Cotreatment-Assisted Anaerobic Digestion of Poultry Manure and Switchgrass
Open Access
- Author:
- Mc Vey, Thomas Matthew
- Graduate Program:
- Agricultural and Biological Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 30, 2024
- Committee Members:
- Suat Irmak, Program Head/Chair
Paul Patterson, Outside Unit & Field Member
Ali Demirci, Major Field Member
Tom Richard, Major Field Member
Daniel Ciolkosz, Chair & Dissertation Advisor - Keywords:
- anaerobic digestion
poultry litter
poultry manure
switchgrass
biogas
renewable natural gas
technoeconomic analysis
TEA
ADM1
methane
cotreatment
intermittent milling
Anaerobic Digestion Model No. 1
MATLAB
SuperPro
ruminant - Abstract:
- Poultry litter, i.e., the waste mixture mostly composed of manure and bedding resultant from the raising of broiler birds, is a significant waste stream within the United States, especially in heavily concentrated areas of production such as the southeastern region of the country, particularly in states like Arkansas. A potential alternative use of poultry litter could be anaerobic digestion, a biological technique for converting organic waste streams into energy-rich biogas and nutrient-rich digestate. The biogas can be sold as renewable natural gas (RNG) or used by the poultry farms to supplement and/or replace their existent fuel source. The digestate, which has the potential for key nutrients to be concentrated into separate phases after phase separation (phosphorous into the solid phase; nitrogen into the liquid phase), can be sold as a nutrient supplement or further processed into other chemicals and/or products. While anaerobic digestion is often an attractive option for utilizing organic waste streams and converting them into useful products, poultry litter possesses two qualities that make it less than ideal for traditional anaerobic digestion: 1) the high nitrogen content of chicken manure, and 2) the lignocellulosic, and therefore recalcitrant, nature of the bedding material. The fact that poultry litter is a mixture of high (bedding) and low (manure) carbon-to-nitrogen ratio substrates gives litter an advantage over mono-digestion of two substrates separately, but with considerably little research on digestion of poultry litter conducted, many gaps in the knowledge still exist. Therefore, this research is undertaken to investigate the potential for anaerobic digestion of poultry litter, both experimentally and economically, and the potential to enhance that digestion via cotreatment. The anaerobic digestion of poultry litter, considered for this research study as a mixture of chicken manure and switchgrass, was studied. Two mixtures of manure-to-switchgrass were digested in two continuously operating (semi-continuously fed) 1.5-L reactors, with Reactor 1 containing 75% manure-to-25% switchgrass, and Reactor 2 containing 25% manure-to-75% switchgrass. Both semi-continuous reactors were operated at a mesophilic temperature of 37°C, a hydraulic retention time of 20-days, and a total solids (TS) loading of 6%. The semi-continuous reactors resulted in methane production of 312.76 and 177.24 mL CH4 per g of volatile solid for Reactor 1 and Reactor 2, respectively. The material from these reactors was then milled (or left un-milled as a control) via 6-passes through a colloid mill set to the lowest gap size and an operating speed of 3000 rpm, and the material was then digested in batch format for subsequent periods of either 3-, 6-, or 24-days. Milling resulted in maximum increases of total gas production for Reactor 1 and Reactor 2 material of 3.54% and 9.10%, respectively. In addition to the experimental work, two models were built and tested, one being a process model developed using Anaerobic Digestion Model No. 1 (ADM1) as the framework, and the other being a technoeconomic analysis (TEA). The process model was able to accurately replicate the results of the experimental work and was then used to assess additional scenarios such as reactor failure, retention time, and various feedstock compositions. Reactor 1 is predicted to fail at a TS loading of 10%, while Reactor 2 fails at a TS loading of 30%. Retention times of 10 days or less resulted in digester failure for both reactors, with a 15-day retention resulting in higher biogas productivity. For the TEA, the net present value (NPV) of a hypothetical digestion facility was analyzed as a function of scale (i.e., one to four large-scale poultry farms providing feedstock), TS content within the digester, and several critical economic inputs (i.e., acquisition of a grant, RNG selling price, and ranges of selling prices for various other potential revenue streams). Sensitivity analysis shows the NPV being most susceptible to the grant and RNG selling price. Assuming an average RIN price and successful acquisition of the REAP grant, the facility has an NPV of approximately $3.3 million at a scale of four poultry farms and a TS of 6%. Cotreatment resulted in a 20% increase in project NPV (NPV w/o cotreatment = $1,397,753; NPV w/ cotreatment = $1,688,611) at a scale of 3 poultry farms, assuming successful acquisition of the REAP grant and average RIN price. The results of this research study suggest the potential for cotreatment as a strategy for increasing, however slightly, the biogas production from anaerobic digestion of poultry litter. The results also point out the potential to increase the efficiency of cotreatment by increasing the overall total solids within the digester and the amount of switchgrass in the feedstock ratio, as milling only directly affects the switchgrass (i.e., the lignocellulosic biomass) within the feedstock mixture, and increasing the amount of switchgrass in turn allows for increasing the total solids loading rate feasible (i.e., without issues of inhibition which lead to digester failure) within the digester.