Aerosol Generation and Drift

Jan. 25, 2018

As water becomes an increasingly scarce resource, more of it is being recycled and reused. Regulation and policies to protect human and environmental health are of ever-greater importance. Dr. Edo McGowan has 40 years of experience in the development of local, regional, and international programs relating to health aspects of water quality, vector control, and the analyses and disposal of hazardous materials. Here he offers his perspectives on the potential for pathogen transfer from recycled water aerosols.

Recycled wastewater is increasingly used on the public-access landscape, often applied through sprinklers. Depending on the degree of its treatment, recycled water, although meeting government standards, has been shown to carry a variety of pathogens, a significant number of which demonstrate antibiotic resistance (1,2). 

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Even assuming tertiary treatment, California Title 22 recycled wastewater can carry (and can thus release) a wide variety of pathogens, many of which are drug resistant. This carriage of pathogens and their genes represents a potentially adverse and growing public health issue (3).

Recently, a growing number of reports and peer-reviewed papers have come out discussing the contamination of municipal parks with superbugs associated with the concomitant use of recycled wastewater as an irrigation source (4,5,6). That recycled water can be a serious source of pathogens and their genes is no longer disputed (7).

Because sprinklers are typically used for this type of irrigation, their ability to disperse pathogens into the air, including the potential for a considerable down-wind drift, warrants discussion. 

The principal determinants controlling initial droplet size are hydraulic pressure and nozzle orifice shape and dimensions. Temperature and humidity come into play next as they affect the released droplet. Release height will also affect the drift distance, hence time of evaporative effect.

Xenobiotics can drift considerable distances (8,9). Using examples of drift studies in other areas will give you some idea of distances. As an Ag Engineering grad student five decades ago, I ran drift studies. We evaluated the down-wind movements of aerially applied herbicides, finding, for example, that Propanil damaged prune trees 50 miles away. 

The USGS has studied drift of pathogens. In one USGS example, pathogens coming off the African continent were found to cause respiratory disease in the Caribbean (10). Looking at spray (sprinkler) irrigation, several papers discuss a variety of distances for down-wind movement of pathogens, certainly more distant than just to the cars parked at the curbs adjacent to a municipal park or dwellings just across the street.

Pathogens also have the capacity to remain viable and infective for extended periods, often weeks. The pathogens arising from Africa and crossing the Atlantic to fall out on the Caribbean take about two weeks, are lofted to the upper atmosphere, subjected during that time to intense UV radiation, and still cause respiratory disease.

Aerosols are suspensions of particles in the air that, because of small size and low settling velocity, remain airborne for prolonged periods. Using an array of droplet sizes of unit density, and a fall height of 1 meter, we have the following approximate settling times: 100 μm—3 sec, 20 μm—80 sec, 10 μm—5.6 min, 5 μm—20 min. Particles under a diameter <3 μm essentially do not settle. (11).

Settling times can be further affected by air turbulence and droplet evaporation. Release at greater heights will, of course, see droplets move farther down-wind.

If we assume a fall of one meter and a 5 mile-per-hour cross-breeze moving through the area, we have the following potential travel distances. Using the above droplet sizes and settling times, we can make some calculations on travel distances. We have 100 μm going 22 feet, 20 μm moving 586 feet, 10 μm at 2463 feet, and 5 μm moving 8795 feet.

Picture the typical interurban municipal park. Irrigation with recycled water is often done during the night. In this neighborhood, cars are parked at the curb and across the street are residential structures. The larger aerosol droplets moving from spray near the periphery of the park may impact the surfaces of parked cars and smaller droplets can waft into the surrounding residential areas. In the warmer areas of the nation, windows are often left open at night, thus allowing aerosol movement into dwellings.

Pathogen transfer to humans is thus a serious concern. And it has become an issue that those in the regulatory community need to acknowledge.











About the Author

Dr. Edo McGowan

Dr. Edo McGowan has forty years of experience in the development of local, regional, and international programs relating to health aspects of water quality, vector control, the analyses and disposal of hazardous materials.