Solution Strengths, Weaknesses and Critical Indicators
Screw press technologies:
- Produces fiber for bedding, soil amendment and compost
- Used extensively on dairies across the country to reduce concerns associated with storage and application
- Do not significantly reduce nutrients from the manure stream
- Can utilize many types of technology like slope screen, screw press, rotary drum and others
- Used before many other technologies to remove coarse solids
- Minimal maintenance and operator time required
- Proven technology for nitrogen recovery, phosphorous recovery, storage reduction, GHG reduction, and odor control but with limited impact in all cases
- Screw press solid-liquid separation is used to separate coarse solids with greater than 3% total solids (TS) content and perform better with manure that is higher in TS, like free stall barn slurry at ~ 10% TS.
- 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.
- Screw presses can be used with all bedding types, although they are not intended to separate bedding sand from manure.
- When located outside, screw presses are limited to temperate climates if intended for daily use year-round. In cool/cold climates, screw presses need to be in a climate-controlled space for use year-round.
Economic/Return on Investment Considerations
- The capital cost is higher compared to other methods of primary separation.
- Operating costs are also higher compared to other methods of primary separation with the main costs being electricity and replacement of screens.
- 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 recycle 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 course solid organic matter results in little to no crust on the storage and this can affect daily farmstead odor emissions.
- This technology can produce a bedding material when managed correctly
- Screw press technology is a mature technology with application across multiple industries, including manure treatment.
- The most common manure treatment technology used in the US with thousands on dairy farms.
- Models are available from multiple technology providers, each with decades of commercial application.
- Removal of organic matter in theory can contribute to manure storage odor reduction overall but in practice daily odor emission can be more noticeable.
- Volume reduction of 5 to 15% less volume entering the lagoon which reduces costly lagoon clean out and preserves volume for liquid storage.
- Reduce organic matter loading to storages/lagoons and total solids (TS) recovery of 20 – 50%.
- Reduce GHG emissions from long-term storages 30-40% because some of the volatile solids are captured and prevented from going into the lagoon, with subsequent solids storage more readily managed for aerobic conditions and control of GHG emissions occurring under anaerobic conditions.
- Separated fiber can be used for bedding.
- The solids / fiber fraction of the manure can be stacked and handled dry.
- If fiber is land applied, the carbon content and moisture retention of the soil is improved.
- Nutrient separation – about 10-30% 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?
- Thicker 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 top of the barn screw press separator. Screw press separators are normally elevated, so the separated solids can drop by gravity and accumulate in a pile for a short time, up to a few days.
- Raw or digested manure is forced into a wedge wire screen cylinder by an auger. A gate/door controls the discharge of manure from the cylinder and creates the back pressure required to force the liquid in the manure through the screen.
- The pressure created by the gate/door is controlled by weights or hydraulics, with the amount of pressure created in the cylinder by the gate/door determines solid dryness.
- Influent is evenly distributed over the slope screen using a distribution weir.
- 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.
Pretreatment and/or Post-treatment Required
- Does not require any pretreatment.
- Does not require any post-treatment.
- Does not work well with dilute manure.
- Does not work well with raw manure containing sand bedding.
- Does not destroy pathogens, with a fraction of pathogens remaining with solids.
- Additional storage and management considerations required for separated solids.
- Operational costs required to operate the electric motors and replace screens that wear out.
- Simple daily maintenance required for screw press and managing stacked solids.
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