Part II: Macrofouling
Macrofouling occurs when debris large enough to interfere with flow (leaves, sticks, plastic bags, etc.) finds its way into a cooling water system. Biological organisms in seawater (barnacles, bivalves, seaweed, etc.) and freshwater (fish and algae) also qualify as macrofoulants. Most seawater cooling system designs include mechanisms for removing traditional macrofoulants at the source.
For U.S. designers of freshwater cooling (and fire protection) systems, however, keeping the system free of macrofoulants is an uphill battle. Their archenemies are Asiatic clams and zebra and quagga mussels, invasive species that may not have made the scene when the plant was built.
Asiatic clams. Corbicula fluminea have been in the U.S. the longest of the three species. They began arriving on the West Coast from Southeast Asia in the 1920s. By the 1970s, they had become strongly established in the Midwest. By the 1980s, they had reached the East Coast. Figure 8 shows their current distribution.
8. Eighty years of migration. The current distribution of Asiatic clams in the U.S. Courtesy: U.S. Geological Survey
The Asiatic clam is a bivalve that can be as large as 1.5 inches across, although most are a bit smaller. At that size, they fit snugly into most heat exchanger tubes and fire protection piping, where they enjoy the flow of water—until they cause it to stop. That's exactly what happened at a nuclear plant in Arkansas in 1980, and the event triggered mandatory inspections of all such plants in the U.S. Today, most freshwater-cooled American power plants (both nuclear and fossil-fueled) run ongoing programs to control infestations of Asiatic clams.
Asiatic clams begin reproducing when in the spring, when the water temperature rises to about 60F. Reproduction continues unless the water temperature is above 86F. During the spawning period, a single hermaphroditic (both sexes in the same animal) adult clam can produce hundreds or thousands of larvae per day.
Zebra mussels. Dreissena polymorpha are a newer arrival. They were first found in Lake St. Clair (between Detroit and Windsor, Ontario) in 1988. Figures 9 and 10 show the distributions of zebra mussels in the U.S. in 1988 and 20005, respectively. America's first zebra mussels were international stowaways in the ballast water of cargo ships from the Black Sea. After the ships made their way down the St. Lawrence Seaway, the mussels were discharged along with the ballast water.
9. Then . . . Zebra mussel distribution in the U.S. in 1988. Courtesy: U.S. Geological Survey
10. . . . and now. Zebra mussel distribution in the U.S. in 2005. The stars indicate isolated waterways and cooling systems where mussels have been found recently. Courtesy: U.S. Geological Survey
The maritime shipping industry has tried to prevent zebra mussels from becoming established west of the Continental Divide by stepping up inspections of ships' bottoms, boat cleaning, and other methods. To date, these efforts have been only partially successful. The yellow stars in Figure 10 pinpoint where zebra mussels have recently been found, both east and west of the Mississippi.
Zebra mussels have a lifespan of two to five years and are slightly larger than Asiatic clams—up to 2 inches across. At that size, they can completely block water intake structures on lakes and rivers. Zebra mussels attach to the structures (and to each other, forming colonies) by means of byssal threads. Colonies have been reported achieving densities of 750,000 to 1,000,000 mussels per square meter. Figure 11 shows a particularly bad infestation.
11. Submarine attack. Zebra mussels blanketing an underwater structure at a power plant. Courtesy: M&M Engineering
Zebra mussels are filter feeders and remove plankton and other small organisms from the water. Zebra mussels in the Great Lakes compete with native species for the food supply. Another environmental impact has been an increase in water clarity. Although this may seem like a good thing, the increased water clarity means that sunlight penetrates deeper and accelerates the growth of algae. Among other things, this has led to taste and odor problems in the Great Lakes. Power plant and municipal intake structures have been fouled by some of these algae (e.g., Cladophora). This has caused blocked flow and reductions in plant output.
Quagga mussels. Dreissena bugensis, close relatives of the zebra mussels, are the most recent biological macrofoulant arrival. They were first detected in the U.S. in 1989. According to one source, the density of quagga mussels on the bottom of Lake Michigan increased from 899 per square meter in 2000 to 7,790 per square meter in 2005. By January 2007, quagga mussels had been discovered in Lake Mead, Lake Mohave, and Lake Havasu (Figure 12).
12. Go west, young mussel. Quagga mussels' current distribution in the U.S. The red dots indicate recent outbreaks. Courtesy: U.S. Geological Survey
Quagga mussels cause most of the same problems as zebra mussels, but they have an even greater negative environmental impact. Quagga mussels are active at lower temperatures than zebra mussels and thus inhabit deeper areas of a lake. Locating an intake pipe as deep as 500 ft below the surface may not keep quagga mussels out of it. Like zebra mussels, quagga mussels are filter feeders, but they are even bigger eaters. In fact, quagga mussels have almost completely displaced zebra mussels in many parts of Lake Michigan.
The life cycles of Asiatic clams, zebra mussels, and quagga mussels are similar. During their early larval and veliger stages, they are all planktonic (disbursed in the water column). During later stages, the veligers (now equipped with a velum to help them swim and feed) begin to settle on surfaces as they grow to adulthood.
War on underwater terror. Strategies for controlling Asiatic clams, zebra mussels, and quagga mussels are likewise similar. They include physical removal, applying a metallic coating or polymer to the interior of pipes and the exterior of structures, heating piping and structures, and—the subject of this article—the use of oxidizing and nonoxidizing biocides.
Some control strategies target the clams and mussels when they are vulnerable larvae and veligers. Others don't try to kill the organisms, but rather prevent them from settling down as veligers. Still others focus on killing or removing adults.
Methods for physically removing mollusks (clams, mussels, and other bivalves) may be as simple as scraping or dredging the population from impacted areas. Some processes require draining structures; others are conducted underwater using equipment or divers.
Coatings have long been used to reduce the accumulation of marine organisms on boat hulls and other underwater structures. Most coatings contain a metal (copper or tin) or another substance (such as a polymer) that makes it harder for larva/veliger to cling to the surface.
Thermal treatment has been used successfully to control zebra mussels because they are essentially cold-water creatures. Although their maximum survival temperature depends on several factors, water hotter than 90F is usually fatal to zebra mussels if they are exposed to it long enough. Temperatures exceeding 95F produce rapid mortality. Quagga mussels are even less tolerant of heat; 86F water is enough to kill even the hardiest adult. By contrast, Asiatic clams are much more heat-tolerant than zebra or quagga mussels, so thermal treatment for Asiatic clam control usually doesn't work.
Oxidizing biocides such as chlorine gas, sodium hypochlorite (bleach), chlorine dioxide, or potassium permanganate can be used to kill both veligers and adult mollusks. Veligers can be controlled with intermittent doses, but killing adults takes about two weeks of continuous feed of an oxidant; the adults can literally "clam up" for that long to avoid it. Some studies have shown that a pulsed-feed approach (with alternating on and off periods) works just as well. Finally, it's worth mentioning that using chlorine dioxide to kill adult clams or mussels requires less time (about six days) to be effective.
Nonoxidizing biocides, such as certain quaternary ammonium compounds, are very effective molluscicides. They can kill off an entire population within 48 hours. However, their use usually requires a detoxification step, such as adsorption with bentonite clay, before discharge. These biocides, which have a strong cationic (positive) charge, interfere with mollusk respiration. So do some conventional water treatment polymers (dispersants and flocculants), which eventually clog the gills of mollusks if applied at elevated dosages.
—David Daniels (david_daniels@mmengineering.com) is a senior consulting scientist for M&M Engineering and a POWER contributing editor. Tony Selby (tony_selby@mmengineering.com) is a principal scientist for M&M Engineering.
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