Discussion
419
Alan H. Molof and Cheng-nan Weng, USA
The number of disks in stage 1, 2 and 3 was increased from 48 for the base period to 60, 71 and 71
respectively at the same time as the atmosphere was enriched with oxygen. This increase in the
number of disks increased the surface area but in contrast decreased the distance between disks from
about 1.11 to 0.64 cm. The authors attempted to compensate for the increased surface by assuming
that the removal of organics was essentially proportional to the surface area for the same contact
period. However, no provision was made for the disk spacing change.
(a) The increase in the number of disks will influence the substrate removal rates in two ways.
First, the increase in area provided by an increase in the number of disks per stage will yield a lower
substrate concentration. Since the rate of the reaction is proportional to the amount of substrate
remaining, the substrate utilization rate per unit disk surface area would, therefore, be lower for the
stage with an increase in the number of disks. Secondly, as the disk spacing decreases, the rotational
speed of the waste water between the disks increases and, therefore, the velocity of the wastewater
relative to the disk slime decreases. This in turn reduces mass transfer and thereby decreases the
substrate utilization rate.
In order to evaluate the effect of an increase in the number of disks per stage the discussors used
mathematical models developed for both the organic and the ammonia removal efficiencies of a
Biological Fixed-Film Rotating Disk (BFFRD) reactor system using laboratory experimental data and
multiple regression analysis
3 4 . The six stage single and double disk reactor system was fed a synthetic
sewage.
The data reported by the authors (1) were put into the mathematical models developed in order to
compare the performance of the laboratory BFFRD model with that of the authors' pilot plant. Three
comparisons were made of the authors' results for the stage 3 effluent. First, the BOD fraction
removed experimentally, 0.255, was 28.1 percent of that predicted by the model (0.903). Secondly,
the COD fraction removed experimentally, 0.126 was 17.9 percent of that predicted by the model
(0.702). Thirdly, the NH 3 N fraction removed experimentally, 0.225, was 22.6 percent of that
predicted by the model (0.998).
The inefficiency of the authors' unit as shown in the above comparisons is felt to be due
principally to an excessive number of disks in each stage and to inefficient spacing between the disks.
It is therefore recommended that both the number of disks and the disk spacing be considered in
the design of biological fixed film rotating disk units.
REFERENCES
3 . WENG, C.N. "Biological Fixed-Film Rotating Disks for Wastewater Treatment" Ph.D. Thesis, New
York University, New York, N.Y. (1972).
4 WENG, C.N. and MOLOF, A.H. "Nitrification in the Biological Fixed-Film Rotating System"
presented at the 45th Annual Water Pollution Control Federation Conference, Atlanta, Georgia, Oct.
8-13, 1972.
W. Engel bar t, Germany
The attempt to increase the metabolism of micro-organisms by increasing the oxygen concentrations to over 100% saturation value seems to be questionable, because most aerobic microbes do
not need higher values than 10% for maximum growth rates and for many microbes high oxygen
concentration is toxic. With the rotating pipe filter which differs from the bio disc in that it is an
effective screw pump avoiding short circuit flow, we found that the digestion rate is independent of
oxygen supply up to BOD digestion rates of more than 100g/m
2 d. Using the rotating pipe filter with
air, digestion rates per square unit were more than 600% compared with 250% for the authors' results
with bio discs in oxygen enriched air.
B. Storch, UK
Do the authors envisage any scale up problems to full size operation as for example, flow
distribution, Which would appear difficult when perpendicular to the shaft axis? Although there were
no biological upsets during two years operation was there not a fall off in quality during de-sliming
operation due to solids being removed and insufficient organisms remaining on the discs during the
eighteen hour recovery period?
419
Alan H. Molof and Cheng-nan Weng, USA
The number of disks in stage 1, 2 and 3 was increased from 48 for the base period to 60, 71 and 71
respectively at the same time as the atmosphere was enriched with oxygen. This increase in the
number of disks increased the surface area but in contrast decreased the distance between disks from
about 1.11 to 0.64 cm. The authors attempted to compensate for the increased surface by assuming
that the removal of organics was essentially proportional to the surface area for the same contact
period. However, no provision was made for the disk spacing change.
(a) The increase in the number of disks will influence the substrate removal rates in two ways.
First, the increase in area provided by an increase in the number of disks per stage will yield a lower
substrate concentration. Since the rate of the reaction is proportional to the amount of substrate
remaining, the substrate utilization rate per unit disk surface area would, therefore, be lower for the
stage with an increase in the number of disks. Secondly, as the disk spacing decreases, the rotational
speed of the waste water between the disks increases and, therefore, the velocity of the wastewater
relative to the disk slime decreases. This in turn reduces mass transfer and thereby decreases the
substrate utilization rate.
In order to evaluate the effect of an increase in the number of disks per stage the discussors used
mathematical models developed for both the organic and the ammonia removal efficiencies of a
Biological Fixed-Film Rotating Disk (BFFRD) reactor system using laboratory experimental data and
multiple regression analysis
3 4 . The six stage single and double disk reactor system was fed a synthetic
sewage.
The data reported by the authors (1) were put into the mathematical models developed in order to
compare the performance of the laboratory BFFRD model with that of the authors' pilot plant. Three
comparisons were made of the authors' results for the stage 3 effluent. First, the BOD fraction
removed experimentally, 0.255, was 28.1 percent of that predicted by the model (0.903). Secondly,
the COD fraction removed experimentally, 0.126 was 17.9 percent of that predicted by the model
(0.702). Thirdly, the NH 3 N fraction removed experimentally, 0.225, was 22.6 percent of that
predicted by the model (0.998).
The inefficiency of the authors' unit as shown in the above comparisons is felt to be due
principally to an excessive number of disks in each stage and to inefficient spacing between the disks.
It is therefore recommended that both the number of disks and the disk spacing be considered in
the design of biological fixed film rotating disk units.
REFERENCES
3 . WENG, C.N. "Biological Fixed-Film Rotating Disks for Wastewater Treatment" Ph.D. Thesis, New
York University, New York, N.Y. (1972).
4 WENG, C.N. and MOLOF, A.H. "Nitrification in the Biological Fixed-Film Rotating System"
presented at the 45th Annual Water Pollution Control Federation Conference, Atlanta, Georgia, Oct.
8-13, 1972.
W. Engel bar t, Germany
The attempt to increase the metabolism of micro-organisms by increasing the oxygen concentrations to over 100% saturation value seems to be questionable, because most aerobic microbes do
not need higher values than 10% for maximum growth rates and for many microbes high oxygen
concentration is toxic. With the rotating pipe filter which differs from the bio disc in that it is an
effective screw pump avoiding short circuit flow, we found that the digestion rate is independent of
oxygen supply up to BOD digestion rates of more than 100g/m
2 d. Using the rotating pipe filter with
air, digestion rates per square unit were more than 600% compared with 250% for the authors' results
with bio discs in oxygen enriched air.
B. Storch, UK
Do the authors envisage any scale up problems to full size operation as for example, flow
distribution, Which would appear difficult when perpendicular to the shaft axis? Although there were
no biological upsets during two years operation was there not a fall off in quality during de-sliming
operation due to solids being removed and insufficient organisms remaining on the discs during the
eighteen hour recovery period?