Analysis of the Possibility of Disinfecting Surfaces Using Portable Foggers in the Era of the SARS-Cov-2 Epidemic

energies

Article Analysis of the Possibility of Disinfecting Surfaces Using Portable Foggers in the Era of the SARS-CoV-2 Epidemic

Marek Ochowiak 1,*, Andzelika˙ Krupi ´nska 1 , Sylwia Włodarczak 1 , Magdalena Matuszak 1, Szymon Woziwodzki 1 and Tomasz Szulc 2

1 Department of Chemical Engineering and Equipment, Poznan University of Technology, Plac Marii Skłodowskiej-Curie 5, 60-965 Poznan, Poland; [email protected] (A.K.); [email protected] (S.W.); [email protected] (M.M.); [email protected] (S.W.) 2 Łukasiewicz Research Network–Industrial Institute of Agricultural Engineering, Staroł˛ecka31, 60-963 Poznan, Poland; [email protected] * Correspondence: [email protected]

Abstract: The SARS-CoV-2 pandemic has resulted in the need for increased surface disinfection. For this purpose, biocides, UV-C radiation, or ozonation can be used. The most commonly used are biocides that can be deposited on the surface with the use of various devices, including foggers. The disinfection efficiency is related to the size of the aerosol droplets formed and their distribution. This paper specifies the distribution of droplet diameters and mean droplet diameters obtained during the use of a commercial fogger. It was shown that the droplet diameters formed were mainly in the range of up to 30–40 µm. A ceramic filter allowed for a larger number of smaller droplets and a limitation in the number of droplets with larger diameters. The results are important in the context of fighting the virus in hard-to-reach places where battery devices can be used. Citation: Ochowiak, M.; Krupińska, A.; Włodarczak, S.; Matuszak, M.; Keywords: SARS-CoV-2; COVID-19; portable foggers; atomization; droplet size distribution; Sauter Woziwodzki, S.; Szulc, T. Analysis of the Possibility of Disinfecting mean diameter Surfaces Using Portable Foggers in the Era of the SARS-CoV-2 Epidemic. Energies 2021, 14, 2019. https:// doi.org/10.3390/en14072019 1. Introduction Coronavirus disease (COVID-19) has become a global pandemic and has affected Academic Editor: Stelios Rozakis many aspects of life, including economic and social systems, health care, transportation, and the energy industry [1–3]. A serious problem that has arisen in connection with the Received: 11 March 2021 ongoing SARS-CoV-2 virus epidemic is the aspect of surface decontamination [4,5]. Surface Accepted: 1 April 2021 disinfection has been recommended as one of the most effective means to combat the spread Published: 6 April 2021 of the coronavirus (SARS-CoV-2) that causes coronavirus disease (COVID-19). Surface disinfection is particularly important because this virus can survive in places such as a door Publisher’s Note: MDPI stays neutral handle, table, or bench at a bus stop from 2 h to 9 days. These are average values, which with regard to jurisdictional claims in are influenced by many external factors, including temperature or air humidity. The fact published maps and institutional affil- that the virus can survive outside the living organism makes it necessary to use effective iations. disinfection. However, disinfection is connected with problems such as exposure of people to chemicals. This can happen when coming into contact with disinfected surfaces or during hand washing. Virucidal ingredients in these products include alcohols, quaternary ammonium salts, phenolic compounds, diols, and biguanides. Copyright: © 2021 by the authors. There are several methods of disinfecting rooms and flat surfaces: physical, chemical, Licensee MDPI, Basel, Switzerland. and thermal-chemical. Physical disinfection of surfaces can be carried out with the use of This article is an open access article steam at a temperature of 100–105 ◦C under reduced pressure or by means of UV rays with distributed under the terms and a wavelength of 256 nm. Chemical disinfection uses quaternary ammonium salts; ethyl al- conditions of the Creative Commons cohol and isopropyl alcohol; aldehydes such as formaldehyde and glutaraldehyde; phenols Attribution (CC BY) license (https:// such as cresol and resorcinol; biguanides, especially chlordexidine; heavy metal compounds creativecommons.org/licenses/by/ (e.g., silver, copper, and mercury); halogen compounds such as iodine, chloramine, and 4.0/).

