Algae flocculation in reservoir water - Semantic Scholar
Algae Flocculation in Reservoir Water
P. Sridhar, C. Namasivayam, and G. Prabhakaran R&D Centre, Southern Petrochemical Industries Corporation Ltd. (SPIC), Spicnagar, Tuticorin-628 005, India Accepted for publication August 6, 1987 Removal of algae i n the reservoir water was studied by electroflocculation using a bipolar cell w i th aluminum electrodes and flocculation by treatment with commercial alum. Comparison of both the methods is discussed.
INTRODUCTION Southern Petrochemical Industries (SPIC) manufactures fertilizers such as urea and diammonium phosphate. The total water requirement is about 5 X 106-6 X lo6 gal/day. The treated water is received into the plant through closed pipes. In order to overcome water shortages occurring in the lean months, SPIC has put up a water storage reservoir with a capacity of 90 X lo6 gal. This water is led into the plant battery limits without any further treatment except pressure filtration. The large surface area of the reservoir and the fairly high residence time of the water flow through the reservoir make it extremely difficult to have a meaningful chlorination to control the biological growth. An undesirable increase in the population of the biological matter has sometimes led to operating problems, such as biofilm formation in the cooling tower system and choking of filters in the demineralization units. Studies were undertaken to characterize the biological growth in the reservoir and determine possible means to control and/or remove them. In an earlier paper,' the use of copper sulfate as an algicide to control the algal blooms in the reservoir has been discussed. Removal of algae can be effected using organic polyelectrolytes such as polyacrylamide,' ~ h i t o s a n etc., , ~ using inorganic flocculants such as aluminum sulfate,' ferric chloride,' etc., or by electroflocculation. Electrocoagulation using aluminum electrodes for water treatment has been reported by Vik et al.4 The possibilities of using electroflocculation as a means of algal harvesting has been discussed by Gaur et a1.5 Contreras et a1.6 have tried an electrolytic cell using carbon electrodes for the flocculation of Alphanothece species from aqueous cultures. The present article deals with the removal of algae from water using flocculation-electroflocculation methods in order to avoid the operating problems faced in the water treatment facility. MATERIALS AND METHODS Reservoir water samples were collected periodically and analyzed for algal content. Algal density and diversity of Biotechnology and Bioengineering, Vol. 32, Pp. 345-347 (1988) 0 1988 John Wiley 81 Sons, Inc.
the populations were studied with the aid of a microscope and a hemocytometer. Different species belonging to the genera of blue green algae occurring predominantly in the reservoir water samples were isolated and identified. They are Anabaena cylindrica, Anabaenopsis sp., Arthrospira sp., Phormidium tenue, and Spirulina sp. Apart from these, diatoms and desmids were found to be present in the reservoir water. The algal population present in water was determined by estimating chlorophyll a' and referred to by the same concentration. In a typical case, the original reservoir water contained 9.2 mg/m3 chlorophyll a. In order to obtain algal concentrate, the reservoir water was ultrafiltered using Amicon DC- lOLA. Water samples containing various algal loads were prepared by mixing the algal concentrate with the original reservoir water and measuring the turbidity at 680 nm. A plot of turbidity vs. concentration of algae was found to be linear. Electroflocculation studies were carried out using a bipolar cell containing aluminum electrodes. The bipolar stack is comprised of 31 electrodes. The electrode has an area of 36.5 X 9.7 cm' and a thickness of 4 mm. The interelectrode gap was kept at 4 mm. The capacity of the electrolytic cell is 7.3 L. Electroflocculation of the reservoir water was carried out by connecting the end electrodes to the terminals of a rectifier. Current through the cell was observed using a clip-on ammeter. The cut off point for electrolysis was determined every 5 min by analyzing the amount of algal population