Rotary Screen

Overall Summary

Primary Application

• Rotary screen solid-liquid separation is used to separate coarse solids from diluted raw dairy manure.
• Solid-liquid separation is generally performed as a pre-treatment for subsequent manure treatment processes, reduce organic matter loading to storages/lagoons, reduce GHG emissions from long-term storages, facilitate pumping to distant fields/storages, and/or use of separated solids for stall bedding/compost media.
• Rotary screens fit on any dairy that can economically justify flush cleaning of barns and/or manure flume conveyance.
• Rotary screens can be used with all bedding types, although they are not intended to separate bedding sand from manure.
• When located outside, rotary screens are limited to temperate climates if intended for daily use year-round. In cool/cold climates, rotary screens need to be in a climate-controlled space for use year-round.

Economic/Return on Investment Considerations

• The capital cost is moderate compared to other methods of primary separation.
• Operating costs are also moderate comparted to other methods of primary separation with the main costs being electricity to run influent pit mixers, influent pumps, and motor(s) to rotate the rotary screen.
• Optimized influent pit design reduces mixing equipment needs and corresponding operating costs.
• As with all forms of primary solid-liquid separation, there is the potential for reduction of soil health due to diverting separated organic matter away from recycling to the land base, thus reducing the overall potential return on investment from a whole-farm system perspective.
• Another possible unintended consequence is increased odor emission from a long-term storage; removal of coarse solid organic matter results in little to no crust on the storage and this can affect daily farmstead odor emissions.

Industry Uptake

• Rotary screen solid-liquid separation technology has been used by dairy farms that employ flush cleaning of barns located in temperate climates but more so by farms to provide primary solids separation as part of an integrated manure treatment system.

Technology Maturity

• Rotary screen separation of dilute manure is a mature technology.

• Rotary screens are all basically the same with some variation in the screen size, solids conveyance fighting, and drum speed.

Primary Benefits

• Removal of organic matter in theory can contribute to manure storage odor reduction overall, but in practice daily odor emission can be more noticeable.
• Reduce organic matter loading to storages/lagoons.
• Reduce GHG emissions from long-term storages.
• Use of separated solids for stall bedding/compost media.

Secondary Benefits

• Pre-treatment for subsequent manure treatment processes.
• Nutrient separation – about 20% of the mass of the original manure nutrient mass is contained in the separated solids.
• Solid-liquid separation provides the opportunity for farms to more easily pump manure long distances thus reducing manure hauling with tanker trucks.

How it Works

• Rotary screen solid-liquid separation only works on very dilute manure slurries.
• Dilute slurry is aggregated from source barns in an influent pit.
• Agitators (pump or impeller) are used to homogenize pit contents.
• Submerged or shaft-driven centrifugal pumps convey pit contents to the inlet of a rotary screen separator.
• Rotary screen separators are normally placed at higher elevation, so the separated solids can drop by gravity and accumulate in a pile for a short time, up to a few days.
• Influent is injected either on the outer face or the inner face of the rotary screen based on the over design of the separator.
• The included screen bar allows liquids to fall through the screen while separated solids are conveyed by flighting affixed to the screen to a designated discharge location.
• The separated liquid gravity flows or is pumped to long-term storage of next treatment step.
• Separated solids are removed from pile normally by a payloader.
• Some rotary screens are outfitted with fresh water spray nozzles to prevent clogging of the screen with solids. They may also include brushes used for the same reason.

Pre-treatment and/or Post-treatment Required

• No pre-treatment is required if the manure is dilute.
• No post-treatment is required but many times is employed based on-farm goals/needs.

Limitations

• Manure must be dilute; rotary screen will not work with raw manure.

Other Considerations

• Rotary screens work best with continuous flow as residual manure solids left on the screen tend to dry and cake between screen uses. A washing system is sometimes used to combat this problem and if used, it can aid in the separation process.
• Manure solids generated by rotary screen separation are very wet; attention is needed to ensure the meaningful leachate volume is collected.