1. 0
0.05
0.1
0.15
0.2
0.25
0.3
0 10 20 30 40 50 60 70
CumulativeYield
(LCH4/gVSfed)
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 Pugela*, Ruth Spierlingab, Tryg Lundquistab
aCalifornia Polytechnic University San Luis Obispo, bMicrobio Engineering
*Corresponding Author: kpugel@calpoly.edu (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 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 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
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
FinalYield
(LCH4/gVSfed)
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
FinalYield
(LCH4/gVSfed)
SPH (5) SPL (6) DPL (8)DPH (7)Reclaimed
water (4)
as they areresolubilized 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 intosludge have been found toreduce methanogen activity by up to
50% with an Al(OH)3 dose of1000 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.