0 0.05 0.1 0.15 0.2 0.25 0.3 0 10 20 30 40 50 60 70 Cumulative Yield (L CH 4 /g VS fed ) Time (days) Recycled water + digestate (1) Recycled water + fertilizer (2) Primary wastewater (3) Reclaimed water, no polymer (4) SPH (5) SPL (6) DPH (7) DPL (8) on the Anaerobic Digestion of Algae Biomass Kimberly Pugel a *, Ruth Spierling ab , Tryg Lundquist ab a California Polytechnic University San Luis Obispo, b Microbio Engineering *Corresponding Author: [email protected] (530) 615-9319 Background Native polyculture algae was harvested from the Cal Poly Algae Field Station (33 m 2 , 4-day HRT raceway ponds) and was batch-digested for 70 days. All digesters were assembled in duplicate according to volume (Figure 2), had their headspace purged with nitrogen gas, and were stored in an incubator at 35±3 °C. Gas composition was measured using a gas chromatograph and gas production was measured using an inverted graduated cylinder. To test water and nutrient recycling, digesters sets #1-4 were assembled: To test the effect of coagulants, digesters #5-8 were assembled using algae biomass from ponds fed reclaimed water (#4, above) and dosed with two types of coagulants: These research findings are based upon work supported by the Department of Energy and the National Science Foundation through the Research Experience for Undergraduates (REU) program at Cal Poly. The authors acknowledge the support of coordinators Dr. Gregg Fiegel, Dr. Hanson and Dr. Yessilier, whose dedication made the program possible. Additional thanks to Dr. Tryg Lundquist, Ruth Spierling, Matt Hutton, Lili Gevorkian, and other Cal Poly Environmental Engineering graduate students for their guidance, support, and patience. Figure 2. 2.5 L Serum bottle digester volumetric breakdown. Algae and seed were diluted to 1% Volatile Solids (VS). 1) To determine whether recycling clarified water and nutrients in algae cultivation decreases overall biogas production of anaerobically digested algae biomass. 2) To investigate the effect of coagulants on the anaerobic digestibility of algae. Figure 1. Process flow diagram for algae biogas production with emphasis on the recycle and coagulant addition steps. Objectives Acknowledgements [1] Sialve, Bruno, Nicolas Bernet, and Olivier Bernard. (2009) "Anaerobic digestion of microalgae as a necessary step to make microalgal biodiesel sustainable." Biotechnology advances 27.4: 409-416. [2] Chen et al., Y. (2008) Inhibition of anaerobic digestion process: A review, Bioresour. Technol., 10 (2008), pp. 4044–4064 [3] Jackson-Moss, C.A. and Duncan, J.R. (1991). The effect of aluminum on anaerobic digestion. Biotechnol. Lett., 13 (2), 143–148. [4] Novak, Park. (2010) “Effect of aluminum and iron on odors, digestion efficiency, and dewatering properties.” Water Environ. Res. Foundation. [5] Dentel, S. and Gossett, J. (1982). “Effect of chemical coagulation on anaerobic digestibility of organic materials”, Water Research. [6]McCarty, P. L. (1964). “Anaerobic Waste Treatment Fundamentals.” Public Works. Effect of Coagulant Addition Polymer Name Polymer Composition VS (g/L) Concentration Added and Digester Abbreviation (#) High Dose Low Dose "Starch Polymer" Organic: starch-based 370 7200 ppm SPH (5) 3200 ppm SPL (6) "Delhi Polymer" Inorganic: aluminum chlorohydrate-based 260 7200 ppm DPH (7) 3200 ppm DPL (8) Table 2. Description for polymers used in this study. Doses were determined through a mass balance with an initial dose of 100 and 300 ppm, a mixture volume of 2.5 L, and assuming that all coagulant partitions and settles with the algae. Final doses were added to the 1 L of digestate, and volume change was negligible. Methodology Effect of Water and Nutrient Recycling Chart 1. Average cumulative methane production, without final headspace added. For both polymers, low doses experienced a growth phase as steep as the control, however, the high doses underwent a more gradual exponential phase. Chart 4. Average total methane production from digestion of algae grown on various water/nutrient sources after 70 days of digestion. Error bars represent one standard deviation from the mean. The control (3) showed the lowest methane yield. 