Environmental Engineering Reference
In-Depth Information
Wind
Turbulent mass transfer
VOC
volatilization
Air
Surface aerator
Oxygen
absorption
Water
Surface impoundment
FIGURE 4.7 Transport of a VOC from a surface impoundment. Note the two distinct areas
of mass transfer, the turbulent zone where surface aerators enhance the volatilization, and
the quiescent area where natural processes drive the evaporation of the chemical from water.
Surface winds carry the VOC away from the source. Simultaneous to volatilization of VOCs
from water, absorption of oxygen from the atmosphere also occurs.
E XAMPLE 4.16 F LUX OF B ENZENE FROM A T YPICAL S URFACE I MPOUNDMENT
A surface impoundment of total area 2
10 4 m 2 is 5 m deep and is well mixed by a
submerged aerator. The average concentration of benzene in water was 10
×
g/L, and the
background air concentration was 0.1 μ g/L. If the induced mass transfer is described
by k w = 50 m/h and k a = 2500 m/h, estimate the mass of benzene emitted during an
8-h period.
1 /K w = 1 /k w + 1 /k a K aw = 1 / 50 + 1 /( 0.23 )( 2500 ) = 1 / 46 h/m. Hence K w =
46 m/h. N i = 4600 ( 10 × 10 9
μ
0.1 × 10 9 / 0.23 ) = 4.4 × 10 5 g/cm 2 h. Amount
emitted in an 8-h period = 4.4 × 10 5 (10 4 )(2 × 10 4 )( 8 ) = 70,400 g = 70.4 kg.
The above calculation is for a given volume of water where there is no additional
input of benzene with time. This is called a batch process. In practice, most surface
impoundments receive a continuous recharge of material via a given volumetric inflow
and outflow of wastewater. This is called a continuous process. We shall revisit these
aspects later in Chapter 6, where applications of kinetics in environmental reactors will
be discussed.
Another wastewater treatment process that involves air-water contact is flotation.
Flotation is the process where tiny air bubbles are forced into a tall water column
(Figure 4.2). Bubbles can be formed either by pressurizing or depressurizing the
solution ( dissolved air flotation ) or by introducing air under pressure through a small
orifice in the form of tiny bubbles ( induced air flotation ). The purpose of bubble
 
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