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R.D. Pomeroy and J.D. Parkhurst
actual consumption in streams more than a few inches deep in a warm climate, or more
than 2 or 3 feet deep where sewage temperatures are low, will establish steady-state
concentrations near zero.
In line 18J (Table 2) the oxygen concentration averaged 1.95 mg/1, but even with a
stream velocity of 7.1 fps it was losing oxygen at a rate of 8.5 mg/l-hr;it would reach a
steady-state concentration of 1.2 mg/1. Seven of the ten sets of data in Table 2 foretell
steady-state concentrations less than 0.5 mg/1.
Oxygen concentrations higher than the steady state levels calculated from the
prevailing flow conditions and observed R r are common, since surface aeration in
uniform-flow reaches is not the only source. Junctions and other points of turbulence
make significant contributions. There are also many sewers in which the slime layer is not
100% effective as an oxygen sink.
Where the oxygen supply is adequate, oxidation of the sewage in transit may be a
significant factor. The South Slope Trunk of the Los Angeles County Sanitation Districts
(Fig. 6) is illustrative. The oxygen supply is augmented by turbulence from steep
tributary lines, so that the stream is fairly well aerated along much of its length. Estimates
made on the basis of the data for January in Fig. 5 indicate that the total flow reacts with
about 67 mg/1 of oxygen by the time it reaches the pump station. In September, more
oxygen is used by the stream, but use by the slimes is less, the total being about 85 mg/1.
Another tributary flow enters just ahead of the Whites Point pump station, and then the
total flow is aerated by compressed air in the force main, consuming 16 mg/1 of oxygen
under average conditions. The oxygen absorbed in the force main partially compensates
for the flow added near the pump station. Thus it appears that about 60 to 75 mg/1 of
oxygen, depending upon the season, react with the sewage from the time it enters the
trunk until it discharges from the force main. This is under daytime flow conditions. The
effect would be somewhat greater for the 24-hour average flow.
Six samples of sewage taken at the end of the force main in March and May showed
BOD concentrations averaging 142 mg/1. An average BOD determined on a set of samples
from the tributary flows, taken at similar hours and seasonal conditions, showed 195
mg/1. It appears well established that the sewage did receive a substantial degree of
biological oxidation in the trunk and force main. Furthermore, some oxidation
undoubtedly occurs in the tributary lines, adding to the total in-sewer BOD reduction
that occurs in this instance. The trend toward regional sewerage systems means that an
increasing number of large, long trunks will be built. Detention times will be long enough
in some of these trunks to permit major BOD reductions, but this does not occur in such
trunks because of oxygen starvation. If oxygen were supplied in sufficient amount,
substantial reduction could be made in the loads reaching downstream treatment plants.
Summary
1. Oxygen reaction rates in sewages vary widely, being low near the origin of the sewage,
increasing for several hours if aerobic conditions prevail, up to rates as high as
20 mg/l-hr, then declining. This behavior was demonstrated both in laboratory
experiments and in sewers.
2. By using a predictive equation for reaeration and measuring the rates of use of oxygen
by the sewage, calculations were made of the rate of oxygen utilization by the slime
layer on the pipe wall. It was hypothesized that in the sewers tested the
slime layer acted as an efficient sink for the oxygen reaching it. The
R.D. Pomeroy and J.D. Parkhurst
actual consumption in streams more than a few inches deep in a warm climate, or more
than 2 or 3 feet deep where sewage temperatures are low, will establish steady-state
concentrations near zero.
In line 18J (Table 2) the oxygen concentration averaged 1.95 mg/1, but even with a
stream velocity of 7.1 fps it was losing oxygen at a rate of 8.5 mg/l-hr;it would reach a
steady-state concentration of 1.2 mg/1. Seven of the ten sets of data in Table 2 foretell
steady-state concentrations less than 0.5 mg/1.
Oxygen concentrations higher than the steady state levels calculated from the
prevailing flow conditions and observed R r are common, since surface aeration in
uniform-flow reaches is not the only source. Junctions and other points of turbulence
make significant contributions. There are also many sewers in which the slime layer is not
100% effective as an oxygen sink.
Where the oxygen supply is adequate, oxidation of the sewage in transit may be a
significant factor. The South Slope Trunk of the Los Angeles County Sanitation Districts
(Fig. 6) is illustrative. The oxygen supply is augmented by turbulence from steep
tributary lines, so that the stream is fairly well aerated along much of its length. Estimates
made on the basis of the data for January in Fig. 5 indicate that the total flow reacts with
about 67 mg/1 of oxygen by the time it reaches the pump station. In September, more
oxygen is used by the stream, but use by the slimes is less, the total being about 85 mg/1.
Another tributary flow enters just ahead of the Whites Point pump station, and then the
total flow is aerated by compressed air in the force main, consuming 16 mg/1 of oxygen
under average conditions. The oxygen absorbed in the force main partially compensates
for the flow added near the pump station. Thus it appears that about 60 to 75 mg/1 of
oxygen, depending upon the season, react with the sewage from the time it enters the
trunk until it discharges from the force main. This is under daytime flow conditions. The
effect would be somewhat greater for the 24-hour average flow.
Six samples of sewage taken at the end of the force main in March and May showed
BOD concentrations averaging 142 mg/1. An average BOD determined on a set of samples
from the tributary flows, taken at similar hours and seasonal conditions, showed 195
mg/1. It appears well established that the sewage did receive a substantial degree of
biological oxidation in the trunk and force main. Furthermore, some oxidation
undoubtedly occurs in the tributary lines, adding to the total in-sewer BOD reduction
that occurs in this instance. The trend toward regional sewerage systems means that an
increasing number of large, long trunks will be built. Detention times will be long enough
in some of these trunks to permit major BOD reductions, but this does not occur in such
trunks because of oxygen starvation. If oxygen were supplied in sufficient amount,
substantial reduction could be made in the loads reaching downstream treatment plants.
Summary
1. Oxygen reaction rates in sewages vary widely, being low near the origin of the sewage,
increasing for several hours if aerobic conditions prevail, up to rates as high as
20 mg/l-hr, then declining. This behavior was demonstrated both in laboratory
experiments and in sewers.
2. By using a predictive equation for reaeration and measuring the rates of use of oxygen
by the sewage, calculations were made of the rate of oxygen utilization by the slime
layer on the pipe wall. It was hypothesized that in the sewers tested the
slime layer acted as an efficient sink for the oxygen reaching it. The