Discussion by C. D. Parker Water Science Laboratories, Australia
The performance of ponds could be modified by stimulation of either bacteria or algal activity or
both. Effectiveness of bacterial activity can be increased by modification of nutrient ratio or increase
of total nutrient supply but this involves the addition of chemicals to the ponds.
The level of algal growth and photosynthetic production of oxygen is basically related to nutrient
supply but is also related to solar irradiation and temperature. The objective of achieving more rapid
purification is dependent on a more rapid supply of oxygen and it is pertinent to consider whether
this is related to rate of cell multiplication or total number of cells present. Certainly it is related to
the amount of chlorophyll exposed to solar influence per unit area of pond.
It would be of interest to know whether information is available from this investigation relating
rate of oxygen production or B.O.D. removal to either rate of cell multiplication or total cell number.
In the paper the weight of algal cell mass is taken as the weight of volatile suspended solids. At the
velocity of flow in the circuit (1.0 ft./sec.) it is stated that algal cells are retained in suspension and
pass off in the effluent while bacterial floe and presumably the original organic solids of the raw
sewage settle to the bottom as sludge, stated to be 5 cm. thick. At this velocity of flow it is doubtful
whether any such clear separation of different forms of solids could be achieved. In fig. 4 curves are
drawn to support the relationship propounded based on differences of 30-40% in the stated rate of
production. In view of the practical difficulties of field measurement of rates it is doubtful whether
these differences are significant.
The values for total cell content related to time, as shown in Fig. 5 show a direct relationship
between time and concentration. It would be of interest to have similar data for B.O.D., nitrogen and
phosphate removal. It is not clear what detention time was related to the effluent data shown in Table
2. Results of investigations carried out in Australia to determine the influence of detention time and
pond depth on algal growth and the removal of nitrogen and phosphate from purified trickling filter
effluent are shown in our Table 1. These results also show increase in concentration of cells with
detention time but there is no consistent increase in removal of phosphate even when cells were
filtered out. Nor was phosphate removal related to increase in pH.
It would be of interest to hear Dr. Shelef s views on the practical application of his findings to the
design of pond systems. Does he claim that moving the contents around the circuit at 1 ft./sec. will
achieve increased algal growth rate, or is it incidental to performance?
Is it suggested that with Jerusalem sewage a pond detention of 1.82 days in June and 2.64 days in
December is optimum for treatment? What treatment?
Obviously effluent requirements need to be considered and met in the design of any particular
installation, whether it be removal of B.O.D. nutrients, coliforms or the growth of algal cell mass. To
which of these objectives are the results applicable?
In particular I should like to ask the authors whether they consider the B.O.D. values shown, after
filtration, indicate complete stabilization of the sewage? With a high suspended solids content of the
raw sewage it is feasible that a significant B.O.D. of these solids may be removed by filtration. Have
the authors developed an analytical procedure for positively measuring the proportion of algal cell
material in the suspended solids content of such effluents?
How was the depth of 5 cm. of bacterial floe measured, was it uniform over the whole of the
bottom of the structure? Were two different means of agitation used?
191
The performance of ponds could be modified by stimulation of either bacteria or algal activity or
both. Effectiveness of bacterial activity can be increased by modification of nutrient ratio or increase
of total nutrient supply but this involves the addition of chemicals to the ponds.
The level of algal growth and photosynthetic production of oxygen is basically related to nutrient
supply but is also related to solar irradiation and temperature. The objective of achieving more rapid
purification is dependent on a more rapid supply of oxygen and it is pertinent to consider whether
this is related to rate of cell multiplication or total number of cells present. Certainly it is related to
the amount of chlorophyll exposed to solar influence per unit area of pond.
It would be of interest to know whether information is available from this investigation relating
rate of oxygen production or B.O.D. removal to either rate of cell multiplication or total cell number.
In the paper the weight of algal cell mass is taken as the weight of volatile suspended solids. At the
velocity of flow in the circuit (1.0 ft./sec.) it is stated that algal cells are retained in suspension and
pass off in the effluent while bacterial floe and presumably the original organic solids of the raw
sewage settle to the bottom as sludge, stated to be 5 cm. thick. At this velocity of flow it is doubtful
whether any such clear separation of different forms of solids could be achieved. In fig. 4 curves are
drawn to support the relationship propounded based on differences of 30-40% in the stated rate of
production. In view of the practical difficulties of field measurement of rates it is doubtful whether
these differences are significant.
The values for total cell content related to time, as shown in Fig. 5 show a direct relationship
between time and concentration. It would be of interest to have similar data for B.O.D., nitrogen and
phosphate removal. It is not clear what detention time was related to the effluent data shown in Table
2. Results of investigations carried out in Australia to determine the influence of detention time and
pond depth on algal growth and the removal of nitrogen and phosphate from purified trickling filter
effluent are shown in our Table 1. These results also show increase in concentration of cells with
detention time but there is no consistent increase in removal of phosphate even when cells were
filtered out. Nor was phosphate removal related to increase in pH.
It would be of interest to hear Dr. Shelef s views on the practical application of his findings to the
design of pond systems. Does he claim that moving the contents around the circuit at 1 ft./sec. will
achieve increased algal growth rate, or is it incidental to performance?
Is it suggested that with Jerusalem sewage a pond detention of 1.82 days in June and 2.64 days in
December is optimum for treatment? What treatment?
Obviously effluent requirements need to be considered and met in the design of any particular
installation, whether it be removal of B.O.D. nutrients, coliforms or the growth of algal cell mass. To
which of these objectives are the results applicable?
In particular I should like to ask the authors whether they consider the B.O.D. values shown, after
filtration, indicate complete stabilization of the sewage? With a high suspended solids content of the
raw sewage it is feasible that a significant B.O.D. of these solids may be removed by filtration. Have
the authors developed an analytical procedure for positively measuring the proportion of algal cell
material in the suspended solids content of such effluents?
How was the depth of 5 cm. of bacterial floe measured, was it uniform over the whole of the
bottom of the structure? Were two different means of agitation used?
191