Discussion
193
Reply
Most of Mr. Parker's comments relate to conventional lagoon or stabilization pond wastewater
treatment processes of which Australia has gathered invaluable experience. There, the combination of
pond depth, prolonged detention period, decreased organic loadings and relative stagnancy, create
conditions in which nutrient supply and oxygen availability, together with solar irradiance, are determining factors in treatment performance and efficiency. Such conventional lagoons or stabilization
ponds can hardly be described as full photosynthetic processes since the conditions there are far from
optimal for maximizing algal production and photosynthetic oxygenation.
The accelerated photosynthetic system described in the paper can be regarded as a controlled
photosynthetic reactor in which the combination of pond depth, BOD and nutrient surface loadings,
day-time agitation and night-time mixing-aeration, together with the applied detention period, are all
designed to maximize algal net production and therefore maximize photosynthetic oxygenation and
nutrient removal. Under these conditions solar irradiance becomes the principle limiting factor for
design purposes, while nutrient supply is in a relative abundance and constitutes only a secondary
limiting factor. Temperature also constitutes only a secondary limiting factor under most climatic
conditions in which intensive photosynthetic systems will be used.
The velocity of 1.0 ft/sec is applied only for night-time mixing-aeration. During day-time, the
velocity is 5 cm/sec which would not allow the bacterial biomass to remain in suspension.
Table 2 clearly indicates the respective detention periods of 2 days in June and 4 days in
December.
It was briefly indicated in the paper that the photosynthetic pond is followed by a coagulationflotation process aimed at the removal of the algal biomass from the effluent. Data summarized
recently indicates that the final effluent is of a tertiary quality in which BOD levels are below 10 mg/1,
nitrogen removal is between 70 to 90 percent and phosphorus levels are below 1.0 mg/1. Detention
period of the pond ranges between 2.5 to 4.0 days according to solar irradiance levels.
Except for direct microscopic determination, no practical quantitative method to differentiate
between algal and bacterial suspended matter has been found yet. Microscopic evaluation indicated
that over 95 percent of the suspended matter at day-time is composed of algal cells.
The bacterial biomass had been settled almost uniformly throughout the channels during day-time
agitation. Most of the bacterial biomass is resuspended during night-time mixing-aeration.
DJ. FarchiU, USA
The Lancaster (California) reclamation project, indicated that the PO4 level should be reduced to
less than 0.15 M8/L (as P) to effectively limit algal growth in the receiving body. If this is to be done
by chemical methods, why optimize algal growth, only to remove it chemically, with a residual P level
of 2 or more mg/1?
Reply
The use of coagulation — flotation process is aimed primarily to remove algal suspended matter.
The simultaneous removal of phosphate is incidental, although most beneficial.
Maximizing algal production and its subsequent removal are essential for removal of both organic
matter and algae growth potential factors of which phosphorus is only one factor.
I. Unger, Canada
The Beer-Lambert law cannot be used for the calculations given in this paper, since this law is not
valid for broad spectrum adsorption.
The Beer-Lambert Law is applicable only for monochromatic light. In order to use it for polychromatic light a proper integration of the extinction coefficient should be made according to the
spectral distribution of the given light source, (see also Reference 18).
Reply
R. Mitchell, USA
The relationship between productivity rate and light intensity is not simple. We have shown that
the dominant alga in the ecosystem and the productivity rate are both dependent on a combination of
light intensity, nitrogen and phosphorus concentration. Frequently we will find the same productivity
rate at high nitrogen, low light as at low nitrogen, high light intensity. In the prediction of algal
productivity and predominence it is essential to take these interactions into account.
193
Reply
Most of Mr. Parker's comments relate to conventional lagoon or stabilization pond wastewater
treatment processes of which Australia has gathered invaluable experience. There, the combination of
pond depth, prolonged detention period, decreased organic loadings and relative stagnancy, create
conditions in which nutrient supply and oxygen availability, together with solar irradiance, are determining factors in treatment performance and efficiency. Such conventional lagoons or stabilization
ponds can hardly be described as full photosynthetic processes since the conditions there are far from
optimal for maximizing algal production and photosynthetic oxygenation.
The accelerated photosynthetic system described in the paper can be regarded as a controlled
photosynthetic reactor in which the combination of pond depth, BOD and nutrient surface loadings,
day-time agitation and night-time mixing-aeration, together with the applied detention period, are all
designed to maximize algal net production and therefore maximize photosynthetic oxygenation and
nutrient removal. Under these conditions solar irradiance becomes the principle limiting factor for
design purposes, while nutrient supply is in a relative abundance and constitutes only a secondary
limiting factor. Temperature also constitutes only a secondary limiting factor under most climatic
conditions in which intensive photosynthetic systems will be used.
The velocity of 1.0 ft/sec is applied only for night-time mixing-aeration. During day-time, the
velocity is 5 cm/sec which would not allow the bacterial biomass to remain in suspension.
Table 2 clearly indicates the respective detention periods of 2 days in June and 4 days in
December.
It was briefly indicated in the paper that the photosynthetic pond is followed by a coagulationflotation process aimed at the removal of the algal biomass from the effluent. Data summarized
recently indicates that the final effluent is of a tertiary quality in which BOD levels are below 10 mg/1,
nitrogen removal is between 70 to 90 percent and phosphorus levels are below 1.0 mg/1. Detention
period of the pond ranges between 2.5 to 4.0 days according to solar irradiance levels.
Except for direct microscopic determination, no practical quantitative method to differentiate
between algal and bacterial suspended matter has been found yet. Microscopic evaluation indicated
that over 95 percent of the suspended matter at day-time is composed of algal cells.
The bacterial biomass had been settled almost uniformly throughout the channels during day-time
agitation. Most of the bacterial biomass is resuspended during night-time mixing-aeration.
DJ. FarchiU, USA
The Lancaster (California) reclamation project, indicated that the PO4 level should be reduced to
less than 0.15 M8/L (as P) to effectively limit algal growth in the receiving body. If this is to be done
by chemical methods, why optimize algal growth, only to remove it chemically, with a residual P level
of 2 or more mg/1?
Reply
The use of coagulation — flotation process is aimed primarily to remove algal suspended matter.
The simultaneous removal of phosphate is incidental, although most beneficial.
Maximizing algal production and its subsequent removal are essential for removal of both organic
matter and algae growth potential factors of which phosphorus is only one factor.
I. Unger, Canada
The Beer-Lambert law cannot be used for the calculations given in this paper, since this law is not
valid for broad spectrum adsorption.
The Beer-Lambert Law is applicable only for monochromatic light. In order to use it for polychromatic light a proper integration of the extinction coefficient should be made according to the
spectral distribution of the given light source, (see also Reference 18).
Reply
R. Mitchell, USA
The relationship between productivity rate and light intensity is not simple. We have shown that
the dominant alga in the ecosystem and the productivity rate are both dependent on a combination of
light intensity, nitrogen and phosphorus concentration. Frequently we will find the same productivity
rate at high nitrogen, low light as at low nitrogen, high light intensity. In the prediction of algal
productivity and predominence it is essential to take these interactions into account.