present in the subnatant sample. Flocculation experiments using alum were carried out with ajar test apparatus. Metal ions in water samples were determined using a Varian AA575 atomic absorption spectrometer. RESULTS AND DISCUSSION Electroflocculation For an applied voltage of 50 V, a current of 0.04 A was observed using fresh electrodes. After a certain time of electrolysis, a drop in current was noticed. This may be due to the dissolution of aluminum electrodes and the possibility of formation of passive aluminum oxide layer over the electrodes. Application of an electric field through the algal suspension causes the formation of buoyant algal flocs, and they move to the surface forming a green mat at
CCC 0006-3592l881030345-03$04.00
the surface of the water. Alterations in the surface charge of algae cells is offered as the possible cause for the formation of flocs.' The effect of algal load on the power consumption was also studied, and the correlation was found to be linear. The results are presented in Figure 1. Analysis of the original reservoir water before and after treatment is shown in Table I. In the treated water, aluminum ion concentration was found to be higher (1.8 ppm) than in the untreated water (0.2 pprn). This increase is due to the dissolution of aluminum from the anodes. However, the aluminum ions present in the treated water can be removed using ion exchange columns. Continuous studies were also carried out with a flow rate of reservoir water at 0.5 L/min in the electrolytic cell. Power consumption (0.068 kWh) was observed to be the same as that for batch studies for an algal load of 9.2 mg/ m3. Aluminum ion concentration (2.0 ppm) in the outlet of the electrolytic cell was found to be almost the same as that observed for batch studies. The treated water was found to be completely free of algae, both in the batch and continuous studies.
Flocculation Experiments Commercial alum, which is the cheapest among the available flocculants in India, was found to be effective for the flocculation of algae in the reservoir water. In a typical case 500 mL original reservoir water containing 9.2 mg/m3 algae in terms of chlorophyll a was completely flocculated using 1.0 mL 1% alum solution. Completion of floccula-
Table I. Analysis of original reservoir water before and after electroflocculation .a Parame ter
Before electroflocculation
PH p-alkalinity as CaCO, rn-alkalinity as CaCO, Total hardness as CaCO, Chloride Sulfate NH3-N Total phosphate Nitrite-N Nitrate-N Conductivity TDS Si02 Sodium Potassium Aluminum
9.82 4.0 48.0 72.0 36.0 32.0 NT NT 0.05 NT 213 182.0 4.2 18.0 3.0 0.2
8.75
NT 52.0 60.0 35.0 23.0
-
0.10
264 167.0 1.o 18.0 3.0 1.8
a Concentration in parts per million and conductivity in micromhos per centimeters; NT, no trace.
tion occurred in 10-15 min, and settling occurred in 2-3 h. After settling of algae water was found to be completely free from algae, Fe3+,and A13+ ions. The sulfate concentration remained the same before and after flocculation. Since algae growth in the open reservoir was substantial over a period of time, the effect of algal load on the consumption of alum was also studied. The results are plotted in Figure 2. Analysis of the original reservoir water before
Figure 1. Variation of power consumption for flocculation of algae with concentration of chlorophyll a .
346
After electroflocculation
BIOTECHNOLOGY AND BIOENGINEERING, VOL. 32, JULY 1988
i
0
1
1
1
30
1
1
1
’
1
80
1
1
’
1
I
CONC. CHLOROPHYLL-a,mgI M3
1
133
1
1
1
8
17l
1.
Figure 2. Variation of alum requirement for flocculation of algae with concentration of chlorophyll a.
and after treatment is shown in Table 11. Analytical values for various water samples containing different algal load after flocculation were found to be more or less the same.
CONCLUSIONS Algal population in the reservoir water can be effectively removed by either electroflocculation using alu-
minum electrodes or flocculation using alum. In the case ion, of electroflocculation, there was a carry-over of whereas in the flocculation using alum, there was no canyover of A13+,Fe3+,as well as SO:- species in the treated water. For the removal of algae, alum treatment works out to be cheaper than electroflocculation.