Energies 2021, 14, 2019. https://doi.org/10.3390/en14072019 https://www.mdpi.com/journal/energies Energies 2021, 14, 2019 2 of 10

iodophores; crystal violet (dye); ethacridine lactate, i.e., the popular Rivanol; oxidants in the form of hydrogen peroxide (hydrogen peroxide H2O2); and permanganates. Chemical disinfection, however, does not eliminate all of the microbes. The thermal-chemical method combines the chemical method with the action of high temperature. The most popular type of disinfection is based on the use of biocides [6]. These are usually preparations based on alcohol, with a particular emphasis on the appropriate concentration of this component. The concentration of alcohol determines the effectiveness of the mixture. The strongest biocidal effect is demonstrated by water solutions of alcohols with a concentration of 60%–90%. The disadvantage of alcohols is their flammability, lower performance in the presence of organic dirt, and quick evaporation. Most of the available preparations require 1–10 min of contact with the surface to be fully effective. Rapid evaporation makes them ineffective in disinfecting large surfaces [7]. The advantage of these preparations is, above all, their easy availability and low cost. In addition, many disinfectants, in addition to alcohol, include chlorine, formaldehyde, glutaraldehyde, ortho- phthalaldehyde, hydrogen peroxide, iodophors, peracetic acid, phenolics, and quaternary ammonium compounds [8]. The effectiveness of disinfection processes depends on the surface onto which the disinfectant is dispensed. In the work [9], an agent based on peracetic acid (PAA) was used for disinfection. The purpose of the disinfection was to remove the spores of Geobacillus stearothermophilus. Disinfection was performed on anti-bacterial, oligodynamic copper and brass materials, and on stainless steel, aluminum, polyvinyl chloride, wood, and ceramics. In the case of stainless steel, aluminum, ceramics, and polyvinyl chloride, the disinfection of spores on a surface of 40 cm2 was achieved with the use of 60 mL of the preparation per m3. A 30% higher dose was needed to disinfect oligodynamic copper and brass materials. This is most likely due to the accelerated degradation of PAA in the case of these materials. The effectiveness of a disinfectant depends on its structure. Porous materials such as wood have a great tendency to absorb the disinfectant, which can lead to incomplete coverage of the surface and thus insufficient disinfection. In the case of polymers that exhibit low surface energies, it is difficult to evenly distribute the disinfectant droplets, which may also result in insufficient disinfection. In rooms that additionally have antibacterial surfaces, it should be taken into account that these surfaces often consist of so-called oligodynamic metals. Such materials have the ability to self-disinfect, which is due to the emission of cations from their surfaces [10]. The use of disinfectants in excessive amounts and in an inappropriate manner may result in negative effects, both directly on human health and on environmental risks [11,12]. Therefore, it is necessary to continuously search for optimal disinfectant compositions that allow for the ecological management of the waste they generate, and the use of an appropriate amount of the preparation in an appropriate, effective form [13]. The coronavirus pandemic has forced the cleaning industry to simultaneously dis- seminate effective and relatively quick methods of disinfecting surfaces. The requirements of the application methods are to ensure personal safety without harmful effects on the cleaned spaces. One approach that meets these requirements is fogging. Foggers contribute to fighting viruses and bacteria on all types of surfaces. For a device of this type to fulfill its function, it should allow to obtain drops of the desired diameters, and the sprayed stream should have a defined range, allowing the preparation to reach places that are difficult to reach. Moreover, the desired effect is that the generated droplets achieve the same size distribution regardless of the distance from the atomizer. The production of a specific spray, which will be properly used, is related to obtaining the intended movement of the liquid [14]. This will allow for the effective operation of the applied preparation, while maintaining its minimum consumption. The work [15] compares the efficiency of two devices—an electrostatic sprayer (SanoS- tatic) and a fogger (SanoFog) delivered by Sanondaf Malta (Sanoserv International Franchis- ing Ltd., Naxxar, Malta). The disinfectant used was a 5% hydrogen peroxide solution with trace amounts of silver ions. Silver ions act as a stabilizer, simultaneously increasing the Energies 2021, 14, 2019 3 of 10

