Difference Between TDS and Hardness

Main Difference – TDS vs Hardness

Water is a substance composed of H₂O molecules. It is one of the most essential compounds for life on earth. Humans use water in many different ways; for drinking, washing, other household needs, industrial needs, etc. Hence, measuring the water quality is very important. There are various chemical and physical parameters measured regarding water quality. Some chemical parameters include pH, electrical conductivity, total solids, total dissolved solids (TDS), and hardness. Some physical parameters include color, odor, etc. TDS or total dissolved solid in water refers to inorganic salts and some trace amounts of organic matter that are dissolved in water. Hard water is water that has a high mineral content. The main difference between TDS and hardness is that TDS include inorganic and organic substances that cannot be filtered through a filter paper whereas hardness is due to the presence of magnesium and calcium salts of carbonate, sulfate and chloride.

Key Areas Covered

1. What is TDS
     – Definition, Explanation, Methods to Measure, Uses
2. What is Hardness
     – Definition, Temporary and Permanent Hardness, Units of Measurement
3. What is the Difference Between TDS and Hardness
     – Comparison of Key Differences

Key Terms: Calcium, Carbonate, Hardness, Hard Water, Magnesium, Minerals, Permanent Hardness, Sulfate, TDS, Temporary Hardness, Total Dissolved Solids

What is TDS

TDS stands for total dissolved solids. Dissolved solids or DS refers to any mineral, salts, metals, cations and anions that are dissolved in water. Total dissolved solids refer to DS along with trace amounts of organic matter that is dissolved in water. TDS is a chemical parameter regarding water quality.

TDS include anything other than pure H₂O molecules present in water. These dissolved solids should not be filtered through a filter paper having around 2 micrometer pores. The unit used to measure TDS is ppm (parts per million). In general, cations and anions dissolved in water account for TDS.

How to Measure TDS

There are two methods to measure TDS.

1. Gravimetric Method
2. Electrical Conductivity Method

Gravimetric Method

This method includes evaporating the liquid and measuring what is left. The residue that is left after evaporating water equals TDS. This residue can be weighed. Although this method is time-consuming, it gives an accurate measurement.

Electrical Conductivity Method

The electrical conductivity of a water sample is directly proportional to the amount or the concentration of ions present in water. It measures the ability to conduct electricity through the solution. This measurement can be obtained either by conductivity meter or TDS meter.

Figure 1: TDS Meter

Measuring TDS is used for several purposes. It is used as an indicator for river, lake and stream testing, swimming pool and spa maintenance, agriculture and hydrophonic cultivations, aquarium maintenance, etc.

What is Hardness

Hardness refers to the presence of a high mineral content. Hard water is formed when water flows through limestone and chalk. These rocks are made up mainly of magnesium and calcium carbonates. There are two types of hardness: temporary hardness and permanent hardness.

Figure 2: Hard Water Calcification

Temporary Hardness

Temporary hardness is due to the presence of bicarbonate minerals that can dissolve in water, mainly magnesium and calcium carbonates. These minerals form magnesium and calcium cations (Ca⁺², Mg⁺²) along with carbonate and bicarbonate anions (CO₃^2- and HCO₃^–) when dissolved in water. Temporary hardness can be removed by boiling water or by adding lime.

Permanent Hardness

Permanent hardness is due to sulfates and chlorides of magnesium and calcium. These minerals do not precipitate when heated. Therefore, permanent hardness cannot be removed simply by boiling. It can be removed using water softeners or ion exchange columns.

Measuring Water Hardness

The total hardness of water is the sum of concentrations of calcium and magnesium cations in mol/L or mmol/L units.

Difference Between TDS and Hardness

Definition

TDS: TDS stands for total dissolved solids.

Hardness: Hardness refers to the presence of a high mineral content.

Causes

TDS: TDS is caused by inorganic salts and dissolved organic matter.

Hardness: Hardness is caused by either magnesium and calcium carbonates or their sulfates and chlorides.

Units

TDS: TDS is measured from ppm units.

Hardness: Hardness is measured from mol/L or mmol/L units.

Measurement

TDS: TDS is measured by either gravimetric method or electrical conductivity analysis.

Hardness: Hardness is measured using laboratory experiments or hardness testing kits.

Effects or Uses

TDS: TDS measurements are useful for river, lake and stream testing, swimming pool and spa maintenance, agriculture and hydrophonic cultivations, aquarium maintenance, etc.

Hardness: Hardness causes the formation of a white precipitate on container surfaces, alter the taste of water, the formation of soap scum, etc.

Conclusion

TDS and hardness are water quality parameters. The main difference between TDS and hardness is that TDS include inorganic and organic substances that cannot be filtered through a filter paper whereas hardness is due to the presence of magnesium and calcium salts of carbonate, sulfate and chloride.

Reference:

1. “TDS.” Lab 2: Determining TDS, Available here.
2. “TDS total dissolved solids water quality testing TDS meters testers.” Water filters Online, Available here.

Image Courtesy:

1. “Measuring water purity with TDS meter” by Your Best Digs (CC BY 2.0) via Flickr
2. “hard water” by Graeme Maclean (CC BY 2.0) via Flickr