298
R.D. Pomeroy and J.D. Parkhurst
A sample that had had acetate added reached an R r peak of 38 mg/l-hr at" 27°C. After
aerating it for an hour more, it was allowed to stand overnight without air and then was
aerated six hours more, at which time R r was 6.2. Acetate was again added in the amount
of 70 mg/1. Immediately thereafter R r was 42 mg/l-hr at 26.3°.
The strong evidence that acetate oxidation accounts for the peak was clouded by a test
run on sewage from the source that gave curve E in Fig. 6. A sample was analysed for
volatile organic acids by distillation and titration, yielding a result of 0.76 me/1. The
sewage was aerated until the peak was passed, then analysed again. The organic acid
content was the 0.35 me/1. Gas chromatography of the acids before and after oxidation
showed the proportions by equivalents to be 63% and 61% acetic, and 19% and 17%
propionic. Small amounts of the butyric, valeric and caproic acids made up the
remainders.
The findings from these exploratory experiments may be sufficiently challenging so
that others will pursue the subject further. The results have a bearing on oxygen
utilization in sewers, and on the possibilities of in-sewer treatment, and may throw light
on the apparent differences of treatability of sewages of different ages and in different
climates, and on the effects of digester liquors returned to the activated sludge process.
The biological oxidizing culture develops more rapidly in the sewer than it does in an
aerated vessel in the laboratory. In some sewers the culture tends to be flocculent in
character, settling rapidly and leaving the sewage with a low R r value, but in other cases it
settled poorly. The samples aerated in the laboratory settle very poorly. It appears that
the biological floe grown in the sewer is, in part, slime that has sloughed from the pipe
wall.
R r can be determined from oxygen balance data in sewers, provided R e is reduced to
zero. This was done by adding sodium hydroxide to the sewage stream to raise the pH to
12 or above for about a half hour followed by sodium hypochlorite added in high
concentration to assure that the biological slime layer was not only killed but also well
oxidized. Hypochlorite treatment alone did not suffice unless continued at a high dosage
for a long time. After the cessation of chlorination, oxygen concentration was measured
at both ends of a test reach, along with measurements of velocity, discharge and
temperature. The test procedures were as described previously. The oxygen exchange
coefficient, f, was calculated by Eq. 3. Since the changes of oxygen concentration in the
reaches were usually small in comparison with the average deficit, the differential
equation was usually replaced by the approximation
Δ(0 2 )
fID
- ^ — = R r - _
(4)
At
d m
in which (0 2 ) is the oxygen concentration in mg/1.
Results are shown in Table 1. It is believed that the largest source of error causing
discrepancies between the values of R r obtained by the two methods is in the
determination ofA(0 2 )/At, especially when the change in a time as short as 7 minutes
must be converted to a per-hour rate. There appears to be a tendency for the sewer results
to be lower than the pot tests, but the difference is not statistically significant.
Oxygen Consumption by Slimes
Oxygen balance tests in sewers in their normal condition, together with determinations
of R r on samples withdrawn from the sewer, allow estimates of oxygen usage by
the slime layer.
R.D. Pomeroy and J.D. Parkhurst
A sample that had had acetate added reached an R r peak of 38 mg/l-hr at" 27°C. After
aerating it for an hour more, it was allowed to stand overnight without air and then was
aerated six hours more, at which time R r was 6.2. Acetate was again added in the amount
of 70 mg/1. Immediately thereafter R r was 42 mg/l-hr at 26.3°.
The strong evidence that acetate oxidation accounts for the peak was clouded by a test
run on sewage from the source that gave curve E in Fig. 6. A sample was analysed for
volatile organic acids by distillation and titration, yielding a result of 0.76 me/1. The
sewage was aerated until the peak was passed, then analysed again. The organic acid
content was the 0.35 me/1. Gas chromatography of the acids before and after oxidation
showed the proportions by equivalents to be 63% and 61% acetic, and 19% and 17%
propionic. Small amounts of the butyric, valeric and caproic acids made up the
remainders.
The findings from these exploratory experiments may be sufficiently challenging so
that others will pursue the subject further. The results have a bearing on oxygen
utilization in sewers, and on the possibilities of in-sewer treatment, and may throw light
on the apparent differences of treatability of sewages of different ages and in different
climates, and on the effects of digester liquors returned to the activated sludge process.
The biological oxidizing culture develops more rapidly in the sewer than it does in an
aerated vessel in the laboratory. In some sewers the culture tends to be flocculent in
character, settling rapidly and leaving the sewage with a low R r value, but in other cases it
settled poorly. The samples aerated in the laboratory settle very poorly. It appears that
the biological floe grown in the sewer is, in part, slime that has sloughed from the pipe
wall.
R r can be determined from oxygen balance data in sewers, provided R e is reduced to
zero. This was done by adding sodium hydroxide to the sewage stream to raise the pH to
12 or above for about a half hour followed by sodium hypochlorite added in high
concentration to assure that the biological slime layer was not only killed but also well
oxidized. Hypochlorite treatment alone did not suffice unless continued at a high dosage
for a long time. After the cessation of chlorination, oxygen concentration was measured
at both ends of a test reach, along with measurements of velocity, discharge and
temperature. The test procedures were as described previously. The oxygen exchange
coefficient, f, was calculated by Eq. 3. Since the changes of oxygen concentration in the
reaches were usually small in comparison with the average deficit, the differential
equation was usually replaced by the approximation
Δ(0 2 )
fID
- ^ — = R r - _
(4)
At
d m
in which (0 2 ) is the oxygen concentration in mg/1.
Results are shown in Table 1. It is believed that the largest source of error causing
discrepancies between the values of R r obtained by the two methods is in the
determination ofA(0 2 )/At, especially when the change in a time as short as 7 minutes
must be converted to a per-hour rate. There appears to be a tendency for the sewer results
to be lower than the pot tests, but the difference is not statistically significant.
Oxygen Consumption by Slimes
Oxygen balance tests in sewers in their normal condition, together with determinations
of R r on samples withdrawn from the sewer, allow estimates of oxygen usage by
the slime layer.