TECHNICAL, ECONOMIC, AND ENVIRONMENTAL FEASIBILITY OF WASTEWATER-DERIVED DUCKWEED BIOREFINERIES
Open Access
- Author:
- Calicioglu Sengul, Ayse Ozgul
- Graduate Program:
- Environmental Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 14, 2018
- Committee Members:
- Rachel Brennan, Dissertation Advisor/Co-Advisor
Rachel Brennan, Committee Chair/Co-Chair
Charles T Anderson, Committee Member
John Michael Regan, Committee Member
Deborah L. Sills, Outside Member
Tom Richard, Dissertation Advisor/Co-Advisor - Keywords:
- ethanol fermentation
anaerobic digestion
volatile fatty acids
acidogenic digestion
life cycle assessment
techno-economic analysis
mixed anaerobic cultures
value cascade
carboxylate platform - Abstract:
- Duckweeds (Lemnaceae) are efficient aquatic plants for wastewater treatment due to their high nutrient uptake capabilities and resilience to severe environmental conditions. Combined with their rapid growth rates, high starch, and low lignin contents, duckweed could be used as a viable feedstock for bioprocessing into fuels and chemicals in a biorefinery system. In this study, several of the knowledge gaps preventing the establishment of integrated wastewater-derived duckweed biorefineries were addressed. Technical, economic, and environmental evaluations were performed to enable the simultaneous utilization of duckweed as a reliable nutrient recovery tool and as a feedstock for bioenergy generation. A naerobic bioprocesses (bioethanol fermentation, acidogenic digestion for volatile fatty acid (VFA) production, and methanogenic digestion for biomethane production) were sequentially integrated to maximize the carbon-to-carbon conversion of wastewater-derived duckweed biomass into bioproducts. Duckweed was fed to reactors raw (dried) after liquid hot water pretreatment or enzymatic saccharification. At the end of each bioprocess, the target bioproduct (i.e., bioethanol, VFAs, or methane) was separated from the reactor liquor (i.e., by vacuum extraction of ethanol, or membrane separation of VFAs) and the remaining reactor components were subjected to further anaerobic bioprocesses. The highest total bioproduct carbon yield of 0.69±0.07 grams per gram of duckweed carbon was obtained by sequential acidogenic and methanogenic digestion. Nearly as high yields were achieved when three bioprocesses were integrated sequentially (0.66±0.08 grams of bioproduct carbon per duckweed carbon). For this three-stage value cascade, yields of each process in conventional single-stage units were: 1) 0.186±0.001 grams ethanol per gram duckweed; 2) 611±64 mg acetic acid equivalent of volatile fatty acids per gram of volatile solids; and 3) 434±0.2 ml methane per gram of volatile solids. Following experimental studies, the techno-economic analysis of a hypothetical large-scale duckweed production/wastewater treatment and biorefinery system was performed. Annual duckweed yield was simulated as 51 dry Mg per hectare after losses, over an area of 141 ha fed with municipal wastewater primary effluent, when 80% of the mat is harvested weekly. Discounted cash flow analysis results revealed that minimum biomass selling price of $25 per dry Mg with a 10% internal rate of return could be achieved if the system boundaries consider wastewater treatment as a credit. Modification and downscaling of the National Renewable Energy Laboratory 2011 Report on lignocellulosic biorefineries revealed a minimum ethanol selling price of $8.2 per U.S. gallon, with a 2.45% internal rate of return. For the calculation of a more realistic minimum ethanol selling price, a rigorous mass and energy balance must be performed. Life cycle assessment of the base case scenario used in techno-economic analysis showed that the recovery of nutrients from wastewater into duckweed biomass produced a net benefit on reducing eutrophication potential. The environmental impacts of duckweed biorefinery products to substituted products (i.e. gasoline, natural gas, and chemical fertilizers) were found to generally depend on biorefinery size: the larger the biorefinery, the smaller the environmental impacts. In terms of global warming potential (GWP), distillation for ethanol production appears to cause the highest environmental burden; however, a credit for marketing of process residues as a synthetic fertilizer substitute results in a net 13% reduction in GWP, more than compensating for the distillation burden.