0.8 L 0.2 L Headspace Algae Biomass (1% VS) Seed (1% VS) 1.5 L # Water Source Nutrient Source Nutrient Form 1 Primary wastewater Primary wastewater NH 3 2 Recycled water Digestate NH 3 3 Recycled water Fertilizer Urea NH 3 4 Reclaimed water Reclaimed water NO 3 - Biogas Production • Addition of coagulants decreased methane yield from 10-30%. • Inorganic polymer decreased methane production more than the organic starch polymer . • Digesters with high doses of coagulant experienced a more gradual exponential biogas production phase. Organic polymer digesters recovered from this decrease over the digestion. • The high-dosed inorganic digesters experienced a lag phase 7 days longer than the others. • The VS destruction of the organic polymer (39, 41%) and inorganic polymer (28, 31%) compared to without polymer (37%) digesters suggests that the VS fraction of the starch polymer is more digestible while the inorganic polymer is less digestible. Nutrient Solubilization • Nitrogen solubilization appeared to be unaffected by coagulant addition. • Phosphorus solubilization was about 35% for high coagulant doses and about 50% for low coagulant doses, compared to 0% without coagulant. This indicates that reactive phosphorus was initially trapped by coagulant (according to dose) but was then released over digestion. Suggestions for future research • Add coagulant to algae grown on a different water and nutrient sources (digesters #1-3). • Regrow algae on inorganic polymer digestate to test the effect of Al metal accumulation. Chart 6. Average total methane production for each coagulant-added digester pair after 70 days of digestion. Error bars represent one standard deviation from the mean. Coagulant addition decreased methane yield according to dose. Inorganic polymer is more detrimental to digestion than organic starch polymer. Overall Results Biogas Production • Recycling water had no apparent effect on methane yield . Algae grown on recycled supernatant (1,2) showed no decrease in CH 4 yield compared to primary wastewater-grown algae. • Recycling nutrients reduced methane yield . Adding fertilizer as a nutrient source instead of recycled digestate increased algae methane yield by 28%. This could be due to: Digestate containing higher bacterial and organic load, encouraging additional heterotrophic growth. These bacteria may be less digestible due to their lower surface area to volume ratio. Reinocculation of ponds with digestion-resistant algae. However, a high VS destruction (38%) indicates that organic matter was digested but resulted in less biogas creation. • Reclaimed-water digesters experienced an initial lag phase of 7 days. This could be caused by: Low initial pH (6.4) below the optimal range of 7.3-7.6. [6] Algae in the pond flocculating more easily. Clumped algae may prolong digestion because of physical blocking of nutrients for bacteria to break down. [1] However, they still produced almost twice the methane of the control. Explanations include: Less heterotrophic biomass was present, due to lower organic content in reclaimed water. Nitrogen source was nitrate. Nitrate uptake may require different cell wall structure than ammonia, possibly allowing algae to be broken down more readily. Nutrient Solubilization • Nitrogen solubilization appeared to be unaffected by recycling. • Phosphorus solubilization when recycling digestate (59%) was about 10% less than when adding fertilizer (71%) and than the control (70%). • Reclaimed water digesters did not solubilize any phosphorus. However, the initial percent reactive phosphorus was 70% (compared to the control’s final percent of 77%), suggesting that phosphorus was mostly solubilized when digestion began. Suggestions for future research • Digest algae grown on recycled water with nitrate provided as nutrient source. • Regrow algae on digestate from digester #1, as this would be the 2 nd round of nutrient recycling. 