Effectively prevent legionella in the cooling tower

Legionella are rod-shaped bacteria. They live mainly in fresh water and are found worldwide. These microorganisms were first detected in 1977. At that time, there was an epidemic in the USA that claimed 29 lives. In Germany, Legionella infections have been notifiable since 2001.

Legionella bacteria can enter technical water systems in small quantities from their natural habitat. Under favorable conditions, they grow there. The microorganisms prefer to multiply in water at temperatures between 20 °C and 50 °C. Popular places for them to settle are:

• water pipes,
• swimming pools, and
• evaporative cooling systems.

These aquatic organisms are transmitted to humans through inhalation (e.g., from contaminated shower water). They can also spread through the atmosphere via suspended particles, known as aerosols (e.g., when the exhaust air from open cooling towers is contaminated). 

Legionella bacteria are considered to be the cause of Pontiac fever. The disease is accompanied by flu-like symptoms and resolves within a few days without treatment.

Far more dangerous is Legionnaires' disease, also known as Legionella pneumonia. This severe form of pneumonia can be life-threatening. If left untreated, it leads to death in approximately 15 percent of all healthy people. The disease occurs with striking frequency in older people, smokers, and men.

Symptoms of Legionella pneumonia include:

• a dry cough,
• headaches and aching limbs, and
• high fever.

Several weeks of antibiotic therapy are required to combat the infection. The considerable treatment costs can sometimes be charged to the person responsible for causing the epidemic.

A cooling tower extracts heat from the water in the cooling circuit. This is often done by means of an artificially generated air flow. Cooling towers are installed outdoors and are therefore constantly exposed to various environmental conditions. Organic material (e.g., leaves or pollen) enters the cooling system and continuously supplies the cooling water with nutrients. Microorganisms find ideal living conditions and optimal growth conditions here.

Over time, a "biofilm" can develop throughout the entire cooling system. Such deposits in the water-carrying areas consist of a multitude of microorganisms. Their excretions form a slime layer. This protects the Legionella bacteria from environmental influences and can limit the effectiveness of countermeasures.

As a renowned cooling system manufacturer with many years of experience, MULTI Kühlsysteme knows that cleaning an evaporative cooling system is time-consuming and costly. It is important to operate the system as hygienically as possible in order to prevent the formation of biofilm from the outset. Chemical and physical disinfection methods (such as the use of biocides) as well as regular maintenance and servicing help to achieve this. Legislators also place an obligation on system operators in this regard. 

The 42nd Ordinance for the Implementation of the Federal Immission Control Act (42. BImSchV) has been legally valid since August 19, 2017. Operators of evaporative cooling systems and cooling towers are required to take extensive measures. For example, they must regularly check the concentration of Legionella bacteria in the cooling water. The aim of the ordinance is to ensure the hygienic and safe operation of the relevant systems. Information on this can also be found in VDI Guideline 2047 Sheet 2.

The background to the 42nd BImSchV is the frequent occurrence of Legionella infections. For example, epidemics occurred in Ulm in 2010 and in Warstein in 2013, some of which were fatal. The bacteria presumably spread via the exhaust air from open cooling towers. The improper operation of an evaporative cooling system poses a serious risk to everyone in the surrounding area. Contaminated cooling tower exhaust air can spread up to 15 kilometers and cause illness within this radius.

The legislator responded to the increasing number of infections with the 42nd BImSchV. We have summarized the most important contents of the regulation for you in the next question. 

The 42nd BImSchV states literally: Evaporative cooling systems and cooling towers "must be designed, constructed, and operated in such a way that contamination of the service water by microorganisms, in particular Legionella, is prevented in accordance with the state of the art." (Section 3 (1) 42nd BImSchV)

The legislator imposes numerous requirements on the hygienic operation of the systems. The key points of the 42nd BImSchV are as follows:

• Evaporative cooling systems must be reported to the competent authority.
• Before a system is (re)commissioned, a risk assessment must be carried out by a person with expertise in hygiene.
• The parameters for Legionella and the general colony count must be determined regularly for the service water.
  The general colony count includes all microorganisms in the cooling water and assesses its hygienic quality.
• If a limit value is exceeded, the operator of the cooling tower must take certain measures.
• The operator must keep an operating log.
  This log must document, among other things, the results of laboratory tests and any measures taken.
• An inspection of the system is mandatory every five years.
  This must be carried out by a sworn expert or a type A inspection body.
• MULTI Kühlsysteme GmbH helps you, as a plant operator, to fulfill your legal obligations.

