Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 299

photosynthesis), surface-active substances (oil/grease at the air-water interface), and
other pollutants that affect BOD. Typical values of k r vary from 0.1-0.23 d − 1 in small
ponds to 0.7-1.1 d − 1 for swift streams. Corrections to k r and k d for the temperature of
the stream are obtained using the following equation:

(k at θ ◦ C ) = (k at 20 ◦ C )( ω ) ( θ− 20 ) ,

(6.94)

where ω is a temperature coefficient that is 1.056 for k d and 1.024 for k r .
Temperature influences the oxygen sag in a manner such that with increasing tem-
perature, the Δ c value is reached faster. Whereas the rate of aeration decreases with
temperature, the rate of deoxygenation increases with temperature. Therefore, we can
substantiate the faster response of the stream at higher temperature. Diurnal variations
in dissolved oxygen result from increased CO 2 due to algal respiration in the daytime
and decreased dissolved oxygen levels at nighttime. Thus dissolved oxygen levels are
largest during the afternoon and least at night.

6.2.2 W ATER P OLLUTION C ONTROL
This section describes the applications of chemical kinetics and reactor models for
selected wastewater pollution control operations.

6.2.2.1 Air Stripping in Aeration Basins

Wastewater is treated in lagoons into which air is introduced in either of two ways: (a)
air bubbles at the bottom of the lagoon or (b) mechanical surface aerators placed
at the water surface to induce turbulence. In either case, transfer of VOCs from
water to air with simultaneous transfer of oxygen from air to water is the objec-
tive. The analysis of oxygen uptake (absorption) and VOC desorption (stripping)
are complementary to one another (Matter-Muller, Gujer, and Giger, 1981; Munz,
1985; Valsaraj and Thibodeaux, 1987). In this section, we will illustrate the kinet-
ics of desorption of VOCs from wastewater lagoons by diffused aeration using air
bubbles.
A typicalVOC stripping operation using air bubbles is depicted in Figure 6.19. Let
us consider first a batch operation ( Q 0 = 0). If the reactor is completely mixed, the

concentration in the aqueous column C w is the same everywhere. Consider a single
air bubble as it rises through the aqueous column. It continuously picks up solute as it
moves up and exits the column carrying the solute to the atmosphere. Obviously, there
is a trade-off between a more contaminated medium (water) and a less contaminated
medium (air). The rate of transfer is controlled by diffusion across the thin boundary
layer surrounding the bubble. The overall water-phase mass transfer coefficient is K w .
The rate expression for the mass transfer to a single gas bubble is given by

V b K w C w −

C w ,

d C w
d t =

A b

(6.95)