Formal Discussion by Ralph Porges
Sewers have truly been one of the forgotten features of a sewerage system as related to biological
oxidation. A scan of the literature reveals little beyond concern for sewer ventilation, and the production of odors, especially hydrogen sulphide, resulting from absence of dissolved oxygen.
Self-purification in sewers has long range significance. The economic implications of utilizing
sewers as treatment devices has stirred many an engineer but real progress has been sparse. Much too
little attention has been directed in the past to the phenomenon of oxidation that may occur in
collection systems.
This report, ''Self-Purification in Sewers" is a real step forward. The authors studied those factors
having an affect on the oxygen balance in a stream of sewage - reaction in the liquid, loss to wall
slimes, and surface reaeration. It is interesting to note that the reaction rate of oxygen in sewage was
independent of oxygen concentration until the oxygen levels are low, generally a few tenths of a
milligram per liter. The authors correctly draw a parallel with oxygen utilization in the activated sludge
process.
The wall slimes may be considered akin to the slimes that develop on the media of the usual
trickling filter. These slimes utilize oxygen for growth and, as concluded by the authors, bring about a
reduction in BOD. Significantly, high reactive slime layers occurred even though sewage velocities were
as high as 7.1 fps (2.2 m/sec).
In a previous paper, the authors developed a predictive equation for reaeration. They point out,
however, that surface aeration is not the only source of oxygen as junctions and other points of
turbulence make significant contributions. In one trunk sewer where the oxygen supply was implemented by agitation by flows from steep tributory sewers, as much as 60 to 75 mg/1 of oxygen
reacted with the sewage.
The BOD of six samples of tributary sewage flows taken in March and May averaged 195 mg/1
while the BOD at the end of the trunk sewer averaged 142 mg/1, a reduction of 53 mg/1, or over 27
percent. The authors have substantiated the fact that considerable biological treatment can take place
in the sewer.
I would like to emphasize the practical application of this study. The Delaware River Basin
Commission has a regionalization policy that requires all waste treatment facilities to be evaluated in
terms of the regional concept. Where a regional plant is available, satellite facilities must become part
of the regional system unless overriding considerations militate against the regional hook-up. In
addition, the Commission may sponsor, and will certainly encourage, new regional systems. Such
systems, already under construction or being considered, may involve interceptors as long as 25 to 30
miles (40.2 - 4 8 . 3 km).
A future system is being projected that may necessitate interceptors of 50 miles (80.5 km) or
more. This latter facility is directed to protecting a vital recreational development with a 37 mile (59.5
km) lake and population densities may not be great for many years, if at all. It will be necessary,
therefore, to closely evaluate the economic aspects so as to assure a viable system. A step-wise
development has some merit with several interim treatment plants that will be phased-out as the
population load increases. However, the recreational lake may not be afforded, by this approach, the
protection required or desirable for maximum beneficial use. Another consideration to provide an
economical solution might be a high pressure system with relatively small pipe sizes to keep interceptor costs low. This might be activated on a limited access basis which intakes through specific
pumping stations. Experience with such a facility handling sewage is quite limited although other
materials are transported very economically in this fashion.
Consideration should be given to utilization to the maximum of the self-purification characteristics
that may be inherent to sewers when properly engineered to nurture these characteristics. The authors
point out that by designing sewers in a way to maximize aeration of the sewage stream, or by
ssupplemental oxygen in other fashions, biological oxidation may be induced to effect substantial
reductions in BOD. Economics in treatment of the reduced load would permit expenditures on more
extensive and possibly, more elaborate, interceptor facilities.
More study and effort to develop maximum efficiencies from biological oxidation in sewers should
be undertaken. It is hoped that the authors and others will pursue this aspect and ascertain the
practicality of economical application.
M. Spector, USA
Do you think that BOD can be completely stabilized within the sewer line?
Reply
Complete treatment is possible in a sewer if the requirements of time, culture and oxygen were
met. However, there are very few places where this is likely to be attempted. It is more probable that
partial BOD reduction, which is more readily attainable, will receive more attention.
