Understanding Water Hardness and Treatment Methods

Posted on Jun 26, 2024 in Chemistry

What is Water Hardness?

Hardness of water is a property that prevents the lathering of soap. This is due to the presence of certain salts of calcium, magnesium, and other heavy metals dissolved in the water.

When hard water is treated with soap (sodium or potassium salt of higher fatty acids like oleic, palmitic, or stearic acid), it doesn’t produce lather but forms a white precipitate. This precipitate is formed due to the formation of insoluble soaps of calcium and magnesium.

Types of Hardness

There are two types of water hardness:

  1. Temporary/Carbonate Hardness: Caused by the presence of dissolved bicarbonates of calcium, magnesium, and other heavy metals, as well as carbonates of iron. The salts responsible for temporary hardness are Ca(HCO3)2 and Mg(HCO3)2. Temporary hardness is generally removed by boiling the water, which decomposes the bicarbonates, yielding insoluble carbonates or hydroxides that deposit as a crust at the bottom of the vessel.
  2. Permanent/Non-Carbonate Hardness: Due to the presence of dissolved chlorides and sulfates of calcium, magnesium, iron, and other heavy metals. The salts responsible for permanent hardness are CaCl2, MgCl2, MgSO4, FeSO4, Al2(SO4)3, etc. Unlike temporary hardness, permanent hardness is not destroyed on boiling.

Units of Hardness

Hardness of water is expressed in various units, including:

  1. Parts per million (ppm): Parts of calcium carbonate equivalent hardness per 106 parts of water.
  2. Milligrams per liter (mg/L): Milligrams of CaCO3 equivalent hardness present per liter of water.
  3. Clarke’s degree (°Cl): Parts of calcium carbonate equivalent hardness per 70,000 parts of water.
  4. Degree French (°Fr): Parts of calcium carbonate equivalent hardness per 105 parts of water.

Relationship Between Units

1 ppm = 0.1 °Fr = 0.07 °Cl = 1 mg/L

EDTA Method for Hardness Determination

The EDTA (Ethylenediaminetetraacetic acid) method is a complexometric titration used to determine the hardness of water.

Principle

The method relies on the formation of a stable complex between EDTA and the hardness-causing metal ions (mainly Ca2+ and Mg2+) in a 1:1 ratio. Eriochrome Black T (EBT) is used as an indicator, which forms a wine-red complex with the metal ions. During titration with EDTA, the EDTA first complexes with the free metal ions. Once all free ions are consumed, EDTA displaces the EBT from the metal-EBT complex, causing a color change from wine-red to blue, indicating the endpoint.

Scale and Sludge Formation in Boilers

In boilers, the continuous evaporation of water leads to an increase in the concentration of dissolved salts. When these salts reach their saturation point, they precipitate out on the inner walls of the boiler, forming either sludge or scale.

Sludge

Sludge is a soft, loose, and slimy precipitate formed in the colder parts of the boiler. It is formed by substances with higher solubility in hot water than in cold water, such as MgCO3, MgCl2, CaCl2, and MgSO4.

Disadvantages of Sludge

  • Poor conductor of heat, leading to heat wastage.
  • Can get trapped in scales, worsening the problem.
  • Disturbs boiler operation by settling in areas of poor water circulation.

Scales

Scales are hard deposits that adhere firmly to the inner surfaces of the boiler. They are difficult to remove and are a major source of problems in boilers.

Causes of Scale Formation

  • Decomposition of calcium bicarbonate.
  • Deposition of calcium sulfate.
  • Hydrolysis of magnesium salts.
  • Presence of silica.

Disadvantages of Scales

  • Reduced heat transfer due to low thermal conductivity, leading to fuel wastage.
  • Overheating of boiler tubes, compromising safety.
  • Decreased boiler efficiency.
  • Risk of explosion due to uneven expansion and cracking of scales.

Boiler Corrosion

Boiler corrosion is the deterioration of boiler material due to chemical or electrochemical reactions with its environment.

Causes of Boiler Corrosion

  • Dissolved Oxygen: Attacks boiler material at high temperatures.
  • Dissolved Carbon Dioxide: Forms carbonic acid, which corrodes boiler material.
  • Acids from Dissolved Salts: Hydrolysis of magnesium salts releases acids that corrode the boiler.

Caustic Embrittlement

Caustic embrittlement is a type of boiler corrosion caused by highly alkaline water. Sodium carbonate (Na2CO3), often present in softened water, decomposes in high-pressure boilers to form sodium hydroxide (NaOH), increasing the water’s alkalinity.

NaOH can seep into minute cracks in the boiler, where it concentrates and attacks the surrounding iron, forming sodium ferroate. This weakens the boiler material, particularly at stressed points, and can lead to failure.

Priming and Foaming in Boilers

Priming

Priming is the carryover of water droplets with the steam during rapid boiling.

Causes of Priming

  • High dissolved solids concentration.
  • High steam velocities.
  • Sudden boiling.
  • Improper boiler design.
  • Sudden increase in steam production rate.

Foaming

Foaming is the formation of persistent bubbles in the boiler water.

Causes of Foaming

  • Presence of oils that reduce surface tension.

Disadvantages of Priming and Foaming

  • Deposition of salts on superheater and turbine blades, reducing efficiency.
  • Damage to machinery due to salt carryover.
  • Difficulty in maintaining boiler pressure due to inaccurate water level readings.

