Assessing the potential of unmanned aerial vehicle spraying of aqueous ozone as an outdoor disinfectant for SARS-CoV-2

Abstract

The COVID-19 pandemic has revealed gaps in our understanding of safe, effective and efficient means of disinfecting high use public spaces. Whilst this creates an opportunity for development and application of innovative approaches such as unmanned aerial vehicle (UAV) based disinfection, unregulated outdoor disinfection using chlorine has led to environmental and public health risks. This study has quantified the efficiency, safety and efficacy of UAV-based spraying of aqueous ozone. Optimised UAV flight characteristics of 4.7 km/h at 1.7 m elevation spraying 2.4 L/min were able to provide >97% and >92% coverage of a 1 m and 2 m wide swath respectively. During spraying operations using 1 mg/L aqueous ozone, atmospheric concentrations of ozone remained within background levels (<0.04 ppm). Highly efficient inactivation of two different isolates of SARS-CoV-2 virus was achieved at aqueous ozone concentrations of 0.75 mg/L after an incubation period of only 5 min, with 0.375 mg/L achieving 82-91.5% inactivation in this time. Exposure of diamondback moth larvae and parasitic wasps to 1 mg/L aqueous ozone did not significantly affect their survivorship. These results indicate for the first time that aqueous ozone may provide the required balance between human and environmental safety and viral inactivation efficacy for targeted application in high risk outdoor settings.

Keywords: COVID-19; Disinfection; Environmental impacts; Outdoor spraying; UAV.

Graphical abstract

Fig. 1

Disinfectant spraying of public spaces in response to COVID-19 outbreaks in: (A) Kolkata, India and (B) Moscow, Russia (Source: Shutterstock).

Fig. 2

Spatial coverage assessment of UAV based spraying across 5 m × 6 m test strips. Flight height (h) and speeds (v) of different spatial coverage assessments were recorded. A) h = 2 m, v = 5.8 km/h, and 4 sprinkler nozzles, B) h = 2 m, v = 4.7 km/h, and 4 mist nozzles, C) h = 1 m, v = 4.7 km/h, and 4 coarse nozzles, D) h = 1.7 m, v = 4.7 km/h, and 2 mist +2 coarse nozzles, E) h = 1.7 m, v = 4.7 km/h, and 4 offset mist nozzles, F) h = 1.7 m, v = 4.7 km/h, and 4 on boom coarse nozzles, G) h = 1.7 m, v = 4.7 km/h, and 4 on boom mist nozzles, H) example of UAV spraying red food dye on geofabric during coverage trials. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3

Aqueous ozone stability in different water preparations. Ozone was dissolved in: Municipal water (dark blue, circle), deionised water (red, square), municipal water pre-filtered through Tersano cation exchange resin (purple, circle), deionised water pre-filtered through Tersano cation exchange resin (green, triangle). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4

SARS-CoV-2 inactivation by aqueous ozone. Viral titres (log₁₀FFU/mL) for QLD02 and QLD935 SARS-CoV-2 isolates remaining after 5 min (A) or 30 min (B) incubation at room temperature with different concentrations of aqueous ozone. Degassed ozone solution was used as a control. Average values from 14 to 24 individual wells for each treatment from two independent experiments are shown. The numbers above the bars show average log₁₀ reductions in viral titres for each ozone concentration. P values were calculated using a nonparametric and Mann-Whitney test. The limit of detection (LOD) was 400 FFU/mL (2.9 log₁₀/mL). No virus was detected in any of the wells for 0.75 m/L and 1.5 mg/L of aqueous ozone. The percentage of reduction (P) for 0.375 mg/L ozone concentrations based on the mean log₁₀ reduction (L) was calculated using the formula: P= (1-10^−L) x 100.

Fig. 5

A) Atmospheric ozone concentrations from Southport monitoring station during May 2020. B) Atmospheric ozone concentrations at both Southport and in-situ logging during spraying operations on May 20, 2020. Southport station data retrieved from QLD Government air quality monitoring database. Field logger data collected in-situ during UAV spraying operation using Aeroqual Inc., Series 500 – Portable Ozone Monitor.

Fig. 6

(A) Survival (72 h) of diamondback moth larvae and (B) Survival (24 h) of Diadegma semiclausum males after spraying with water, aqueous ozone (1 mg/L), bleach (4000 ppm), hydrogen peroxide (0.25%) and insecticide (deltamethrin, 10 ppm). Bars indicate standard error, and asterisks above bars indicate statistical significance (*P < 0.05; **P < 0.01; ***P < 0.001).