Controlling Scale To Prevent Legionella
Legionella is a water-based organism that causes infection in humans when they inhale it in aerosol form. It can cause a severe, progressive form of pneumonia that is fatal in up to 15% of the cases. The disease was first identified in 1978 after an epidemic outbreak among American Legion conventioneers in Philadelphia in 1976 infected 221 and killed 34. The agent was described as a rod-shaped bacteria which was named legionella pneumophila.
According to Jan De Baat Doelman, president of Scalewatcher North America, Inc., Oxford, PA, besides cooling towers, legionella can be found in a number of systems, including potable hot and cold water systems, recirculating water humidifiers, whirlpool spas, and others.
CRITICAL CONDITIONSConditions that affect the proliferation of legionella, noted De Baat Doelman, include:
- The presence of scale deposits or algae growth in the water;
- Deadlegs in the pipework or stagnation due to low use of outlets;
- Low temperature in potable hot water heaters and distribution systems;
- Stratification of water in water heaters; and
- Inappropriate water treatment.
SCALE DEPOSITSScale is a problem in both hot and cold water systems. “In hot systems, scale can trap legionella and biofilms,” De Baat Doelman stated. “This provides a perfect growth medium, which disinfectants cannot penetrate.”
Scale deposits that contain legionella can continuously recontaminate a system, even after disinfection, he said. The biofilms trapped are a food source for the bacteria and can also lead to odor problems from the products of their metabolism.
In addition, scale is a major cause of inefficiency in hot water systems. Scale on heat exchange surfaces significantly reduces heat transfer efficiency and promotes corrosion.
“Descaling of a hot water system can be time consuming and expensive,” De Baat Doelman said. Water softeners can reduce scale, but there is concern over possibly creating high levels of sodium in the water.
MAINTAINING COOLING TOWERSA cooling tower can operate at temperature ranges “ideal for bacteria and algae growth,” said De Baat Doelman, and if not maintained, can supply substantial nutrients for their generation. Legionella can readily thrive in this environment.
“However, in a tower that is well designed and maintained, chances of problems with legionella are greatly reduced,” he continued. “Most cases of outbreaks have occurred in towers which were badly designed, or had little or no maintenance.”
In cooling towers, temperature, water hardness, pH, scale, and corrosion are all factors that encourage the growth of biofilm, algae, and legionella. Methods used to control these factors include scale and corrosion inhibitors, dispersants, and biocides.
Periodic chlorination and descaling of cooling towers is the recommended practice, he noted. “However, chlorine is not always compatible with other treatment chemicals like corrosion inhibitors, is not effective in alkaline water, and can itself cause corrosion.”
Some biocides are effective against legionella if used in sufficient concentration. Strains of legionella and other bacteria, however, can become resistant to particular biocides; therefore, dual or alternating biocides are used.
ELECTRONIC WATER TREATMENTWhat is required in all water systems, according to De Baat Doelman, is a method of continuously controlling scale deposition and a water treatment program that prevents the growth of biofilms, bacteria, and in particular, legionella. This is available in the form of electronic scale treatment of water to prevent scale deposition, together with the chlorination of all water supplied to the facility.
“This involves the application of electronic water descaling equipment at strategic points in the water system,” he said. With an average cooling tower, two units will be required for treating the water feed and the recirculating pipe. Larger cooling towers with more than one feed require additional units. “Water treated by such systems will prevent scale from forming in the pipework and on heat transfer surfaces, and will also, over a period of time, remove existing scale deposits.”
The electronic system uses a solenoid coil(s) wrapped around the pipe; a signal generator continuously changes the frequency supplied to the coils. The pulse-shaped current creates an induced electric field. This field serves to stop the formation of mineral scale in the water passing through the pipe. Power, frequency, and coil configuration can be adjusted for optimum performance.
The advantages to this system, explained De Baat Doelman, are:
- Energy use is substantially reduced due to heat exchange surfaces remaining free of scale deposits. (He pointed out that just 1¼4 in. of scale increases energy costs by approximately 40%.)
- Corrosion caused by scale deposits is eliminated.
- The downtime and labor cost involved in conventional descaling is eliminated.
- Water distribution pressure and efficiency are increased by the removal of scale, which can reduce pipe diameters.
- A source for the establishment of biofilms and legionella is removed.
For more information, contact Jan De Baat Doelman at 610-932-6888; 610-932-7559 (fax); firstname.lastname@example.org (e-mail).
Publication date: 01/14/2002