Laboratory tests are used to detect Legionella bacteria in service water. In accordance with the 42nd BImSchV, sampling must be carried out on site by a certified sampler. The cooling system should be in normal operation and the last use of biocides should be as long ago as possible.

According to the regulation, the water samples must be analyzed in an accredited laboratory. The number of Legionella bacteria in the cooling water is determined in a complex procedure. This involves culturing the bacteria on special nutrient media. The result shows how many "colony-forming units" (CFU) can be detected per 100 ml of water.

As a general rule, the higher the concentration of Legionella bacteria in service water, the more drastic the measures that must be taken. No action is required for values up to 100 CFU (colony-forming units) per 100 ml of water.

If this limit is exceeded in an evaporative cooling system, an additional laboratory test is required first. If the values remain elevated, the legislator prescribes the following measures:

• Determination of the causes,
• Technical procedures to reduce the number of Legionella bacteria in the service water
• Weekly internal checks and monthly laboratory tests (until the values fall below the limit),
• From 1000 CFU/100 ml, immediate measures to reduce the microbial load (e.g., use of biocides).

If the values exceed 10,000 CFU per 100 ml, the operator must take immediate action. In addition to the measures already mentioned, a report to the competent authority is required. The authority may then even order the shutdown of the affected system. For example, a contaminated evaporative cooling system in Düsseldorf was shut down in October 2017.

The uncontrolled spread of Legionella and other microorganisms in evaporative cooling systems can be prevented. To do this, the following points should be observed:

• Appropriate design and layout of the cooling system to meet effective cooling requirements
• sensible choice of location for the cooling tower (as little nutrient input as possible, discharge of aerosols into safe areas),
• Compliance with the parameters recommended by the system manufacturer for service water (pH value, temperature, etc.),
• Regular inspection of surfaces for biofilms and cleaning if necessary
• Regular maintenance and servicing of the cooling system.

Our systems are effectively designed, robust, and very durable. We are also happy to take care of their maintenance and servicing to prevent Legionella contamination. 

Microorganisms in cooling water can be combated chemically or physically. Chemical methods rely on biocides. These substances prevent, inhibit, or stop microbial growth.

Chlorine dioxide has proven particularly effective in destroying Legionella bacteria. The biocide removes the biofilm in the pipes of the systems and kills viruses, bacteria, and fungal spores. Chlorine dioxide has a long-lasting effect and works independently of the pH value of the water. However, it must be produced directly on site. In addition, the chemicals required for this are comparatively expensive.

In addition to chemical methods such as the use of chlorine dioxide, physical techniques can also be used. For example, the cooling system can be irradiated with UV-C light. This eliminates all microorganisms in the cooling water. However, the irradiation is locally limited and cloudy water reduces its effect.

The use of catalysts is also a physical disinfection method. This destroys microbial biofilms and simultaneously produces surfactants. The latter additionally clean the cooling system. In most cases, such water treatment makes it possible to dispense with biocides.

In the event of a Legionella infestation, the operator should first contact the system manufacturer. Experienced suppliers such as MULTI Kühlsysteme GmbH know the technology of their systems down to the smallest detail. They are also informed about restrictions on possible disinfectants.  

MULTI Kühlsysteme GmbH supports you in implementing the 42nd BImSchV. To this end, we have expanded our range of services. In addition to our proven portfolio, we now offer our customers the following services:

• Notification of the system to the competent authority,
• Preparation of a risk assessment,
• Preparation of an operating log,
• Regular sampling by our own certified samplers,
• Sample analysis in an accredited laboratory, and
• Implementation of the prescribed measures if limit values are exceeded.

Your competent contact for hygiene safety in evaporative cooling systems is Dr. Thomas Spaller. You can reach him by phone at 03771 246722 or by email at t.spaller@multi-kuehlsysteme.de. Dr. Spaller looks forward to your inquiry!

• Legionella: History and Significance.pdf
• 42 Federal Immission Control Ordinance—Hygienic Operation of Evaporative Cooling Systems.pdf