Are you planning to buy a new water purifier? One word that you encounter frequently is TDS. What is TDS?
Read on to know everything about TDS. We shall also look at ways how to measure TDS. We will also discuss the acceptable levels of TDS and how to eliminate TDS from the water.
What is TDS?
TDS stands for “Total Dissolved Solids.” Our Chemistry teacher teaches us that the molecular formula for water is H2O. Therefore, every molecule of water contains two atoms of Hydrogen and one atom of Oxygen. It is true of pure water.
However, the water available on this planet contains a lot of salts like calcium, magnesium, potassium, fluorides, carbonates, and sulfates, and many others. It is a long list.
Also, the groundwater and other water sources like rivers, lakes, and ponds contain heavy metal impurities like arsenic, lead, and mercury. These chemicals are dangerous. Therefore, you need appliances like water purifiers to eliminate these contaminants.
At the same time, excessive salt intake can also be harmful to health. There is a need to regulate the levels of TDS in water.[/su_note]
Is TDS necessary?
Ideally, you should consume pure water. However, pure water does not have any taste. Salts like calcium, magnesium, and potassium are necessary to impart the required flavor and nutrition value. However, you should ensure that you maintain the levels of TDS. Let us now see the acceptable levels of TDS.
TDS – Acceptability Levels
The Bureau of Indian Standards (BIS) decides the acceptability levels of TDS in water. According to BIS, the TDS up to 500 mg/liter is acceptable. The following table can make things clear.
| TDS Levels mg/liter | Reasons for Acceptability/Non-acceptance |
|---|---|
| Less than 50 | This level is unacceptable because water with such a low proportion of TDS does not contain the required minerals for healthy growth. |
| 50 to 150 | It is an acceptable level because there are industrial and other types of pollution involved |
| 150 to 250 | This level of TDS is the healthiest as far as cardiovascular health is concerned |
| 250 to 350 | There are places in India where the ground level water has TDS levels in this range. It is also an acceptable level |
| 350 to 500 | The BIS certifies water with TDS levels up to 500 as acceptable for human consumption |
| Above 500 and up to 1200 | Any level over 500 is not acceptable for human consumption. However, you can purify this water using RO and other kinds of water purifiers. It can result in the elimination of excess TDS thereby reducing it to below 500 and making it fit for consumption. |
| Above 1200 | Water having TDS levels over 1200 is utterly unfit for human consumption. |
TDS – Palatability Levels
There is a difference between the acceptability and palatability levels of TDS in water. We shall now look at the palatability levels of TDS before proceeding with measuring the amounts of TDS in water.
| TDS Levels mg/liter | Palatability Quotient |
|---|---|
| Any level less than 300 | Excellent for drinking |
| More than 300 and up to 500 | These levels are acceptable |
| 500 to 900 | As you can eliminate the excess TDS by using water purifiers, it is a fair level. |
| Above 900 to 1200 | High-quality water purifiers can reduce TDS levels in this range as well. However, water having a TDS level in this range constitutes poor palatability. |
| Above 1200 | It is an unacceptable range. |
How do you measure TDS levels in water?
You can measure TDS levels in water using three methods. Let us look at them in brief.
1. Using Digital TDS Meter (Easiest)
You must have noticed that the water purifier technician uses a small hand-held device similar to a thermometer to check out whether the water you get is pure or not. This hand-held device indicates the level of TDS in any solution like water.
Using a TDS meter is very easy. We explain the procedure involved in brief.
- Collect the sample water in a clean cup
- Switch on your TDS meter and remove the cap.
- You have two electrodes at the end of the meter. Insert the electrodes into the water. Never submerge the entire meter as you can risk damaging it.
- Ensure that there are no bubbles attached to the ends of the electrodes. You can shake it off by stirring the water lightly with the meter.
- Wait for about 5 seconds so that the reading stabilises. This is your TDS reading
- Remove the meter from the water and shake off the excess liquid from the ends of the electrode. Place the cap over the meter and store it safely.
The result is just a number that flashes on the screen as soon as you place the instrument in contact with the water. This number indicates the quantity of dissolved solids in the water.
Any value in the range of 300 mg/litre (ppm) to 500 mg/litre is good. TDS levels above 500 mg/litre indicate that the water is hard and hence requires purifying.
Any value below 50 mg/litre increases the acidity content in the water thereby making it unfit for consumption. Levels over 2000 mg/litre are not at all potable.
It is very important to note that the TDS meter indicates the amount of TDS in water. It does not give any indication of the nature of contaminants present in it. It can happen that the TDS levels of water could be 300 ppm.
Is it safe?
Logically speaking, it should be but do you know what the 300 ppm comprises of? It could be arsenic and mercury alone. Now, is the water safe?
This brings the importance of the water purifier into the equation.
2. The Electrical Conductivity Meter Method
We have seen that TDS constitutes both organic and inorganic salts. The Electrical Conductivity Meter method is the most common one in use today. Follow these steps to calculate the level of TDS in water.
Apparatus required for measuring the TDS.
