The proposed full-scale design fully complies with the Code of Federal Regulations (CFRs) Title 29: Labor, that mandate specific requirements related to:

•operator use of farm machinery and accident prevention•safety of processes on the full-scale unit•worker protection during manure collection and separation

•29 CFR 1200 specifically requires worker safety training and hazard communication for operators associated with the full-scale unit

Worker Health and Safety

Task 1Heidi Eckman, Jesse Hawkins, and Benjamin Yeamans

WERC: A Consortium for Environmental Education

and Technology Development

Design Contest 2005

Concentrated Animal Feeding Operations (CAFOs) in New Mexico produce over 25 million tons of wet manure annually. This manure can be anaerobically digested to generate methane for power production. However, current collection and separation technology does not adequately remove rocks, dirt, sand, and fibrous solids found in manure. These inert solids negatively impact anaerobic digestion. The overall design objective was to:

•Combine collection and separation processes into a more efficient, cost effective manure management system

•Remove inert solids from the manure before final processing in an anaerobic digester

Problem Summary

• Decision matrices were used to evaluate collection and separation options based on design specifications

• Design parameters were given a weighting value that represented the effect of the parameter on the overall full-scale design

• Vacuum collection was chosen over mechanical scrapers due to greater efficiency of collection and ease of implementation

• Rotary drum vacuum filtration (RDVF) was chosen over inclined screens, centrifuges, screw presses, and belt presses due to adaptability to a mobile platform and overall operation and performance

Current Technology and Process Selection

•The full-scale design features:

•The full-scale design produces two output streams

•A concentrated Liquid Stream of soluble organics as feed for an anaerobic digester

•A Solid Stream of rocks, dirt, sand, and insoluble organic fibers for subsequent composting or application to agricultural land

Full Scale Design

Slurry Tank Filtrate Tank

Solids Output Storage

Feed Pump

Return Pump

VacuumVacuum

Rotary Drum Vacuum Filter

Raw InputRaw Liquids & Solids

Solids Stream

Liquid Stream

3 way valve

Raw input

Overflow Return

Liquid Stream

Liquid Output

Maceration

.

•Vacuum filtration tests were conducted at different percent solid concentrations and vacuum differentials to estimate the specific resistance and coefficient of compressibility of as-collected manure:

•Specific resistance of 2.62 x 107 s2/g•Coefficient of compressibility of 1.12

•Laboratory data indicated a good dewaterability for manure solids

•A dried solids yield of 26.8 lb/ft2/hr was based on specific resistance data and used to estimate the size of the RDVF for the full-scale design

Experimental

A Finite Element Analysis (FEA) was used to assess the structural integrity of the chassis design. The loads applied to the chassis are equal to the forces applied by the weight of the processing equipment and support structures. Contour plots for deformation, Von Mises equivalent stress, and safety factor were generated. The safety factor for the weakest point in the chassis is greater than 2.0, indicating that the design is structurally sound.

Finite Element Analysis

•The economic analyses indicate that the proposed design would increase the net present worth (NPW) of the selected CAFO by $155 per head.

•The following assumptions were made for the proposed full-scale design:

•1000 head CAFO herd size

•Seven-year MACRS depreciation

•Higher annual CAFO revenue

•Higher initial capital equipment cost

•Lower annual anaerobic digestion cost

Economic Analysis

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Proposed Design

Conventional Design

The bench-scale model uses the same processes as the full-scale design for collection and separation of the as-deposited manure

Bench-scale Development and Testing

•Anaerobic digestion breaks down manure collected from CAFOs into methane, which is an efficient power source for running other operations

•However, hydrolysis of fibrous solids found in manure is the major rate-limiting step in anaerobic digestion and requires a long detention time for conversion to methane

•Overall efficiency of conversion of fibrous solids is low

•Elimination of fibrous solids from the digester influent will increase overall efficiency, reduce digester size, and lower operation and maintenance costs

Anaerobic Digestion of Manure

CH4 and CO2Hydrolysis Acid Formation Methane ProductionInsoluble Organics

Soluble Organics Organic Acids

•Vacuum collection

•Rotary Drum Vacuum Filtration

•In-line maceration

•In-tank agitation

•On-board liquid storage

•On-board solids storage

•Power take-off drive train

•User friendly interface