Physical Manure Treatment
Physical manure treatment is the process of separating suspended solids from slurry or liquid. Separation is achieved by trapping the particles on a mechanical screen or sieve or by centrifugation; or in the case of gravity separation, with a basin that provides the proper conditions of velocity/retention to allow particles to settle.
Solid separation is used to:
- reduce the frequency with which solids must be removed from a storage or treatment facility
- facilitate the operation of a sprinkler irrigation system for land application of the separated liquid fraction
- allow the utilization of separated solids for such uses as and bedding
- remove some nutrients and volatile solids which can be an environmental concern
How Does This Practice Work?
Different physical mechanisms are involved in the separation process, and their effectiveness depends on the shape, density and size of the solid particles involved. The separation process normally involves screening, centrifugation or gravity. Sometimes pressure, such as with a series of rollers, is used to remove additional liquid from the solids.
Screening is a process where the liquid passes through the screen, and solids slide or are brushed/scraped off the screen surface. The size and shape of the opening in the screen determines the size of the particles separated. The stationary, inclined screen or a variation is the most common screening separator. The liquid waste to be separated is spread over the top of the screen. The liquid falls through the screen, and the solids slide off the bottom edge.
Other screen based separators, including the vibrating screen, drag flight conveyors, screw presses and rotary drums, rely on a motor to move the screen or solids to keep the solids moving. Centrifuge separators use centrifugal force to separate the liquid and solid fractions according to density, allowing the different fractions to exit the separator at different points.
Settling basins may be of earthen construction or constructed of concrete or concrete block. In the basin, the waste stream is slowed to allow gravity to settle the suspended particles. The denser the particles, the more quickly they settle. Normally the settling basin needs sufficient volume for the water velocity to slow to a maximum of 1.5 feet per second. The settling basin can be equipped with a device called a “weeping wall”, which is often constructed with wood, to allow for automatic dewatering, or liquid from the basin may flow by gravity into additional basins for further separation or into larger containment areas for longer term storage. Solids must be removed from the basin on a regular schedule to maintain the basin’s effectiveness.
Where This Practice Applies and Its Limitations
Solid separation applies wherever manure and other agricultural-related residuals are collected and either treated or stored. The separation process can occur either before treatment and/or storage or after treatment and/or storage. The solid separation process must be matched with pumps, pipelines and other components to ensure continued operation and maintain operational requirements, such as maintaining the necessary flow rates for optimal solid removal.
Some solid separators work well with high solid content liquid/slurry, and others perform best with low solid content. The method of separation must be carefully matched with the consistency and fibrous nature of the material. Separated solids may still retain a lot of moisture, and provisions must be made for retention of drained liquid, as well as the safe storage of the solids.
The effectiveness of mechanical separators is variable and is based on the type of separator, type of animal producing the liquid/slurry stream, total solids content of the stream and other less defined factors.
In dairy liquid/slurry systems, sloped screen separators remove 30-35% solids for single stage and up to 50% for two stage systems. Rotary drum and screw press separators remove 25% of solids, while centrifuge and belt press separators can remove up to 50% of solids. Settling basins and weeping walls can remove 50-80% of solids. In beef liquid systems, which is primarily runoff from lots, screen separators and roller presses remove up to 16% of solids, while centrifuge systems can remove up to 50% solids. Some settling basins for beef feedlots have been noted to remove up to 90% of total solids. In swine liquid systems, screens can remove 10 to 60% of solids, roller presses and belt presses can remove 20-30% of solids and centrifuges can remove 30 to 60% of solids. The addition of flocculating agents can improve the separation efficiency of most separators; however they can create a product that cannot obtain organic certification, which may be a concern in some cases.
The effectiveness of solid separation in removing nutrients in the solid fraction is not as well documented. From the limited data available, it appears mechanical separation can remove 20-30% of nitrogen (N) and up to 35% of the phosphorus (P) from the manure stream. Again, the actual amount of N and P removed will vary by separator, type of animal producing the manure stream, solids content of streams and whether a flocculating agent was used.
