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Fundamentals of Water Purification (Part 1 of 2)

by Brian Hagopian, CIPE
January 2000

Reprinted with permission from Plumbing Engineer

Pure water is becoming more important to an increasing number of businesses. More and more contaminants are being found in water supplies, making water purification more difficult and complex. Many industries, particularly the semiconductor, power generation, and pharmaceutical industries, are demanding water with greater levels of purity than ever before.

As a result, there is an increasing need for plumbing engineers to be able to properly design water purification systems. Prior to doing this, we all need to understand the nature of the contaminants, or undesirable materials, found in water. We also will learn that the source of the water and the treatment done on a municipal level will determine how difficult a given water source is to purify.

This article is divided into five sections: 1) the contaminants found in water supplies, 2) contaminant variations based upon water source, 3) water purification, or contaminant removal processes, 4) how to properly sequence water purification processes into an effective water purification system, and 5) the importance of controls in making the components of a water purification system operate as a synchronized whole. This article will focus primarily on the first three sections, touching on the final two, which will be covered in a Part 2 of this article. The article will conclude by discussing methods of monitoring water purity to determine if a system is working properly.

Water as a Chemical

First we must understand the water molecule, which means we have to understand a bit of chemistry. Water is considered the universal solvent. It is a molecule that contains a positive and a negative end. Molecules such as water are called "polar" molecules. This polarity allows water to easily dissolve materials that are also polar. Nonpolar molecules, such as oils, hydrocarbons, etc., are much more difficult to dissolve in water. It is important to remember this when considering water purification processes.

Water also possesses unique properties because of its molecular structure. The polarity of water allows it to line up with itself (positive end to negative end, a chemical property called hydrogen bonding) as well as with other polar molecules. When water freezes, hydrogen bonding effects force water molecules to form a stable structure where its molecules are further apart than when it was in a liquid state. For this reason, ice is lighter than water, which causes it to float. Water is the only molecule that behaves this way. Hydrogen bonding plays an important role in many water purification processes.

Contaminants Found in Water Supplies

Five basic groups of contaminants are present in water: particles, dissolved salts, organics, colloids, and bacteria.

Particles are suspended solid materials that do not dissolve in water. These materials are silt, dirt, rust, and other suspended matter. If particles are large enough and/or dense enough, they may remove themselves from water by settling out naturally in to carry a slight negative charge, which cause them to repel each other like two south poles of a magnet. Although they do not readily dissolve in the water, they repel each other and form a stable suspension in water. Colloids represent one of the most difficult group of contaminants to remove from water.

Bacteria are living organisms present in water supplies. Al- though most are benign, some are harmful. Bacteria can grow and proliferate in pure water systems because the chemicals used to hold them in check are usually removed early in the water purification process. The presence of living bacteria also means that there will be dead bacteria, along their decomposition products in water systems.

The Importance of Water Sources

Water sources determine the amount and type of contaminants that a water purification system will have to be designed to handle. Water travels through several states in its cycle through our environment. Water exists as water vapor, in the form of clouds, condenses to rain, flows through the ground, travels to us for use, flows through municipal treatment plants, flows into the ocean and evaporates into water vapor to complete the cycle. Water is in its purest state when it exists as water vapor. Once it condenses into rain, it begins to pick up contaminants, dissolving small bits of everything that it comes into contact with.

Surface water supplies. If water becomes part of a surface reservoir or river water supply, the water will come into contact with topsoil and naturally occurring vegetation. Consequently, waters from surface supplies typically contain low levels of dissolved minerals and higher levels of particulate, organic, and colloidal contaminants. Surface waters may contain higher levels of a particular contaminant based upon local conditions (i.e., nitrates from fertilizer runoff).

Well water supplies. If water has to travel 500 feet into the ground to become part of a municipal well water supply, it will dissolve a lot of the minerals that it encounters on the way. Waters from deep well supplies have been in contact with the ground for a longer period of time and typically contain higher levels of the dissolved minerals. Calcium and magnesium salts, which represent the hardness minerals, can be found in high concentrations in deep well waters.

Hardness minerals can be easily converted to less soluble compounds with the addition of heat, leading to precipitation and scale formation. Hardness represents a major problem where water temperature needs to be raised, or where some water purification processes are employed. Deep well waters also may contain high levels of dissolved iron salts. Iron is particularly troublesome because it exists in two different states. Ferrous iron (Fe+2) is found in some well waters and imparts a metallic taste to the water, but does not discolor it. After exposure to air, or another oxidant, it is converted to ferric (Fe+3) iron, or rust, which is insoluble and precipitates from the water. Deep well waters are more highly filtered, containing lower levels of particles when compared to surface water supplies. Deep well waters spend little time at the surface of the earth, and contain lower levels of organics and colloidal materials than surface waters. All of these factors play an important role in determining the optimum water purification system for a particular need. A responsible water system designer will begin with a water analysis, which will indicate the types and abundance of various contaminants. In addition to the standard laboratory tests, a test for colloids (such as a silt density index test) and total organic carbon is highly recommended.

Part 2 of this article will be printed in the next issue of the newsletter.

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