Drum Composter / Bedding Recovery

Overall Summary

Primary Application

• More and more dairies are using recycled manure solids (RMS), also known as fiber, for bedding in free stall barns to reduce costs and complexity associated with manure treatment when sand or other bedding is used.
• Rotary drum composters are typically integrated with a solids separator that separates recycled manure solids (RMS) / fiber from raw slurry.
• Units are sized for dairies with hundreds to thousands of cows that harvest manure as raw slurry with typical solids content of ~10%.
• Vendors typically provide a few different sizes / capacities of screw presses or slope screens and rotary composters, depending on the number of cows.
• Once the bedding is spent, it is land applied.
• There are other uses for RMS such as being used as a replacement for peat moss in potting soils.

Economic/Return on Investment Considerations

• Capital costs are high.
• Operating costs are low because low horsepower motors are used for the solids separation and for turning the drum and heat is generated naturally from the biological process of composting.
• Return on investment can often be realized in a few years by cost savings from avoiding the purchase of sand for bedding, sand separation equipment and reduced manure handling costs like the periodic clean out of sand from slurry storage tanks and/or basins.
• Typically, producers use ~60% of RMS for bedding, leaving 40% that can be sold to other producers for bedding or for sale, usually to the nursery industry to be mixed with potting soil.

Industry Uptake

• Separated solids can be used either directly, after stacking for storage, after further treatment in a rotary drum, or through long-term dedicated composting.
• Thousands of dairies are separating solids, but we estimate that hundreds are using drum composters for bedding recovery / composting, but the adoption rate is increasing.

Technology Maturity

• Mature technology for both solid separation and drum composting.
• Recent advancements have been made for integrating the two technologies to improve the efficiencies of operation and throughput of RMS.

Primary Benefits

• Cost savings from avoided purchase of sand or other type of bedding such as sawdust.
• Decreased cost and complexity of managing sand laden manure.
• Decreased GHG emissions by up to 2/3rds because solids are kept out of the basin or lagoon where they would decompose and form methane and there are no GHG emissions associated with trucking of sand or other bedding material to the farm.
• Decreased pathogens by up to 2/3rds compared to RMS that are not composted.
• Slight storage reduction benefits (10%) by taking the RMS from ~ 30 – 35% dry matter to ~40 – 45% dry matter.

Secondary Benefits

• Potential revenue stream is created if a market can be found for composted RMS.
• Decreased carbon footprint of the dairy because manure is separated and composted for bedding.
• Some dairymen report improved cow comfort and cleanliness using RMS.

How it works

• RMS are captured after raw slurry goes through a screw press, slope or vibrating screen separator with rollers and typically comes out at ~30-35% dry matter.
• RMS drop onto a conveyor belt that feeds into a rotary drum composter.
• Residence time in the drum is usually 18 – 24 hours which is the time required for biological (natural) heating to occur and get the solids up to at least 55 ⁰C.
• Some vendors recommend that the solids stay in the drum until they reach 70 ⁰C.
• The drum drying process will lead to modest reduction in moisture (usually resulting in dry matter content of 40 – 45%), only a slight increase in dry matter content, but leading to significant reduction in pathogens and, therefore, presumably reductions in cow health concerns.
• This reduction in moisture results in both a slight volume reduction and improved bedding quality.
• RMS come out of the drum and are conveyed to a concrete holding area until being loaded and spread into the free stall barns.
• Newer systems are completely automated and require less than an hour daily for maintenance and oversight.

Pretreatment and/or Post-treatment Required

• No pretreatment is required for the raw slurry before going through solids separation.

Limitations

• These systems emit exhaust directly into the air with no emission control devices.
• Research into long term manure composting shows significant potential for volatilization of ammonia nitrogen (8-43%). Presumably volatilization of ammonia occurs with drum composting.
• Odor release is probable through emissions of volatile carbon and sulfur compounds.

Other Considerations

• Keys to success are a commitment to make RMS bedding work. There are many dairies that have converted successfully to RMS bedding and have not encountered significant increases in mastitis or somatic cell count.
• Justification for installation of the technology most often comes via cost savings and reduced complexity of manure management.