Advanced cooling water treatment concepts (Part 3)

This installment provides an overview of common microbiological fouling issues, with discussion also about macro-biological fouling.

Editor's note: This is the third installment of a multi-part series by Brad Buecker, President of Buecker & Associates, LLC.

Read Part 1 here.

Read Part 2 here.

Parts 1 and 2 of this series examined issues related to scaling and corrosion in open recirculating cooling systems, i.e., those systems with cooling towers, and it also provided an overview of modern chemistry methods for scale/corrosion control. An issue that frequently dwarfs these concerns is microbiological fouling. Without proper control, microbes can rapidly establish colonies at many locations within cooling systems. These colonies can degrade heat transfer, restrict fluid flow and induce under-deposit corrosion. Cases are known where heavy microbiological fouling and accumulation of silt and other debris caused partial collapse of cooling towers. This installment provides an overview of common microbiological fouling issues, with discussion also about macro-biological fouling. Parts 4 and 5 will examine current and evolving microbial control methods.

An enormous number of microbes exist in our environment, and space limitations prevent extensive discussion of the varieties here. For cooling water systems, the three general types of microorganisms that can be problematic are bacteria, algae, and fungi. Bacteria can form colonies in many locations, fungi will attack cooling tower wood, and algae can grow prolifically in sunlit areas such as cooling tower decks. Advanced microorganisms including amoeba and protozoa may appear in established bacterial colonies. These more complex organisms may in turn harbor Legionella bacteria.

Bacteria

Bacteria are generally classified into three types: aerobic, anaerobic and facultative. Aerobic bacteria require oxygen for metabolism, whereas anaerobic bacteria extract oxygen from oxygen-containing molecules such as sulfate and nitrate. Facultative bacteria utilize oxygen if it is present, but can extract oxygen from other sources when oxygen is absent.

Common organisms include:

Bacteria that are free floating in water are known as planktonic organisms, whose concentration can be readily measured. However, if the organisms settle on surfaces to form sessile colonies, problems may arise very rapidly. Some bacteria exude a polysaccharide film (slime) that protects the organisms and allows development of complex colonies that may include many of the organisms listed above. The slime in turn will capture silt to form heavy deposits that often resemble mud.

As is clearly obvious, the slime in this exchanger must have greatly restricted flow and energy transfer.

Microbial deposits can also induce serious corrosion. For starters, the deposits allow differential oxygen cells to form, where the metal underneath the deposit becomes anodic to clean surfaces. Localized corrosion and pitting may result. Beyond that difficulty, some organisms produce harmful compounds as part of their metabolic processes. Sulfate-reducing bacteria are a prime example, whose metabolic byproduct is sulfide. Sulfides will attack many metals including iron and copper.

This attack is commonly referred to as microbiologically-induced corrosion (MIC). The author observed a situation once where a 15,000-tube (316L stainless steel) steam surface condenser developed thousands of pinhole leaks during a month-long maintenance outage. Water was left standing in the tubes, which allowed microbes to settle and produce harmful byproducts that damaged most of the tubes. The subsequent condenser re-tubing was quite expensive.

Another location that can suffer heavy deposition and fouling is cooling tower fill.

Again, the deposits restrict fluid flow and inhibit heat transfer. Deposition can also add enormous weight to the fill, as is illustrated.

Fungi

With the development of plastic and metal cooling tower structural components and less reliance on wood, fungi attack is perhaps not as broadly serious as in the past. However, wood has not disappeared as a cooling tower material, and fungi control remains important in many applications. Some fungi utilize wood as a nutrient, and the organisms can degrade wooden components. Some species attack the cellulosic wood fibers, while others attack the lignin binder. Names for the various types of attack include surface rot, white rot, and brown rot. Fungi thrive in acidic environments and are less active in modern cooling water systems per typical operation within a mildly basic pH range.

Algae

Algae are photosynthetic organisms that can grow as large masses in areas exposed to sunlight.

A common location for algae growth is on the decks of crossflow cooling towers. The organisms can plug the perforations to the fill below and reduce cooling capacity and tower efficiency.

As was noted in Parts 1 and 2 of this series, phosphorus is a limiting nutrient for algae, so in those systems that have been converted from phosphate-phosphonate scale/corrosion control chemistry to non-P programs, algae growth may be somewhat limited.

Legionella

If sessile colonies become well established, higher life forms including amoeba and protozoa may emerge. While some amoeba species directly cause human health problems, well known is an established relationship between amoeba, protozoa and Legionella bacteria. Legionella is the bacteria first discovered in 1976 when it infected people attending an American Legion convention (and other guests including the author's parents) at a Philadelphia hotel. (4) Nearly three dozen people died, and many more became ill. The outbreak was traced to water vapor containing the bacteria in the exhaust plume of a cooling tower on the roof of the convention hotel. The hotel's air handling system circulated some of the vapor through the building.

The link between Legionella and amoeba was first reported by Rowbotham (5) who showed that amoeba could serve as hosts for the Legionella.

Of critical importance for Legionella control is proper design and operation of a biocide feed system that minimizes all forms of microbiological fouling. Another important measure is finding and eliminating, if possible, all dead leg piping. Stagnant water can allow organisms to proliferate. When workers enter cooling systems for regular maintenance work, such as cleaning condenser waterboxes, masks should be a part of their personal protective equipment (PPE).

Macrofouling

Much larger organisms including mussels, clams, and barnacles can cause severe cooling system fouling. Fresh water clams first became a problem in the United States in the late 1970s when the Asiatic clam, Corbicula flominea, entered the country. A problem with these creatures is that they are often the perfect size to fit into steam condenser and heat exchanger tube inlets.

Then in 1986 Dreissena polymorpha, the zebra mussel, and its close relative the quagga mussel were introduced to the Great Lakes in the ballast water discharge of a foreign ship. These mussels are native to the Black and Caspian Sea areas. The mussels attach to surfaces, including each other, with string-like filaments known as byssal threads. Colonies can become massive, with thousands of mussels per square foot.

Zebra mussels have been a primary focus in recent years following their spread from the Great Lakes through various waterways in the eastern and midwestern United States. Besides spreading naturally through waterways, the organisms can also survive for two to three weeks out of water. Thus, if they attach to a recreational boat in one water body and the boat is transferred to another water body within a relatively short time, the mussels can infiltrate the next source.

As this installment suggests, micro- and macro-biological fouling can occur rapidly in cooling systems, and it can cause severe problems to the point of perhaps partial or total plant shutdown. Critical to prevent such fouling are well-designed, operated, and maintained treatment systems in which the selected chemistry maximizes biocide efficacy. We will explore these topics in Parts 4 and 5 of this series. And, as we shall see, some of the treatment methods are also appropriate for once-through cooling systems.

This discussion represents good engineering practice developed over time. However, it is the responsibility of plant owners, operators and the technical staff to implement reliable programs based on consultation with industry experts. Many additional details go into the design and subsequent use of these technologies than can be outlined in a single article.

References

About the Author: Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing. Most recently he served as Senior Technical Publicist with ChemTreat, Inc. He has over four decades of experience in or supporting the power and industrial water treatment industries, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company's (now Evergy) La Cygne, Kansas station. Buecker has a B.S. in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry. He has authored or co-authored over 250 articles for various technical trade magazines, and has written three books on power plant chemistry and air pollution control. He may be reached at [email protected].