bactericidal effect of the disinfecting mixture. The fogger provides non-contact and uniform coverage of the surface with a disinfectant. The electrostatic sprayer allows the object to be surrounded with a spray, which is permanently deposited on the object’s surface, due to giv- ing the mixture a negative electrostatic charge. The effectiveness of the disinfecting mixture against pathogens of Enterococcus faecalis (vancomycin-resistant enterococci VRE: clinical isolate), Klebsiella pneumoniae (extended-spectrum β-lactamase producer ESBL: ATCC 700,603—American Type Culture Collection), K. pneumoniae (carbapenemase-producing Enterobacterales CPE: NCTC 13,438—National Collection of Type Cultures), Staphylococcus aureus (Meticillin-resistant Staphylococcus aureus MRSA: NCTC 12,493—National Collection of Type Cultures), and Clostridioides difficile was analyzed. For pathogens E. faecalis (VRE: clinical isolate), K. pneumoniae (ESBL: ATCC 700603), K. pneumoniae (CRE: NCTC 13438), and S. aureus (MRSA: NCTC 12493), the bacterial count was reduced to undetectable levels using both devices. A fogger was found to be a better solution to combat the C. difficile pathogen. However, the SanoSpray combined with the SanoFog provided the highest effectiveness. Depending on the specific application, it is expected that the resulting aerosol will be characterized by droplets of a certain size and distribution. The finer the droplets, the better the spraying, because a larger total surface area can be obtained [14,16]. When describing the spraying process and in the sprayer design procedure, the mean droplet diameters are important. This concept was introduced for the purposes of the analysis of the jet forming process and the evaluation of the spraying effect. It is more advantageous to use a droplet with a constant diameter, characteristic of the given conditions, rather than a collection of droplets with various diameters. In the literature, several conventional droplet diameters are described that provide different information and are used for various applications [17]. All of these can be expressed using the general formula: v u m p u ∑ D ∆ni = p−q i=1 i Dpq t m q (1) ∑i=1 Di ∆ni

where the p and q exponents are used simultaneously to denote a given diameter. The Sauter mean diameter is, by deﬁnition, the ratio of the third to the second moment of the probability density function: R dmax d3 p(d)dd dmin D32 = (2) R dmax d2 p(d)dd dmin or for any size distribution of discrete entities:

i=n 3 ∑ Nid D = i=1 i (3) 32 i=n 2 ∑i=1 Nidi

The literature also uses designations, e.g., Dv(10), Dv(50), Dv(90), which are the measured droplet diameters below which 10%, 50%, and 90% of the drop set volume can be found, respectively. The aim of this study was to analyze the possibility of disinfecting surfaces using portable foggers, taking as an example the RY18FGA-0 battery fogger by RYOBI (TechTronic Industries, Kwai Chung, New Territories, Hong Kong). The volumetric and numerical distributions of the droplet diameters, and the values of the selected substitute diameters for the fogger, were determined depending on the ﬁlter used and the distance from the outlet of the device.

2. Materials and Methods The object of the research was to characterize the generated spray jet by a commercial device, i.e., the RY18FGA-0 battery fogger by RYOBI. Figure1 presents a photo of the device. Energies 2021, 14, x FOR PEER REVIEW 4 of 10

Energies 2021, 14, x FOR PEER REVIEW 4 of 10

diameters for the fogger, were determined depending on the filter used and the distance diametersfrom the foroutlet the of fogger, the device. were determined depending on the filter used and the distance from the outlet of the device. 2. Materials and Methods 2. MaterialsThe object and ofMethods the research was to characterize the generated spray jet by a commercial device,The objecti.e., the of RY18FGA-0 the research battery was to foggercharacte byrize RY theOBI. generated Figure 1 spraypresents jet bya photo a commercial of the de- Energies 2021, 14, 2019 4 of 10 device,vice. i.e., the RY18FGA-0 battery fogger by RYOBI. Figure 1 presents a photo of the device.

