Is the Govt. being misled about the effectiveness of Drone based Disinfectant Spraying?

Over the past few weeks, we have been reading in the news on how different drone companies are claiming to have come up with a solution of Drone based Disinfectant Spraying in effected areas. Some companies claim to have deployed numerous drones across the country and the numbers with regards to work-rate they claim baffled us. For instance, some companies are claiming that Spraying Drones can cover 15-20 Km per Day distance-wise and Tens of Thousands of sq.m. per day Area-wise. But nobody seems to be measuring or questioning the effectiveness.

We have been sceptical right from the start when the news came out regarding drone-based spraying of disinfectant to counter COVID-19. We gave the benefit of doubt of their effectiveness as other countries like China, Brazil, Spain etc have used drones to deliver disinfectant. So, when we got an enquiry regarding drone-based spraying, we took it and we went to study the effectiveness of drone-based spraying compared to vehicle mounted spraying in locations in and around Hyderabad. Below are a few images of our team and our in-house designed and built 10 Litre Agricultural Spraying Drone spraying disinfectants in and around Hyderabad in the last 2 weeks.

Drone capacities, Spray Volumes and Practical Aspects

For starters, the drones that are used in these scenarios are basically used for agricultural spraying of pesticides, fungicides among other agricultural chemicals. These drones usually carry a 10L/15L/20L tank and are designed to deliver chemicals at ultra-low volume and low volume (anywhere between 4 to 30 litres per acre). They typically have an average flight time of 8 to 15 mins per battery in which time they can deliver a maximum of 15 to 20 litres taking into account take off & landing time and an average discharge of 2.6L to 3.5L per minute which is typical for a drone of this category.

Now let’s talk about the spray quality. The size of droplets that come out of these drone nozzles typically range between 100 and 300 microns considering the nozzles that are commonly used on Agricultural Drones. The drone has a spray width of 3 -6 meters where 6 meters is on the higher side. According to the international guidelines and best practices on weather, wind speed, humidity, drone speed and height with regards to Agricultural Spraying, Drone should be used for spraying when the temperature is below 30C, wind speed at below 10kmph, humidity at above 30%, maximum speed of 6m/s and Drone height at 1-3 meters above the crop canopy for proper effectiveness. Considering all these numbers an average of 60 litres can be sprayed in an hour considering the time required for tank refills and battery change time.

Since we have the numbers in place, lets reconstruct a typical scenario where a drone sprays disinfectant in urban environment:
  • In an urban environment considering the roadside trees, buildings, electric lines crisscrossing and other obstacles, it’s not safe to fly beyond speed of 4m/s (although maximum 10m/s is possible).
  • Line of sight operation is limited to 200 meters since you are not spraying on an open field. This range might sometimes be even less if there is a cell phone tower nearby as it interferes with drone radio signal.
  • You must keep moving from point to point with the drone and chemical to be sprayed considering the obstacles. Whereas in an Agricultural Spraying scenario, refills can happen at a single point without having the Operator to move around carrying the chemical, that may not be possible in the urban setting. This grossly effects the quantity of chemical that can be sprayed.
  • Considering the size of the droplet and dry Indian summer conditions most of these droplets evaporate by the time they reach the ground thus rendering the operation ineffective.
  • The drones that are currently being deployed can spray only on horizontal surfaces and not on walls or other vertical surfaces.

For those who are not aware, Water Sensitive Paper (WSP) is used to test Spray Coverage Area and to assess Spray Droplet Sizes. WSP turns from Yellow to Blue upon contact with liquids. Blue area in the pictures above denotes the area that received the spray. Observe the difference in the coverage area in both images. Image 1 above showing the relatively scarce droplets is perfectly fine when it comes to Agricultural Spraying and the same has been tested and proven in multiple field tests by us and other companies across the World. However, when it comes to Disinfectant Spraying, the effect of Spraying should be as in Image 2 or even better. Only when the entire surface is covered by the disinfectant is the virus controlled effectively. The empty areas (yellow areas) as shown in Image 1 can still harbour the virus when done the same way for COVID-19. Add to this the conditions (hot temperatures and high altitudes that can result in evaporation of the spray or drifting of the spray to non-target areas) in which Drone Spraying is done for COVID-19, the effectiveness is reduced even further.

