Composting

Overall Summary

Primary Application

• More and more dairies are using recycled manure solids (RMS) or composted solids, for bedding in free stall barns to reduce costs and complexity associated with manure treatment when sand or other bedding is used.
• Compost operations are typically integrated with a solids separator that separates manure solids / fiber from raw slurry.
• Operations are sized for dairies of all sizes that harvest manure as raw slurry with typical solids content of ~10%.
• When not used as bedding, compost is typically land applied or sold.
• There are other uses for composted solids, such as being used as a replacement for peat moss in potting soils.

Economic/Return on Investment Considerations

• Capital costs are moderate to high depending on the technology selected.
• Operating costs can be high because tractors and specialized turning equipment are used for turning the solids and there is usually one person who is responsible for this operation on a medium to large size dairy.
• When used for bedding, ~60% of the composed solids are required, leaving 40% that can be sold to other producers for bedding or for sale typically to the nursery industry to be mixed with potting soil.

Industry Uptake

• Composted solids require 4-6 weeks of curing before being used.
• Thousands of dairies are separating solids, but it is estimated that hundreds are composting for bedding recovery.

Technology Maturity

• Mature technology for both solid separation and composting.

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 up to 33%+ 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 (0-33%+) 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 and other uses.
• Some dairy operators report improved cow comfort and cleanliness using RMS solids for bedding.

How it works

• RMS are captured after raw slurry goes through a screw press, slope or vibrating screen separator with rollers.
• RMS are transported to the composing area and windrowed or passed through a rotary drum composter.
• Residence time in the windrow is usually 3-6 weeks depending on climate conditions, this is the time required for biological (natural) heating to occur and get the solids up to at least 55 °C. Primary composting through a rotary drum may be 2-3 weeks or less.
• Some vendors recommend that the solids stay in the windrow until they reach 70 °C.
• The composting 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.
• Some producers may pass the compost through a secondary composting cycle for additional treatment, further pathogen reduction and more stable product.

Pretreatment and/or Post-treatment Required

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

Limitations

• These operations 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%).
• Odor release is probable through emissions of volatile carbon and sulfur compounds during the turning process.

Other Considerations

• Keys to success are a commitment to making a composting operation 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 composting most often comes via cost savings and reduced complexity of manure management.