Cost of Implementing the Practice
The cost of mechanical separators can vary widely, ranging from $3,000 to greater than $1,000,000 depending on separator type, separation efficiency, size of the facility, and number of technologies put in place. Size is normally discussed in terms of throughput or volume per unit of time, such as gallons per minute. Ancillary components such as pumps, pipelines, stacking pads and reception pits are often needed in addition to the separation technology itself.
Settling basins are generally less expensive than mechanical separators due to their simplicity. Costs for earth settling basins are lower than those constructed with concrete and concrete block. However, costs can also vary based on local labor and materials pricing. Therefore, it is critical to ensure sizing meets the needs of the operation. Oversized settling basins may add additional temporary storage, but they will cost more to build and maintain. A settling basin will also usually require more land.
Mechanical separators, in general, produce a relatively consistent solids fraction that can be used as bedding in a dairy setting. Both the solid and liquid fraction can be land-applied to improve soil conditions and to supply the nutrient needs of a crop. Often the greatest return from the solid separation process is measured in the reduction of the cost to remove solids from the storage or treatment facility, such as the lagoon or storage pit.
Operation and Maintenance
Maximum flow rates are critical for the proper operation of mechanical separators and settling basins. If the flow rate in a settling basin exceeds the rate assumed in design, the hydraulic residence time may not be adequate to settle the solid particles. If the flow rate exceeds the screening capacity of a mechanical separator, the efficiency of the separator will be reduced, resulting in a too-wet solids fraction. Generally, the manufacturer’s information discusses minimum and maximum flow rates. Both the settling basin and mechanical separator operation are sensitive to the total solids content of the liquid/slurry stream. Typically, settling basins work best with a total solids content of 1 percent or less. Manufacturer’s literature for mechanical separators should detail the appropriate range of total solids in the waste stream.
Settling basins should be cleaned out regularly. The frequency of clean out is normally identified during the design phase and should be timed with opportunities for land application. The solid material should be tested for nutrient content and applied either on a N or P basis, depending on current rules and regulations. Many times, settling basins are built in parallel so one can dry and be cleaned out while the other is filling.
Solids, including precipitation of minerals, sometimes adhere to screening devices, and if allowed to dry can clog the screen, reducing the efficiency of the separation process. Rinsing the screen after each use will prevent this problem. Stacked solids beneath mechanical separators will often need to be removed to another location before end use. Life expectancy of solid separation components will vary with type of material, climate and frequency of use. With proper maintenance, a 10-year life should be expected.
Maintenance of settling basins should be similar to the maintenance of other earthen or structural components of the system. Concrete and concrete block structures should be checked frequently for cracks and premature degradation, and if problems are discovered, they should be corrected. Earthen basins should be checked for signs of leakage or slope failure, and corrective action taken if needed. Vegetation along the edges should be kept mowed, and weeds and woody vegetation removed. Any wood components should be checked for both natural deterioration and damage caused by humans or livestock and replaced when necessary. Screening devices are normally constructed using various kinds of metal. These devices should be checked regularly for deterioration of protective coatings and repaired as necessary. Many mechanical separators also involve the use of electric motors, pumps and gears. These should be routinely maintained as recommended by the manufacturer.
Manure-related facilities are associated with varying levels of safety concerns. Fences and warning signs should be employed as appropriate. Safety guards should be installed and maintained on all exposed moving parts, and all electrical components should be grounded. Accumulation of gases in waste facilities can lead to explosions or asphyxiation. No one should enter a confined space associated with manure without taking proper safety precautions.
More information on solid separation can be found in USDA-NRCS’s Agricultural Waste Management Field Handbook (AWMFH), Chapters 10 and 13. The AWMFH can be found on the Web at https://directives.sc.egov.usda.gov/viewerfs.aspx?hid=21430
Normally each land grant university will have information on solid separation techniques used in the local area.
For Further Information
Contact your local soil and water conservation district, USDA-NRCS or Cooperative Extension Service office. To find your local USDA Service Center, visit https://www.nrcs.usda.gov/contact/find-a-service-center.
North Carolina State University
Editing and Design
NC State University
University of Tennessee
Leytem, A. and S. Kulesza. 2023. Physical Manure Treatment (Solids Separation). SERA17 Phosphorus Conservation Practices Fact Sheets. https://sera17.wordpress.ncsu.edu/physical-manure-treatment/