Discussion by G. Shelef
Environmental Engineering Research Centre
Technion - Israel Institute of Technology, Haifa, Israel
An excellent example of the complete recycling of waste matter is seen in the conversion of both
the carbon and nutrient elements in wastewater into an algal proteinaceous biomass, which, after
proper processing and separation, can be used as animal feed. A photosynthetic wastewater treatment
system can be designed to provide a high quality effluent which can be used for irrigation as well as
saleable protein matter, while at the same time can solve immediate public health problems and aid in
protecting the environment. It is conceivable that such a wastewater treatment system would be less
costly to operate than alternative systems, provided that there is a demand for effluent and that the
algal byproduct can be marketed for a price at least covering the cost of the separation, drying and
processing. Such systems are economically feasible only in warm regions with ample sunlight.
The conditions in Southeast Asia, however, are not as favorable for the realization of such a
system's full potential as they would be in more developed arid regions such as the South-western
United States (Arizona, Texas, New Mexico, Southern California), Israel, South Pueto Rico, etc. In
Southeast Asia, not only is the demand for high quality effluent for irrigation negligible, but the local
communities are not yet aware of the need to cope with the growing pollution problems that could
benefit from an algal treatment system.
While stressing the high yield and concentration of the algal biomass itself, the authors have failed
to sufficiently emphasize the capacity of the High Rate Pond to produce a high quality effluent, after
the algae has been separated. It should be noted that in addition to the efficient BOO removal, the
high rate algal system also provides for the removal of nitrogen from the wastewater, both by the
utilization of the nitrogen by the algae, and by the evolution of ammonia into the atmosphere due to
high levels of PH at the ponds' surface. Values of less than 20 mg/1 5-days BOO, and the removal of
more than 80% of the nitrogen, are common in the final effluent after the algae has been separated.
(Shelef et al 1972). Chemical flocculation, described by the authors as the method of algae separation,
is also most effective in the removal if phosphorus and pathogenic bacteria from the effluent. Various
other algal micronutrients are incorporated in the algal biomass that once separated from the effluent,
reduces the capacity of that same effluent to sustain a second crop of algae. (Shelef and Halperin
1970).
One of the most beneficial advantages of the High Rate Pond system, the substantial reduction of
the Algae Growth Potential in a given effluent, is a possibility of the effluent increasing eutrophication
trends in the receiving bodies of water. This important advantage is unfortunately rarely taken into
consideration when justifying the establishment of a high rate algal system in economically
underdeveloped countries. Without taking into careful consideration the advantages of reuseable
effluent and environmental protection, it is inconceivable that a community would invest in the
establishment and maintenance of an algal wastewater treatment system - the only return being then
the harvested algae used for animal feed. In developing countries, the algae harvest alone cannot
possibly justify the establishment of such an algal system, especially in light of the recent agricultural
revaluation resulting from the introduction of high yield crops such as rice, soybeans, and wheat.
However, the authors work on minimizing the costs of algae separation and processing, may eventually
make the algal system economically feasible based principally on the algal by-product without having
to consider the economic advantages of the effluent.
In most algal high rate wastewater systems, light irradiance levels, rather than temperature or
nutrient levels, pose the principle growth limitation factor. Only under unusual conditions, where high
solar irradiance levels are coupled with low temperatures, does temperature becomes the growth
limiting factor. In most domestic wastewater, nutrients become growth limiting only in the
light-saturated zones of the pond cross section. (Shelef et al 1968).
The attempt of the authors to predict algal yields and concentration based on total radiant energy
per liter of pond water calls for comment. Figure 3, compiling data of various experiments, shows the
strong relationship between total solar radiation, Sa, and algal concentration in the pond system, Cc. A
regression analysis of the data yielded the following formulation of Equation2:
Cc = 57.4 +5 Sa - 0.0225 Sa
2
This equation is later used to predict algal yields. It seems, however, that the regression analysis
was strongly influenced by data points reflecting unusually low solar radiation levels during one of the
experiments. The general form of Figure 3 strongly suggests that, under such experimental conditions
as described by the authors, the relationship between algal concentration and solar radiation is actually
linear or very close to it. This relationship is especially probable under the conditions of near legal
starvation under which the experimental pond was operated.
Since the experiments were conducted under conditions of uniform pond depth, it is difficult to
predict either the algae concentration or yield in ponds of different depths.
