Controlling Scale in Water Systems Helps Prevent Legionnaires’ Disease

By Jan De Baat Doelman | December 12, 2012

Category:
  • Scale formation can be a breeding ground for Legionella bacteria.
  • A method of controlling scale deposition is required continuously.
  • An electronic descaling system prevents scale from forming.
Pipe with legionnaires disease

The Legionella bacterium Legionella pneumophila, the fundamental agent of Legionnaires’ disease, is a water-based organism that can cause infection when inhaled. Legionnaires’ disease acquired its name in 1976 when an outbreak of pneumonia occurred among attendees of a convention of the American Legion in Philadelphia. Later the bacterium causing the illness was named Legionella (see
Figure 1).

Normally associated with cooling towers and evaporative condensers, as well as mist machines, humidifiers, whirlpool spas, and showers, the bacteria Legionella pneumophila is most commonly associated with the disease outbreak (legionellosis). It travels by air and is caused by the inhalation of contaminated water in the form of aerosol spray droplets smaller than 5μm.

Legionella bacteria thrive in stagnating water in tanks, reservoirs, dead legs in piping systems, and poor-flow areas. The bacteria require temperatures between 68 and 113 degrees F and a supply of nutrients found in scale, algae, rust, and sludge.

Many air-conditioning and refrigeration plant systems are water-cooled. The heat generated by cooling coils is cooled by water passing through a water-cooling tower. These are recirculating systems that operate at temperatures ideal for bacterial and algal growth and have plentiful supplies of nutrients. They have been identified as a major possible source of Legionnaires’ disease mainly
because of the large number of people that can be affected.

Conditions for Proliferation

Legionella pneumophila
Figure 1
Legionella pneumophila is most commonly associated with the disease legionellosis. It travels by air and is caused by the inhalation of contaminated water.

Health agencies continually warn of the risks of Legionella proliferation in cooling towers and evaporative condensers in cooling water treatment. Conditions that facilitate the proliferation of Legionella include:

  1. The presence of scale deposits or algae growth in water.
  2. Dead legs in pipework or stagnation as a result of very low use of outlets.
  3. Low temperatures in potable hot water heaters and distribution systems.
  4. Stratification of water in water heaters.
  5. Inappropriate water treatment.

In cooling towers, temperature, water hardness, pH, scale, and corrosion are all factors, which increase the incidence of biofilms, algae, and Legionella colonization (see Figure 2).

Legionella pneumophila may be able to colonize in certain types of water fittings, pipework, and materials such as plastic and steel used in the construction of water systems. The presence of such materials, and of large quantities of sediment, may provide nutrients for Legionella and can make eradication difficult. In practice, Legionella pneumophila is found in many recirculating hot and
cold water systems, particularly in large, complex ones such as those found in factories, as well as hospitals, hotels, and office blocks.

Managing the risks from Legionella in water systems requires a holistic approach and a suite of control measures. These measures must be underpinned by a suitable and sufficient risk assessment specific to the risk system in question. In hard water areas, scale formation can be a problem unless properly managed, and it can increase the likelihood of Legionella persisting.

Problems With Scale

Legionella proliferation in cooling towers
Figure 2
Health agencies continually warn of the risks of Legionella proliferation in cooling towers and evaporative condensers in cooling water treatment. Conditions that facilitate the proliferation of Legionella include the presence of scale deposits or algae growth in water.

Scale, or lime scale, is a hard, rocklike deposit of calcium or magnesium salts that forms in heat exchangers, cooling tower packing, and other water-fed equipment as a result of heat and increased concentration.

Scale formation impairs heat transfer, interferes with flow and cooling, and can be a breeding ground for Legionella bacteria.

A water supply’s scaling tendency depends on the hardness of the water. If it is not adequately treated, even relatively soft water can become highly scaled when it is concentrated by evaporation. Poor control not only puts production-cooling processes at risk, it can squander thousands of dollars in wasted energy, chemicals, and water charges.

Legionella, Biofilms. Scale is a major problem in both hot and cold water systems. Scale can deposit in and around dripping taps, and in high ambient room temperatures, taps can be an ideal growth medium for Legionella pneumophila.

In hot water systems, including steam humidifiers, scale can trap Legionella and biofilms. This provides a perfect growth medium that disinfectants cannot penetrate. Scale deposits that Legionella colonize can continuously recontaminate a system, even after disinfection. Trapped biofilms are a source of nutrients for Legionella and can lead to taste and odor problems from the byproducts of
their metabolism.

