302
R.D. Pomeroy and J.D. Parkhurst
Table 2 shows results of 48 tests in 8 sewers. Results for each sewer were averaged,
except that when a sewer was tested under two quite different flow conditions, or at two
times separated by an interval during which the hydraulic condition changed because of
cleaning or some other circumstance, the results were divided into two groups. Thus
Table 2 shows averages of ten sets of runs.
It may be noted that Δ(0 2 ) was negative in all cases. The reaches tested were
necessarily the maximum available distances presenting uniform conditions, with no
junctions. In all cases there were junctions not far upstream, resulting in points of
turbulence raising dissolved oxygen to levels that could not be sustained by the reaeration
rates in the test reaches.
It seems reasonable to suppose that the net rate at which the slime layer can use
oxygen will be influenced by oxygen concentration. In view of the rich food supply, the
slimes may develop a capacity to use oxygen as fast as it arrives. If this is true, then the
amount of oxygen used should be proportional to the oxygen concentration in the water.
The flux of oxygen to the slime layer, 0 e , is equal to R e r, in which r is the hydraulic
radius. If the flux is proportional to the oxygen concentration (0 2 ), one may define a
transfer coefficient, e:
e =
=
(5)
(0 2 )
(0 2 )
e is analogous to the exchange coefficient from air to water, and will be expressed in
oxygen balance calculations as m/hr.
If the rate at which the slime layer uses oxygen is dependent upon the rate of arrival of
oxygen, then it should be proportional not only to the concentration of oxygen in
solution, but also to the rate of renewal of water adjacent to the slime layer by eddy
diffusion. Thus, it may be a function of the rate of energy dissipation. If it be assumed
that it is a simple power function, one may write
e = C E (su)q
(6)
0.0005
0.001
0.002
0.005
0.01
0 0 2
su,
m/sec.
Fig. 7
Effect of rate of energy dissipation on coefficient for transfer of oxygen to the slime layer.
R.D. Pomeroy and J.D. Parkhurst
Table 2 shows results of 48 tests in 8 sewers. Results for each sewer were averaged,
except that when a sewer was tested under two quite different flow conditions, or at two
times separated by an interval during which the hydraulic condition changed because of
cleaning or some other circumstance, the results were divided into two groups. Thus
Table 2 shows averages of ten sets of runs.
It may be noted that Δ(0 2 ) was negative in all cases. The reaches tested were
necessarily the maximum available distances presenting uniform conditions, with no
junctions. In all cases there were junctions not far upstream, resulting in points of
turbulence raising dissolved oxygen to levels that could not be sustained by the reaeration
rates in the test reaches.
It seems reasonable to suppose that the net rate at which the slime layer can use
oxygen will be influenced by oxygen concentration. In view of the rich food supply, the
slimes may develop a capacity to use oxygen as fast as it arrives. If this is true, then the
amount of oxygen used should be proportional to the oxygen concentration in the water.
The flux of oxygen to the slime layer, 0 e , is equal to R e r, in which r is the hydraulic
radius. If the flux is proportional to the oxygen concentration (0 2 ), one may define a
transfer coefficient, e:
e =
=
(5)
(0 2 )
(0 2 )
e is analogous to the exchange coefficient from air to water, and will be expressed in
oxygen balance calculations as m/hr.
If the rate at which the slime layer uses oxygen is dependent upon the rate of arrival of
oxygen, then it should be proportional not only to the concentration of oxygen in
solution, but also to the rate of renewal of water adjacent to the slime layer by eddy
diffusion. Thus, it may be a function of the rate of energy dissipation. If it be assumed
that it is a simple power function, one may write
e = C E (su)q
(6)
0.0005
0.001
0.002
0.005
0.01
0 0 2
su,
m/sec.
Fig. 7
Effect of rate of energy dissipation on coefficient for transfer of oxygen to the slime layer.