS worldwide. The best thing is, as the solution is dilute, in these cases, these two units are equivalent.

Therefore, 1 ppm = 1 mg/L. So, if we say, a sample of water has a TDS of 500, that means you can assume, it has an equivalent TDS of 500 mg/L or 500 ppm.

How is the TDS of water measured?

Now, that we know the basics of TDS in drinking water, let’s find out how we can measure it.

We can do it in two ways.

• Boiling the water sample, weighing the rest, and averaging them over.
• Using a TDS meter.

So, let’s discuss how we can do this.

Way No: 01 (Boiling, Weighing & Averaging)

The first way is pretty simple but time-consuming. You take 250 ml of water as a sample and keep boiling it until all the water molecules get evaporated.

As all the water molecules have turned into vapors, all that remains are those organic and inorganic compounds that came in dissolved with the water.

Now, you measure the weight of the remaining sample. So, suppose you measure a weight of 10 milligrams.

So, assume 250 ml of water contains 100 milligrams of residue.

Now, using the unitary method, 1000ml or 1L of water will contain (100 X 1000/250) = 400 mg of residue.

Therefore, the TDS of the sample is 400. Now, you might want to take 2 or 3 samples like that and average the result over to get the accurate estimate.

For example, you find out TDS of 400 in the first sample, then TDS of 370 in the second and TDS of 390 in the third. So, the average TDS would be 387 approximately.

In this fashion, you can easily measure the TDS of the tap water. But, you can realize this might take some time.

Way no. 02: Using a TDS meter

The second way is easy and fast. But, you will need an additional tool in this case.

In this case, you will use a TDS meter. You can check one out at Amazon.

So, how does this work? As minerals and organic compounds mix with the water, certain characteristics of water changes.

One of those is the conductivity of water. You probably know that pure water is an insulator. That means it does not pass electrical charges. 

Now, as more and more minerals dissolve into the water, there will be more and more ions in the water. 

As a result, the water starts to conduct to some extent. The more the minerals, the higher the conductivity.

So, the TDS meter works on this principle. They find out the resistance of a sample of water, and then, accurately estimate the TDS of the sample.

Other types of TDS meters take into account the change of salinity to determine the TDS of water.

If you’d like to find out the TDS quantity of your tap water, you can use any of the TDS meters out there.

Here, you can take a look at Amazon.

How can you interpret the TDS value?

Now, that we know the ways to measure the TDS of water, let’s focus on something else.

So, you find out the TDS value of tap water. It might have a value of 500 ppm, 800 ppm, or 1000 ppm. What do you do with this value?

That’s why you need to know what to make out of the TDS values. In normal cases, the tap water should have a TDS value below 500.

Now, if you’re using reverse osmosis (RO) water filter, the TDS value will come down to almost 20 to 50 ppm. So, RO purifiers wipe out most of the dissolved materials from the water.

If the TDS value becomes too low, the water loses out the mineral contents. In that case, you can add an alkaline filter to store back some of the minerals.

On the other hand, if the TDS value is way over 500 ppm, it might cause some health concerns. 

In this case, the recommended action would be to install a water filter to keep the value in your desired range.

So, in short, you should take the following actions after finding out the value of TDS of your drinking water. This is approved by the World Health Organisation (WHO).

Are you looking for a water filter? If you’re, then check out our recommended list of water filters in our latest post here.

Conclusion

So, in this post, we tried to cover everything you need to know about the term TDS of water. 

Interpreting the value of TDS can be very important and gives you insight into the quality of your water.

If you still have some queries left, you can let us know in the comment section. Also, you can reach out to us through our contact page.

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