Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)


Agricultural and Biological Engineering

Committee Chair

Jiqin Ni

Committee Member 1

Douglas Smith

Committee Member 2

Chi-Hua Huang

Committee Member 3

Yongping Yuan

Committee Member 4

Indrajeet Chaubey

Committee Member 5

Laura Bowling


The adoption of anaerobic digestion (AD) at agricultural systems as an alternative technology for waste management and renewable energy production has raised the concern regarding the value of AD digestate as fertilizer and the environmental impact of the digestate land-application. This Ph.D. research project aimed to gain new knowledge into the impact of AD on digestate characteristics in comparison with the digester feedstock, and the impact of digestate as fertilizer on water quality in comparison with inorganic fertilizer and raw manure. The project was divided into four different studies: a digestate characterization study and three digestate application studies. The digestate characterization study was conducted by sampling and analyzing feedstock and digestate at three farm-scale digester systems every two months for two years. The three digestate application studies focused on water and nutrient runoff and leaching under three different experimental conditions: (1) outdoor study using soil boxes under natural rainfall for 180 days, (2) indoor study using soil trays under a single artificial rainfall, and (3) outdoor study using field plots under single artificial rainfall. Characteristics of digestate from the three digesters varied during the two years but with no identifiable seasonality. Solid separation resulted in light reduction in total solids from digestate without changing its nutrient and metal concentrations. Results from the three application studies demonstrated that the impact of digestate on cropland application was similar to raw manure. Most of the variations in nutrient losses were due to the different digester sources. Nitrogen losses were mainly via NO3-N leaching and phosphorus losses were mainly via runoff. Results showed that digestate can substitute raw manure without increasing the nutrient losses and the risk of water pollution.