Environmental Engineering Reference
In-Depth Information
microorganisms
waste organics
DO
o
CO
new cell growth
2
This relation is an oversimplification of the extremely complex biochemical reactions which take place.
In the following subsections simple first-order and zero-order kinetics models describing the oxygen
demand, stream reaeration, and photosynthesis processes are presented. These models are of the form of
the Streeter-Phelps (1925) model and its modifications and extensions (O'Connor, 1967; Thomann, 1972).
More complex and theoretically correct models are available, as described in topics, such as Thomann
and Mueller (1987), Chapra (1997), and Di Toro (2001), and are used in water-quality models, such as
QUAL2E (Brown and Barnwell, 1987) and WASP (Ambrose et al., 1988). However, the extended
Streeter-Phelps type models are presented here because they simply illustrate the overall DO balance for
a river, can still be used for some DO analyses, and the concepts often are included in the fundamentally
more complex models.
9.2.2
Streeter and Phelps Model
In streams, the interplay between deoxygenation resulting from the consumption of biodegradable
organic wastes and reaeration produces a dissolved oxygen profile called the oxygen sag (Fig. 9.2).
Streeter and Phelps (1925) approximated both the deoxygenation (related to the consumption of BOD)
and reaeration processes as first-order processes.
Biochemical Oxygen Demand (BOD)
—The first-order process describing the reduction of BOD
concentration along a flowing stream has the following form:
K ux K t
x
LL L
(9.1)
where
L
x
= the BOD concentration at point
x
in milligrams per liter,
L
0
= the initial BOD concentration at
x
= 0 in milligrams per liter,
t
=
x
/
u
= travel time to point
x
in days,
K
11
=
K
1
+
K
3
= rate of BOD removal
in 1/day,
K
1
= the deoxygenation rate (i.e. rate of removal of BOD due to oxidation), also referred to
as
K
d
in the literature, in 1/day,
K
3
= rate of BOD lost due to sedimentation in 1/day, and
u
= average
streamflow velocity in the reach in kilometers per day.
The distance x is measured in kilometers from the upstream end of the reach, typically a point of waste
discharge. Streeter and Phelps (1925) assumed the rate of sedimentation was negligible, thus, the reduction
in BOD along a flowing stream has the form:
(
/
)
e
e
11
11
0
0
x
LL
(9.2)
which is directly related to the deoxygenation of the stream. This form is used for the purposes of
illustration in this chapter.
As noted earlier both carbonaceous material and oxidizable nitrogen exert an oxygen demand and, thus,
are part of BOD. The growth of nitrifying bacteria lags behind the growth of microorganisms which
perform the carbonaceous reaction (Clark et al., 1977, p. 289). Further, Davis and Masten (2004, p. 281)
note that few of the nitrifying organisms occur in untreated sewage, but the concentration in well treated
effluent is high. Thus, at the time the Streeter-Phelps equations were developed most sewage was untreated
and carbonaceous BOD (CBOD) dominated the BOD-DO relation, whereas today in the U.S. and other
developed countries most effluent is well treated and both CBOD and nitrogeneous BOD (NBOD) are
important. This section focuses on CBOD and the next section focuses on NBOD.
BOD is the amount of molecular oxygen required to stabilize the biodegradable organic (carbon or
nitrogen) waste present in a water body by aerobic biochemical action. BOD and CBOD often are measured
using a 5-day test resulting in the 5-day BOD or CBOD (BOD
5
and CBOD
5
). The 5-day BOD was chosen
as the standard value for most purposes because the test was devised by sanitary engineers in England, where
the River Thames has a travel time to the sea of less than 5 days, so there was no need to consider oxygen
K t
0
e
Search WWH ::
Custom Search