Figure 1. Device under test. FigureFigure 1. 1. DeviceDevice under under test. test. The basic properties of the spray effect were determined, such as volume distribu- tionsTheThe of basic basicdroplet properties diameters, ofof the thenumerical sprayspray effect effect distribu were weretions determined, determined, of droplet such such diameters, as volumeas volume and distributions distribu-substitute tionsofvalues droplet of dropletof diameters,mean diameters, droplet numerical diameters. numerical distributions The distribu meas ofurementstions droplet of droplet diameters,were made diameters, and on a substitute specially and substitute valuesprepared of valuesmeantest stand, dropletof mean the diameters. dropletmain element diameters. The measurementsof which The measis an urementsop weretical made unit–the were on amade Spraytec specially on a droplet preparedspecially size prepared test analyzer stand, testtheby stand, mainMalvern element the Instrumentsmain of element which (Malvern, is of an which optical Unitedis unit–thean op Kingdom).tical Spraytec unit–the In droplet Spraytecpreparing size droplet analyzerthe Spraytec size by analyzer Malvern particle byInstrumentssize Malvern analyzer, Instruments (Malvern, a 300 mm United (Malvern, lens was Kingdom). selected United In Kingdom).for preparing the measurements, In the preparing Spraytec which the particle Spraytec is suitable size analyzer,particle for use sizeawith 300 analyzer, mmaerosols lens a with was300 mm selecteddiameters lens forwas in the selectedthe measurements, range for 0.5–600 the measurements, whichμm. The is suitablebasic which stage for isin use suitable carrying with aerosolsfor out use the withwithmeasurement aerosols diameters with inprocess thediameters range was 0.5–600 in the rangeµcreationm. The 0.5–600 basicof μa stagem. standard The in basic carrying stagemeasurement out in thecarrying measurement procedure out the measurementprocess(SOP—Standard was the process creation Operating was of a standardProcedurethe creation measurement Software of a by procedurestandard Malvern (SOP—StandardInstruments)measurement [18]. procedure Operating The con- (SOP—StandardProceduretinuous measurement Software Operating by Malvern mode Procedure was Instruments) selected, Software with [18 ].by a The dataMalvern continuous collection Instruments) measurement frequency [18]. of mode The1 Hz. con- was The tinuousselected,measurement measurement with a duration data collection mode was was defined frequency selected, as 60 of with 1s. Hz.Th ais Thedata was measurementcollection done on frequencythe duration basis of of wasmultiple 1 Hz. deﬁned The ob- measurementasservations 60 s. This of was duration the done test measuremen onwas the defined basis oft, as and multiple 60 60s. Ths is observationsis a wastime done interval ofon the thethat test basis allows measurement, of formultiple full stabili- ob- and servations60zation s is aof time ofthe the intervalmeasurement test measuremen that allows and reliablet, for and full 60 measur stabilizations is a timeement interval data of the to that measurement be obtained.allows for To andfull ensure reliablestabili- re- measurement data to be obtained. To ensure repeatability of the results, the samples zationpeatability of the ofmeasurement the results, the and samples reliable were measur sprayedement from data a to designated be obtained. place, To maintainingensure re- were sprayed from a designated place, maintaining a constant distance of the aerosol peatabilitya constant of distance the results, of the the aerosol samples from were the sprayed measurement from a designatedzone. The characteristic place, maintaining param- from the measurement zone. The characteristic parameters were selected from the list a etersconstant were distance selected of from the aerosol the list from of possible the measurement options and zone. compared The characteristic to fully characterize param- of possible options and compared to fully characterize the spray. Measurements were etersthe spray.were selected Measurements from the were list ofrepeated possible 10 options times. Figureand compared 2 shows toa diagramfully characterize of the test repeated 10 times. Figure2 shows a diagram of the test stand. thestand. spray. Measurements were repeated 10 times. Figure 2 shows a diagram of the test stand.

FigureFigure 2.2. Test stand: 1—tested 1—tested device, device, 2—Spraytec 2—Spraytec analyzer, analyzer, 3—computer 3—computer with with the the appropriate appropriate software. Figuresoftware. 2. Test stand: 1—tested device, 2—Spraytec analyzer, 3—computer with the appropriate software. MeasurementsMeasurements werewere mademade atat 33 differentdifferent distancesdistances betweenbetween thethe outletoutlet ofof the the tested tested devicedeviceMeasurements and and the the laser laser were beam beam made ( x(x),), which whichat 3 different were were 0.2, 0.2, distances 0.5, 0.5, and and 1 1between m. m. the outlet of the tested deviceTwo and filters the laser were beam used (x during), which the were tests 0.2, (Figure 0.5, and3): 1 plastic m. (F1; black filter; part number 5131043669; materials: PC+PBT, FKM F170B, stainless steel) and ceramic (F2; white filter; part number 5131043728; materials: sintered porous, zinc alloy, carbon spring steel). The filter was located in the liquid tank and mounted with a clip on the hose carrying the liquid to the fogger spray system. The mass flow rate for the plastic filter (F1) was (4.81 ± 0.04) × 10−3 kg/s and for the ceramic filter (F2) was (2.32 ± 0.02) × 10−3 kg/s. Energies 2021, 14Energies, x FOR 2021PEER, 14 REVIEW, x FOR PEER REVIEW 5 of 10 5 of 10

