Hard Water - What is it?

The rain water, devoid of any minerals, picks up various types of naturally occurring dissolved minerals or inorganic salts as it passes over different surfaces and percolates down the ground. The types and quantum of minerals present in the water depends on the source from which it is drawn.

Water that contains hard salts, namely calcium (Ca) and magnesium (Mg), more than the prescribed limits is called Hard Water. When the concentration of these hard salts exceeds its ‘solubility constant’, they start precipitating which leads to hard scaling.

Calcium and Magnesium may be present in the water in both carbonate and non-carbonate form. Carbonate Hardness is known as temporary hardness and non-carbonate hardness is known as permanent hardness.

While the hardness content in surface water (Rivers, Dams etc) is generally (few exceptions are there) within the prescribed limits, in ground water (Eg. Deep Bore Wells) the hardness can be excessive and the level of hardness is based on the sub-soil characteristics.

Around 75% of ground water in India is considered hard water since the water contains more than 150 ppm of total hardness. As per the WHO standards, the permissible hardness level in drinking water is 50 ppm (parts per million).

Continuous usage of water with excessive hardness content for general purpose and consumption, may lead to various types of practical difficulties and health hazards in the long term.

Hazards of Hardness

Hard water is generally not suitable for utility, consumption and industrial purposes. A few of the challenges posed by hard water usage is here below.

• Excessive hardness in water causes scale deposits in pipes, faucets, showers, sinks, bathtubs and other plumbing acces sories leading to clogging and galvanic corrosion in the long term
• Scale build-ups can severely affect the efficiency of process equipment such as cooling towers, boilers and other industrial machineries that use water.
• Hardness can hamper the functioning of appliances such as washing machines, water heaters, dish washers and any water-using equipment due to the scale build-up it causes.
• Soaps and detergents do not easily dissolve in hard water thereby preventing sufficient lathering. This in turn drastically reduces the cleaning power of soaps and detergents resulting in partially washed clothes carrying residual detergents. Hard Water also adversely affects the life of fabrics.
• Continuous usage of hard water for bathing leads to craggy hair, hair loss, dry and itchy skin on account of soap scums.
• Leaves spots and deposits on kitchen utensils and expensive crystal/silver wares
• If used for consumption, hard water can lead to various health ailments including kidney stone formation. Hard water also spoils the natural taste of food and beverages.

Technology and Process Description

Ion-exchange based water softening is the conventional way of reducing hardness content in water and is a proven and time-tested method used globally. Synthetic polystyrene material in the form of very small spherical-shaped beads called cationic resins are used in this method to facilitate the exchange process.

In this water softening process, the salts that induce hardness in the water (Ca,Mg) are substituted by a soft salt (Sodium-Na), a process termed as ion exchange. The crosslink structure of the resins contains negatively charged ‘exchange sites’ that can hold the ions.

When the feed water containing hard salts is passed through a bed of cationic resins supersaturated with sodium, these resins act as an exchange point where calcium and magnesium ions in the feed water stick to the ‘exchange sites’ on the resin surface and the sodium ions are knocked off. Calcium and Magnesium both being divalent easily displace the monovalent sodium ions and get attached to the resin. Feed water will continue to get softened until the resins get saturated with hard salts.

After the ‘Service Cycle’ is completed, the resins will have to be again regenerated with brine(sodium-rich) solution that is stored in the brine tank. This brine solution is injected into the softener vessel using an ejector. The concentrated sodium in the brine solution replaces the calcium and magnesium ions trapped on the resin surface and the water softener will be ready for the next ‘Service Cycle’. Excess brine and other impurities are washed out during the rinsing phase.

Note: Brine Solution need to be prepared by dissolving salt in water at prescribed concentrations.

In case the feed water has turbidity, suspended solids and other related characteristics more than the permissible limits, suitable pre-treatment modules need to be used to ensure the long-term efficacy of the Water Softener Plant.

Maintenance of Water Softeners

Regeneration (Brine Injection) Process: After completion of the service cycle (softening process), the resins will be filled up with Hard Salts and brine solution needs to be injected into the softener unit to remove the trapped hard salts and again saturate the resins with sodium. This is called regeneration, a vital process to ensure the continuity of the softening process. The injection of brine (NaCl) solution is done in the reverse direction. Based on water characteristics and end use applications, the regeneration can be done either by using raw water or the softened water.

Rinse: Once the regeneration is completed, fresh water is allowed to pass through the resin bed in the regular direction. This helps in gently washing out the excess salt, other impurities and resets the resin bed for the next service cycle.

These maintenance processes need to be performed at prescribed intervals based on the recommended OBR (Output Between Regeneration) for optimal performance and longer life of resins in the water softener plant.

The polystyrene resins used in water softeners tend to break after prolonged use due to factors such as continuous water flow, high pressure inside the vessel etc. Such breakages are termed as attrition loss and is normally around 5-7% of the total resin quantity per annum. These resin fines (broken resins) if not removed from the vessel will choke up the bed and increase the pressure drop resulting in reduced flowrate of the water softener unit.

The fines on top of the resin bed are removed in normal backwash but some fines that go deep into the resin bed remain there. To remove these fines, extended backwash along with air scouring is used. This method is recommended based on the unit assessment done by our technical personnel but generally it is done - unless warranted under special circumstances - once in a year.

Product Features and Technical Information of Water Softeners

Unless warranted under special conditions, Water Softeners and Water Softening Plants generally comprise the following major parts

• iVessel
• iiValve
• iiiFood-Grade Cationic Resins
• ivBrine Tank

The basic design and dimensions of Water Softeners are highly formula-driven and should be primarily based on the following factors.

• Total Hardness present in the water
• Other related feed water characteristics
• Hourly Flow Rate (LPH) required
• Volume of water to be softened per day
• Desired OBR (Output between Regeneration)
• Site Conditions and end use applications

General Applications of Water Softener Plants

• Used to reduce high level of hardness content in ground water for domestic-residential applications.
• Used in commercial segments. ie., Hotels, Hospitals, Commercial Laundries etc
• Used as Pre-treatment to DM Units and RO Plants
• Used for the recirculation of industrial and process water.ie., Boiler Feed, Textile dyeing, Cooling Water Towers etc