Can it be effective at all?

We are not saying that drones are not useful in spraying disinfectants. The question is whether it is an effective method. The volume that’s required to disinfect an area effectively using drones is around 80L+ for an area of 0.004 (1 acre). If this is the case, it would typically take around 90 mins to 120 mins to cover such a small area in an urban environment. The above numbers are based on studies and pilots done by Thanos Technologies. These numbers are simply not viable (financially or practically) neither for a drone company nor the authorities. A vehicle based misting machine is far more effective, faster and easier to deploy.

Another claim being made by some is that drones are useful to disinfect rooftops of high-rise buildings or any building rooftops. The problem with this scenario is that it is difficult to cover 100% of the area on the rooftops as pilot is controlling the drone from the ground. Even with a camera attached to drone, it’s difficult to cover the complete area. If you increase the height of the drone so as to make it more visible from the ground, then most of the droplets evaporate by the time they hit the surface. The best scenario would be to get onto the rooftop and spray. If you do that, then the whole concept of drone-based spraying becomes pointless. More importantly, why would you spray rooftops using drones? For High-Rise buildings, it’s far more sensible to disable access to the terrace. This is something that is done as a standard practice anyway in quite a few such apartment complexes and buildings during normal times too. And for individual house rooftops, why would you need to spray at all when nobody other than the residents have access to the rooftop. And even residents in such COVID-19 positive-case houses and quarantine houses are not supposed to come out anyway. There is also no clear evidence from anywhere in the World that Drones are recommended or have been proven to be effective for disinfecting surfaces. A quick search on google reveals the same.

Our take

A typical drone company might charge anywhere between Rs. 10,000 to 25,000 per day to deploy a drone. These figures are not relevant to those kind-hearted companies which are doing it for free. Now considering the above cited factors with regards to the effectiveness of drone spraying, we believe that it’s a waste of taxpayers’ money and other resources for the government in this time of crisis. Drone companies which are actively pushing drone-based spraying of disinfectants in such Open Areas, Buildings, Roads or High-Rises are either ignorant about the above issues or are trying to mislead the government authorities to consider drone based disinfectant spraying as an effective method. We’d like to believe it is the former. While we understand the apparent benefit of Contact-less Spraying by Drones thus avoiding Manual Spraying, we believe that the benefit is not significant enough in most cases and the Spraying itself is ineffective owing to the low volumes & conditions of spraying. It would be far more effective and faster to use vehicle mounted misting machines or even temporarily employing migrant workers who are stranded all over country by giving them proper PPE and a Manual Battery-Operated sprayer to spray disinfectants. We are willing to bet that quite a few other companies in the Drone Industry share a similar opinion.

Our Background & Credentials

We, Thanos Technologies, have been designing and manufacturing Agricultural Spraying Drones (and various custom drones) for the past 3 years. We have won national and international awards for our drone design and are gold medallists at the Inaugural Drone Olympics 2019. We are part of one of the biggest ever exercises in the world in studying the effectiveness of drone based spraying in an agriculture scenario with BAYER where we have studied close to 30+ different chemicals with 50+ combinations at different volume, height and weather conditions so that we could recommend the same for safe operations. All the observations and agricultural spraying related numbers cited in this article are based on our own experiences over the last 3 years.

Note: Our idea is not to discredit anybody’s hard work. We are only attempting to share our understanding and dispel any myths about what Spraying Drones can or cannot do in this crisis. We are always eager to be proven wrong and curious to learn from others. So, anybody, who’d like to argue against the above may please get in touch with us for a healthy discussion so that the Indian Drone Industry can contribute in a positive way in this time of crisis.