Zeolite Process of Water Softening

The zeolite process is a water softening method that uses zeolites, naturally occurring or synthetic hydrated sodium aluminosilicate minerals, to exchange sodium ions (Na+) with hardness-causing ions (Ca2+, Mg2+).

Process

Hard water is passed through a bed of zeolite. The zeolite exchanges its sodium ions with the calcium and magnesium ions in the water, thus removing hardness. The exhausted zeolite is regenerated by passing a brine (NaCl) solution through it, replacing the calcium and magnesium ions with sodium ions.

Advantages

  • Effective removal of both temporary and permanent hardness.
  • Automatic adjustment to varying water hardness.
  • Compact equipment.
  • No sludge formation.

Disadvantages

  • Ineffective for turbid water.
  • Sensitive to mineral acids.
  • Can be fouled by colored ions like iron and manganese.
  • Increases sodium content in treated water.

Reverse Osmosis (RO) Desalination

Reverse osmosis is a water purification process that uses a semi-permeable membrane to remove impurities from water.

Principle

Osmosis is the natural tendency of water to move from a less concentrated solution to a more concentrated solution across a semi-permeable membrane. Reverse osmosis applies pressure to the more concentrated side, forcing water molecules to move from the concentrated to the dilute side, leaving impurities behind.

Process

Impure water is forced through a semi-permeable membrane under high pressure. The membrane allows water molecules to pass through but blocks larger ions and molecules, effectively removing salts, minerals, and other contaminants.

Advantages

  • Removes a wide range of impurities, including ionic, non-ionic, colloidal, and organic matter.
  • Simple and reliable process.
  • Relatively low capital and operating costs.

Disadvantages

  • High energy consumption.
  • Requires high pressure.
  • Can remove beneficial minerals, making the water acidic.

Understanding the different aspects of water hardness, its effects, and treatment methods is crucial for various applications, from domestic use to industrial processes. Choosing the appropriate treatment method depends on the specific water quality requirements and the intended use.

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Automatic Shut Off Valve (SOV): To conserve water, the RO system has an automatic shut off
valve. When the storage tank is full, the automatic shut off valve closes to stop any more water from
entering the membrane and blocks flow to the drain. Once water is drawn from the RO faucet, the 
pressure in the tank drops; the shut off valve then opens to send the drinking water through the
membrane while the contaminated wastewater is diverted down the drain.
Check Valve: A check valve is located in the outlet end of the RO membrane housing. The check
valve prevents the backward flow of treated water from the RO storage tank. A backward flow 
could rupture the RO membrane.
Flow Restrictor: Water flowing through the RO membrane is regulated by a flow restrictor. There
are many different styles of flow controls, but their common purpose is to maintain the flow rate
required to obtain the highest quality drinking water (based on the gallon capacity of the
membrane). The flow restrictor also helps maintain pressure on the inlet side of the membrane.
Without the additional pressure from the flow control, very little drinking water would be produced
because all the incoming water would take the path of least resistance and simply flow down the 
drain line. The flow control is most often located in the RO drain line tubing.
Storage Tank: The standard RO storage tank holds from 2 – 4 gallons of water. A bladder inside
the tank keeps water pressurized in the tank when it is full. The typical under counter Reverse
Osmosis tank is 12 inches in diameter and 15 inches tall. 
Faucet: The RO unit uses its own faucet, which is usually installed on the kitchen sink. Some areas
have plumbing regulations requiring an air gap faucet, but non-air gap models are more common
Drain line: This line runs from the outlet end of the Reverse Osmosis membrane housing to the
drain. The drain line is used to dispose of the wastewater containing the impurities and
contaminants that have been filtered out by the reverse


 osmosis membrane. 

Working of RO system:
There Are Generally Four Stages in The Reverse Osmosis Process:
Sediment Filter: This pre-filter stage is designed to strain out sediment, silt, and dirt and is
especially important as the sediment filter protects dirt from getting to the delicate RO membranes
that can be damaged by sediment.
Carbon Filter: The carbon filter is designed to remove chlorine and other contaminants that affect
the performance and life of the RO membrane as well as improve the taste and odour of your water.
Reverse Osmosis Membrane: The semipermeable RO membrane in your RO system is designed
to allow water through, but filter out almost all additional contaminants.
Polishing Filter: In a four-stage RO System, a final post filter (carbon filter) will “polish” off the 
water to remove any remaining taste and odour in the water. This final filter ensures you will have
outstanding drinking water.
The process of reverse osmosis is also known as super or hyper-filtration.
Advantages:
(1) Reverse osmosis can remove ionic as well as non-ionic, colloidal and high molecular weight
organic matter.
(2) Colloidal SiO2 can be removed by reverse osmosis which even cannot be removed by
dimeneralisation.
(3) It is simple and reliable process.
(4) Capital and operating cost are low.
(5) The life of the semi-permeable membrane is about 2 years and it can be easily replaced within a
few minutes, thereby nearly uninterrupted water supply can be provided.
Disadvantages:
(1) A lot of energy is required for the entire process.
(2) There is a lot of pressure that is needed so that deionization can occur.
(3) The water becomes acidity because it has been deionised of all its mineral content. It is not


advisable drinking water from the process because naturally, the water must possess some minerals which help in the functioning of the body.