Desalination – Technical Information

When dealing with desalination equipment, a number of common terms are used to describe the process and the various water conditions, such terms have been defined below and include:

• Reverse Osmosis
• Desalination
• Salinity
• Hard Water
• Soft Water
• Iron Staining

Reverse Osmosis

What is it?

Reverse osmosis is a filtration process that is often used for water. It works by using pressure to force feedwater through a membrane, retaining the unwanted particles on one side and allowing the pure water to pass to the other side. This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied.*

What is it?

Desalination refers to any of several processes that removes excess salt and other minerals from water. There are many instances where desalination can be applied, in particular groundwater and/or bores.*

Reverse Osmosis systems can be adjusted to address what minerals and/or salts are present in your feedwater. In most cases this is measured by Total Dissolved Salts or TDS. The level of TDS will dictate the type of treatment together with what other minerals or impurities you need removed.

Salinity

What is it?

Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates and bicarbonates. Salinity in Australian English and North American English may also refer to the salt content of soil (see soil salination).*

What problems does it cause?

Salinity is an ecological factor of considerable importance, influencing the types of organisms that live in a body of water. As well, salinity influences the kinds of plants that will grow either in a water body, or on land fed by water (or by a groundwater). Salt can be expensive to remove from water, and salt content is an important factor in water use (such as potability).*

Hard Water

What is it?

Hard Water is water that has high mineral content (mainly calcium and magnesium ions) (in contrast with soft water). Hard water minerals primarily consist of calcium (Ca²⁺), and magnesium (Mg²⁺) metal cations, and sometimes other dissolved compounds such as bicarbonates and sulfates. Calcium usually enters the water as either calcium carbonate (CaCO₃), in the form of limestone and chalk, or calcium sulfate (CaSO₄), in the form of other mineral deposits. The predominant source of magnesium is dolomite (CaMg(CO₃)₂). Hard water is generally not harmful to one's health.*

The simplest way to determine the hardness of water is the lather/froth test: soap or toothpaste, when agitated, lathers easily in soft water but not in hard water. More exact measurements of hardness can be obtained through a wet titration. The total water 'hardness' (including both Ca²⁺ and Mg²⁺ ions) is read as parts per million (ppm) or weight/volume (mg/L) of calcium carbonate (CaCO₃) in the water. Although water hardness usually only measures the total concentrations of calcium and magnesium (the two most prevalent, divalent metal ions), iron, aluminium, and manganese may also be present at elevated levels in some geographical locations.*

What problems does it cause?

Soft Water

What is it?

Soft water the term used to describe types of water that contain few or no calcium or magnesium metal cations. The term is usually related to hard water, which does contain significant amounts of these ions.*

Soft water usually comes from peat or igneous rock sources, such as granite but may also derive from sandstone sources, since such sedimentary rocks are usually low in calcium and magnesium. Water softened by sodium ion exchange will have a higher sodium ion content than the natural water it was derived from.*

What problems does it cause?

Water treated through a reverse osmosis unit will be very "soft" because all, or most, of the ions have been removed. Typically this water will need some hardness correction or blending with hard water before it can be used in normal reticulation systems. It is also found in other types of reticulation systems. Soft water is often mixed with Epsom salt.*

Iron Staining

What is it?

Iron is the metal that is most abundant on Earth and is therefore very prevalent in soils and groundwater. Dissolved iron occurs naturally in groundwater in concentrations of up to around 50 milligrams per litre. Iron salts become increasingly soluble as groundwater becomes more acidic.

In anoxic and acidic groundwater (pH values below 5), iron concentrations of one to 20 milligrams per litre are common (usually as stable carbonates). Iron is normally found dissolved in groundwater in the reduced ferrous form (Fe2+) and oxidises to relatively insoluble ferric form (Fe3+) when the water pH is raised and with exposure to oxygen in the air.

When acidic iron-rich groundwater is extracted and mixes with air, carbon dioxide and hydrogen sulphide (rotten egg gas) is frequently released, the pH rises and the iron precipitates as ferric hydroxide (rust) on any flat surface where it rests as water evaporates. Over time this oxide coating builds up causing discolouration to light coloured surfaces.

Iron may also be present in groundwater in organic complex and slimy, but harmless, bacterial forms. These are unlikely to settle out, but may discolour water.

What problems does it cause?

Iron deposits cause problems in irrigation systems, especially those that rely on small orifices for pressure control or delivery via water drippers, and may also affect heat transfer in hot water systems.

Iron staining is unsightly but shouldn’t cause serious harm to plants, animals or humans or structural damage. Although high concentrations of iron (more than 20 parts per million) some plants with heavy iron staining may experience a reduction in photosynthesis and vigour.

*Source: http://en.wikipedia.org/