209 566 199 615 210 599 360 812 349 773 342 823 336 772 333 777 906 913 903 984 1001 1045 1084 1142 1151 1188 1080 1220 1175 1273 1238 1207 0 200 400 600 800 1000 1200 1400 mg –N / L Recycled water + digestate (1) Primary wastewater (3) Reclaimed water (4) initial Recycled water + fertilizer (2) final SPH (5) SPL (6) DPL (8) DPH (7) Total Ammonia Nitrogen Organic Nitrogen References 46 108 58 135 44 121 150 156 139 146 178 169 168 158 213 220 0 50 100 150 200 250 mg – P / L Recycled water + digestate (1) Primary wastewater (3) Reclaimed water (4) initial Recycled water + fertilizer (2) final Total Reactive Phosphorus Organic Phosphorus 150 156 117 161 134 175 112 155 125 174 213 220 239 239 246 228 242 239 244 235 0 50 100 150 200 250 300 mg – P / L SPH (5) SPL (6) DPL (8) DPH (7) Reclaimed water (4) initial final Total Reactive Phosphorus Organic Phosphorus Chart 2. Total nitrogen composition of digesters before and after digestion. Nitrogen forms are ammonia and organic nitrogen. Nitrogen solubilization for all digesters was 40-55%. Final organic N percent was 30-40%. Similar initial and final TN confirm the mass balance. Chart 7. Total phosphorus (TP) composition of digesters #4-8 before and after digestion. Phosphorus forms are total reactive phosphorus (TRP) and organic phosphorus. Similar initial and final TP concentrations confirm the mass balance. Chart 5. Total phosphorus (TP) composition of digesters 1-4 before and after digestion. Phosphorus forms are total reactive phosphorus (TRP) and organic phosphorus. Similar initial and final TP concentrations confirm the mass balance. Table 1. Description for water and nutrient sources of algae used in this study. Discussion / Conclusions Results Discussion / Conclusions Results Volatile Solids Destruction Digester Average Std. Dev 1 38% 1.4% 2 44% 0.6% 3 44% 1.4% 4 37% 1.1% 5 39% 0.4% 6 41% 1.1% 7 28% 1.1% 8 31% 1.6% Chart 3. Average Volatile Solids (VS) destruction over 70 days of digestion. Average and standard deviation calculated from digester duplicates. Effects of Water and Nutrient Recycling and Coagulant Addition Anaerobic digestion of microalgae produces biogas that can be burned to release heat and energy as part of the biorefinery process [1] . Two components that contribute to the sustainability and productivity of the biorefinery system (Figure 1) are water and nutrient recycling and a high harvest efficiency. The recycling of water and nutrients allows for efficient use of resources. Likewise, coagulants are frequently added at algae-based wastewater treatment plants as they result in a high biomass harvest at a relatively low cost. A potential drawback to both the addition of coagulants and the recycling of water and nutrients is a reduction in methane production and nutrient resolubilization from anaerobic digestion. Water can be reused from tertiary wastewater treatment effluent (“reclaimed”) or from supernatant after settling (“recycled”, Figure 1). For the current experiment, recycled water-fed ponds were initially primary-fed. Fifty days prior to harvesting algae, the influent was turned off and water recycling began. Nutrients can also be recycled from digester effluent, 0.247 0.280 0.231 0.376 0.00 0.10 0.20 0.30 0.40 Final Yield (L CH 4 / g VS fed ) Recycled water + digestate (1) Recycled water + fertilizer (2) Primary wastewater (3) Reclaimed water (4) 0.376 0.322 0.340 0.282 0.328 0.00 0.10 0.20 0.30 0.40 Final Yield (L CH 4 / g VS fed ) SPH (5) SPL (6) DPL (8) DPH (7) Reclaimed water (4) as they are resolubilized during digestion ( Figure 1). The current study tested the effect of reusing these water and nutrient sources on the anaerobic digestion of the algae biomass. Inorganic coagulants dosed into sludge have been found to reduce methanogen activity by up to 50% with an Al(OH) 3 dose of 1000 ppm [2] and one study found anaerobic digesters failed at a dose of 2500 ppm. [3] Some research suggests this inhibition is due to toxicity and metal accumulation [4] , while others suggest inhibition is caused by the physical enmeshment and chemical interactions during flocculation. [5] In the current study, both inorganic and organic coagulants were added to digesters to further explore the cause of decreased digestibility.
