Practicalities of Electrostatic Spraying for Large Scale Viral Control

Electrostatic spraying has been used successfully for decades to improve spray coating applications. There are many claims that this technique can be deployed to help improve the application of the antiviral agents being used to combat the covid19 pandemic.

This article looks at the practicalities of electrostatic spraying for large scale viral control and draws some lessons from the application of these techniques to large scale spraying in agriculture.

Electrostatics basics

The use of electrostatic spraying for disinfection is an interesting concept. The theory is to charge the particles before they are ejected from the nozzle. This is done by using a strong positively charged spray.

This charged spray will then adhere better to any negatively or neutrally charged surface it comes into contact with. This technique has been used successfully in spray painting for many years.

One of the big issues with electrostatics is that liquid sprays will rapidly lose their charge as they exit the nozzle. Charging a particle means stripping away electrons from the molecules it consists of.

The resulting ions are then “hungry” for electrons as so will absorb them rapidly to return to an electrically neutral state. As the spray interacts with the air it will rapidly return to an electrically neutral state. This means that after a short distance the charged spray is no longer charged.

In spray painting and other coating applications the electrostatic nozzle is kept close to the target for this reason. In addition, the surfaces the spray is coming into contact with are consistent, well defined and within a clear line of sight of the nozzle.

The environment is also stable and controlled. Whilst electrostatics are a tried and tested technique in this type of application there are few if any studies showing that this type of spraying will be beneficial to widescale room-sized decontamination.

Mass to charge ratio

In order to work, the electrostatic charge on each droplet needs to be above a certain threshold. Research shows [1] that if we have below 1.5 µC / g electrostatic spraying gives little benefit. As the mass of a droplet is a function of its size it is clear larger droplets will require larger charges. The overall charge carrying capacity of a droplet, the Rayleigh limit, is given by the following equation.

We can immediately see that as the mass (radius) of the droplet increases the maximum charge to mass ratio will decrease because the mass increases with an exponent of 3 and the Rayleigh limit increases only with an exponent of 2/3. The conclusion will be that larger droplets will struggle to achieve sufficient mass to charge ratios to make electrostatic spraying effective.

Most electrostatic paint sprayers will produce fine droplets in the 20-40 micron range. This is achieved by air atomisation (using compressed air to atomise and direct the spray).

This also means the droplets are small enough to have a sufficient charge to mass ratio for effective electrostatic coating. Unless these small droplet sizes can be replicated by the electrostatic sprayer being used for disinfection then it is unlikely to give much benefit.

The studies that do exist on electrostatic disinfection spraying tend to be in quite controlled spraying situations. One study [2] by the EPA showed good results when decontaminating PPE equipment.

Electrostatic spraying was compared against backpack spraying of PPE. Both decontaminated the suit but the electrostatic set up did so with far less runoff (i.e. less chemical was wasted).

It should, however, be noted that in this trial the droplet sizes from the two sprayers were very different with the electrostatic sprayer having a fine 40-micron spray and the backpack sprayer having an unmeasured drop size but likely to be 150 microns plus.

It is a well-known fact in spray engineering that spray consisting of smaller droplets will give superior coverage of a surface for any given volume of spray.

This means that a small droplet spray will be far more efficient at covering a surface. So, much of the increase in efficiency demonstrated in the EPA study could be down to this drop size difference rather than the electrostatic effect itself.

Humidity

The relative humidity of the environment will also have a large effect on whether electrostatic sprays will work. Humid air will tend to discharge the sprays more quickly.

In very dry environments electrostatic spraying may offer benefits but in more humid environments the same spraying equipment will offer little benefit as the charging is dissipated to the atmosphere too rapidly.

For this reason, the humidity levels are often strictly controlled in electrostatic spray-painting applications. This, however, is not possible in the large-scale spraying of public places.

Lessons from agriculture

Electrostatic spraying in agriculture has been seen as a potential great innovation that will save chemical usage. The theory is that by charging crop sprayers we can reduce wasteful run off and get more of the chemical to act on the plant. This means farmers can use less chemicals and get the same results. The cost benefits of this are clear as well as the environmental benefits.

Despite this promise, up take on electrostatic spraying in agriculture has not been widespread. There are a number of practical problems when translating technology successfully deployed for decades in coating and painting to the farm.

Firstly, the droplet sizes from agricultural sprayers are much larger, typically crop sprayers will produce droplet in the 100-300 microns in size. This is because the use of compressed air (air atomising nozzles) is impractical on such a large scale.

Similarly, the pressures required to produce fine droplets by hydraulic nozzles are impractical. So, getting the required charge to mass ratio is problematic in larger scale spraying that relies only on low pressure (sub 10 bar) nozzles.

The second issue is humidity. It impossible to control humidity in an agricultural environment and, in all likelihood, it is likely to be higher than in a factory setting.

