Is clear water pure? Some may think so. They feel that as long as the water is colourless then it means it is pure. Others can talk about the purity of water by tasting. You will be amazed that in as much as those are some of the determinants in pure water other factors ought to be considered too. We are all aware that water is formed by two basic components; hydrogen and oxygen. However, water as a solvent can dissolve quite several solids. These solids make up what is referred to as total dissolved solids (TDS).
What are Total dissolved solids in the water?
Total dissolved solids are the number of inorganic minerals and organic substances dissolved in water. The minerals and salts from the cations and anions.
Inorganic minerals include:
The above salts form the cations.
Anions components include:
Organic solids dissolved in water comprise humic substances from dead plants and some forms of hydrocarbons.
Water also dissolves other solid compounds such as lead, arsenic, manganese, copper, iron and mercury.
All of these amount to total dissolved solids in water.
Where do these dissolved solids originate? There are both natural and human sources.
Natural sources include:
- Rocks that contain mineral and salt deposits, may find their way into the water
- Springs are natural reservoirs containing minerals from rocks and chemicals from runoffs.
- Lakes collect water from rivers which may contain total dissolved solids.
- Oceans contain high levels of total dissolved solids like sodium.
In some studies, it was found that groundwater contained higher levels of total dissolved solids in comparison to surface water.
Human activities increase the levels of TDS in water. How is this possible? Consider the following:
- Agriculture: inputs such as fertilizers, pesticides and herbicides. These may find their way into water through runoffs.
- Industrial activities lead to chemical and physical wastes which contain solids that dissolve in water.
- Salt used to de-ice roads.
- Wastewater from households and sewage runoffs. Boreholes and wells near septic pits have been found to have high levels of dissolved solids.
- Storage tanks, boilers and pipes act as sources of soluble solids.
- Chemicals used in water treatment plants.
How to measure TDS in water
The amount of total dissolved solids is measured in mg/litre according to set standards. In some instances, it can also be calculated in terms of parts per million (ppm).
There are different devices used to measure the quantity of total dissolved solids in water:
1- Use of digital TDS meter
Digital TDS meters are easy-to-use devices used in calculating the amount of total dissolved solids. All you need is a handheld TDS meter. This can be purchased at affordable prices online or in medical stores.
How do you use a TDS meter?
- Take a sample of water in a glass
- Press the hold button on your TDS meter and immerse it in water
- Press the hold button again, remove the meter from water and take the readings.
That is very easy and quick, isn’t it? Most TDS meters are calibrated to read amount in parts per million (ppm).
The TDS meter is an effective available device in households. You can easily obtain readings for your tap water or borehole water.
2- Using filter paper and weighing scale
This is also considered an easy procedure that can be conducted at home. To be successful you will need the following:
- Filter paper
- Weighing scales
- Evaporating dish
Then do the following:
- Make sure your evaporating dish has no particles while weighing. Get the initial weight.
- Filter the water at least thrice.
- Take the filtrate and place it in the evaporating dish and get the final weight.
To calculate the amount of total dissolved solids in the sample water, calculate the difference between the initial weight of the evaporating dish and its final. The value obtained is the amount of TDS.
3- Electrical conductivity
Ions present in water makes it a good conductor of electricity. This method is used to determine the number of TDS by detecting the strength in picking and transmission of electric currents. A simple example is sample water with sodium chloride. Compared to pure water, sodium chloride water records a higher reading in electrical conductivity than pure water due to the presence of sodium ions. Hence the higher the reading the larger the levels of total dissolved solids.
Why is it important to measure TDS?
When was the last time you measured TDS in the water? The answer depends on whether you deem it necessary. In some countries such as the USA, measuring TDS is a guideline in determining accepted levels for safe drinking water. This varies from one region to another due to factors including geological locations. It has been noted that water with high levels of TDS has a bitter taste, while one with extremely low TDS is flavourless.
Measuring TDS is considered in investigating pollutants. Sources such as industrial waste and other harmful chemicals contaminate water. Polluted water has high levels of total dissolved solids. However, measuring TDS does not individually distinguish the type of pollutants dissolved in water.
Understanding PH: Studies have shown that there is a direct correlation between TDS and PH. The latter tackles the acidity or alkalinity of water. A higher level of TDS is associated with low ph. Acidic water causes corrosion of metal equipment.
Measuring TDS helps in understanding hard water. This is because inorganic cations such as calcium and magnesium increase hard water possibilities. Readings of higher levels of TDS may be used to explain scaling in boilers, storage tanks and water pipes.
Is high TDS in drinking water harmful?
After understanding TDS some may ask if it is still safe to drink water with high TDS levels. In-depth studies have been conducted regarding the issue. According to WHO, previous studies undertaken had concluded that there was a correlation between TDS and complications such as cancer, cardiovascular disease and arteriosclerotic. Nonetheless, these conclusions have not been regarded as indicators to determine if TDS in water is harmful to both animals and humans.
This has prompted WHO to regard TDS as an aesthetic factor as far as safe drinking water is concerned. Many countries around the globe give different variations to the levels of TDS considered to be safe in drinking water.
It is accepted that levels of TDS in water are beneficial to the body, due to the presence of minerals such as magnesium. Furthermore, WHO conducted experiments ascertaining if TDS in water is related to its taste. It was reported that moderate levels of TDS made water palatable, while extremely high TDS made it bitter and low levels made it insipid.
The presence of contaminants which include heavy metals like mercury and lead can pose some health risks to human and animal life.
How to lower the TDS level in drinking water
If you realize that your drinking water has high-level TDS what can be done? It is simple; all you need to do is demineralize your water through the following methods:
This is a procedure that applies the biological process of osmosis. It is done by allowing water to pass through a semi-permeable membrane which is selective as it allows some particles to pass through while others are restrained.
To achieve this effectively, the pressure is applied as water passes through the membrane. This makes most TDS particles to be held back as water particles pass through.
RO has proven to be a successful method of demineralization since more than 90% of TDS are restrained. It has been used in the majority of water filters.
This method uses the technique of boiling and condensation. We can easily understand this from the natural hydrological cycle.
As the water boils turning into steam it evaporates leaving behind TDS particles. Steam is condensed and collected separately as pure water. This method has been utilized in making seawater palatable in areas where there are no other sources of water.
As explained earlier, TDS in water consists of inorganic substances that are cations and anions. The process of deionization ensures that water is freed of these ions. In this procedure, TDS ions are exchanged by hydrogen and hydroxide ions. Hydrogen ions for the cations while hydroxide ions for anions.
Two separate resin beds are used, one with hydrogen ions and the other with hydroxide ions. When water is pumped in, cations such as magnesium, calcium and potassium are exchanged and replaced with hydrogen ions. When water continues to flow in the second resin bed, anions such as carbonates and Sulphates are exchanged with hydroxide ions. The result is demineralized water.
However, in using this method, the hydrogen and hydroxide ion resin beds have to be regenerated by using dilute hydrochloric acid and dilute hydroxide respectively. This is to ensure that they maintain the hydrogen and hydroxide ions.
In as much as deionization is a successful method in demineralization, the user should be aware of the following limitations:
- Cost: the resin beds and the installation can be very expensive.
- Deionization does not guarantee the removal of heavy metals such as lead, mercury or chromium in water.
Paying attention to what has been discussed so far; how can you tell pure water? How does your water taste? The answers are very clear if we understand TDS.