MRO Magazine

Controlling scale in water systems helps prevent bacterial outbreaks


Industry

March 4, 2013
By PEM Magazine

The legionella bacterium (L. pneumophila) is a water-based organism, which causes infection when inhaled in an aerosol form. Normally associated with cooling towers and evaporative condensers, mist machines, humidifiers, whirlpool spas and showers, the bacteria L. pneumophila is most commonly associated with the disease outbreak (legionellosis), which travels by air and is caused by the inhalation of contaminated water under the form of aerosol spray that is smaller than 5 μm. 

The bacteria thrive in stagnating water (tanks, reservoirs, dead legs in piping systems, poor flow areas). The bacteria require temperatures between 68°F and 113°F (under 68°F they survive over 140°F they are killed) and a supply of nutrients found in algae, rust, sludge and scale.

PREVENTION
Health agencies continually draw attention the risks and good practice concerning cooling towers and evaporative condensers in cooling water treatment. Conditions that affect the proliferation of legionella include:

1. The presence of scale deposits or algae growth in the water
2. Deadlegs in the pipework or stagnation due to very low use of outlets
3. Low temperature in potable hot water heaters and distribution systems
4. Stratification of water in water heaters
5. Inappropriate water treatment

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L. pneumophila may be able to colonize certain types of water fitting, pipework and materials 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, L. pneumophila is found in many recirculating hot and cold water systems particularly in larger, complex systems such as those found in hospitals, hotels, office block and factories.

Managing the risks from legionella in water systems requires a holistic approach and a suite of control measures 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 can increase the likelihood of legionella persisting.

SCALE
Scale or lime-scale is a hard, rock-like deposit of calcium or magnesium salts that forms in heat exchangers and cooling tower packing and other water-fed equipment as a result of heat and increased concentration factor.  Scale formation impairs heat transfer, interferes with flow and cooling, and can be a breeding ground for legionella bacteria.  The scaling tendency of a water supply will depend on the hardness of the water but if not adequately treated even relatively soft waters can become highly scaling when concentrated by evaporation.  Poor control not only puts your cooling process at risk, but can also squander $1,000s in wasted energy, chemicals and water charges.

Scale is a major problem in both hot and cold water systems. Dripping taps can deposit scale in and around the tap, and with high ambient room temperatures, provide an ideal growth medium for LP. In hot systems, scale can trap Legionella and biofilms. This provides a perfect growth medium, which disinfectants cannot penetrate. Scale deposits colonized by Legionella can continuously re-contaminate a system, even after disinfection. Biofilms trapped are a source of nutrients for LP and can lead to taste and odor problems from the products of their metabolism.

Scale is a major cause of inefficiency in hot water systems. Scale on heat exchange surfaces dramatically reduces the heat transfer efficiency and promotes corrosion in the calorifiers and pipework. Descaling of a hot water system is time consuming and expensive. Water softeners can reduce scale, but there is growing concern over the increase to sometimes-high levels of sodium in the water.

AIR CONDITIONING AND REFRIGERATION WATER SYSTEMS
Many air conditioning and refrigeration plant systems are water-cooled. The heat generated by cooling coils is removed by water, which is passed through a water-cooling tower. These are recirculating systems, which operate at temperatures ideal for bacterial and algal growth, and have plentiful supplies of nutrients. They have been highlighted as a major possible source of Legionnaires disease mainly because of the large number of people that can be affected.

However, in a tower that is well-designed and maintained, chances of problems with L. pneumophila are greatly reduced. Most cases of outbreaks have occurred in towers, which were badly designed, and had little or no maintenance. In cooling towers, temperature, water hardness, pH, scale and corrosion are all factors, which increase the chance of biofilms, algae and Legionella colonization. Many agents are used to control these factors, including scale and corrosion inhibitors, dispersants and biocides. Water softeners are sometimes used for soft water, which can cause a problem with foaming.

Biofilms are a major problem in cooling towers. Biofilms and scale can reduce the efficiency of cooling systems to the point where the system no longer functions with regard to heat transfer. Health & 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 L. pneumophila if used in sufficient concentration. Strains of L. pneumophila and other bacteria may become resistant to particular biocides hence dual or alternating biocides are used.

ALTERNATIVES
What is required in all the systems, cold, hot and process is a method of continuously controlling scale deposition and a water treatment regime which prevents the growth of biofilms, bacteria and, in particular, L. pneumophila. This method is now available in the form of electronic scale treatment of water to prevent scale deposition together with the chlorination of all water supplied to a building or factory both hot and cold.

ELECTRONIC WATER TREATMENT
This involves the fitting of electronic water descaling equipment, such as the patented Scalewatcher, at strategic points in the water system. Water treated by such systems will prevent scale from forming in pipework and on heat transfer surfaces, and will also, over a period of time, remove existing scale deposits. There are many advantages to this non-intrusive engineering solution:

1. Energy use is greatly reduced due to heat exchange surfaces remaining free of scale deposits (just a ¼ inch of scale increases energy costs by around 40 per cent).
2. Corrosion caused by scale deposits is eliminated.
3. Extensive downtime and labor cost involved in descaling systems is eliminated.
4. A source of colonization by biofilms and L. pneumophila is removed.
5. Water distribution efficiency and pressure is increased by removal of scale deposits which can reduce pipe diameters considerably.


Jan De Baat Doelman is the president of Scalewatcher North America Inc. For more information, visit www.scalewatcher.com.