Treatment System Designed to Protect Cooling Water Systems
The fouling of intake structures, piping systems and heat exchangers by clams, mussels and other macroorganisms is a major global problem for industrial facilities.
By Ray Post and Roy Manley
The fouling of intake structures, piping systems and heat exchangers by clams, mussels and other macroorganisms is a major global problem for industrial facilities. In electric utilities, condenser biofouling can account for as much as a 3 percent loss of unit availability, with approximately 40 percent of the loss contributed by macroorganisms.
Asiatic clam and other larvae have passed through this intake screen and grown to maturity on the other side.
The principal fresh water macrofouling organisms are the zebra mussel and Asiatic clam, small, hard-shelled bivalve filter feeders that are widespread throughout lakes and rivers in the United States, as well as various hydroid species. Mussels predominate in saltwater macrofouling, predominantly the blue mussel in colder waters and the green mussel in tropical waters. Other prolific macrofoulers found in seawater cooled facilities include barnacles, tube worms, bryozoans, tunicates and sponges. Approximately 12 percent of US generating stations are seawater cooled.
The zebra mussel and Asiatic clam can out-compete and dominate other organisms in the environment and in power cooling circuits. Their advantages include a lengthy spawning season, prolific reproductive capacity, broad temperature tolerance band, and a high growth rate. As a non-indigenous (introduced) foreign species, they have few natural predators, and unlike native mollusks their larval stage has no requirement for a host fish population. Consequently, their proliferation within a power plant cooling system is unrestrained by the absence of fish.
Screens and typical strainers are not fine enough to prevent microscopic larvae and juvenile mollusks from entering cooling systems and growing into adulthood. As a result, various physical and chemical measures have been employed to combat macrofouling, including thermal shocking with hot water, no-foul coatings, scraping and removal, and chlorination. Although all of these methods have worked in certain applications, chemical control measures are the most widely practiced.
Severe mussel fouling of condenser intake screen before beginning treatment program.
Chlorine, bromine and other oxidizing agents are the most commonly applied biocides for the control of slime-forming microorganisms in industrial cooling systems. However, they are not as effective in controlling mollusks and other macroorganisms that close their shells after sensing the chemical, thus limiting exposure. It is believed that oxidizing biocides may cause mortality through asphyxiation over a prolonged period rather than from direct toxic effect, so that any interruption of the treatment allows the macroorganisms to recover.
Condenser intake screen after 7 1/2 months of treatment with Spectrus.
The continuous or semicontinuous application of oxidizing biocides for the many days or weeks required to overcome this resistance can have adverse environmental impacts, destroying aquatic life and producing harmful chemical by-products like THMs. In addition, continuous chlorination accelerates the corrosion of copper alloys and the large quantity of chlorine is costly.
In 1985, GE Betz researchers screening long-established antimicrobial products for efficacy against Asiatic clams found that certain amines were effective in low concentrations against a broad range of marine organisms in fresh and salt water. Subsequent testing determined that the amines were undetected by the organisms and produced 100 percent mortality with short exposures of only a few hours. Compared to chlorine, the amines were highly selective, affecting the target organisms while leaving fish and other species unharmed. They are rapidly adsorbed onto suspended solids, sediments, and other naturally occurring materials and rapidly degraded by microorganisms.
After extensive environmental studies, GE Betz obtained broad USEPA registration to use these amines for macrofouling control in once-through cooling systems, influent systems and auxiliary water and wastewater systems under the registered trade name ClamTrol™. The name was later changed to Spectrus™.
The first commercial fresh water application of ClamTrol was in 1986 at the Lake Hubbard Station of Texas Utilities, a gas-fired facility with chronic fouling problems and forced outages from the pluggage of main condenser tubes by Asiatic clams. The first commercial application to control the growth of blue mussels occurred in 1989 at the Salem Harbor Station of New England Power, approximately 20 miles north of Boston. The product was first used against the green mussel in 1992 in Trinidad.