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0
0.05
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Cu
mu
lati
ve Y
ield
(L C
H4/g
VS fe
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Time (days)
Recycled water + digestate (1)Recycled water + fertilizer (2)Primary wastewater (3)Reclaimed water, no polymer (4)SPH (5)SPL (6)DPH (7)DPL (8)
on the Anaerobic Digestion of Algae BiomassKimberly Pugela*, Ruth Spierlingab, Tryg Lundquistab
aCalifornia Polytechnic University San Luis Obispo, bMicrobio Engineering*Corresponding Author: [email protected] (530) 615-9319
Background
Native polyculture algae was harvested from the Cal Poly Algae Field Station (33 m2, 4-day HRT raceway ponds) and was batch-digested for 70 days. All digesters were assembled in duplicate according to volume (Figure 2), had their headspace purged with nitrogen gas, and were stored in an incubator at 35±3 °C. Gas composition was measured using a gas chromatograph and gas production was measured using an inverted graduated cylinder.
To test water and nutrient recycling, digesters sets #1-4 were assembled:
To test the effect of coagulants, digesters #5-8 were assembled using algae biomass from ponds fed reclaimed water (#4, above) and dosed with two types of coagulants:
These research findings are based upon work supported by the Department of Energy and the National Science Foundation through the Research Experience for Undergraduates (REU) program at Cal Poly. The authors acknowledge the support of coordinators Dr. Gregg Fiegel, Dr. Hanson and Dr. Yessilier, whose dedication made the program possible. Additional thanks to Dr. Tryg Lundquist, Ruth Spierling, Matt Hutton, Lili Gevorkian, and other Cal Poly Environmental Engineering graduate students for their guidance, support, and patience.
Figure 2. 2.5 L Serum bottle digester volumetric breakdown. Algae and seed were diluted to 1% Volatile Solids (VS).
1) To determine whether recycling clarified water and nutrients in algae cultivation decreases overall biogas production of anaerobically digested algae biomass.
2) To investigate the effect of coagulants on the anaerobic digestibility of algae.
Figure 1. Process flow diagram for algae biogas production with emphasis on the recycle and coagulant addition steps.
Objectives
Acknowledgements
[1] Sialve, Bruno, Nicolas Bernet, and Olivier Bernard. (2009) "Anaerobic digestion of microalgae as a necessary step to make microalgal biodiesel sustainable." Biotechnology advances 27.4: 409-416.
[2] Chen et al., Y. (2008) Inhibition of anaerobic digestion process: A review, Bioresour. Technol., 10 (2008), pp. 4044–4064 [3] Jackson-Moss, C.A. and Duncan, J.R. (1991). The effect of aluminum on anaerobic digestion. Biotechnol. Lett., 13 (2), 143–148.[4] Novak, Park. (2010) “Effect of aluminum and iron on odors, digestion efficiency, and dewatering properties.” Water Environ. Res. Foundation.[5] Dentel, S. and Gossett, J. (1982). “Effect of chemical coagulation on anaerobic digestibility of organic materials”, Water Research.[6] McCarty, P. L. (1964). “Anaerobic Waste Treatment Fundamentals.” Public Works.
Effect of Coagulant Addition
Polymer Name Polymer CompositionVS
(g/L)
Concentration Added and Digester Abbreviation (#)
High Dose Low Dose
"Starch Polymer" Organic: starch-based 3707200 ppm
SPH (5)3200 ppm
SPL (6)
"Delhi Polymer"Inorganic: aluminum chlorohydrate-based
2607200 ppm
DPH (7)3200 ppm
DPL (8)
Table 2. Description for polymers used in this study. Doses were determined through a mass balance with an initial dose of 100 and 300 ppm, a mixture volume of 2.5 L, and assuming that all coagulant partitions and settles with the algae. Final doses were added to the 1 L of digestate, and volume change was negligible.
Methodology
Effect of Water and Nutrient Recycling
Chart 1. Average cumulative methane production, without final headspace added. For both polymers, low doses experienced a growth phase as steep as the control, however, the high doses underwent a more gradual exponential phase.