- A sterilized glass beaker free of dust
- The water sample should be around 250C
- An electrical conductivity meter
The electrical conductivity meter works by releasing an electrical current into the water thereby measuring its resistance.
Measure the conductivity
Ensure that you keep the beaker on a flat surface. Switch on the conductivity meter and insert the measuring lead into the water.
- The reading will fluctuate a bit for a few seconds before stabilizing. You should note down the reading after it stabilizes.
- This measurement is the purity of water. The reading is in µS (micro Siemens). The lower the value, the purer is the water. Water in its purest H2O form has 0 µS.
Enter the data into the following formula to arrive at the TDS level in the water.
The basic formula for arriving at the TDS level in water is as follows.
TDS (in mg/litre) = KE (Co-relation factor) X EC (Conductivity)
KE, the Co-relation factor depends on various factors such as the nature of the liquid and the temperature of the water. It also depends on the atmospheric conditions. The value of KE ranges between 0.55 and 0.8. The reading from the electrical conductivity meter is EC.
- The temperature of the water sample and the current pressure conditions are essential for determining the KE factor.
- Water at around 250C and atmospheric pressure results in a KE factor of approximately 0.67
- Water with less than 500 mg/litre is acceptable as per BIS.
- However, water with higher levels of TDS in the range up to 1200 can be subject to water purification using RO water purifiers to make it fit for consumption.
Using this formula, you can measure the TDS levels in almost all liquids. However, each liquid will have a specific KE.
3. Using Filter Paper and a Weighing Scale
This method of measuring TDS levels in water is a conventional one. You can do it easily at home. You need not buy the electrical conductivity apparatus. Let us look at this method in brief.
Apparatus Required
- A clean and sterilized glass beaker
- Water sample for measuring TDS levels
- Filter paper
- Evaporating dish
- A sterile stirring stick
- Pipette large enough to collect a minimum of 50 ml
- A weighing scale
Weigh the empty evaporating dish
Using the weighing scale, weigh the empty evaporating dish. Ensure that the dish is completely dry and free from any extraneous particulate matter. Note down the reading in mg.
Stir the water sample
Stir the water sample vigorously to ensure that the particulate matters, if any, distribute evenly throughout the sample.
Collect the water sample
Using the pipette, collect a minimum of 50 ml of water sample. Ensure that you stir the liquid continuously while collecting the sample. The particulate matter should not settle at the bottom. It will not give an accurate reading. In case you find it difficult to do so, ask your companion to keep stirring the liquid while you obtain the sample.
Extract the filtrate
Pour the water sample through the filter paper into another beaker. Repeat the procedure at least three times to ensure the collection of the entire particulate matter in the filter paper.
Weigh the evaporating dish with the filtrate
Transfer the filtrate to the evaporating dish and allow it to dry completely. Weigh the evaporating dish along with the filtrate and note down the reading in mg.
Use this formula
TDS = [(A-B) X 1000]/ml sample
(A = weight of the evaporating dish and the filtrate and B= the weight of the evaporating dish alone.)
Note that you had used 50 ml of the water sample. Hence, the ml sample value would be 50 in this case. The final value of TDS is mg/liter
Ideally, the TDS level should be below 500 for the water to be fit for human consumption. However, you can use RO and other water purifiers to eliminate TDS if the TDS levels are more than 500. Any value over 1200 mg/liter is unacceptable according to BIS.
Why should you remove excess TDS?
We have seen that TDS is not a single contaminant. It is a mixture of various salts such as calcium, magnesium, and potassium among others. These salts are necessary for our good health.
However, the water sources in our country contain other undesirable impurities like human and industrial waste. There are instances of the water sources being contaminated by arsenic, mercury, and lead as well.
These impurities can pose a health hazard. Similarly, TDS levels of more than 500 ppm can also have a detrimental effect on our overall health. Therefore, you need to remove the excess TDS from the water before consuming it.
How do you eliminate excessive TDS?
You can eliminate excess TDS using three methods. Let us discuss them, in brief, to understand them better.
Distillation
This process involves boiling the water and allowing ten vapors to cool down. The resultant water is bereft of TDS as the salts remain in the boiling water solution. It is the easiest method to remove TDS.
Reverse Osmosis
Reverse Osmosis (RO) is a popular method of eliminating TDS. Almost all the water purifiers available on the market use this technique. This process involves the passage of water through a semipermeable membrane containing microscopic pores that allow molecules smaller than 0.0001 microns to pass through.
The TDS molecules are comparatively larger, and hence they get trapped in the membrane. RO is one of the best methods as it ensures the complete elimination of TDS from the water.
Deionization
The deionization process involves water passing through two electrodes (positive and negative). The ion-selective membranes enable the positive ions to separate from the water as they move towards the negative electrode. The resultant water is pure water.
However, it is better to use this procedure after subjecting the water to RO treatment. It helps to remove non-ionic organic contaminants.
Inference
We have seen the concept of TDS and learned how to measure the TDS levels as well. We have also looked at the aspects of eliminating excessive TDS and ensuring that the resultant water has acceptable levels of TDS.