Centrifuge

Overall Summary

Primary Application

• Dairy farms with over 1,500 cows or smaller dairies with a need for fine solids and phosphorus separation.
• Sand bedding should be removed prior to centrifugation to avoid premature equipment wear.
• Recommended for manure slurries with total solids concentrations 6% TS or less.

Economic/Return on Investment Considerations

• The capital cost of the technology is in the medium range compared to other solid liquid separation technologies and in the low range for phosphorus and fine solids separation equipment.
• Operating costs are in the medium range compared to other separation technologies, comprised primarily of preventative maintenance and electricity.
• Using polymer to enhance phosphorus separation increases the operating costs, but the extent is variable depending on the polymer dosage and cost.

Industry Uptake

• It is estimated that approximately 50 systems are installed on dairy farms in the U.S.
• Applications of the technology are primarily for phosphorus and fine solids removal from liquid manure.
• Solids can also be used as a thermochemical energy source (i.e. gasification and pyrolysis).

Technology Maturity

• Centrifuge is a commercially available, mature technology.
• Worldwide, thousands of centrifuge systems are installed for concentrating and dewatering solids in municipal, industrial and agricultural waste streams.
• Systems can be configured with or without polymer. Systems using polymer have higher phosphorus and solids removal rates, but with a significant increase in operating costs.

Primary Benefits

• The primary benefit of centrifugation is the removal of phosphorus and solids from liquid manure.
• Centrifuge systems can remove 40 to 60% of phosphorus and total solids without polymer and up to 80% or more of the phosphorus with polymer additions.
• Total nitrogen removal from the liquid manure is in the range of 20 to 40%, with lower end without polymer and higher end with polymer.
• Due to the removal of solids, the volume of the liquid manure (centrate) is reduced by 10 to 20% following centrifugation.
• Removal of solids from the liquid manure reduces greenhouse gas (GHG) and odor emissions, as less organic matter is held under anaerobic conditions during storage.
• Pathogens are to some extent partitioned into the solids, thereby reducing overall pathogen count in the liquid, although pathogens are not destroyed in the process.

Secondary Benefits

• Centrifuge removes finer manure solids compared to other primary solid-liquid separators.
• Manure fiber has been used by many dairies as a bedding material.
• Relatively low moisture content coupled with high carbon, phosphorus, and nitrogen content makes separated manure fiber a valuable fertilizer, soil amendment, and/or compost input.
• Coarse and fine solids removal, achieved by centrifuge, is an essential component of clean water systems.

How it works

• The manure slurry is pumped into the centrifuge where it enters the bowl and is exposed to high centrifugal forces resulting from the bowl of the machine spinning at 2,000 to 4,000 RPM. Due to the high-speed rotation and centrifugal forces, manure solids are forced to the outermost edge of the bowl and are moved to the discharge by a stationary scroll or auger. Liquid flows through the machine and is discharged.
• Centrifuges operate in batch, semi-continuous and continuous modes depending on style and manufacturer.
• For batch or semi-continuous operations, the centrifuge will stop or slow to allow for the discharge of capture manure solids.
• Separated manure solids are typically referred to as manure solids or manure fiber and are generally stored as a solid prior to reuse as bedding, land application, or export for other purposes.
• Liquid effluent from the centrifuge is called centrate or ‘tea water’ and moves on to liquid storage or further treatment.

Pretreatment and/or Post-treatment Required

• The only required pretreatment is sand separation for sand bedded dairy farms.
• Some systems may have grit separation installed beforehand to remove larger debris such as stone, concrete, and metal that could damage the centrifuge.
• Post-treatment requires storage of the products, a thin manure liquid and a stackable manure solid.

Limitations

• Limitations of the technology are primarily operation cost (energy, maintenance, and chemicals) and complexity.
• Centrifugation is not considered an effective tool for separating nitrogen or potassium.

Other Considerations

• Successful operation of a centrifuge requires a dedicated maintenance plan.