A Brief Primer on Microorganisms

By Sam R. Owens | October 26, 2006
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There are many microorganisms that cause problems in industrial water systems, such as cooling towers and closed loops. These include bacteria, algae, fungi and yeasts.

Bacteria can live and grow in extreme conditions: with or without oxygen, temperatures from 12 degrees to 220 degrees Fahrenheit, pressures from zero to 25,000 pounds per square inch, salinity from zero to 300,000 parts per million and pH from 1.0 to 10.2. General problems created by bacteria include: primary corrosion (direct), secondary corrosion (protect and/or produce other bacteria), plugging, or fouling and scale (from bacteria by-products).

Bacteria can also be useful. Common examples of useful bacteria are the yeast for bread, bacteria for cheese and bacteria for beer and wine.

Identification is difficult since they are not visible to the naked eye and they must be grown in laboratory conditions or using bacteria media. Bacteria are commonly associated with disease in humans, animals and plants. Some of the health hazards include viruses and Legionnaire's disease.

Testing methods include growing cultures or using prepared bottles for anaerobic or sulfate reducing bacteria. Test strips, or paddles, and "bio-film" monitors are also available. Sometimes bacteria are on the surface of pipes or equipment not in the bulk water; when this occurs, a bacteria test will not show any bacteria in the water.

There are two types of bacteria, both of which can cause fouling and corrosion problems. One type, aerobic bacteria, needs air to live. The other type, anaerobic bacteria, does not need air. Aerobic bacteria are the most often encountered species of bacteria, and the type listed in most reports. They are frequently controlled with biocide.

Anaerobic bacteria, or desulfovibrio bacteria, are also known as "sulfate-reducing" bacteria (SRB). These bacteria should always be controlled. Anaerobic bacteria may exist even when a bacteria test cell registers zero. An indicator of anaerobic bacteria is a swamp gas or methane smell.

SRB use sulfur or sulfates as a nutrient. This strain of bacteria is prolific in ideal conditions, multiplying or doubling in number every 30 minutes. The bacteria secrete a sticky substance, called polysaccharide slime, which provides a protective environment for colony growth and clings to anything. Once attached to metal, a by-product from the colony's lifecycle—hydrogen sulfide gas—is trapped against the surface by the slime mass, where corrosion pitting begins. The trapped gas can form sulfuric acid. Anaerobic bacteria can also produce weak acids, which are corrosive over time and attack metal, wood and sealing caulking material.

Denitrifying bacteria, or pseudomonas, cause the loss of nitrite inhibitor in closed-water systems. Because this type of bacteria can utilize nitrite as a nutrient, positive counts of this bacteria will result in a "present" status under the denitrifying bacteria category.
Iron-depositing bacteria grow best in low oxygen environments but are common in open-circulation systems. Gallionella and sphaerotilus use soluble, or ferrous, iron as an energy source, and convert it to an insoluble oxide or hydroxide form. These deposits create fouling and set up concentration corrosion cells and conditions under which anaerobic bacteria flourish. Gallionella frequently leave spiderweb-like deposits on metal surfaces. The deposit looks like black iron. Severe corrosion is usually evident under the deposit.

Algae are very simple plants. They can be one-celled organisms or multi-celled organisms with minor cell specialization. Algae can live in colonies, usually referred to as "blooms." Both types—single- and multi-celled algae—are common in cooling towers. They can live in still or moving water. Algae growth is dependent on water conditions like light, temperature, oxygen, carbon dioxide and mineral content. Algae can exist in extreme conditions, such as water temperature to 185 degrees Fahrenheit and sea water depths to 300 feet. Algae can act as a filter to trap and concentrate minerals to form ore deposits. They can also form peat bogs, which eventually become coal deposits. Algae produce food for other life forms, including higher life bacteria and fungi.

Many biomasses containing both algae and bacteria are difficult to control. Each type of organism tend to protect the other from chemicals. Algae require control because the biomass can break loose and cause exchanger fouling. When this happens, slimy, rubbery masses form, which cause plugging and decrease the tower efficiency. The sludge also provides food for bacteria, which causes corrosion.

The most common types of algae are blue-green, green, brown, red and red-brown. Certain types of red and red-brown algae are known as "red tide." Although algae are a problem for cooling towers, they are beneficial in fish food, fertilizer, soups, gelatin and cosmetics. It is also beneficial as a source of iodine and as a filter medium.

Yeast and fungi are generally considered to be plants, but they don't form roots, stems or leaves. They vary in size, from microscopic yeasts to mushrooms, and mold is the most common form. Some yeast and fungi live on dead plant or animal remains while others are parasites living on live animals. There are more than 75,000 species of yeast and fungi, which include mold, smut, rust and mildew. They may be colorless or cover the entire color spectrum. Most grow best in warm, dark, moist places. Most are aerobic with low oxygen demand. A few, including yeast, are anaerobic. Fungi can grow on almost any surface and are considered an attributing factor to wood deterioration. Both yeast and fungi are commonly transported by air currents.
Controlling microorganisms is important. To learn what methods of treatment are best for a particular system, call a water treatment professional.

Sam R. Owens is with Corpus Christi, Texas-based Chemico International Inc. Reach him at (361) 883-8255 or samowens@intcomm