Two filters wereTwo usedfilters during were usedthe tests during (Figur thee tests3): plastic (Figur (F1;e 3): black plastic filter; (F1; partblack num- filter; part number 5131043669;ber 5131043669;materials: PC+PBT, materials: FKM PC+PBT, F170B, FKMstainless F170B, steel) stainless and ceramic steel) and(F2; ceramicwhite (F2; white filter; part numberfilter; part 5131043728; number 5131043728;materials: sintered materials: porous, sintered zinc porous, alloy, carbonzinc alloy, spring carbon spring steel). The filtersteel). was The located filter inwas the located liquid inta nkthe and liquid mounted tank and with mounted a clip on with the ahose clip car-on the hose car- Energies 2021, 14, 2019 5 of 10 rying the liquidrying to thethe liquidfogger to spray the fogger system. spray The system.mass flow Th erate mass for flowthe plastic rate for filter the plastic(F1) filter (F1) −3 −3 was (4.81 ± 0.04)was ×(4.81 10−3 ±kg/s 0.04) and × 10 for thekg/s ceramic and for filter the ceramic (F2) was filter (2.32 (F2) ± 0.02) was ×(2.32 10−3 ±kg/s. 0.02) × 10 kg/s.

Figure 3. Applied filters: F1 (black filter) and F2 (white filter) and their construction. FigureFigure 3. Applied 3. Applied filters: filters: F1 (black F1 (black filter) filter) and F2 and (white F2 (white filter) filter) and their and theirconstruction. construction.

3. Results 3. Results 3. Results To characterizeTo characterizethe filters used, the filtersthe pressu used,re thedrop pressu as a refunction drop as of a thefunction volumetric of the volumetric To characterize the filters used, the pressure drop as a function of the volumetric flow rate wasflow studied rate was(Figure studied 4). The (Figure spray 4). medium The spray was medium distilled was water. distilled To measure water. theTo measure the flow rate was studied (Figure4). The spray medium was distilled water. To measure the pressure drop,pressure the filter drop, was the immersed filter was in immersed the water intank the to water a depth tank of to 10 a cm.depth The of probe 10 cm. The probe pressure drop, the filter was immersed in the water tank to a depth of 10 cm. The probe for for measuringfor themeasuring differential the pressuredifferential wa pressures mounted wa downstreams mounted downstreamof the filter. of The the filter. The measuring the differential pressure was mounted downstream of the filter. The pressure pressure waspressure measured was relative measured to the relative ambient to pressure.the ambient Valves pressure. were Valvesused to were control used the to control the wasflow. measured The rotameters relative VA40 to the produced ambient pressure. by Krohne Valves Messtechnik were used GmbH to control & Co the KG flow. (Duis- The flow. The rotametersrotameters VA40 VA40 produced produced by by Krohne MesstechnikMesstechnik GmbH GmbH & & Co Co KG KG (Duisburg, (Duis- Germany) burg, Germany)burg, were Germany) used towere measure used theto measure liquid flow the rate.liquid The flow pressure rate. The drop pressure was drop was weremeasured used tousing measure a digital the liquid pressure flow rate.gauge The Di pressuregiComb drop1900 wasproduced measured by usingTecsis a GmbH digital measured usingpressure a digital gauge pressure DigiComb gauge 1900 producedDigiComb by 1900 Tecsis produced GmbH (Offenbach by Tecsis amGmbH Main, Germany). (Offenbach am(Offenbach Main, Germany). am Main, Germany).

Figure 4. DependenceFigure 4.4. of Dependence pressure drop ofof pressure pressureon the volumetric dropdrop onon thethe flow volumetricvolumetric rate for the flowflow testedrate rate for filters:forthe the tested tested filters: filters: F1—plastic F1—plastic filter, F2—ceramic filter. F1—plastic filter,filter, F2—ceramic F2—ceramic filter. filter.

