Discussion
535
The data presented in Fig. 3 was, in fact, drawn from experiments under conditions of various
depths (25-45 cm) as illustrated on the figure this data has been previously published (McGarry and
Tonkasame, 1971). The strong relationship between algal concentration and total solar radiation
appears to be relatively independent of depth within this range. Algal concentrations referred to
therein relate to averaged data drawn from many samples taken in close proximity to the pond's
effluent. This method of sampling was purposefully employed so that yields might be accurately
predicted. As the discussor may be aware Equation 2 is valid only within the range presented (0-55
kcal/1) it is not suggested that it be used outside of this range. However, optimal yields are achieved
within this range. Accuracy of prediction is directly related to variance of the data about the line and
the number of data used in the regression. The strong correlation between the variables is well
illustrated by the close proximity of data to the line.
H. Leclerc, France
Is the author aware of the cultures of Spirullius prepared for experimental purposes, particularly in
Mexico? Industries based on algae may be useful in sunny countries but are they of any use in
northern climates? Our experimental work in the north of France shows that algae make little
contribution towards reoxygenation, still less as a source of production.
Reply
The author is aware of Spirulina culture in France and Mexico by the Institute de Petrole of
France. The product algae is specifically aimed at the human food market, however. I concur that
industries based on algae may be useful in sunny-countries. This is the major reason for our presence in
Thailand. Reoxy generation of sewage oxidation pond water by algae is a well-known technique of
introducing oxygen into the pond. Contrary to experience in Northern France this technique has
worked well in both Northern Canada and Alaska.
Israel Unger, Canada
What techniques were employed in the determination of light absorbed? The figure showing algae
yield vs k cal/1 of light absorbed would be better expressed as a quantum yield. The algae can absorb
one, two or more quanta of a given wavelength; it does not absorb 20 k cal or 40 k cal. I should also
like to point out that the Beer Lambert Law does not hold for this type of system.
Reply
Light was determined by use of a solarimeter measuring the total input energy cal/cm"
2 day"
1 . I
agree that light absorbed would be, in theory at least, a more meaningful measurement. However, it
was our intention to utilize a practical unit of measurement which is commonly in use throughout the
world so that yields could be predicted in locations in which the algae system might be proposed.
Aaron Moron, Israel
To increase algal yields how successful are depth, mixing and nutrient balance in addition to light?
Reply
It appears, in accord with Figure 3 of the paper, that light is a factor involved in determining algal
yield. Discussion is given in the paper as related to mixing. There are some indications that carbon
might be limiting to this system.
N.A. Lever, South Africa
What was the nitrogen content of the treated effluent from the high rate ponds, after algal growth
but before irrigation? Did the authors use artificial fertilizers, particularly nitrogen, to increase the
yield of algae?
Reply
The ammonia N component of the effluent was in the order of 20 mg/'
1 . This nitrogen could be
reduced by its incorporation into the photosynthesis process without much difficulty. No artificial
fertilizers were used to increase algae yield.
535
The data presented in Fig. 3 was, in fact, drawn from experiments under conditions of various
depths (25-45 cm) as illustrated on the figure this data has been previously published (McGarry and
Tonkasame, 1971). The strong relationship between algal concentration and total solar radiation
appears to be relatively independent of depth within this range. Algal concentrations referred to
therein relate to averaged data drawn from many samples taken in close proximity to the pond's
effluent. This method of sampling was purposefully employed so that yields might be accurately
predicted. As the discussor may be aware Equation 2 is valid only within the range presented (0-55
kcal/1) it is not suggested that it be used outside of this range. However, optimal yields are achieved
within this range. Accuracy of prediction is directly related to variance of the data about the line and
the number of data used in the regression. The strong correlation between the variables is well
illustrated by the close proximity of data to the line.
H. Leclerc, France
Is the author aware of the cultures of Spirullius prepared for experimental purposes, particularly in
Mexico? Industries based on algae may be useful in sunny countries but are they of any use in
northern climates? Our experimental work in the north of France shows that algae make little
contribution towards reoxygenation, still less as a source of production.
Reply
The author is aware of Spirulina culture in France and Mexico by the Institute de Petrole of
France. The product algae is specifically aimed at the human food market, however. I concur that
industries based on algae may be useful in sunny-countries. This is the major reason for our presence in
Thailand. Reoxy generation of sewage oxidation pond water by algae is a well-known technique of
introducing oxygen into the pond. Contrary to experience in Northern France this technique has
worked well in both Northern Canada and Alaska.
Israel Unger, Canada
What techniques were employed in the determination of light absorbed? The figure showing algae
yield vs k cal/1 of light absorbed would be better expressed as a quantum yield. The algae can absorb
one, two or more quanta of a given wavelength; it does not absorb 20 k cal or 40 k cal. I should also
like to point out that the Beer Lambert Law does not hold for this type of system.
Reply
Light was determined by use of a solarimeter measuring the total input energy cal/cm"
2 day"
1 . I
agree that light absorbed would be, in theory at least, a more meaningful measurement. However, it
was our intention to utilize a practical unit of measurement which is commonly in use throughout the
world so that yields could be predicted in locations in which the algae system might be proposed.
Aaron Moron, Israel
To increase algal yields how successful are depth, mixing and nutrient balance in addition to light?
Reply
It appears, in accord with Figure 3 of the paper, that light is a factor involved in determining algal
yield. Discussion is given in the paper as related to mixing. There are some indications that carbon
might be limiting to this system.
N.A. Lever, South Africa
What was the nitrogen content of the treated effluent from the high rate ponds, after algal growth
but before irrigation? Did the authors use artificial fertilizers, particularly nitrogen, to increase the
yield of algae?
Reply
The ammonia N component of the effluent was in the order of 20 mg/'
1 . This nitrogen could be
reduced by its incorporation into the photosynthesis process without much difficulty. No artificial
fertilizers were used to increase algae yield.