Organism Concentration in Wastewater and Downwind Aerosol

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Organism

Standard plate count

Total coliforms

Fecal coliforms

Coliphage

Fecal streptococci

Pseudomonas

Klebsiella

Clostridium perfringes

Wastewater Concentration [(No./100 mL) x 106]

Aerosol Concentration at Edge of Sprinkler Impact Circle (No./m3 of air sampled)

Source: Sorber, C.A. and Sagik, B.P., in Wastewater Aerosols and Disease, EPA 600/9-80-078, U.S. Environmental Protection Agency, Health Effects Research Laboratory, Cincinnati, OH, 1980, 23-35.

3.4.5 Aerosols

Aerosol particles may be up to 20 |im in diameter, which is large enough to transport bacteria or virus. Aerosols will be produced any time that liquid droplets are sprayed into the air, or at the boundary layer above agitated water surfaces, or when sludges are moved about or aerated. Aerosol particles can travel significant distances, and the contained pathogens remain viable until inactivated by desiccation or ultraviolet light. The downwind travel distance for aerosol particles depends on the wind speed, turbulence, temperature, humidity, and presence of any barrier that might entrap the particle. With the impact sprinklers commonly used in land application of wastewater, the volume of aerosols produced amounts to about 0.3% of the water leaving the nozzle (Sorber et al., 1976). If no barrier is present, the greatest travel distance will occur with steady, nonturbulent winds under cool, humid conditions, which are generally most likely to happen at night. The concentration of organisms entering a sprinkler nozzle should be no different than the concentration in the bulk liquid or sludge. Immediately after aerosolization, temperature, sunlight, and humidity have an immediate and significant effect on organism concentration. This aerosol shock is demonstrated in Table 3.11.

As the aerosol particle travels downwind, the microorganisms continue to die off at a slower, first-order rate due to desiccation, ultraviolet radiation, and possibly trace compounds in the air or in the aerosol. This die-off can be very significant for bacteria, but the rates for viruses are very slow so it is prudent to assume no further downwind inactivation of viruses by these factors. Equations 3.27 and 3.28 form a predictive model that can be used to estimate the downwind concentration of aerosol organisms:

where

Cd = Concentration at distance d (number/ft3; number/m3).

Cn = Concentration released at source (number/s).

Dd = Atmospheric diffusion factor (s/ft3; s/m3).

= -0.023 for bacteria (derived for fecal coliforms).

a = Downwind distance d/wind velocity (ft-ft-s; m-m-s),

B = Background concentration in upwind air (number/ft3; number/m3).

The initial concentration Cn leaving the nozzle area is a function of the original concentration in the bulk wastewater (W), the wastewater flow rate (F), the aero-solization efficiency (E), and a survival factor (I), all as described by Equation 3.28:

where

Cn = Organisms released at source (number/ft3; number/m3).

W = Concentration in bulk wastewater (number/100 mL).

E = Aerosolization efficiency.

= 0.003 for wastewater.

= 0.0004 for sludge spray guns.

= 0.000007 for sludge applied with tank truck sprinklers.

I = Survival factor.

= 0.34 for total coliforms.

= 0.27 for fecal coliforms.

= 0.71 for coliphage.

= 3.6 for fecal streptococci.

= 80.0 for enteroviruses.

The atmospheric dispersion factor (Dd) in Equation 3.27 depends on a number of related meteorological conditions. Typical values for a range of expected conditions are given below; USEPA (1982b) should be consulted for a more exact determination:

Field Condition

Dd

(s/m3)

Wind < 6 km/hr, strong sunlight

176

X

10-6

Wind <6 km/hr, cloudy daylight

388

X

10-6

Wind 6-16 km/hr, strong sunlight

141

X

10-6

Wind 6-16 km/hr, cloudy daylight

318

X

10-6

Wind > 16 km/hr, strong sunlight

282

X

10-6

Wind > 16 km/hr, cloudy daylight

600

X

10-6

Wind > 11 km/hr, night

600

X

10-6

The following example illustrates the use of this predictive model. Example 3.6

Find the fecal coliform concentration in aerosols 8 m downwind of a sprinkler impact zone. The sprinkler has a 23-m impact circle and is discharging at 30 L/s, fecal coliforms in the bulk wastewater are 1 x 105, the sprinkler is operating on a cloudy day with a wind speed of about 8 km/hr, and the background concentration of fecal coliforms in the upwind air is zero.

Solution

1. The distance of concern is 31 m downwind of the nozzle source, and the wind velocity is 2.22 m/s, so we can calculate the a factor:

Downwind distance 31 _ .

Wind velocity 2.22

2. Calculate the concentration leaving the nozzle area using Equation 3.28:

= 2430 fecal coliforms released per second at the nozzle

3. Calculate the concentration at the downwind point of concern using Equation 3.27:

= 0.54 fecal coliforms per m3 of air, 8 m downwind of the wetted zone of the sprinkler

This is an insignificant level of risk.

The very low concentration predicted in Example 3.6 is typical of the very low concentrations actually measured at a number of operational land treatment sites. Table 3.12 provides a summary of data collected at an intensively studied

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