As can be seen, in the case of the ceramic filter, the obtained pressure drop values were practically twice as high as those of the plastic filter. The flow resistance (pressure drop) increases with the decrease in the cumulative cross-section of the filter channels. The smaller cross-sectional area of the filter channels contributes to a lower flow rate of the

liquid supplied to the atomization system. The less liquid in relation to the volumetric ﬂow rate of the atomizing gas, the greater the kinetic energy of the gas per unit volume of the sprayed liquid. This results in a better breakup of the liquid jet into droplets, which directly affects the reduction in the size of droplets produced. Energies 2021, 14, x FOR PEER REVIEW 6 of 10

As can be seen, in the case of the ceramic filter, the obtained pressure drop values were practically twice as high as those of the plastic filter. The flow resistance (pressure drop) increases with the decrease in the cumulative cross-section of the filter channels. The smaller cross-sectional area of the filter channels contributes to a lower flow rate of the liquid supplied to the atomization system. The less liquid in relation to the volumetric Energies 2021, 14, 2019 flow rate of the atomizing gas, the greater the kinetic energy of the gas per unit volume of6 of 10 the sprayed liquid. This results in a better breakup of the liquid jet into droplets, which directly affects the reduction in the size of droplets produced. HistogramsHistograms and andcumulative cumulative curves curves were were used used to describe to describe the droplet the droplet populations populations and theirand theirsizes sizes in the in sprayed the sprayed stream stream of liquid. of liquid. Figures Figures 5 and5 and6 present6 present the thevolumetric volumetric and and numericalnumerical distributions distributions obtained obtained when when spraying spraying water water with with the use the of use both of both filters. ﬁlters.

Energies 2021, 14, x FOR PEER REVIEW 7 of 10 FigureFigure 5. Histograms—F1 5. Histograms—F1 filter, 20 ﬁlter, cm 20distance: cm distance: (a) volume (a) volume distribution distribution of the recorded of the recorded droplet droplet diameters,diameters, (b) numerical (b) numerical distribution distribution of recorded of recorded droplet droplet diameters. diameters.

Figure 6. Histograms—F2 ﬁlter, 20 cm distance: (a) volume distribution of the recorded droplet Figure 6. Histograms—F2 filter, 20 cm distance: (a) volume distribution of the recorded droplet diameters,diameters, (b) numerical (b) numerical distribution distribution of recorded of recorded droplet droplet diameters. diameters.

In addition to the recorded droplet size distribution, the graphs also show the log-normal distribution used for the interpretation. For the log-normal distribution, the analysis provides the parameters of D—geometric mean and N—geometric deviation for this distribution. The equation for the log-normal distribution is as follows:

1 (ln(𝑑) ln(𝑥)) 𝐹(𝑑) = exp( ) (4) ln(𝑛)√2𝑛 2(ln(𝑛))

where: F(d) is the relative percentage of the dimension d, and x and n are the characteristic dimension and width of the distribution, respectively. The cumulative distribution is the integral of F(d) [16,18]. The differences that can be observed between the volume and numerical distribution of the obtained droplet diameters result from the calculation methodology. Based on the graphs (Figures 5 and 6) it can be determined that all of the recorded drops had a diameter less than 150 μm. A substantial numerical share had droplets with a diameter smaller than 10 μm. According to the presented numerical distribution, it can be seen that about 90% are drops with diameters of 2–10 μm. Figure 7 shows the numerical distributions obtained when spraying water at a distance of 1 m for a plastic filter and a ceramic filter. On the basis of the presented dependencies, it can be observed that in both cases droplets with a diameter of up to 40 μm mainly occur. Above this value, only a few droplets are present. However, in the case of using a ceramic filter, a larger number of drops are present with a diameter of up to approx. 10 μm compared to a plastic filter. Furthermore, there are fewer droplets with diameters greater than 20 μm.

Energies 2021, 14, 2019 7 of 10

In addition to the recorded droplet size distribution, the graphs also show the log- normal distribution used for the interpretation. For the log-normal distribution, the analysis provides the parameters of D—geometric mean and N—geometric deviation for this distribution. The equation for the log-normal distribution is as follows: ! 1 −(ln(d) − ln(x))2 F(d) = √ exp (4) ln(n) 2n 2(ln(n))2