Table 11. Analysis of reservoir water before and after flocculation with alum.a ~~
Parameter
Before flocculation
After flocculation
PH p-alkalinity as CaCO, m-alkalinity as CaCO, Total hardness as CaCO, Chloride Sulfate NH,-N Total phosphate Nitrite-N Nitrate-N Conductivity TDS SiO, Sodium Potassium Aluminum Iron
9.82 4.0 48.0 72.0 36.0 32.0 NT NT 0.05 NT 273 182.0 4.2 18.0 3.0 0.2 NT
7.92 NT 40.0 80.0 38.0 24.0
a
Concentration and conductivity as in Table I; NT, no trace.
-
0.05 -
299 239.0 3.2 18.0 3.0 0.2 -
The authors are grateful to SPIC management for use of the facilities and the permission to publish this work. Thanks are also due to P. Authimoorthi for his encouragement. The authors wish to acknowledge V. N. Raja Rao, University of Madras, for his help in identifying the algae species and Mohan and V. Nammalvar for their assistance.
References 1. P. Sridhar and G. Prabhakaran, Chem. Eng. World, 21, 52 (1986). 2. V. Moravcova, Vodni Hospod B , 35, 315 (1985). 4. 3. V. S. Govindan, Asian Environ., 7,4 (1985). E. A. Vik, D. A. Carlson, A. S. Eikum, and E. T. Gjessing, War. R e s . , 18, 1355 (1984). 5 . J. P. Gaur and H. D. Kumar, “Electrical Flocculation as a Means for Algae Harvesting,” presented at the National Workshop on Algal System, October 3 and 4, 1980, Madras, sponsored by Indian Society of Biotechnology. 6. S . Contreras, M. Pieber, A. D. Rio, M. A. Soto, J. Toha, and A. Veloz, Biorechnol. Bioeng., 23, 1165 (1981). 7. “Determination of Photosynthetic Pigments in Sea Water,” Monograph on Ocean Methodology, UNESCO, 1966, p. 69. 8. E. Matijevic and L. H. Allen, Environ. Sci. Technol., 3, 264 (1969).
SRIDHAR, NAMASIVAYAM, AND PRABHAKARAN: ALGAE FLOCCULATION IN RESERVOIR WATER
347
P. Sridhar, C. Namasivayam, and G. Prabhakaran R&D Centre, Southern Petrochemical Industries Corporation Ltd. (SPIC), Spicnagar, Tuticorin-628 005, India Accepted for publication August 6, 1987 Removal of algae i n the reservoir water was studied by electroflocculation using a bipolar cell w i th aluminum electrodes and flocculation by treatment with commercial alum. Comparison of both the methods is discussed.
INTRODUCTION Southern Petrochemical Industries (SPIC) manufactures fertilizers such as urea and diammonium phosphate. The total water requirement is about 5 X 106-6 X lo6 gal/day. The treated water is received into the plant through closed pipes. In order to overcome water shortages occurring in the lean months, SPIC has put up a water storage reservoir with a capacity of 90 X lo6 gal. This water is led into the plant battery limits without any further treatment except pressure filtration. The large surface area of the reservoir and the fairly high residence time of the water flow through the reservoir make it extremely difficult to have a meaningful chlorination to control the biological growth. An undesirable increase in the population of the biological matter has sometimes led to operating problems, such as biofilm formation in the cooling tower system and choking of filters in the demineralization units. Studies were undertaken to characterize the biological growth in the reservoir and determine possible means to control and/or remove them. In an earlier paper,' the use of copper sulfate as an algicide to control the algal blooms in the reservoir has been discussed. Removal of algae can be effected using organic polyelectrolytes such as polyacrylamide,' ~ h i t o s a n etc., , ~ using inorganic flocculants such as aluminum sulfate,' ferric chloride,' etc., or by electroflocculation. Electrocoagulation using aluminum electrodes for water treatment has been reported by Vik et al.4 The possibilities of using electroflocculation as a means of algal harvesting has been discussed by Gaur et a1.5 Contreras et a1.6 have tried an electrolytic cell using carbon electrodes for the flocculation of Alphanothece species from aqueous cultures. The present article deals with the removal of algae from water using flocculation-electroflocculation methods in order to avoid the operating problems faced in the water treatment facility. MATERIALS AND METHODS Reservoir water samples were collected periodically and analyzed for algal content. Algal density and diversity of Biotechnology and Bioengineering, Vol. 32, Pp. 345-347 (1988) 0 1988 John Wiley 81 Sons, Inc.