Inefficiency. Scale is also a major cause of inefficiency in hot water systems. Scale on heat-exchange surfaces dramatically reduces heat transfer efficiency and promotes corrosion in the calorifiers and pipework. Descaling a hot water system is time-consuming and expensive. Water softeners can reduce scale, but the accompanying sodium levels in the water, which can become
very high, can cause concern.

Biofilms in cooling towers are a major problem. Combined with scale, they can reduce the efficiency of cooling systems to the point where heat transfer is impaired.

However, in a well-designed and -maintained tower, the risks of Legion-ella pneumophila are greatly reduced. Most outbreaks have occurred in cooling towers that were badly designed and had little or no maintenance.

Some Methods of Controlling Scale

Many agents are used to control these factors, including scale and corrosion inhibitors, dispersants, and biocides. Sometimes water softeners are used to soften water, which can cause foaming problems.

Chemicals often are used to prevent scaling, but they do not always solve the problems completely. Additional treatment utilizing electric and magnetic fields results in the dissolved minerals coming out of solution (becoming molecules) in the bulk fluid flow. This will prevent further scaling and corrosion because fewer free mineral ions are in the water.

Health and safety officers recommend the periodic chlorination and descaling of cooling towers. 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 may become resistant to particular biocides, hence, dual or alternating biocides are used.

What is required in all the systems—cold, hot, and process—is a method of controlling scale deposition continuously and a water treatment regime that prevents the growth of biofilms, bacteria, and, especially, Legionella pneumophila.

Electronic Water Treatment

Electronic Scale water treatment
Figure 3
Electronic scale treatment of water, both hot and cold, is a method of continuously controlling scale deposition that, combined with the chlorination of all water supplied to a factory or other facility, effectively prevents scale deposition.

Electronic scale treatment of hot and cold water is a method of continuously controlling scale deposition that, combined with the chlorination of all water supplied to a factory or other facility, effectively prevents scale deposition.

Electronic scale removal is a noninvasive system utilizing a solenoid coil or coils wrapped around the pipework to be treated. A signal generator with continuously changing frequency supplies current to the coils. The pulse-shaped current creates an induced electric field, concentric around the axis inside the pipe. As a consequence of this arrangement, any charged particle or ion moving
within the field experiences a so-called Lorentz force that is generated by the interaction between charged particles and magnetic and electric fields.

Research at a major university in Philadelphia confirmed that the Lorentz force is unchanged regardless of flow rate. Generated magnetic fields have been measured and found to be less than 1 gauss, lower than the earth magnetic field strength. This technology overcomes one major disadvantage of permanent magnet devices: They work well only within a certain flow rate window, and that at higher
and lower flows, the performance drops to zero.

Electronic water treatment affects the formation of scale by increasing the homogeneous precipitation rate of calcium carbonate and certain other minerals. The ability to adjust power, frequency, and coil configurations on-site enables performance to be optimized with no downtime and no pipe replacement.

Using electronic scale elimination requires the fitting of electronic water descaling equipment at strategic points in the water system.

An electronic descaling system prevents scale from forming in water-filled pipework and on heat-transfer surfaces. Over time it removes existing scale deposits as well.

Advantages, Disadvantages of Electronic Descaling

There are many advantages to this nonintrusive engineering system:

  1. Removes a source of colonization by biofilms and Legionella pneumophila.
  2. Reduces energy use, because scale deposits on heat-exchange surfaces increase energy demands (just ¼ inch of scale increases energy costs by about 40 percent).
  3. Eliminates corrosion caused by scale deposits.
  4. Eliminates extensive downtime and labor cost involved in descaling systems.
  5. Increases water distribution efficiency and pressure by enlarging pipe diameters.

Electronic devices are not flow rate-dependent and can be built to fit pipe diameters up to 60 in. The units are lightweight, easy to install, can be retrofitted, and produce no significant magnetic field.

Manufacturers may realize a short-term payback from the savings resulting from reduced maintenance and energy costs and the premature replacement of capital equipment. Depending on the water hardness, within one year it is possible for more than 2 mm of scale to form, causing a relative fuel loss of 6 percent. This can amount to a significant recurring financial cost.

A disadvantage of electronic water treatment is that sometimes a soft, powdery residue remains at the bottom of a vessel, which may need to be wiped away. However, in pipelines, this soft substance is removed with the water flow.

Electronic water treatment provides a nonintrusive, chemical-free, and maintenance-free method of removing scale from pipes and equipment.

Jan De Baat Doelman is president of Scalewatcher North America Inc., 345 Lincoln St., Oxford, PA 19363, 610-932-6888, sales@scalewatcher.com, www.scalewatcher.com.

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