where: F(d) is the relative percentage of the dimension d, and x and n are the characteristic dimension and width of the distribution, respectively. The cumulative distribution is the integral of F(d)[16,18]. The differences that can be observed between the volume and numerical distribution of the obtained droplet diameters result from the calculation methodology. Based on the graphs (Figures5 and6) it can be determined that all of the recorded drops had a diameter less than 150 µm. A substantial numerical share had droplets with a diameter smaller than 10 µm. According to the presented numerical distribution, it can be seen that about 90% are drops with diameters of 2–10 µm. Figure7 shows the numerical distributions obtained when spraying water at a distance of 1 m for a plastic filter and a ceramic filter. On the basis of the presented dependencies, it can be observed that in both cases droplets with a diameter of up to 40 µm mainly occur. Above this value, only a few droplets are present. However, in the case of using a Energies 2021, 14, x FOR PEER REVIEW ceramic filter, a larger number of drops are present with a diameter of up to approx.8 of 10 10 µm compared to a plastic filter. Furthermore, there are fewer droplets with diameters greater than 20 µm.

FigureFigure 7. Numerical 7. Numerical distribution distribution of droplet of droplet diamet diametersers obtained obtained at a distance at a distance of 1 ofm: 1(a m:) plastic (a) plastic filter, ﬁlter, (b) ceramic(b) ceramic filter. ﬁlter.

To determineTo determine the thesize size of the of theformed formed drop droplets,lets, measurements measurements were were also also made made using using the themicroscopic microscopic method. method. The Themeth methodod of trapping of trapping the droplets the droplets on a on flat a ﬂatsurface surface was wasused. used. The method consists in trapping the droplets by placing a glass plate covered with an immersion liquid in the way of the liquid stream. Then the plate was placed under a Nikon Eclipse 50i microscope equipped with an OptaTech camera and a series of photos were taken using the MultiScanBase program from Computer Scanning Systems II. The methodology is described in detail in [19]. Figure 8 shows sample photos from the analysis obtained while spraying the liquid at a distance of 1 m from the measuring point for the F1 and F2 filters. The resulting atomization effect is similar. For each tested filter, the number of drops with diameters in the range of 2–10 μm prevails.

Figure 8. Images of droplets during atomization from a distance of 1 m: (a) with a ceramic filter and (b) with a plastic filter; (c) photo of the scale for a ×10 lens (distance between lines is 0.01 mm).

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Figure 7. Numerical distribution of droplet diameters obtained at a distance of 1 m: (a) plastic filter, (b) ceramic filter. Energies 2021, 14, 2019 8 of 10 To determine the size of the formed droplets, measurements were also made using the microscopic method. The method of trapping the droplets on a flat surface was used. The method consists in trapping the droplets by placing a glass plate covered with an The method consists in trapping the droplets by placing a glass plate covered with an immersion liquid in the way of the liquid stream. Then the plate was placed under a immersion liquid in the way of the liquid stream. Then the plate was placed under a Nikon Eclipse 50i microscope equipped with an OptaTech camera and a series of photos Nikon Eclipse 50i microscope equipped with an OptaTech camera and a series of photos were taken using the MultiScanBase program from Computer Scanning Systems II. The were taken using the MultiScanBase program from Computer Scanning Systems II. The methodologymethodology is described inin detaildetail inin [19[19].]. FigureFigure8 shows8 shows sample sample photos photos from from the the analysis anal- ysisobtained obtained while while spraying spraying the liquidthe liquid at a distanceat a distance of 1 m of from 1 m from the measuring the measuring point point for the for F1 theand F1 F2 and ﬁlters. F2 filters. The resulting The resulting atomization atomization effect is effect similar. is similar. For each For tested each ﬁlter, tested the filter, number the numberof drops of with drops diameters with diameters in the range in the of range 2–10 µofm 2–10 prevails. μm prevails.

FigureFigure 8. 8. ImagesImages of of droplets droplets during during atomi atomizationzation from from a a distance distance of of 1 1 m: m: ( (aa)) with with a a ceramic ceramic filter ﬁlter and and ((bb)) with with a a plastic plastic filter; ﬁlter (distance(c) photo betweenof the scale lines for is a 0.01×10 lens mm). (distance between lines is 0.01 mm).

The maximum measured droplet size was about 60 µm. The microscopic analysis also confirmed the lack of significant differences between the obtained atomization effect for the black and white filters. Tables1 and2 summarize the values of the compared, characteristic sizes for the black and white filters, depending on the distance from the measuring beam in which the liquid was sprayed. It is shown that the Ryobi portable fogger enables continuous spraying with a range of up to 4.5 m. The basic mean droplet diameters [16] were compared.