It is especially difficult to predict algal yield without considering the factor of algae net decay in
533
Environmental Engineering Research Centre
Technion - Israel Institute of Technology, Haifa, Israel
An excellent example of the complete recycling of waste matter is seen in the conversion of both
the carbon and nutrient elements in wastewater into an algal proteinaceous biomass, which, after
proper processing and separation, can be used as animal feed. A photosynthetic wastewater treatment
system can be designed to provide a high quality effluent which can be used for irrigation as well as
saleable protein matter, while at the same time can solve immediate public health problems and aid in
protecting the environment. It is conceivable that such a wastewater treatment system would be less
costly to operate than alternative systems, provided that there is a demand for effluent and that the
algal byproduct can be marketed for a price at least covering the cost of the separation, drying and
processing. Such systems are economically feasible only in warm regions with ample sunlight.
The conditions in Southeast Asia, however, are not as favorable for the realization of such a
system's full potential as they would be in more developed arid regions such as the South-western
United States (Arizona, Texas, New Mexico, Southern California), Israel, South Pueto Rico, etc. In
Southeast Asia, not only is the demand for high quality effluent for irrigation negligible, but the local
communities are not yet aware of the need to cope with the growing pollution problems that could
benefit from an algal treatment system.
While stressing the high yield and concentration of the algal biomass itself, the authors have failed
to sufficiently emphasize the capacity of the High Rate Pond to produce a high quality effluent, after
the algae has been separated. It should be noted that in addition to the efficient BOO removal, the
high rate algal system also provides for the removal of nitrogen from the wastewater, both by the
utilization of the nitrogen by the algae, and by the evolution of ammonia into the atmosphere due to
high levels of PH at the ponds' surface. Values of less than 20 mg/1 5-days BOO, and the removal of
more than 80% of the nitrogen, are common in the final effluent after the algae has been separated.
(Shelef et al 1972). Chemical flocculation, described by the authors as the method of algae separation,
is also most effective in the removal if phosphorus and pathogenic bacteria from the effluent. Various
other algal micronutrients are incorporated in the algal biomass that once separated from the effluent,
reduces the capacity of that same effluent to sustain a second crop of algae. (Shelef and Halperin
1970).
One of the most beneficial advantages of the High Rate Pond system, the substantial reduction of
the Algae Growth Potential in a given effluent, is a possibility of the effluent increasing eutrophication
trends in the receiving bodies of water. This important advantage is unfortunately rarely taken into
consideration when justifying the establishment of a high rate algal system in economically
underdeveloped countries. Without taking into careful consideration the advantages of reuseable
effluent and environmental protection, it is inconceivable that a community would invest in the
establishment and maintenance of an algal wastewater treatment system - the only return being then
the harvested algae used for animal feed. In developing countries, the algae harvest alone cannot
possibly justify the establishment of such an algal system, especially in light of the recent agricultural
revaluation resulting from the introduction of high yield crops such as rice, soybeans, and wheat.
However, the authors work on minimizing the costs of algae separation and processing, may eventually
make the algal system economically feasible based principally on the algal by-product without having
to consider the economic advantages of the effluent.
In most algal high rate wastewater systems, light irradiance levels, rather than temperature or
nutrient levels, pose the principle growth limitation factor. Only under unusual conditions, where high
solar irradiance levels are coupled with low temperatures, does temperature becomes the growth
limiting factor. In most domestic wastewater, nutrients become growth limiting only in the
light-saturated zones of the pond cross section. (Shelef et al 1968).
The attempt of the authors to predict algal yields and concentration based on total radiant energy
per liter of pond water calls for comment. Figure 3, compiling data of various experiments, shows the
strong relationship between total solar radiation, Sa, and algal concentration in the pond system, Cc. A
regression analysis of the data yielded the following formulation of Equation2:
Cc = 57.4 +5 Sa - 0.0225 Sa
2
This equation is later used to predict algal yields. It seems, however, that the regression analysis
was strongly influenced by data points reflecting unusually low solar radiation levels during one of the
experiments. The general form of Figure 3 strongly suggests that, under such experimental conditions
as described by the authors, the relationship between algal concentration and solar radiation is actually
linear or very close to it. This relationship is especially probable under the conditions of near legal
starvation under which the experimental pond was operated.
Since the experiments were conducted under conditions of uniform pond depth, it is difficult to
predict either the algae concentration or yield in ponds of different depths.
It is especially difficult to predict algal yield without considering the factor of algae net decay in
533