In 1997, GE Betz received NSF approval to use Spectrus CT1300 in potable water system intakes and desalination facilities producing potable water. Two years later, the first application to control macrofouling in a thermal seawater desalination facility was conducted in Aruba at W.E.B. (Water-en-Energiebedrijf).
This plant, which produced all of the island's potable water, had been plagued throughout its history by macrofouling organisms that obstructed pipes and blocked the tubes on heat exchangers and condensers. This resulted in a steady decline in the performance ratio (distillate produced : steam consumed), and production losses when shells were cleaned and removed from the system. The successful treatment reduced LP-steam requirements, resulting in significant energy savings, reduced downtime for cleaning, lowered maintenance costs, and increased water production.
Various studies have confirmed the rapid biodegradation of Spectrus CT1300 in different environments. In a cooling system with the blowdown closed, the half-life is less than four hours. In most applications, the discharge of Spectrus CT1300 treated water poses no environmental risk to the receiving stream. In cases where the naturally occurring demand does not consume the product rapidly enough, supplemental clay can be added to adsorb and inactivate the product residual prior to discharge.
Application methods for Spectrus CT1300 are primarily dependent on the system to be treated and the macrofouling organism. The object is to apply the product at a frequency that will prevent the macroorganism from ever growing large enough to cause fouling problems. This is determined by the growth rate of the fouling organism and the size of the tubes or passages in the system. An effective treatment will cause the organism to release and pass through the system. A power plant with 1-inch condenser tubes can successfully control zebra mussel or Asiatic clam fouling using only one or two applications per year. However, an industrial facility with 3/4 inch tubes may require dosing every two weeks to control fouling caused by the rapidly growing tropical green mussel.
In general, salt water macrofouling species grow faster and larger, especially in tropical waters, and require more frequent dosing than fresh water species. However, warmer water is associated with accelerated metabolism and siphoning rates, permitting the use of lower product concentrations and shorter exposure times.
Cooling tower systems generally suffer the least from macrofouling and are the easiest to treat successfully. In most cases, the blowdown is discontinued while the treatment is in progress. Product residuals are allowed to decay naturally by adsorption onto suspended solids and surfaces within the system. Effective treatments are usually possible within 6 to 8 hours at the elevated temperatures typical of tower systems. Once-through cooling systems are more difficult and expensive to treat because of the much greater water intake requirement.
With respect to macrofouling control, the intake structure is the most critical component of the cooling system. The conservative water velocities in the screen pit area designed to prevent fish entrainment are ideal for promoting the settlement and growth of macrofouling organisms. Once within the protective confines of the screen pit, the mollusks can thrive in a predator-free environment with a constant, gentle flow of water. In time, they grow shells large enough to block tubes and restrict cooling passages. Shells that break loose are carried into the cooling system where they can lodge against tube sheets and restrict water flow.
Plants with long inlet tunnels are particularly prone to macrofouling problems. Unlike pressurized piping systems in which flow velocities can limit macrofouling growth, gravity flow intake tunnels operate at lower water velocities that are often in the ideal range for settlement and growth. These intake tunnels are frequently greater than 1000 feet long, several feet in diameter, and can accumulate several tons of macrofouling growth in a single season. Treatment of these long intake tunnels requires product application at the inlet.
Years of laboratory and field experience have shown Spectrus CT1300 to be a cost effective and environmentally preferable alternative to chlorination for macrofouling control in both fresh and salt water. Power generating stations, desalination plants and industrial facilities around the world are now using this technology.
About the Authors: Ray Post is a senior consultant with GE Betz, providing technical support to the Americas service team. He has 26 years of experience in water treatment, including 24 years with GE Betz. Roy Manley is manager of GE Betz' Environmental and Regulatory Affairs Department. He has 27 years of experience in water treatment and the environmental and regulatory field, the last 18 of them with GE Betz.