Graduation Term
Spring 2025
Degree Name
Master of Science (MS)
Department
Department of Agriculture
Committee Chair
Liangcheng Yang
Committee Member
Robert Rhykerd
Committee Member
David Kopsell
Abstract
Cover crops may improve soil quality, water quality, soil health, reduce soil fertilizer use, and increase nutrient availability to cash crops. The benefits of cover crops are discussed due to the below ground presence of cover crops, but the aboveground biomass can serve as a source of sustainable energy through anerobic digestion. The nutrients in the above ground biomass can also be recycled and reused through land application of digestion effluent. We have tested anaerobic co-digestion of different biomass feedstocks: cover crops, soybean stalk, and corn stover with swine manure using both small and pilot scale digesters. Nutrients, specifically NPK, were analyzed in different forms and their distributions. It was observed that co-digestion improved biogas production of all feedstocks.
Soybean biomass that was co-digested with swine manure a 60:40 mixing ratio was observed and this system yielded 271.10 L/kg-VS of methane. The liquid fraction of the digestion effluent contained about the same amount of the ammonium nitrogen and organic nitrogen, and about 60-80% of the potassium, while the solid fraction of the digestion affluent contained more organic nitrogen than the ammonium nitrogen. Overall, 94% nitrogen, 94% phosphorus, and 85% potassium recovered in the digestion effluent from the 60:40 soybean-swine manure mixing conditions.
Cover crops including cereal rye, annual ryegrass, and pea, clover, radish, oat (PCRO) mix were also tested in anerobic digesters for energy production and nutrient cycling. It was observed that co-digestion also improves biogas production with the cereal rye yielding 387.21L/kg-VS, annual ryegrass yielding 349.27L/kg-VS, and the PCRO mix yielding 306.92L/kg-VS of methane. An 80:20 cover crop-swine manure mixing conditions was observed for the pilot scale experiment. Nutrients recovered from the cereal rye pilot scale digestion effluent include 95% nitrogen, 94% phosphorus, and 105% potassium. The nutrients recovered in the annual ryegrass pilot scale experiment were 95% nitrogen, 105% phosphorus, and 97% potassium. Nutrients recovered in the PCRO mix pilot scale experiment includes 97% nitrogen, 103% phosphorus, and 94% potassium.
Corn stover was also co-digested with swine manure at a mixing ratio of 80:20 biomass-swine manure. It was observed that co-digestion at a mixing ratio of 40:60 swine manure to corn stover improves biogas production with a yielding 304.64L/kg-VS of methane. Nutrients recovered from the pilot scale digestion effluent include 93% nitrogen, 88% phosphorus, and 102% potassium. The modified Gompertz model was used to evaluate the theoretical methane yield, maximin daily yield, and lag phase of the methane yields from the pilot scale reactors.
A pretreatment experiment was also tested on each of the biomasses. Soybean biomass was pretreated with Sodium hydroxide pellets and the other four biomasses were treated with a Calcium hydroxide powder. For all biomass types a 0%, 5% and 8% concentrations were observed. The only biomass that proved to be statistically significant was the soybean biomass. All other biomass types the results from the ensiling test were not statistically different than the control.
Based on the measured results, we estimated that co-digestion of swine manure with biomass harvested from one acre of land within a three-crop rotation (soybean-cover crop-corn) can produce 5,461–6,823 m3 of methane, and provide 160–211 kg of N, 29-37 kg of P, and 254–288 kg of K. This presents significant economic potential of co-digesting crop residues with swine manure.
Access Type
Thesis-Open Access
Recommended Citation
Stoner, Shelby, "Nutrient Transport and Transformation in Anaerobic Co-digestion of Crop Residues and Swine Manure" (2025). Theses and Dissertations. 2091.
https://ir.library.illinoisstate.edu/etd/2091