the populations were studied with the aid of a microscope and a hemocytometer. Different species belonging to the genera of blue green algae occurring predominantly in the reservoir water samples were isolated and identified. They are Anabaena cylindrica, Anabaenopsis sp., Arthrospira sp., Phormidium tenue, and Spirulina sp. Apart from these, diatoms and desmids were found to be present in the reservoir water. The algal population present in water was determined by estimating chlorophyll a' and referred to by the same concentration. In a typical case, the original reservoir water contained 9.2 mg/m3 chlorophyll a. In order to obtain algal concentrate, the reservoir water was ultrafiltered using Amicon DC- lOLA. Water samples containing various algal loads were prepared by mixing the algal concentrate with the original reservoir water and measuring the turbidity at 680 nm. A plot of turbidity vs. concentration of algae was found to be linear. Electroflocculation studies were carried out using a bipolar cell containing aluminum electrodes. The bipolar stack is comprised of 31 electrodes. The electrode has an area of 36.5 X 9.7 cm' and a thickness of 4 mm. The interelectrode gap was kept at 4 mm. The capacity of the electrolytic cell is 7.3 L. Electroflocculation of the reservoir water was carried out by connecting the end electrodes to the terminals of a rectifier. Current through the cell was observed using a clip-on ammeter. The cut off point for electrolysis was determined every 5 min by analyzing the amount of algal population present in the subnatant sample. Flocculation experiments using alum were carried out with ajar test apparatus. Metal ions in water samples were determined using a Varian AA575 atomic absorption spectrometer. RESULTS AND DISCUSSION Electroflocculation For an applied voltage of 50 V, a current of 0.04 A was observed using fresh electrodes. After a certain time of electrolysis, a drop in current was noticed. This may be due to the dissolution of aluminum electrodes and the possibility of formation of passive aluminum oxide layer over the electrodes. Application of an electric field through the algal suspension causes the formation of buoyant algal flocs, and they move to the surface forming a green mat at
CCC 0006-3592l881030345-03$04.00
the surface of the water. Alterations in the surface charge of algae cells is offered as the possible cause for the formation of flocs.' The effect of algal load on the power consumption was also studied, and the correlation was found to be linear. The results are presented in Figure 1. Analysis of the original reservoir water before and after treatment is shown in Table I. In the treated water, aluminum ion concentration was found to be higher (1.8 ppm) than in the untreated water (0.2 pprn). This increase is due to the dissolution of aluminum from the anodes. However, the aluminum ions present in the treated water can be removed using ion exchange columns. Continuous studies were also carried out with a flow rate of reservoir water at 0.5 L/min in the electrolytic cell. Power consumption (0.068 kWh) was observed to be the same as that for batch studies for an algal load of 9.2 mg/ m3. Aluminum ion concentration (2.0 ppm) in the outlet of the electrolytic cell was found to be almost the same as that observed for batch studies. The treated water was found to be completely free of algae, both in the batch and continuous studies.
Flocculation Experiments Commercial alum, which is the cheapest among the available flocculants in India, was found to be effective for the flocculation of algae in the reservoir water. In a typical case 500 mL original reservoir water containing 9.2 mg/m3 algae in terms of chlorophyll a was completely flocculated using 1.0 mL 1% alum solution. Completion of floccula-
Table I. Analysis of original reservoir water before and after electroflocculation .a Parame ter
Before electroflocculation
PH p-alkalinity as CaCO, rn-alkalinity as CaCO, Total hardness as CaCO, Chloride Sulfate NH3-N Total phosphate Nitrite-N Nitrate-N Conductivity TDS Si02 Sodium Potassium Aluminum
9.82 4.0 48.0 72.0 36.0 32.0 NT NT 0.05 NT 213 182.0 4.2 18.0 3.0 0.2
8.75
NT 52.0 60.0 35.0 23.0
-
0.10
264 167.0 1.o 18.0 3.0 1.8
a Concentration in parts per million and conductivity in micromhos per centimeters; NT, no trace.
