Active Solids Drying

Overall Summary

Primary Application

• Typically, rotary drum dryers or belt/conveyor dryers are used with (RMS) / fiber / fine solids to increase the value and marketability of manure solids by decreasing:
  • Transportation costs.
  • Odor.
  • Pathogens.
• Active drying can also be used to improve the quality of fiber bedding for use in free stall barns but drying is almost always accomplished using heat generated during the natural composting process (See Drum Composter / Bedding Recovery Critical Indicator for more information).
• Drying systems are not common on dairy farms but when used, require significant scale to justify the costs (cow numbers in the thousands).
• For best and most economical results, drying is usually done on separated solids that range from 15-35% dry matter achieving final products with moisture ranges of 65 – 85% dry matter.
• Drying can be done with separated fine solids like those coming from chemical flocculation of dairy manure but this application is not yet common on dairy farms.

Economic/Return on Investment Considerations

• Capital costs for a unit are typically high.
• Operating costs are high because of the required purchase of propane, natural gas or electricity as the primary fuel source for heat for the dryer.
• Some dryers use waste heat from anaerobic digesters with combined heat and power generators and only supplement “purchased” heat when needed.

Industry Uptake

• Active drying on farms is rare with perhaps only a few dozen in operation because of the capital and operating costs and lack of developed markets for dried manure products.

Technology Maturity

• Mature technology that is commonly used in other industries.

Primary Benefits

• Improving the marketability of manure solids by reducing:
  • Odor by 2/3rds or greater.
  • Pathogens by 2/3rds or greater.
  • Costs associated with the transportation of water – removing moisture decreases costs.
  • Storage reduction of 80% plus because of the removal of moisture from manure solids.

Secondary Benefits

• GHG emissions, primarily methane, reduction by up to 1/3^rd.
• Low nitrogen recovery 0-33%.
• Phosphorus content is unchanged.
• The storage reduction volume of 80+% applies to solids, not the pre-separated liquids that go to long term storage.

How it works

• Active drying uses electricity or the combustion of propane or natural gas to create heat.
• Rotary dryers use a slow rotation that lifts the manure that then falls to the bottom of the drum exposing the manure to the hot air and hot metal of the drum, removing moisture, just as a clothes dryer removes moisture from clothing.
• Belt/conveyor dryers work by exposing the manure on top of slowly moving perforated belts to hot air that is moving upward or downward as well as removing moisture via contact with the hot perforated belts.
• The dried manure is typically discharged by gravity to a conveyor belt that transports it to a concrete pad.

Pretreatment and/or Post-treatment Required

• Screw press, slope or vibrating screen separation of fiber solids is typically done before active drying.
• Post-treatment depends on the end use. If retail, the manure is screened and bagged. If another end use, the dried manure is transported in bulk.

Limitations

• The main concern with using dryers is the cost and GHG emissions resulting from using fossil fuels for heat to dry the manure.
• A second concern relates to the gaseous emissions resulting from the drying. Dryers can be incorporated with exhaust control devices including cyclones, filters, scrubbers, and electrostatic precipitators to significantly reduce VOCs (odors), particulates and ammonia emissions. These control devices increase the capital and operating costs.

Other Considerations

• A successful project is where the capital and operational costs are offset by the revenue produced by selling a manure-based product that delivers value.

Development of the market for manure-based products is typically the key limitation.