Discussion by W.A. Pretorius
The use of rotating biological discs (RBD) for the treatment of industrial (Antonie and Welch
1969) and domestic wastes has substantially increased since Hartmann (1965) published his paper on
this subject. By 1971 over 400 installations were in operation (Borchardt 1971). The generally
employed RBD systems operated generally as "plug flow" systems. Owing to this plug flow nature a
natural separation of the different biological processes responsible for waste treatment is obtained
along the direction of flow. Oxidation of the carbonaceous material is followed by the oxidation of
the nitrogenous compounds. The improvements in the oxidation rates noted by the authors were quite
considerable and were generally in agreement with similar work done (Albertsson et al 1970) on the
use of pure oxygen in the conventional activated sludge process.
It is not yet clear whether or not the use of oxygen enriched air for the RBD systems is
economical. The authors mentioned that the dissolved oxygen in the first two compartments was nil
and that to increase the dissolved oxygen the atmosphere was enriched with oxygen. The
concentration of dissolved oxygen in the bulk of the waste liquid in conventional activated sludge
units is of utmost importance for proper biological activity, whereas this seems to be of lesser
importance to the RBD system. Owing to the fact that practically all the active micro-organisms in the
RDB system are attached to the discs and that these discs rotate, a thin layer of waste liquid plus
micro-organisms is continuously exposed to the atmosphere, where saturation with oxygen is reached
almost immediately. Whether the bulk of the liquid itself is aerobic or not is of lesser importance.
However, if one would be interested in maintaining a positive dissolved oxygen level in the bulk of the
liquid this could easily be attained by increasing the rotation rate of the discs (Welch 1968). In this
case the discs would function as both a biological support as well as a mechanical aerator, indicating
the flexibility of the RBD system.
The author's observation that, in the presence of oxygen, nitrification occurred only when most of
the soluble organic matter had been oxidized, points to an important indication that high
concentrations of dissolved organic matter might be an inhibiting factor during nitrification.
This study has again shown the flexibility of the RBD system. Owing to the separation of the
process into individual stages it is possible to carefully follow the different reactions and therefore
produce quite a range of effluents, each suitable for its own particular purpose. In this respect, as the
plant was operated on settled sewage, why operate it as an extended aeration unit to oxidise the sludge
while it could be removed immediately after carbon oxidation? Furthermore, as the authors aimed at
producing a high quality effluent by the inclusion of an algal stage, why should the nitrogen be
oxidised all the way to nitrate when Hemens and Stander (1968) have shown that perhaps better
nitrogen removal rates could be obtained if the nitrogen is present as ammonia during the algal stage?
The authors give no data on the cost aspect of this system as compared with other existing
biological systems.
REFERENCES
ALBERTSSON, J.G., McWHIRTER, J.R. ROBINSON, E.K. and VALDIECK, N.P. (1970). Investigation of the use of high purity oxygen aeration in the conventional activated sludge process. Wat.
Poll. Contr. Res. Series #17050DNW05/70. Federal Water Quality Administration, Washington
D.C.
ANTONIE, R.L. and WELCH, F.M. (1969). Preliminary results of Novel Biological Process for
Treating Dairy Wastes. Proc. of the 24th Ind. Waste Conf., Part I. Purdue University.
BORCHARDT, J.A. (1971). Biological Waste Treatment Using Rotating Discs. Biotechnol. & Bioeng.
Symposium No. 2, 131 - 140. John Wiley & Sons, Inc.
HARTMANN, H. (1965). The Bio-Disc Filter. Oesterreich. Wasswirtsch. 11/12.
HEMENS, J. and STANDER, G.J. (1968). Nutrient Removal from Sewage Effluents by Algal Activity.
Advances in Water Poll. Research. Proceedings of the 4th International Conf., Prague 1969. Edited
by S.H. Jenkins, Pergamon Press pp 701 - 715.
WELCH, F.M. (1968) Preliminary Results of a New Approach in the Aerobic Biological Treatment of
Highly Concentrated Wastes. Proceedings of the 23rd Ind. Waste Conf. Purdue University. Vol. 1.
pp 428-437.
