Dissertation
Microalgae as a nutritious cattle feed supplement to sequester carbon dioxide and recover nutrients from wastewater
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Spring 2021
DOI: 10.17077/etd.006229
Abstract
Today, nearly 1 billion of Earth's 7.6 billion people already lack sufficient nutritious food on a regular basis and the population is predicted to increase by 2-3 billion by the year 2050. Crop and livestock production will have to increase in quantity, and efficiency, using finite resources to meet global demand. Soy crops use freshwater resources, occupied 89.5 million acres of United States agricultural land in 2017, and the seedlings are conventionally supplemented with ammonium fertilizers produced through the energy-intensive Haber-Bosch process. Microalgae are a promising alternative to soy for more rapidly and sustainably produced protein-rich biomass. Microalgae can utilize nitrogen and phosphorus from wastewater, tolerate saline water, and occupy less land footprint per unit of produced biomass than soy. Microalgal biomass, however, is currently less economical than soybean meal due to harvest and processing challenges associated with a dilute product. Rather than separately growing terrestrial crops and treating various industrial and agricultural wastewaters, microalgae can use the nutrients from wastewater and the CO2 from flue gas to produce high-protein animal feed. To sustainably achieve these goals at scale, barriers to microalgal production such as tolerance for waste streams and dramatic improvement in dewatering and settleability of the microalgae must be overcome.Experiments in a 2 L photobioreactor with the ability to control CO2 concentrations and pH demonstrated that S.obliquus growth was not prohibitively reduced even at 35% CO2. The rate of growth exceeded all values in the literature for S. obliquus at concentrations greater than 2.5% CO2, and the amino acid content of the microalgae was equal or superior to that of soy. A substrate inhibition model indicated maximum biomass productivity of 640 ± 100 mg L−1d−1 at 4.5% CO2, which exceeded previously measured biomass productivity values at inhibitory CO2 concentrations. Protein contents of S. obliquus and soy were comparable.
Before proceeding with experiments that used toxic and corrosive flue gas components, a protocol was established to safely cultivate microalgae with flue gas and was published as a methods paper.
Further studies of S. obliquus, sparged with simulated coal-fired power plant flue gas, demonstrated that both biomass productivity and settling rates were increased. The average maximum biomass productivity was 700 ± 40 mg L-1 d-1, which significantly exceeded that of the control culture. Modeled bulk settling showed rapid coagulation and compaction when the cultures were grown with simulated emissions. The stress of simulated-emissions exposure decreased the S. obliquus protein contents and altered the amino acid profiles but did not decrease the fraction of methionine, a valuable amino acid in animal feed.
In subsequent experiments, S. obliquus was cultivated with a fertilizer plant wastewater formula and simulated coal-fired power plant flue gas, then separated through either forward osmosis, with reverse osmosis reject model water as the draw solution, or sedimentation. Microalgal batches grown with simulated wastewater removed NH4+ within 2 days and reached nitrogen and phosphorus limitation simultaneously on Day 5. Sparging with the flue gas caused S. obliquus to produce significantly greater quantities of extracellular polymeric substances (30.7 ± 1.8 µg mL-1), which caused flocculation and enhanced settling to an advantageous extent. Five-hour forward osmosis trials showed no statistically significant difference between water fluxes for cultures grown with simulated flue gas and CO2-supplemented air. Reverse salt fluxes were low for all conditions.
Direct reuse of waste resources to grow microalgal biomass has the potential to be more efficient than conventional agriculture and conventional wastewater treatment. The results will move forward efforts to upcycle waste to high-protein microalgal biomass for cattle feed as a large-scale, economical alternative to soybean meal.
Details
- Title: Subtitle
- Microalgae as a nutritious cattle feed supplement to sequester carbon dioxide and recover nutrients from wastewater
- Creators
- Hannah Rae Molitor
- Contributors
- Jerald L Schnoor (Advisor)David M Cwiertny (Committee Member)Gregory H LeFevre (Committee Member)Craig L Just (Committee Member)Patrick T O'Shaughnessy (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Civil and Environmental Engineering
- Date degree season
- Spring 2021
- DOI
- 10.17077/etd.006229
- Publisher
- University of Iowa
- Number of pages
- xxii, 163 pages
- Copyright
- Copyright 2021 Hannah Rae Molitor
- Language
- English
- Description illustrations
- color illustrations
- Description bibliographic
- Includes bibliographical references (pages 151-163)
- Public Abstract (ETD)
Conventional agriculture places significant demands on natural resources and is generally inefficient. As organisms with tolerance for varied conditions, microalgae (single-cell photosynthetic microorganisms) are a promising alternative to conventionally grown soy for more rapidly and sustainably produced protein-rich animal feed. Microalgae can use the CO2 from combustion emissions and the nutrients from wastewater to produce valuable biomass. However, there are significant barriers to growing nutritious salable microalgae, recovering nutrients from wastewater, and fixing CO2 from power plant emissions in full-scale sustainable operations, which my research seeks to address. High growth rates, beneficial nutritional qualities, and low-energy harvesting methods would all contribute to making sustainable biomass cultivation economically feasible.
A photobioreactor (vessel to grow microorganisms that use light for energy) was key to this work, as it was used to cultivate Scenedesmus obliquus, a nutritious green microalga, in a wide range of experimental conditions. My work showed that this species can tolerate the high CO2 levels characteristic of power plant or industrial emissions, while maintaining favorable protein contents and amino acid profiles. Additionally, my research demonstrated that combustion emissions can be leveraged to enhance the harvestability and growth rates of nutritious microalgae, while sequestering toxic pollutants. To conduct experiments with flue gas, a protocol for safe cultivation of microalgae with toxic gases was also designed and published. After attaining positive results with combustion emissions, simulated wastewater and emissions were used to cultivate microalgae and demonstrate that the protective cell secretions caused cells to aggregate and dramatically improved sedimentation rates. Experiments were also conducted to characterize harvesting potential with forward osmosis, a low-energy membrane separation technology. Finally, I described the potential for carbon negative microalgal wastewater treatment in a co-authored book chapter.
Successful scale-up of this research could reduce fertilizer and freshwater resources use, prevent agricultural runoff contamination to natural water bodies and drinking water reservoirs, offset wastewater treatment costs, and mitigate greenhouse gas emissions.
- Academic Unit
- Civil and Environmental Engineering
- Record Identifier
- 9984188375802771
Metrics
13 File views/ downloads
97 Record Views