Tap to Read ➤

Methods for Water Purification

Aishwarya Nirmal Jan 28, 2019
We always trust the Water Supply Department to provide us with pure drinking water. Ever wondered how exactly the water gets purified? Read on to know more about the methods of purifying water.
Of the 3% freshwater available on earth, 68.7% is frozen in the form of icecaps and glaciers, 30.1% is groundwater, 0.3% is surface water and 0.9% is unaccounted for. It is clear that we use the 0.3% water by purifying it over and over again.
The surface water includes 87% of lakes, 11% of swamps and remaining 2% of rivers.With so much of waste being dumped into them every minute, water pollution is inevitable, and thus the purity cannot be guaranteed. Hence, none of these sources provide us with clean and pure drinking water. In order to make this water consumable, it is processed in many ways.
The water from rivers and lakes is treated in large water treatment plants. These methods give the basic treatment to the polluted water. Following is a 5 step pre-treatment process for water purification.

Pumping and Containment

The tanks in which water is stored and the pipes through which it is brought in, should be constructed with materials that do not dissolve or react with water. The tanks and pipes should be thoroughly cleaned.


In this step, substances like huge stones, plastics, etc. are filtered.


The water is stored in reservoirs for biological processing and also as a backup source in case of drought.


The hardness of water is treated in this step by adding sodium bicarbonate (NaHCO3).


In this step, chlorine is added to the water to eliminate the biological organisms that enter the water from the tanks and pipes.
After the pre-treatment, water has to undergo several more treatments to make it consumable. The following processes are generally carried out in the water treatment plant.

pH Adjustment

The pH level of neutral water is 7. If water traveling through the pipes is acidic, it may corrode the pipes, causing the pipe metal to dissolve in the water. Hence, if water is acidic i.e. having pH less than 7, lime is added to increase its pH.
If water is too alkaline, hydrochloric acid can be added to it, to decrease its pH. Alkaline water can also corrode the pipe metal, but compared to the acidic water, chances of corrosion are lower.


It is the process of removing turbidity present in the water. The water is made clear and colorless. The chemicals used for this process are iron hydroxide and aluminum hydroxide.


In sedimentation, the floc that precipitates from the water during the flocculation process is separated. The sludge formed after the treatment settles at the bottom of the sedimentation basin. The floor of the basin is constantly cleaned.


This is the last step for the removal of dirt. The minutest of dirt particles are filtered in this process. Sand filters are used for the filtration. The topmost layer is of carbon, where the organic impurity is removed. Then the water is passed through the sand layer, where particles are trapped as they pass through.

Removal of Chemical Impurities and Ions

This is a process of removal of hazardous chemical substances and elements like lead, nitrite and arsenic. Hardness of water is treated by removing the ions causing hardness i.e. calcium and magnesium.


Chlorine and other chemicals like ozone and hydrogen peroxide are added to water to kill disease-causing bacteria and viruses. Sometimes, water may be treated with UV radiation to kill the pathogens.
Water from the supply plants travels through large pipes to the residential areas, during which some impurities may get added to it. Hence, many residential complexes have certain purification systems installed locally. Some of the home-based water purification methods are as follows.


It is the most basic form of water purification. Water is boiled for a minimum of 10 minutes. Though this method may kill the pathogens in water, it cannot filter the harmful chemical elements.

Chemical Purifiers

Iodine, potassium and chlorine are some of the chemical purifiers. They are available in the tablet form in the market and have to be added to water as per the quantity to be treated. They are the only portable form of purifier hence, beneficial while traveling.


In distillation, water is heated till it evaporates. The vapor is then cooled and allowed to condense on the cooler surfaces of the apparatus. Water droplets from the condensed vapor are collected to obtain distilled water. It is a very slow process and cannot be implemented for large quantities of water.

Commercial Filter

Individual homes may have some purifying instruments like water filters to ensure further safety. Commercial filters make use of many processes like reverse osmosis, ion exchange, activated-carbon adsorption, micro-porous filtration, ultra-filtration and photo-oxidation to purify water.
Approximately 60-65% of human body is made up of water and it is essential for us to consume pure water to maintain our health. After all, prevention is always better than cure.