Complete guide on how to Detect, Test & Treat Salinity in Water
Have you wondered why we can't drink ocean water? Many a times even at our homes we drink water only to find it being unpleasantly salty. Know all about salinity in water and how to use/get rid of it!
What is Salinity in Water?
Salinity in water is the presence of many dissolved salts in water responsible for its salty taste. It is defined as the number of grams of salts dissolved in per kilogram of water. It is often expressed in ppm or parts per thousand.
We are all familiar with % which is parts per hundred similarly parts per thousand is represented as (%o).
Salinity in ppt = (grams of dissolved salts)/(1,000 grams of seawater)
Factors affecting Salinity in water
The salinity increases with the increase in water temperature. As we know water evaporates, on being subjected to heat (from sunlight). Evaporation tends to decrease the volume of water thus resulting in greater concentration of salt in water. Hence higher the rate of evaporation, greater will be the salinity.
Rainfall increases the volume of water thus diluting the solution. This decreases the concentration of salts in water.
Freezing of water to ice results in lowering the volume of water. Low volume of water has high concentration of dissolved salts, hence high salinity.
Water from river runoffs increases the volume of the water body thus diluting the solution. This decreases the salinity.
Why Analyse Salinity for Water Quality?
Water under the effect of free dissolved salt ions behaves a bit differently than pure water with little to no minerals in it.
Salinity in Water and Conductance
Conductivity in water depends upon the availability of ions. These ions are highly mobile charge carriers and their movement is known as ionic mobility. Ionic mobility in water increases with increase in water salinity. This is because dissolved salt in water ensures the availability of free charge carrying ions. These free ions are rapid moving charge carriers increasing the conductivity of the solution. Electric current flows more readily in saline water than in fresh water.
Measuring the amount of salt dissolved in water helps, categorize the type of saline water. Based on their salt content they are classified as:
- Brackish water : Is a mixture of freshwater and saline water.
- Brine solution : Has a very high concentration of salt content in water. For this reason it is also known as Hyper saline water.
- Distilled water : Has very minimal if any amount of salt concentration.
Knowing the salt concentration in water helps determine the way to utilize the saline water. Eg. using saline water in machinery could choke and dissolve the parts forcing its break down. Hence distilled water is preferred to be used with machinery.
(The concentration in ppt of the above defined solutions is discussed in detail in urban water salinity standard section.)
Seawater is the largest source of Saline water.
There are many causes responsible for raising the salinity in a water body. Releasing untreated salt water can increase the salinity and pollute the water body. Eg. Releasing water used in tanneries to wash away the salt used for treating leather, untreated directly into the water body.
Urban Water Salinity Standards
Salinity (in ppt)
Category of water
0 - 0.001
0 - 0.5
0.5 - 30
30 - 38
Operations: Detect, Measure, Test & Treatment
There is no distinct color of saline water. In fact the ocean water which have its color owes it to the dissolved organic matter which gives it distinct green or brownish tint.
There is no odor detection. Taste is more important a parameter than odor in water salinity detection.
The concentration of salt in water could be determined by the following methods:
- Electrical Conductivity Test (EDT) - Measures the resistance offered by the solution to the flow of electrical current through it, hence determining Conductivity. More the salt in the solution lesser is the resistance offered and hence more easily current can conduct through it.
- Total Dissolved Solids (TDS) - measures the amount of organic and inorganic components dissolved in the solution.
Hydrometer is used to determine Salinity of a solution. It works on the principal of determining the density of the medium in order to calculate the salinity of it.
A Hydrometer is a device that measures the specific gravity or relative density of two different liquids. One of the two liquid is the liquid to be tested and the other is pure water of known standards.
Urban Water Salinity Treatment
Desalination or Desalinization is the process of extracting both the organic and inorganic impurities from Saline water. It can be achieved through three principal ways:
- Temperature - Distillation
- Electrical - Electric Current
- Pressure - Reverse Osmosis or RO
Temperature : Water Distillation System is used to remove the salinity in water. It is based on the concept of separating water from dissolved impurities based on the differences in their boiling points. Water easily reach its boiling point and turns into vapor form from liquid state while the impurities still remain in liquid/solid state. This vapor is collected, condensed to liquid water while the impurities remain behind. Check out the Water Distillation System in detail.
Electric current: is used instead of heating to separate water from impurities. A low voltage electric current is used to charge the salt ions and redirect them into a diversion. Water is thus separated from these salt ions and the process is known as Electrochemically Mediated Seawater Desalination.
Reverse Osmosis: is one of the popular method deployed to separate water from impurities. RO or Reverse Osmosis makes use of the difference in size of the molecules of water and the impurities. An osmotic pressure is applied onto the impure solution. This result in water molecules being smaller compared to that of the impurities passes through the semi-permeable membrane leaving behind the impurities. Check out the working of Reverse Osmosis in detail.
Saline water is beneficial for the survival of various salt water ecosystem. Many salt water fishes survive because their body adapts to the salt content in water. Moreover, Saline water of the ocean completes the minerals and salt requirement of all aquatic organisms which otherwise is an important task for the land animals to fulfil, making their body function properly.
Saline water is beneficial to the human body functions if taken in appropriate quantities.
It supplements our body with healthy minerals like magnesium, sodium and calcium. Sodium for instance is a necessary element in muscles and nerves functioning.
It prevents dehydration and helps keep the digestive tract in good shape.