As such the charges delivered to electrostatic sprays tend to dissipate more rapidly than in the paint shop. These two factors have meant that the promise of electrostatic spraying in agriculture is yet to be realised in practice.

Wet surfaces

If a surface is already damp, then electrostatically charged sprays may have a greatly reduced effect. The presence of micro-films of water on the surface may mitigate any benefit from a positively charged spray.

This is because water is an excellent conductor of electricity and will quickly dissipate any positive charge. Indeed, a common method for removing static is to deploy a fine mist to discharge the area or surface. In electro-static spray-painting great care is taken to ensure the surface is dry for this reason.

Covid19 spraying

When spraying for pathogens like coronavirus electrostatic spraying would seem to offer many benefits. The increased adhesion to surfaces not directly in the line of sight of the sprayer would be advantageous in disinfecting difficult to reach surfaces. But, as with agricultural spraying, there will be mitigating factors.

Many sprayers deployed for disinfection purposes will produce quite large droplets and so electrostatic charges will be ineffective as the charge to mass ratio is too low.

To combat this, air assisted or high-pressure electrostatic sprayers may be deployed but these are often very expensive and bulky pieces of kit meaning they are impractical for large scale spraying.  

Another issue is that the presence of biofilms or just general dirt will prevent disinfection from mists or fogs occurring. If the virus is protected by dirt, then the fog / spray will not be effective.

A 2013 study [3] in Applied Environmental Microbiology compared three method of sanitising: standard hydraulic spraying, air assisted electrostatic spraying and mechanical wiping.

Good old-fashioned wiping performed best; standard hydraulic spraying came next with electrostatic spraying ending up performing worst. The study commented that the increased mechanical action of wiping and the larger droplets from hydraulic spraying was probably the cause of this difference, although they did also note some experimental flaws that might have unfairly influenced the effectiveness of the electrostatic spraying.

It should also be noted that in this study the disinfectant was applied only to an exposed surface so it could not comment on how effective the wrap around effect would be.

Given the need for pre-cleaning of surfaces we can immediately see why this fact may be problematic for the use of electrostatic sprays in viral control.

This essential pre-cleaning will often leave the surface slightly wet. And, as discussed above, the presence of residual moisture in the air (humidity) and on the surface will serve to very rapidly eliminate any benefit from electrostatics.

Conclusions

As far as this author is aware there are no conclusive studies showing that electrostatic spray is more effective in larger scale disinfection spraying. There are limited studies that show electrostatic spraying techniques being used for the disinfection of eggs, PPE and other items of a known shape and in a set spray booth i.e. mimicking an electrostatic paint booth.

This makes sense; logic would dictate that in the targeted spraying of controlled surfaces in controlled environments they will help with spray adhesion as they have done in painting applications for years.

The jury is still somewhat out on whether the electrostatic spraying on a larger scale will offer big benefits. Large scale electrostatic spraying has been attempted in agriculture for many years with limited success.

Farmers tend to be a rather practical and pragmatic bunch so the slow adoption of electrostatic spraying in this industry, despite the theoretical promises, may be telling.

If electrostatic spraying is to be used in large scale spraying it is clear that a low droplet size, similar to that found in spray painting systems, will be required in order for them to be effective.

Indeed, given the environmental factors often stacked against successful electrostatic coating mentioned above, a small droplet size with a corresponding high charge to mass ratio would be essential. As such air atomising nozzle systems with electrostatic charging would probably be a necessity (much like in electrostatic spray painting).    

Over time controlled and independent studies will undoubtably be carried out. Such experiments should mimic the real world and look carefully at the pathogens eliminated.

Controls should be put in place to ensure sprays with similar droplet sizes are being used or direct comparisons made against other types of disinfecting sprays/fogs deployed.

Then and only then will we understand how much more effective electrostatic spraying will be in practice rather than theory. At that point we will be able to determine whether the additional cost of electrostatic spraying is worth it.

The Covid19 pandemic will undoubtably trigger this type of research. At the end of all this we, as a civilisation, will increase our knowledge on all matters of hygiene and disinfection.

No doubt new spraying techniques and chemical formulas will be developed. This will, hopefully, have some long-lasting benefits that stretch beyond the immediate crisis leading to a permanently safer and cleaner world.

References

[1] G. A. MATTHEWS, Electrostatic spraying of pesticides: a review, CROP PROTECTION Vol. 8 February 1989, 3-15

[2] Environment Protection Agency. Evaluation of Electrostatic Sprayers for use in Personal Decontamination Line Protocol for Biological Contamination Incident Response Operations, EPA/600/R-183, September 2018

 [3] Bolton et al, Sanitizer efficacy against murine norovirus, a surrogate for human norovirus, on stainless steel surfaces when using three application methods, Appl Environ Microbiol. 2013 Feb;79(4):1368-77. doi: 10.1128/AEM.02843-12. Epub 2012 Dec 21.