Unit : 10

Water softening with dose calculations : 

Water softening is a process used to remove dissolved minerals, such as calcium and magnesium, from hard water. The most common method of water softening is ion exchange, which uses a resin bed to exchange the dissolved minerals with sodium ions.

The dose of the chemical used in water softening process is an important consideration to ensure that the water is effectively softened and that the resin bed is not damaged or exhausted. The dose calculation is typically based on the hardness of the water, the flow rate of the water, and the capacity of the resin bed.

The following is an example of a calculation for the dose of sodium chloride (salt) used in water softening:

  • Measure the hardness of the water in milligrams per liter (mg/L) of calcium carbonate.
  • Determine the flow rate of the water in gallons per minute (gpm).
  • Calculate the total hardness in grains per gallon (gpg) by multiplying the mg/L of hardness by 17.1 (1 gpg = 17.1 mg/L)
  • Determine the capacity of the resin bed in cubic feet.

Calculate the amount of salt needed in pounds per cubic foot (lb/ft3) of resin by dividing the total hardness in gpg by the capacity of the resin bed in ft3.

For example, if the water has a hardness of 300 mg/L, a flow rate of 50 gpm and the resin bed has a capacity of 1 cubic ft, the calculation would be:

(300 mg/L x 17.1) / 50 gpm = 102 gpg


102 gpg / 1 ft3 = 102 lb/ft3


This calculation would indicate that a dose of 102 pounds of salt per cubic foot of resin is needed to effectively soften the water and regenerate the resin bed.

It is important to note that the dose calculation may vary depending on the type of resin used, the specific contaminants that need to be removed and the regulations and standards that must be met. Additionally, the chemical dose should be adjusted periodically to account for changes in the water quality and flow rate. Consultation with a professional engineer would be necessary to design a cost-effective and environmentally friendly water softening process. 

Iron and Manganese , Arsenic removal : 

Iron and manganese, as well as arsenic, are common contaminants that can be found in water sources. Removal of these contaminants is important to ensure that the water is safe for human consumption and use.

Iron and manganese Removal:

Oxidation: Iron and manganese can be removed from water by oxidizing them into a solid form that can be filtered out. This can be done through chemical oxidation, such as with hydrogen peroxide or potassium permanganate, or through biological oxidation, such as with bacteria.  

Filtration: Once oxidized, the iron and manganese can be removed from the water through filtration, such as with a greensand filter, a manganese greensand filter, or a multimedia filter.  

Arsenic Removal:

Oxidation: Arsenic can be removed from water by oxidizing it into a form that can be filtered out. This can be done through chemical oxidation, such as with potassium permanganate or chlorine dioxide, or through biological oxidation, such as with bacteria.

Adsorption: Once oxidized, the arsenic can be removed from the water through adsorption, such as with an iron-based adsorbent or an activated alumina filter. 

Membrane Filtration: Arsenic can also be removed from water by using a reverse osmosis or ultrafiltration membrane.

It is important to note that the specific method of removal will depend on the type and concentration of the contaminants, the source of the water, and the regulations and standards that must be met. 

Additionally, the removal process should be designed and operated to minimize the environmental impact, ensure safety, and comply with regulations and standards. Consultation with a professional engineer would be necessary to design a cost-effective and environmentally friendly removal process for these contaminants.

Deflouridation systems : 

Defluoridation is the process of removing fluoride ions from drinking water. There are several methods that can be used to defluoridate water, including adsorption, precipitation, and ion exchange. Some common defluoridation systems include activated alumina filters, bone char filters, and reverse osmosis systems. These systems can be installed at the point of use, such as in a household, or at a larger scale, such as in a community water treatment plant. It is important to note that while fluoride is beneficial for dental health, excessive intake of fluoride can lead to health problems such as dental fluorosis and skeletal fluorosis. 

Cascade Aeration for ground water : 

Cascade aeration is a method of treating ground water by passing it through a series of aeration basins. The water is pumped into the first basin, where it is aerated through the introduction of air bubbles. The water then flows through a series of additional basins, with each basin providing additional aeration. The process of cascade aeration is used to remove dissolved gases, such as hydrogen sulfide and methane, from the water.

Additionally, cascade aeration can be used to remove dissolved iron and manganese, which can cause staining and discoloration of water. It can also be used to remove dissolved radon, which is a radioactive gas that can be harmful to human health. Cascade aeration is typically used for ground water treatment in small communities and rural areas.