tion occurred in 10-15 min, and settling occurred in 2-3 h. After settling of algae water was found to be completely free from algae, Fe3+,and A13+ ions. The sulfate concentration remained the same before and after flocculation. Since algae growth in the open reservoir was substantial over a period of time, the effect of algal load on the consumption of alum was also studied. The results are plotted in Figure 2. Analysis of the original reservoir water before
Figure 1. Variation of power consumption for flocculation of algae with concentration of chlorophyll a .
346
After electroflocculation
BIOTECHNOLOGY AND BIOENGINEERING, VOL. 32, JULY 1988
i
0
1
1
1
30
1
1
1
’
1
80
1
1
’
1
I
CONC. CHLOROPHYLL-a,mgI M3
1
133
1
1
1
8
17l
1.
Figure 2. Variation of alum requirement for flocculation of algae with concentration of chlorophyll a.
and after treatment is shown in Table 11. Analytical values for various water samples containing different algal load after flocculation were found to be more or less the same.
CONCLUSIONS Algal population in the reservoir water can be effectively removed by either electroflocculation using alu-
minum electrodes or flocculation using alum. In the case ion, of electroflocculation, there was a carry-over of whereas in the flocculation using alum, there was no canyover of A13+,Fe3+,as well as SO:- species in the treated water. For the removal of algae, alum treatment works out to be cheaper than electroflocculation.
Table 11. Analysis of reservoir water before and after flocculation with alum.a ~~
Parameter
Before flocculation
After flocculation
PH p-alkalinity as CaCO, m-alkalinity as CaCO, Total hardness as CaCO, Chloride Sulfate NH,-N Total phosphate Nitrite-N Nitrate-N Conductivity TDS SiO, Sodium Potassium Aluminum Iron
9.82 4.0 48.0 72.0 36.0 32.0 NT NT 0.05 NT 273 182.0 4.2 18.0 3.0 0.2 NT
7.92 NT 40.0 80.0 38.0 24.0
a
Concentration and conductivity as in Table I; NT, no trace.
-
0.05 -
299 239.0 3.2 18.0 3.0 0.2 -
The authors are grateful to SPIC management for use of the facilities and the permission to publish this work. Thanks are also due to P. Authimoorthi for his encouragement. The authors wish to acknowledge V. N. Raja Rao, University of Madras, for his help in identifying the algae species and Mohan and V. Nammalvar for their assistance.
References 1. P. Sridhar and G. Prabhakaran, Chem. Eng. World, 21, 52 (1986). 2. V. Moravcova, Vodni Hospod B , 35, 315 (1985). 4. 3. V. S. Govindan, Asian Environ., 7,4 (1985). E. A. Vik, D. A. Carlson, A. S. Eikum, and E. T. Gjessing, War. R e s . , 18, 1355 (1984). 5 . J. P. Gaur and H. D. Kumar, “Electrical Flocculation as a Means for Algae Harvesting,” presented at the National Workshop on Algal System, October 3 and 4, 1980, Madras, sponsored by Indian Society of Biotechnology. 6. S . Contreras, M. Pieber, A. D. Rio, M. A. Soto, J. Toha, and A. Veloz, Biorechnol. Bioeng., 23, 1165 (1981). 7. “Determination of Photosynthetic Pigments in Sea Water,” Monograph on Ocean Methodology, UNESCO, 1966, p. 69. 8. E. Matijevic and L. H. Allen, Environ. Sci. Technol., 3, 264 (1969).
SRIDHAR, NAMASIVAYAM, AND PRABHAKARAN: ALGAE FLOCCULATION IN RESERVOIR WATER
347
