4 Advice to Choose a Manure Solid Liquid Separator

Many, if not most, systems designed for manure treatment depend on reliable solid-liquid separation. What is solid-liquid separation for manure and when is it something to consider doing for your farm?

View Details

Defining Manure from a Solid-Liquid Separation Perspective

Manure can be thought of as a mixture of water, minerals, and organic components. Some of the minerals will be soluble and dissolve in the water, while the rest will tend to settle to the bottom or float to the top of the manure storage. The density of the organic components will determine whether they settle to the bottom of the storage unit, remain in suspension, or form a floating crust. The amount of water, source of manure, and manure handling system will determine the extent of these trends. More on solid-liquid separation’s role in manure storage….

The single most important factor affecting the tendency to separate is the amount of water in the manure. With low moisture (solid) manure, very little apparent separation takes place. As the moisture content increases the tendency for separation also increases. When characterizing manure’s physical characteristics it is often thought of as being a solid, semisolid, slurry, or liquid. From a practical perspective separation is limited to liquid, slurry, and sometimes semisolid manure.

When is it Desirable to Separate Solids and Liquids?

Whether the tendency for manure to separate into liquid and solid fractions is desirable depends on the objectives of the manure handing systems. In concrete pits and holding ponds it is usually undesirable because the manure often needs to be remixed to remove settled solids and obtain uniform material for land application.

In other situations this tendency for separation is desirable. If some of the land application sites are significant distances from the manure source, being able to concentrate manure solids reduces the volume and expense of transportation. In some areas of the nation phosphorus application rates are an environmental concern. Separating the phosphorus with the solids while leaving most of the nitrogen with the liquids may be an option. After separation, the liquids could be land applied near the manure source as a nitrogen fertilizer. The solids could be transported and applied to fields in need of phosphorus fertilizer.

Benefits of Separation and Uses for the Components

Even if transportation distance and phosphorus application rates are not a concern using solids separation prior to storage reduces the solids accumulation in holding ponds and lagoons. This increases the time between sludge removal operations and maximizes the liquid storage capacity of the holding pond or lagoon.

Another potential benefit is the reduction in odors. Under the anaerobic conditions found in holding ponds and lagoons microorganisms produce odorous compounds. Reducing the amount of available manure with solids separation tends to reduce the production of objectionable odors.

In animal confinement systems where water from holding ponds and lagoons are used to flush manure from the barns, solids separation can result in a better quality recycle flush water.

The separated manure solids have a reduced moisture content and increased nutrient concentration both of which increase its value as a fertilizer source. In addition, depending on the resulting moisture content it becomes a potential compost ingredient. Where the resulting compost may be used as bedding or potentially marketed off farm.

Manure separation is also likely to play a critical role in preparing manure for conversion in into energy. Different energy conversion technologies will require the manure to have different physical and chemical properties.

Solid-Liquid Separation Options

There are a lot of different options for solids separation. Some, like settling basins, use gravity and time to allow manure solids to settle out of solution. These systems are a non mechanical low tech approach with the resulting solids typically being a wet slurry. In contrast the mechanical approaches such as inclined or vibrating screens, belt or screw presses, centrifuges, and many others result in separated solids that while still having significant moisture can be handled and stored as a solid material.

Often the separation process can be enhanced through the use of chemicals. Some chemicals work by converting soluble mineral compounds into insoluble compounds. Other chemicals work by causing smaller particles to clump together into larger particles. Depending on the situation these chemicals may be used by themselves or combined.

Recommended Reading About Solid-Liquid Separation

• SERA-17 publication, Physical Manure Treatment (Solids Separation) Advantages of Manure Solid-Liquid Separation, Alabama Cooperative Extension
• Mechanical Solid-Liquid Separation of Livestock Manure Literature Review, Ridgetown College – University of Guelph (Canada)
• Solid-Liquid Separation of Animal Manure and Wastewater Texas AgriLife Extension Service

Author: Karl VanDevender, University of Arkansas

Separating manure can increase management options for farmers when they use the products.

In the simplest separations, manure is separated into one liquid and one solid stream. In this case, the solid stream can contain a higher concentration of solids and nutrients, and the liquid stream becomes more dilute.

Separation systems can increase in complexity where additional steps further separate these streams producing multiple products with differing characteristics. With any separation system, the division of the original manure into multiple new streams can be useful as it allows for increased management of the new products or potentially can produce a product with more value.

Sifeng Product Page

When you separate manure, the solids and nutrients within the manure are separated into each of the new streams. However, it is important to know that solids and nutrients do not separate evenly or at the same rate.

For example, when you use a screw press separator, where the manure is separated into one liquid and one solid stream, typically more phosphorus is moved into the solid stream than nitrogen.

This means that the nitrogen to phosphorus ratio from the original manure generally decreases in the solid stream (higher phosphorus concentration in that stream) and increases in the liquid stream.

In a recent assessment of screw press separators in Wisconsin, generally if your original manure stream has a 3:1 nitrogen to phosphorus ratio, the new separated streams produced commonly result in the solid manure with a 1:1 nitrogen to phosphorus ratio and the liquid streams 4:1, or even higher depending upon the systems performance. Therefore, you may want to alter where the manure is applied depending upon the characteristics and crop needs.

In addition to the manure nutrients separating differently, the nutrients in each stream may be in different forms. For example, in the same screw press separation system discussed above, the nitrogen in the solid streams is mostly organic nitrogen.

This means that the nitrogen in the solids may not be immediately available, which is important when determining supplemental nitrogen. The higher available nitrogen in the liquid streams may require additional management, such as injection or incorporation, to reduce ammonia losses and retain the nitrogen for the crops.

The various separation systems available on the market can result in different amounts of nutrients in solids in the separated streams, this is also referred to as separation efficiency as discussed above. It is important when selecting a piece of equipment to ensure you can determine the separation efficiency for each manure component separately.

For example, the separation efficiency for the solids, nitrogen, and phosphorus are all different. Understanding the differences will help you better select the technology that reaches your desired outcomes. Many systems may have very low separation efficiencies for specific manure components that may not result in changes to the product characteristics that do not make the costs of the system worth the investment.

Separation systems can be as simple as a settling basin. In these systems, solids settle, and the liquids can be drained away giving you two streams. Generally, you integrate more system components to increase separation efficiency of solids or a particular nutrient.

A screw press is a mechanical separation system that can produce a solid stream with less liquid that a settling basin. There are other mechanical separation systems, such as a rotating drum, that also separate into one liquid and solid stream but have different separation efficiencies than a screw press (traditionally used with manures that have less solids coming into the separator).

In more complex mechanical systems, you can integrate additional steps. Some systems integrate several separation steps where one of the products in the final separation is clean water.

It is important to remember there are still several other products produced that must be managed with varying characteristics. When selecting a separation system, it is important to identify your goals and find a system that best meets those goals.

For example, if you are trying to separate solids to apply to fields with a high groundwater table, a simple separation system may be the most cost effective. However, if you are trying to separate a significant amount of a specific nutrient something more complex may be necessary.

The key to getting the most out of any separation system is understanding your goals before you purchase, selecting the system that can achieve those goals most cost effectively, and then adjusting the system when installed to increase the efficiency it terms of your desired outcome.

Larson is an Associate Professor and Extension Specialist, Biological Systems Engineering with the University of Wisconsin-Madison.

Want more information on Manure Solid Liquid Separator? Feel free to contact us.