Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 273

Enhanced aeration (using surface agitation) can volatilize organic compounds from
surface impoundments. This process also facilitates the growth of bioorganisms that
are capable of destroying the waste. Surface impoundments also perform the function
of a settling pond in which particulates and other sludge solids are separated from the
waste stream. This is called an activated sludge process . Figure 6.4 depicts the three
primary pathways by which a pollutant is removed from a waste stream entering an
impoundment.
A surface impoundment can be considered to be a CSTR with complete mixing
within the aqueous phase, and the performance characteristics analyzed. If the reactor
is unmixed (i.e., no back mixing) such that a distinct solute concentration gradient exists
acrossthereactor,theimpoundmentwillhavetobemodeledasaPFR.Thethreeprimary
loss mechanisms for a chemical entering the impoundment are: (i) volatilization across
the air-water interface, (ii) adsorption to settable solids (biomass), and (iii) chemical or
biochemical reaction. Each of these loss processes within the reactor can be considered
to be a first-order process. Chemical reactions degrade the pollutant. Exchange between
phases (e.g., volatilization, settling) within the reactor tends to deplete the chemical
concentration; it does not, however, change the nature of the chemical. The overall
mass balance for a chemical A in the impoundment is

d N A
d t = F A0 − F A + V ν A r tot ,
where r tot is the total rate of loss of A by all processes within the reactor.
The rate of volatilization of A is

r voln = k v C A .

Euent Q 0 C A

CSTR

Feed Q 0 C A0

Volatilization

Chemical/
biodegradation

Solids
settling

FIGURE 6.4 Primary loss mechanisms in a surface impoundment modeled as a
CSTR.

continued