Nutrient recovery from anaerobic co-digestion of Chlorella vulgaris and waste activated sludge

Michael Gordon1, Tyler Radniecki PhD2, Curtis Lajoie PhD2

BioResource Research Interdisciplinary Program1, School of Chemical, Biological, and Environmental Engineering2

Oregon State University, Corvallis, OR 97331

Biofuels• Renewable energy sourced from biomass

• Ideally carbon neutral

• Policy mandated

• Energy Policy Act 2005, Energy Independence and Security Act 2007

• Renewable Fuel Standard- 38 billion gal by 2022

http://green.blogs.nytimes.com

climatetechwiki.org iipdigital.usembassy.govgreenwoodresources.com

Algal Biofuels

• Unique advantages of algal biomass

• lipid dense: up to 70% dry wt

• High area productivity (1.25 kg m3/day)

• Doesn’t require arable land

• Water source flexibility

energytrendinsider.com

solixbiosystems.com

Algal Biofuels

Chisti et al., 2007

Algal Biofuels

• Large scale production requires substantial inputs of nutrients• “Nutrients”= Nitrogen and Phosphorus

• 45 kg Nitrogen and 4 kg Phosphorus / 1000 kg biomass

• Nutrient inputs economically sustainable?

mnmtraders.weebly.com

Rock Phosphate

Phosphorus is non-renewable

Viacarri, 2009

Phosphorus• A rise in biofuel production is

expected to increase competition with industrial agriculture for limited resources

Anaerobic Digestion

• Proposed as a means of nutrient recovery and recycling• Digestion releases nutrients from biomass into solution for later recovery

• Proven technology at scale

• Enhanced energy yield from CH4 production

• Provides a way to manage large quantities of

residual biomass

Bill Chambers of Stahlbush Island Farms

Stahlbush.com

Anaerobic Digestion

epa.gov

Backyard-scale digester in Eugene, OR

Sewage Coarse Filter

Primary Settling

Tank

liquidAerobic Growth

primary solids

WASAnaerobic Digester

Robert Esch

Anaerobic Digestion

• Widely used in wastewater treatment plants to treat

sewage

• Produces a nutrient rich effluent

Settling Tank

slurry

grow algae

harvest

drying

lipid extraction

algal debris

lipids

glycerol

methyl esters

MeOH + NaOH

anaerobic digester

biogas

effluent: liquidnutrient-rich

effluent: solids

Research Goal: Quantify recoverable nutrients in liquid phase of anaerobic digester effluents

Questions:

1. How does the digestion of algae compare to WAS?

2. Is co-digestion necessary to maintain digester performance?

3. Does the digestion of lipid-extracted cells differ from the digestion of whole cells?

Hypothesis: digester performance and nutrient recovery will decline as the percentage of

algal substrate increases, and, the digestion of lipid-extracted cells will result in lower

digester performance and nutrient recovery when compared to whole cells

Lab-scale batch anaerobic digesters

• Constant loading rate of 2070 mg VS L-1

• Constant inoculum to substrate ratio of 5.8:1

• Substrate composition varies

• *1 trial w/ whole cells and 1 w/ lipid-extracted algal

debris

Inoculum: Corvallis WWTP

Buffered H20

Digester Substrate: Algae and or WAS

Head space (N2)

Digester Breakdown

Total Liquid =100 mL

Lab-scale batch anaerobic digesters

Monitoring: pH, biogas, CH4, VS reduction

Nutrient quantification

Influent Hach® vials Total N Total P

Nutrient quantification

Effluent Centrifuge Pellet Supernatant

Hach: Total N, Total P

Ion Chromatography:

PO3 NO2

Colorimetric: NH4

• Biogas production provides a measure of digester activity

• Substrate loading standardized by volatile solids (VS) content

• Sig. diff. in biogas yields b/w WAS control and 100% lipid-extracted C. vulgaris (p<0.001)

• respective cumulative biogas yields 657 and 408 mL g-1 VS

• 85 % CH4

Results

Results• As the % of algae increases, a greater reductions in biogas were observed

• [1-(Treatment biogas(mL) / Control biogas (mL)]*100• Sig. diff. in biogas yields b/w WAS control and 100% lipid-extracted C. vulgaris treatments (p<0.001)

Results• Recoverable nutrients

are those that end up in the supernatant

• Reductions in biogas correlated with a decline in recoverable nutrients

• Nutrient recovery is more efficient with WAS than with C. vulgaris

Sig. Diff: nitrogen: p<0.02, phosphorus: p<0.001

Results

• [1-(Treatment nutrients recovered (mg) / Control nutrients recovered (mg)]*100• 100% C. vulgaris treatment sig diff than WAS control, N: p<0.02, P: p<0.001

• No sig. dif. b/w influent nitrogen in WAS control and 100% C. vulgaris treatment (p=0.8)

• Sig. dif. b/w influent phosphorus in WAS control and 100% C. vulgaris treatment (p=0.04)

Results: Co-digestion necessary?

• Ammonia inhibition not observed

• NH4 concentrations well below inhibitory levels (1500 ppm)

• Future experiments: shock loading

Results: Whole cells vs lipid-extracted cell debris?• Whole cells produced significantly more biogas than lipid-extracted cells (p<0.001)

p<0.001

Results: Whole cells vs lipid-extracted cell debris?• Nutrient recovery from whole cells was more efficient than lipid-extracted

p<0.001 for both N and P

Conclusions• Increasing concentrations of C. vulgaris resulted in lower biogas production

• Decrease in biogas production correlated to a decline in recoverable nutrients

• Anaerobic digestion of algal debris as a means of nutrient recovery is possible though not as efficient as nutrient recovery from waste activated sludge

• More data is needed to determine the relationship between % of algal substrate and recoverable nutrients

• More precise analytical tools are needed to quantify nutrients in sludge

Acknowledgements• Support provided through OSU’s USDA funded Bioenergy Education Project

• Collaborators: Brian Kirby and Xuwen Xiang

• City of Corvallis wastewater treatment plant

• Advisors: Dr. Radniecki and Dr. Lajoie