Water disinfection and Legionella bacteria control systems
Most water treatment methods do not ensure microbiological safety, so water disinfection is an important part of the water treatment technology in many applications. The purpose of disinfection is to neutralize all pathogenic microorganisms and protect water against its secondary bacteriological contamination in the mains water system. Disinfection must be applied for surface water treatment and, in the case of groundwater free from pathogens, it should protect treated water from secondary contamination. Disinfection can be carried out by different methods, both physicochemical and chemical. In practice, these methods are often combined:
- thermal - pasteurization, deactivates microorganisms, leading to protein denaturation;
- ultrasonic - carried out by means of waves with frequencies above 20 kHz strongly affecting living bacteria. The mechanism of operation of ultrasounds is based on the phenomenon of cavitation, i.e. tearing of a cell from the inside due to the formation of gas bubbles in the cell;
- UV radiation - this method does not cause chemical changes, but water must be very clear, because otherwise the rays reflect on suspended particles and colloids. Radiation acts locally. A disadvantage of the method is the lack of protection against secondary contamination;
- chemical methods, dosing chlorine dioxide or hypochlorous acid.
Where Legionella bacteria occur
Legionella bacteria are found in aquatic environments, with warm and humid places being most exposed to them.
The following can be sources of Legionella infection in facilities: hot water systems, air conditioning systems, air humidifiers, swimming pools, shower heads, draw-off valves, cooling towers, car washes, dental turbines, breathing aids, etc. Bacterial growth is encouraged by temperatures in the range of 25-45 °C (optimal temperature for their growth is 38 °C), low concentration of disinfectants, water stagnation and biotic factors such as the presence of other microorganisms, biofilm. Currently, controlling Legionella is one of the biggest challenges for managers and owners of many large facilities and buildings.
Controlling Legionella
One of the basic ways of controlling legionella is thermal disinfection, the so-called overheating of the system, unfortunately its effect is short-lived or the cost of its implementation is not commensurate with the effects achieved.
Chlorine dioxide – effective disinfection
One of the most effective methods of potable water disinfection is dosing chlorine dioxide (ClO₂). Chlorine dioxide eliminates bacteria, spores and reliably kills Legionella. Chlorine dioxide has a much stronger disinfecting effect than chlorine over a wide range of pH values. No dangerous by-products (THM, dioxins, chlorophenols, etc.) are produced when using chlorine dioxide.
Chlorine dioxide is widely used in municipal and local potable water treatment plants, often in combination with UV lamps. UV lamps disinfect water “locally”, chlorine dioxide is effective along the entire length of the pipeline and reaches distant water draw-off points. Additionally, it eliminates the biofilm, which is the “habitat and shelter” of millions of various dangerous bacteria. Disinfection with chlorine dioxide has a long-lasting effect; this agent is also ideal for “shock” or one-off disinfection.
It guarantees elimination of single Legionella bacilli even in relatively low concentrations: 0.1 - 0.2 mg ClO2/l for cold water and 0.35 mg ClO2/l for hot water systems.