Hygienic air humidification strategies to combat biofilms

Properties of humidification water

Generally speaking, the standards for drinking water are so high that it can, in principle, be used for humidification purposes. It should be noted, however, that whilst drinking water must not contain pathogenic microorganisms in significant quantities, it is by no means sterile. Furthermore, the minerals present in untreated drinking water will sooner or later lead to salt and limescale deposits, whether on the contact surfaces of the humidification systems themselves, on duct walls or on HVAC components.

In damp and wet areas, these deposits provide exceptionally good living conditions, particularly for the microorganisms carried in with the drinking water. These settle there and can grow into substantial colonies in the form of a biofilm within a very short time.

To prevent such deposits, humidification water is usually softened and/or desalinated. However, the water treatment systems used for this purpose often provide equally favourable conditions for microbial growth. This is discussed in more detail in the following section.

Biofilms in water-bearing systems

Microorganisms occur in water-carrying systems either freely in the water or bound within so-called biofilms. Biofilms consist of microorganisms embedded in a slime-like matrix of extracellular polymeric substances (EPS). This structure enables them to colonise almost any moist surface.

The biofilm establishes itself in the ion exchange units of water softening or full desalination systems, can colonise the modules of reverse osmosis systems and, depending on the design, can even penetrate their membranes and thus reach the permeate side. It then colonises the walls of the downstream pipework and intermediate storage tanks. In the case of open intermediate storage tanks, additional microbial contamination from the ambient air can further promote biofilm formation.

As long as the biofilm is in its growth stage, it often goes unnoticed. Only once it has reached its stationary phase are microorganisms continuously released. At this stage, the microbial count in the water rises dramatically. Simply disinfecting the humidification water is then usually no longer sufficient, as this has only a limited effect on an already established biofilm.

Biofilms in HVAC systems

Once a biofilm has established itself in the water-carrying systems, small fragments of biofilm (bio-envelopes) are constantly breaking off and settling on all damp areas of an HVAC system. This can occur on the contact surfaces of the air humidification system itself, but also on duct walls or any HVAC components. Wherever there is moisture, dampness or standing water in the HVAC system and no suitable countermeasures are taken, new biofilms will inevitably form and bacterial growth will continue unchecked.

Strategies for preventing biofilms

From a hygiene perspective, it is far more effective to prevent the formation of biofilms at the very outset. However, technically effective solutions for completely preventing biofilms are currently only available to a limited extent. Pipe systems that are continuously and uniformly exposed to UV radiation do not yet exist, and other approaches are in some cases restricted by patent or usage rights. In practice, therefore, microbe-reducing substances are used.

According to VDI 6022 Part 1 (4.3.7), the supply and recirculating water in humidifiers may only be disinfected using methods whose efficacy has been proven, which are harmless to health and which leave no residues in the supply air. These strict requirements significantly limit the number of suitable methods.

The agent used must effectively reduce microorganisms and biofilm formation without being present as a biocide in the supply air. In practice, therefore, only a few active substances are currently suitable:

● Hydrogen peroxide

● Chlorine dioxide

● Silver

● Ozone

Oxidative processes such as ozone, hydrogen peroxide or chlorine dioxide react with microorganisms and organic airborne pollutants. However, their safe use requires precise dosing and reliable measurement and control technology to ensure that no residual concentrations posing a health risk remain in the indoor air.

Silver has a broad antimicrobial spectrum of activity. Silver ions or nanoscale silver particles are particularly effective. In contrast to oxidative processes, the effect of silver is based on the targeted influence on enzymatic processes in microorganisms. Even low concentrations of silver ions can inhibit the growth of bacteria and thus prevent the formation of biofilms.

In air humidification systems, for example, silver ions can be generated electrolytically and dosed in accordance with the water flow rate. Suitable separation surfaces, such as ceramic materials, can then be used to remove silver residues from the air and water. This enables hygienic air humidification whilst ensuring that the supply air is low in residues.

Summary

Biofilms pose a significant hygiene risk in adiabatic air humidification systems. They tend to form in water-carrying components and can persist despite disinfection measures.

The requirements of VDI 6022 are therefore designed to minimise microbial growth through both design and operational measures. As the removal of existing biofilms is very labour-intensive, prevention is of crucial importance. Suitable methods for reducing bacteria in the humidifier water – such as oxidative processes or the use of silver ions – can help to effectively limit biofilm formation and ensure the hygienically safe operation of air humidification systems.