Chart 4. Average total methane production from digestion of algae grown on various water/nutrient sources after 70 days of digestion. Error bars represent one standard deviation from the mean.The control (3) showed the lowest methane yield.
0.8 L
0.2 L
Headspace
Algae Biomass (1% VS)
Seed (1% VS)
1.5 L# Water Source Nutrient Source Nutrient Form
1 Primary wastewater Primary wastewater NH3
2 Recycled water Digestate NH3
3 Recycled water Fertilizer Urea NH3
4 Reclaimed water Reclaimed water NO3-
Biogas Production• Addition of coagulants decreased methane yield from 10-30%.• Inorganic polymer decreased methane production more than the organic starch polymer.• Digesters with high doses of coagulant experienced a more gradual exponential biogas
production phase. Organic polymer digesters recovered from this decrease over the digestion.• The high-dosed inorganic digesters experienced a lag phase 7 days longer than the others.• The VS destruction of the organic polymer (39, 41%) and inorganic polymer (28, 31%) compared
to without polymer (37%) digesters suggests that the VS fraction of the starch polymer is more digestible while the inorganic polymer is less digestible.
Nutrient Solubilization• Nitrogen solubilization appeared to be unaffected by coagulant addition.• Phosphorus solubilization was about 35% for high coagulant doses and about 50% for low
coagulant doses, compared to 0% without coagulant. This indicates that reactive phosphorus was initially trapped by coagulant (according to dose) but was then released over digestion.
Suggestions for future research• Add coagulant to algae grown on a different water and nutrient sources (digesters #1-3). • Regrow algae on inorganic polymer digestate to test the effect of Al metal accumulation.
Chart 6. Average total methane production for each coagulant-added digester pair after 70 days of digestion. Error bars represent one standard deviation from the mean. Coagulant addition decreased methane yield according to dose. Inorganic polymer is more detrimental to digestion than organic starch polymer.
Overall Results
Biogas Production• Recycling water had no apparent effect on methane yield . Algae grown on recycled supernatant
(1,2) showed no decrease in CH4 yield compared to primary wastewater-grown algae.• Recycling nutrients reduced methane yield. Adding fertilizer as a nutrient source instead of
recycled digestate increased algae methane yield by 28%. This could be due to: Digestate containing higher bacterial and organic load, encouraging additional heterotrophic
growth. These bacteria may be less digestible due to their lower surface area to volume ratio. Reinocculation of ponds with digestion-resistant algae. However, a high VS destruction (38%)
indicates that organic matter was digested but resulted in less biogas creation.• Reclaimed-water digesters experienced an initial lag phase of 7 days. This could be caused by:
Low initial pH (6.4) below the optimal range of 7.3-7.6.[6]
Algae in the pond flocculating more easily. Clumped algae may prolong digestion because of physical blocking of nutrients for bacteria to break down.[1]
However, they still produced almost twice the methane of the control. Explanations include: Less heterotrophic biomass was present, due to lower organic content in reclaimed water. Nitrogen source was nitrate. Nitrate uptake may require different cell wall structure than
ammonia, possibly allowing algae to be broken down more readily.
Nutrient Solubilization• Nitrogen solubilization appeared to be unaffected by recycling.• Phosphorus solubilization when recycling digestate (59%) was about 10% less than when adding
fertilizer (71%) and than the control (70%).• Reclaimed water digesters did not solubilize any phosphorus. However, the initial percent reactive
phosphorus was 70% (compared to the control’s final percent of 77%), suggesting that phosphorus was mostly solubilized when digestion began.
Suggestions for future research• Digest algae grown on recycled water with nitrate provided as nutrient source.• Regrow algae on digestate from digester #1, as this would be the 2nd round of nutrient recycling.
