We are continuously receiving information about the different fluids for disinfection and existing methods to carry out a correct disinfection. But do you really know which one is the most effective? Hereunder, we will try to answer this question, providing a rationale, based up on our studies and tests performed at the SAMOA Industrial laboratory.
Droplet Size and why it matters for Disinfection
When seeking effectiveness in disinfection methods, the droplet size concept always pops up. There is no certainty as to which droplet size value is most suitable for an adequate disinfection. However, our experience shows that droplet sizes between 10 and 25 microns produce good results. Droplet sizes of less than 10 microns have the inconvenient of staying on the air for a long time and could be inhaled by people, a very harmful event for our health.
When we speak of droplet size generated by atomization, we are not referring to a specific size, but to a probabilistic role of droplet sizes in a specific sample.
With High Pressure Atomization technology having droplet sizes D50 = 10 microns or less is possible. It should be noted though, that when pointing out to D50, it is guaranteed that at such sample, 50% of the particles will be larger than 10 microns and, the other 50% will be less than 10 microns in size. This is one existing way of speaking about particle diameter by atomization and we must embrace it. That is, when we say that we have 10 micron particle diameters at the jet, we are not saying that every particles or drop is actually 10 microns in size.
Photo taken at the SAMOA Industrial Lab using a SFC11-40 nozzle at 150bar
Washing vs Disinfection, can you spot the difference?
At this point, it is necessary to explain the difference between WASHING and DISINFECTION. Most cleaning operations carried out count as washing, although, mistakenly, they can be equated with disinfection.
When we watch a City Council truck clean the streets with a large hose, a solution of water with bleach is being used to wash the road. Such washing operation can drag down the shells from pipes, also organic particles and general garbage, but it does not disinfect. In other words: the jet used only washes the street, because it is not capable of reaching any holes, cracks or hard-to-reach places where any virus or bacteria can be hiding. Moreover, the water flow used does not allow the necessary contact between the disinfectant and the virus to destroy it.
Low pressure, large jet or large droplet size devices with high flow rates are used to perform such WASHING operations, which, in itself is not compatible with DISINFECTION.
Don’t confuse Washing Operations with Disinfection
Some examples of this kind of washing, wrongly branded as disinfection, can be found at stations, public places or buildings exteriors such as railings, street furniture, etc. Sometimes we see cleaning workers carrying low pressure spraying tools on their shoulder while spreading liquid everywhere. Right after a second worker appears with a cloth, drying the surfaces the first went through. This washing method is the complete opposite to disinfection. The use of rags is totally contraindicated when performing disinfection, as virus and bacteria can “hide” among the rag fibers, being spread later on the areas theoretically disinfected when the surfaces dry.
So, what do you need to perform a professional Disinfection? With these ideas in mind it is difficult to believe that one can use a sprayer, or rather a low pressure spray device to disinfect, for example, a hospital room, a meeting room or any interior household room. The reason is clear, in such areas it is necessary to disinfect, not perform wash operation that may ruin any accessories inside.
Characteristics of the equipment needed to carry out professional disinfections
Therefore, a professional disinfection of these spaces must be done using high-pressure atomization systems with the following features:
The pressure and the nozzles used allow the creation of fine drops that facilitate disinfection.
The disinfectants flow in these applications is minimal, therefore the wasted of fluid is minimal and the disinfection is maximum.
Fine particles generated have appropriate size to guarantee the minimum contact time of the disinfectant with the organism to be eliminated, in order to guarantee cell death.
These fine particles also have the peculiarity of a size small enough to guarantee evaporation without leaving any mark on any surface and hence, no need to wipe with a cloth spoiling the whole disinfection process.
Such particles are capable of reaching every nook and cranny in the room, such as internal edges, crevices, slits, angles and corners. Here the application of large drops and large flow rates will not guarantee the total elimination of harmful organisms. – Organisms to be eliminated are on the micron scale and specifically the Coronavirus, has a diameter of approx. 0.12 micron. At such scales, phenomena such as coalescence, capillary action and in general, the consequences of superficial fluid tension make it difficult to reach them.
So what is the optimal droplet size for effective disinfection then?
Some scientific research analyzes the optimal drop diameter range to disinfect spaces related to health. Constructions such as hospitals, chemical, biological or radiological premises as well as nuclear environments. When disinfecting such environments, the use of high pressure systems capable of generating fine droplets with the properties explained above is recommended.
The droplet size spectrum indicated is 10micron <D32 <25micron. In this environment we must consider several important technical aspects for its interpretation. Research agrees on the fact that drop diameters less than 10 microns can be harmful. Because these particles are so small, they remain suspended in the air and can be inhaled by the people performing the disinfection. Moreover, they are considered too small, hence incapable of remaining active for long enough. They evaporate too quickly before breaking down the targeted organism.
Cleaning distance: what is it and how is it calculated?
Another important concept is D32 interpretation. This refers to the SAUTER or SMD mean diameter, named after the German scientist who first used it. In this case, this said diameter corresponds to a spherical drop, having the same surface and volume relationship, as the rest of all drop surfaces and volumes within the sample. This is nothing more than a statistical function confirming that we are dealing with extremely fine drops.
From here, it is important to introduce other concepts, such as the application distance of these fine drops in a disinfection operation. Bear in mind that when a stream of liquid is being sprayed in fine drops, it is necessary for those drops to reach a certain speed so they hit the surface to be cleaned, causing either a bounce, adhesion or breakage of said drop (” sparked ”). In order to determine what type of behavior the drop will have upon reaching the surface, it is crucial to be familiar with the dimensionless Weber and Laplace numbers that define the limits or Critical We numbers for each situation discussed.
Detailed studies lead us to consider the maximum fine droplet jet application distance on the surface to be cleaned. Without going into calculations or rough details, it is considered that the cleaning distance will be between 2 and 3 meters from the surface depending on its type.
In short: difference WASHING and DISINFECTION and discover the ATOMIZATION
In conclusion, it should be considered that it is mandatory to differentiate between WASHING and DISINFECTION. They are completely different phenomena and therefore must be treated differently.
It is a proven fact that high pressure technology with fine droplet atomization is the most suitable for surface DISINFECTION. Generating fine drops is not enough to perform a good disinfection. Taking the exposure TIME of the surface into account, as well as the FLOW RATE supplied is mandatory. Meaning, if you have a fine droplet atomization system, but the application distance as well as the application time, causes the surface to soak and coalesce in the droplets, allowing them to join together creating larger droplets and streams on the wall, the disinfection capacity sought is decreased.
If, after reading this article, you are wondering where and how to get the professional spray equipment necessary for professional disinfection jobs, click here to download information about SAMOA products for surface disinfection.
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