Separating manure can increase management options
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.
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.