307
Sewers have truly been one of the forgotten features of a sewerage system as related to biological
oxidation. A scan of the literature reveals little beyond concern for sewer ventilation, and the production of odors, especially hydrogen sulphide, resulting from absence of dissolved oxygen.
Self-purification in sewers has long range significance. The economic implications of utilizing
sewers as treatment devices has stirred many an engineer but real progress has been sparse. Much too
little attention has been directed in the past to the phenomenon of oxidation that may occur in
collection systems.
This report, ''Self-Purification in Sewers" is a real step forward. The authors studied those factors
having an affect on the oxygen balance in a stream of sewage - reaction in the liquid, loss to wall
slimes, and surface reaeration. It is interesting to note that the reaction rate of oxygen in sewage was
independent of oxygen concentration until the oxygen levels are low, generally a few tenths of a
milligram per liter. The authors correctly draw a parallel with oxygen utilization in the activated sludge
process.
The wall slimes may be considered akin to the slimes that develop on the media of the usual
trickling filter. These slimes utilize oxygen for growth and, as concluded by the authors, bring about a
reduction in BOD. Significantly, high reactive slime layers occurred even though sewage velocities were
as high as 7.1 fps (2.2 m/sec).
In a previous paper, the authors developed a predictive equation for reaeration. They point out,
however, that surface aeration is not the only source of oxygen as junctions and other points of
turbulence make significant contributions. In one trunk sewer where the oxygen supply was implemented by agitation by flows from steep tributory sewers, as much as 60 to 75 mg/1 of oxygen
reacted with the sewage.
The BOD of six samples of tributary sewage flows taken in March and May averaged 195 mg/1
while the BOD at the end of the trunk sewer averaged 142 mg/1, a reduction of 53 mg/1, or over 27
percent. The authors have substantiated the fact that considerable biological treatment can take place
in the sewer.
I would like to emphasize the practical application of this study. The Delaware River Basin
Commission has a regionalization policy that requires all waste treatment facilities to be evaluated in
terms of the regional concept. Where a regional plant is available, satellite facilities must become part
of the regional system unless overriding considerations militate against the regional hook-up. In
addition, the Commission may sponsor, and will certainly encourage, new regional systems. Such
systems, already under construction or being considered, may involve interceptors as long as 25 to 30
miles (40.2 - 4 8 . 3 km).
A future system is being projected that may necessitate interceptors of 50 miles (80.5 km) or
more. This latter facility is directed to protecting a vital recreational development with a 37 mile (59.5
km) lake and population densities may not be great for many years, if at all. It will be necessary,
therefore, to closely evaluate the economic aspects so as to assure a viable system. A step-wise
development has some merit with several interim treatment plants that will be phased-out as the
population load increases. However, the recreational lake may not be afforded, by this approach, the
protection required or desirable for maximum beneficial use. Another consideration to provide an
economical solution might be a high pressure system with relatively small pipe sizes to keep interceptor costs low. This might be activated on a limited access basis which intakes through specific
pumping stations. Experience with such a facility handling sewage is quite limited although other
materials are transported very economically in this fashion.
Consideration should be given to utilization to the maximum of the self-purification characteristics
that may be inherent to sewers when properly engineered to nurture these characteristics. The authors
point out that by designing sewers in a way to maximize aeration of the sewage stream, or by
ssupplemental oxygen in other fashions, biological oxidation may be induced to effect substantial
reductions in BOD. Economics in treatment of the reduced load would permit expenditures on more
extensive and possibly, more elaborate, interceptor facilities.
More study and effort to develop maximum efficiencies from biological oxidation in sewers should
be undertaken. It is hoped that the authors and others will pursue this aspect and ascertain the
practicality of economical application.
M. Spector, USA
Do you think that BOD can be completely stabilized within the sewer line?
Reply
Complete treatment is possible in a sewer if the requirements of time, culture and oxygen were
met. However, there are very few places where this is likely to be attempted. It is more probable that
partial BOD reduction, which is more readily attainable, will receive more attention.
307