417
The use of rotating biological discs (RBD) for the treatment of industrial (Antonie and Welch
1969) and domestic wastes has substantially increased since Hartmann (1965) published his paper on
this subject. By 1971 over 400 installations were in operation (Borchardt 1971). The generally
employed RBD systems operated generally as "plug flow" systems. Owing to this plug flow nature a
natural separation of the different biological processes responsible for waste treatment is obtained
along the direction of flow. Oxidation of the carbonaceous material is followed by the oxidation of
the nitrogenous compounds. The improvements in the oxidation rates noted by the authors were quite
considerable and were generally in agreement with similar work done (Albertsson et al 1970) on the
use of pure oxygen in the conventional activated sludge process.
It is not yet clear whether or not the use of oxygen enriched air for the RBD systems is
economical. The authors mentioned that the dissolved oxygen in the first two compartments was nil
and that to increase the dissolved oxygen the atmosphere was enriched with oxygen. The
concentration of dissolved oxygen in the bulk of the waste liquid in conventional activated sludge
units is of utmost importance for proper biological activity, whereas this seems to be of lesser
importance to the RBD system. Owing to the fact that practically all the active micro-organisms in the
RDB system are attached to the discs and that these discs rotate, a thin layer of waste liquid plus
micro-organisms is continuously exposed to the atmosphere, where saturation with oxygen is reached
almost immediately. Whether the bulk of the liquid itself is aerobic or not is of lesser importance.
However, if one would be interested in maintaining a positive dissolved oxygen level in the bulk of the
liquid this could easily be attained by increasing the rotation rate of the discs (Welch 1968). In this
case the discs would function as both a biological support as well as a mechanical aerator, indicating
the flexibility of the RBD system.
The author's observation that, in the presence of oxygen, nitrification occurred only when most of
the soluble organic matter had been oxidized, points to an important indication that high
concentrations of dissolved organic matter might be an inhibiting factor during nitrification.
This study has again shown the flexibility of the RBD system. Owing to the separation of the
process into individual stages it is possible to carefully follow the different reactions and therefore
produce quite a range of effluents, each suitable for its own particular purpose. In this respect, as the
plant was operated on settled sewage, why operate it as an extended aeration unit to oxidise the sludge
while it could be removed immediately after carbon oxidation? Furthermore, as the authors aimed at
producing a high quality effluent by the inclusion of an algal stage, why should the nitrogen be
oxidised all the way to nitrate when Hemens and Stander (1968) have shown that perhaps better
nitrogen removal rates could be obtained if the nitrogen is present as ammonia during the algal stage?
The authors give no data on the cost aspect of this system as compared with other existing
biological systems.
REFERENCES
ALBERTSSON, J.G., McWHIRTER, J.R. ROBINSON, E.K. and VALDIECK, N.P. (1970). Investigation of the use of high purity oxygen aeration in the conventional activated sludge process. Wat.
Poll. Contr. Res. Series #17050DNW05/70. Federal Water Quality Administration, Washington
D.C.
ANTONIE, R.L. and WELCH, F.M. (1969). Preliminary results of Novel Biological Process for
Treating Dairy Wastes. Proc. of the 24th Ind. Waste Conf., Part I. Purdue University.
BORCHARDT, J.A. (1971). Biological Waste Treatment Using Rotating Discs. Biotechnol. & Bioeng.
Symposium No. 2, 131 - 140. John Wiley & Sons, Inc.
HARTMANN, H. (1965). The Bio-Disc Filter. Oesterreich. Wasswirtsch. 11/12.
HEMENS, J. and STANDER, G.J. (1968). Nutrient Removal from Sewage Effluents by Algal Activity.
Advances in Water Poll. Research. Proceedings of the 4th International Conf., Prague 1969. Edited
by S.H. Jenkins, Pergamon Press pp 701 - 715.
WELCH, F.M. (1968) Preliminary Results of a New Approach in the Aerobic Biological Treatment of
Highly Concentrated Wastes. Proceedings of the 23rd Ind. Waste Conf. Purdue University. Vol. 1.
pp 428-437.
417