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Design and evaluation of an anaerobic digestion system for the treatment of dairy cattle manure: Knigge Farms LLC / Daniel Earl Croisant.

by Croisant, Daniel Earl.

Anaerobic digestion.

Agricultural waste as fuel.

Sewage sludge digestion

138 leaves : ill. ; 29 cm.

Thesis advisor(s): Stephen Arant.

Committee members: Kenneth Kaszubowski, Dr. Carol Diggelman.

Introduction -- Literature review -- Materals and methods -- Results and discussion -- Conclusions and recommendations -- Appendices A: Combined waste stream characterization; B: Sizing the plug flow digester; C: Estimating biogas production; D: Sizing the combined waste stream pumps; E: Design of the solid-liquid separation system; F: Initial heating of the combined waste stream; G: Envelope thermal energy transmission; H: Sizing the heat exchanger system within the plug flow digester; I: Greenhouse gas emissions; J: Financial analysis.

In recent history, the social and environmental effects that dairy producers have on their surrounding community have come under review, revealing the need for a manure management system that not only stores waste, but reduces odors and greenhouse gas emissions. One such manure management system that meets these needs, as well as produces a source of renewable energy, is an anaerobic digestion system.

Anaerobic digestion systems are generally utilized on dairy farms with herds of 500 head or more. It was the objective of this paper to determine the feasibility of installing a system on a farm with a herd size of 100-500 head. As a case study, Knigge Farms of Omro, Wisconsin, having a herd of 205 head, was chosen.

An anaerobic digestion system degrades volatile solids within the dairy cattle waste stream, producing biogas and a stabilized effluent. The biogas is then captured and can be used to generate thermal and/or electrical energy for use on the farm, or sold to the utility for profit.

The system in this project was designed to be reliable, therefore only equipment that has had a long service history in treating agricultural wastes was considered, making the system more predictable and robust. Based on the total waste stream produced at Knigge Farms, the system designed utilized a plug flow digester, screw press, and a microturbine. The plug flow digester was designed for a hydraulic retention time of 22 days at 1000F, producing a stabilized effluent and approximately 14,000 cubic feet of biogas per day. The screw press was sized to separate only enough solids to meet the bedding requirements of the farm. With biogas as the fuel, the microturbine was expected to produce approximately 19.9 kW of electrical power, and enough thermal energy to heat the influent waste, and maintain the operating temperature within the plug flow digester.

With these specifications, the system reduced total greenhouse gas emissions by 276 tons of carbon dioxide equivalents per year, and had a net present value of approximately $83,900 over 20 years. However, this design was not economically feasible for Knigge Farms. This system, with a capital investment of approximately $213,000 and a simple payback period of approximately 20 years, does not provide a favorable return on investment considering the volatility of the dairy market.

Although the particular design proposed was not economically feasible, other designs of an anaerobic digestion system for small dairies may yield more favorable results. As an example, approximately 48% of the capital cost of the entire system was associated with the gas handling equipment. If a water heater was used instead of a microturbine, and a market for excess separated solids could be identified, an anaerobic digestion system may become more economically feasible. Additionally, new advancements in anaerobic digestion technology could have the potential to reduce capitol and/or operation and maintenance costs.

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