Table 1. The results obtained during the measurements for the F1 plastic ﬁlter.

Parameter x = 0.2 m x = 0.5 m x = 1 m

Dv(10)(µm) 14.2 ± 0.4 18.0 ± 0.4 19.3 ± 0.5 Dn(10)(µm) 2.30 ± 0.04 2.61 ± 0.04 2.93 ± 0.04 Dv(50)(µm) 40.6 ± 0.5 48.6 ± 0.6 50.2 ± 0.7 Dn(50)(µm) 3.35 ± 0.04 3.82 ± 0.03 4.37 ± 0.04 Dv(90)(µm) 93.6 ± 0.9 102.4 ± 0.9 103.6 ± 0.8 Dn(90)(µm) 9.8 ± 0.2 11.3 ± 0.2 14.6 ± 0.3 D32 (µm) 29.1 ± 0.3 35.1 ± 0.4 36.7 ± 0.5 %N < 50 µm (%) 99.79 99.56 99.39

Table 2. The results obtained during the measurements for the F2 ceramic ﬁlter.

Parameter x = 0.2 m x = 0.5 m x = 1 m

Dv(10)(µm) 15.1 ± 0.4 16.3 ± 0.4 15.7 ± 0.4 Dn(10)(µm) 2.26 ± 0.03 2.41 ± 0.03 2.63 ± 0.03 Dv(50)(µm) 41.7 ± 0.6 45.6 ± 0.5 44.7 ± 0.7 Dn(50)(µm) 3.30 ± 0.05 3.58 ± 0.07 4.11 ± 0.06 Dv(90)(µm) 94.5 ± 1.1 99.9 ± 0.9 99.2 ± 1.2 Dn(90)(µm) 9.4 ± 0.6 10.9 ± 0.6 11.5 ± 0.4 D32 (µm) 30.5 ± 0.6 32.7 ± 0.5 31.8 ± 0.6 %N < 50 µm (%) 99.79 99.67 99.72 Energies 2021, 14, 2019 9 of 10

4. Conclusions In this work, the influence of the filter and distance on the distribution of droplets in the spray stream, and the average drop diameter, were analyzed. The Ryobi portable fogger enables continuous spraying with a range of up to 4.5 m. It can be used to spray disinfectants, detergents, pesticides, insecticides, fungicides, etc. The main task of the filters is to filter the liquid fed to the atomizing system. In addition, the filters affect the liquid flow rate. The filters had different flow resistance and allowed droplets of different sizes to be obtained. The obtained spray is characterized by droplets with a diameter of about 30 µm, regardless of the distance from the outlet of the device, reaching all hard-to-reach places. Smaller values of mean droplet diameters were obtained for the plastic filter compared to the ceramic filter, at a distance of 0.2 m. However, when the liquid was sprayed at distances of 0.5 and 1 m, smaller mean diameters were obtained for the ceramic filter. For example, the value of Sauter mean diameter equaled 36.7 µm for a plastic filter and 31.8 µm for a ceramic filter, at a distance of 1 m. Furthermore, the microscopic size of the drops means that the sprayed surfaces were not wet after spraying. The lightweight design of the device allows easy movement around the decontaminated space (e.g., office). Regarding measures to address COVID-19, a better solution is to use a ceramic filter, which generates smaller droplets than a plastic filter. These experiments are important in the context of the COVID-19 virus, particularly in locations with remote or no permanent access to electricity, in which devices powered by a battery can be used.

Author Contributions: Conceptualization, M.O., S.W. (Sylwia Włodarczak) and A.K.; methodology, M.O., S.W. (Sylwia Włodarczak), S.W. (Szymon Woziwodzki) and T.S.; formal analysis, M.O. and S.W. (Sylwia Włodarczak); investigation, M.O., A.K., S.W. (Sylwia Włodarczak), M.M., S.W. (Szymon Woziwodzki) and T.S.; writing—original draft preparation, M.O., A.K. and S.W. (Sylwia Włodar- czak); writing—review and editing, M.O., A.K. and S.W. (Sylwia Włodarczak); visualization, M.M.; supervision, M.O. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Ministry of Education and Science of Poland. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The study was supported by the Ministry of Education and Science of Poland. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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