209
566
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906 913 903984 1001
1045 10841142 1151
11881080
12201175
1273 1238 1207
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mg
–N
/ L
Recycled water +
digestate(1)
Primary wastewater
(3)
Reclaimed water (4)
init
ial
Recycled water +
fertilizer (2)
fin
al
SPH (5) SPL (6) DPL (8)DPH (7)
Total Ammonia Nitrogen Organic Nitrogen
References
46
108
58
135
44
121
150 156
139 146
178169 168 158
213 220
0
50
100
150
200
250
mg
–P
/ L
Recycled water + digestate (1)
Primary wastewater (3)
Reclaimed water (4)
init
ial
Recycled water + fertilizer (2)
fin
alTotal Reactive Phosphorus Organic Phosphorus
150 156
117
161
134
175
112
155
125
174
213 220239 239 246
228242 239 244 235
0
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–P
/ L
SPH (5) SPL (6) DPL (8)DPH (7)Reclaimed water (4)
init
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al Total Reactive Phosphorus Organic Phosphorus
Chart 2. Total nitrogen composition of digesters before and after digestion. Nitrogen forms are ammonia and organic nitrogen. Nitrogen solubilization for all digesters was 40-55%. Final organic N percent was 30-40%. Similar initial and final TN confirm the mass balance.
Chart 7. Total phosphorus (TP) composition of digesters #4-8 before and after digestion. Phosphorus forms are total reactive phosphorus (TRP) and organic phosphorus. Similar initial and final TP concentrations confirm the mass balance.
Chart 5. Total phosphorus (TP) composition of digesters 1-4 before and after digestion. Phosphorus forms are total reactive phosphorus (TRP) and organic phosphorus. Similar initial and final TP concentrations confirm the mass balance.
Table 1. Description for water and nutrient sources of algae used in this study.
Discussion / Conclusions
Results
Discussion / Conclusions
Results
Volatile Solids DestructionDigester Average Std. Dev
1 38% 1.4%
2 44% 0.6%
3 44% 1.4%
4 37% 1.1%
5 39% 0.4%
6 41% 1.1%
7 28% 1.1%
8 31% 1.6%
Chart 3. Average Volatile Solids (VS) destruction over 70 days of digestion. Average and standard deviation calculated from digester duplicates.
Effects of Water and Nutrient Recycling and Coagulant Addition
Anaerobic digestion of microalgae produces biogas that can be burned to release heat and energy as part of the biorefinery process[1]. Two components that contribute to the sustainability and productivity of the biorefinery system (Figure 1) are water and nutrient recycling and a high harvest efficiency. The recycling of water and nutrients allows for efficient use of resources. Likewise, coagulants are frequently added at algae-based wastewater treatment plants as they result in a high biomass harvest at a relatively low cost. A potential drawback to both the addition of coagulants and the recycling ofwater and nutrients is a reductionin methane production and nutrientresolubilization from anaerobicdigestion.
Water can be reused from tertiary wastewater treatment effluent (“reclaimed”) or from supernatant after settling (“recycled”, Figure 1). For the current experiment, recycled water-fed ponds were initially primary-fed. Fifty daysprior to harvesting algae, the influent wasturned off and water recycling began. Nutrients can also be recycled from digester effluent,
0.247
0.280
0.231
0.376
0.00
0.10
0.20
0.30
0.40
Fin
al Y
ield
(L C
H4
/ g
VS fe
d)
Recycled water + digestate (1)
Recycled water + fertilizer (2)
Primary wastewater (3)
Reclaimed water (4)
0.376
0.322 0.340
0.2820.328
0.00
0.10
0.20
0.30
0.40
Fin
al Y
ield
(L C
H4
/ g
VS fe
d)
SPH (5) SPL (6) DPL (8)DPH (7)Reclaimed water (4)
as they are resolubilized during digestion ( Figure 1). The current study tested the effect of reusing these water and nutrient sources on the anaerobic digestion of the algae biomass.
Inorganic coagulants dosed into sludge have been found to reduce methanogen activity by up to 50% with an Al(OH)3 dose of 1000 ppm[2] and one study found anaerobic digesters failed at a dose of 2500 ppm.[3] Some research suggests this inhibition is due to toxicity and metal accumulation[4], while others suggest inhibition is caused by the physical enmeshment and chemical interactions during flocculation.[5] In the current study, both inorganic and organic coagulants were added to digesters to further explore the cause of decreased digestibility.
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