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Boiler water treatment philosophy and overview

1 .TYPES OF WATER

General

• Water could generally be described as the most important of all chemical substances. Its chemical designation is H2O; the water molecule is composed of 2 Hydrogen atoms and 1 Oxygen atom.

Natural water

• Raw water is the description of the water to which we have daily access. We can obtain our water from:
  1. The ocean
  2. Surface sources (e.g. from lakes)
  3. Underground sources
• The water will vary in composition. The natural water cycle may be as below:

The natural water cycle

• While it is evaporating from the surface of a lake or the ocean into the atmosphere, we can designate the water vapour H2O. In the atmosphere, clouds will form, and during suitable humidity and temperature, the clouds will deposit water (rain). While the rain is falling towards the earth, it absorbs gases which are in the air, e.g. CO2 (Carbon Dioxide), SO2 (Sulphur Dioxide) and O2 (Oxygen).
• When the water hits the earth, it absorbs additional Carbon Dioxide (from biological degradation). The rainwater which is now slightly acid will dissolve various minerals from the soil.

2. Basic Chemistry

The chemistry of water

For more information on property of fresh water please read [1]

• It is necessary to examine some of the basic theories in order to understand the various problems associated with water treatment.
• While rain is falling through the air, it absorbs gaseous contaminants, e.g. O2 (Oxygen), which solubility in pure water depends on temperature.
• At 20 °C, 9 mg/l O2 may dissolve, and at 50 °C approx. 5.5 mg O2/l, and at 90 °C approx. 1.5 mg O2/l, and at 100 °C approx. 0.0. mg O2/l, so, the higher the temperature, the less O2 can dissolve in water.
• CO₂ (Carbon Dioxide) dissolves in water as follows:
  CO₂ + H₂O > H₂CO₃
  H2CO3 is a very weak acid. In contact with CaCO3 (ordinary lime), it is reactive and the lime dissolves as follows:
• CaCO₃ + H₂CO₃ > Ca⁺⁺ + 2HCO₃ –
• Ca(HCO₃)₂ is called Calcium Bicarbonate.
• SO₂ (Sulphur Dioxide) is an air pollutant which stems from flue gases, so there is usually a high atmospheric content of this gas around industrial areas.
• 2SO₂ + O₂ + 2H₂O > 2H₂SO₄
  H₂SO4 is called Sulphuric Acid, and this acid also dissolves lime (CaCO₃)
  as follows:
  CaCO₃ + H₂SO₄ > CaSO₄ + H₂O + CO₂. CaSO4 is called Calcium Sulphate (gypsum).
• In other words, the gases dissolved in the water will increase the leaching of the subsoil’s minerals, so that we may have solutions in water due to:

• TEMPORARY HARDNESS (Alkaline Hardness) is due to bicarbonates of Calcium and Magnesium which are Alkaline in nature. They are “temporary” because when heated they rapidly break down to form Carbon Dioxide and the corresponding carbonates which deposit as scale.
• PERMANENT HARDNESS (Non-Alkaline Hardness) is due mainly to Sulphates and Chlorides of Calcium and Magnesium which are acid in nature. They are “permanent” and do not break down, but under certain conditions deposit to form scale of varying hardnesses.