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?
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.
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.
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.
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.
Author: Karl VanDevender, University of Arkansas
There are a number of benefits that can be achieved by incorporating one or more solid-liquid separation methods on your dairy.
If your dairy is bedding on sawdust or another similar form of organic bedding, adding a manure separator may allow you to create your own bedding source. This can be achieved with either a Screw Press or Roll Press system. The separated manure fibers can be mixed with your bedding or used by themselves. If used by themselves, adding a Bedding Dryer can really help with reducing moisture and bacteria levels.
Dried manure solids discharging from a McLanahan Manure Bedding Dryer.
Manure can be extremely challenging to process simply because it is so unpredictable. The manure can contain ear tags, long fibers, string and concrete chunks, to name just a few. This foreign material wreaks havoc on pumping systems and downstream processing equipment. Installing a manure separator removes all this unwanted debris, as well as manure fibers, immediately and in a more manageable manner. With the manure fibers and debris removed, the remaining effluent can be transferred via pumping system to trucks or fields with reduced risk of plugging pipes, pumps and nozzles. In some cases, it allows for the irrigation of water and nutrients that wasn’t possible before.
In many cases, supernatant (the cleaner portion of the manure water) is recycled back from storage lagoons for flushing cow lanes, cow holding areas and flume pipes. Adding manure separation equipment prior to manure storages reduces the organic loading in the lagoon, which yields higher quality return water to the barns. Often, water can be reused immediately from the separation equipment, eliminating the need to pump water back and forth from barn to storage.
Processing your manure with a form of solid-liquid separation can reduce the required lagoon size for your dairy. It also reduces the organic loading going into the lagoon, which makes it less energy intensive to remove. Cornell University’s Pro-Dairy program published a paper studying several mechanical solid-liquid separation methods and concluded that separation can reduce required lagoon size by up to 30%.
Incorporating solid-liquid separation allows nutrients to be more strategically placed. By removing the large fibers and unwanted debris from the manure, the remaining liquid containing the bulk of the nutrients can be transported, transferred and applied with greater flexibility. The liquid effluent no longer needs to be hauled to fields with tankers and trucks; it can be pumped and applied strategically with irrigation and injection systems. The separated fibers, due to their lower bulk density, can be hauled further at a lower cost.
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There are also some disadvantages to solid-liquid separation that can't be overlooked.
The separated fibers will need to be periodically moved and stacked. This is accomplished on most dairies with either a skid-steer or large loader. The amount of time needed depends on the size of the dairy but can sometimes take multiple hours per day. Also, instead of hauling the manure with a single style piece of equipment, usually a manure tanker, a box spreader is needed to haul and spread the separated fibers. If the dairy doesn’t already own one, this can be an additional capital expense.
Adding separation to your dairy will increase your operation and maintenance costs. In most cases, it means adding at least one electric motor, usually two when an additional pump is needed. There’s also the additional labor that’s required to periodically clean, inspect and fix the components in the system. These increased costs cannot be ignored.
This capital cost goes further than just the equipment. There are concrete, structures, supports, plumbing and electrical costs to account for too. In cold climates that get below freezing, a building with additional heat is required to keep the equipment operational during the winter months.
There are multiple ways to accomplish manure separation. These methods range from simple to very complex ways to remove fibers and nutrients. Several of the more common and relatively simpler methods are described below.
This technology can be used across a wide range of industries. It can be found processing food waste, screening wastewater and sorting minerals. This relatively simple technology uses a rotating drum composed of mesh or perforated plate. The material to be processed can be introduced either to the inside (internally fed) or outside (externally fed) of the screen. Particles are captured on the screen while liquid and fines travel through. Spray bars are common, and brushes can be added to help keep the screen clean. These screens provide great separation for manure systems with potential for bypass sand.
A McLanahan Rotary Screen.
Inclined screens, sometimes called sidehill screens, banana screens or slope screens, are, as the name implies, large, flat or curved screens that are set on a specific angle close to vertical. Material is pumped through the top and allowed to cascade down the screen. Fibers are separated and dewatered on the face of the screen before falling off. One of the advantages of this screen is no moving parts. Some screens incorporate wash bars and vibrating motors to aid in screen cleaning. The large advantage is no electrical requirements.
There are multiple variations to this type of screener, but they all use a vibrating motor to move the screen in either a circular or elliptical motion to aid in particle separation and impart movement to the separated material. These screens have a high processing capacity for the relatively small footprint. These screens require periodic cleaning.
These systems use a drag chain located within an inclined trough, the bottom of which is built from screen material. The chain is slowly pulled upward toward the discharge. As it is pulled upward, it cleans away fibers that are captured on the screen and drags them to the discharge.
Roll Press separators are a secondary separator used in series with a Rotary Drum to dewater thickened manure. Material is fed between two rotating rolls, which apply a high pressure zone between them. One of the rolls contains small openings, which allow water to drain through while retaining fibers. These machines have a small overall screening area, so they aren’t used for primary separation.
A McLanahan Roll Press.
Screw Presses are compact separation and dewatering machines. They contain a rotating screw shaft within a screen. As material is conveyed into the body of the machine, the screw advances it forward past the screening surfaces. Fibers that are captured on the screen are wiped off and advanced forward by the screw. As these fibers accumulate at the discharge, they form a “plug” of material that is highly compressed and dewatered. These versatile separation machines provide great separation in a compact footprint. Due to high tolerances and high pressure zones, they are prone to abrasive wear.
A McLanahan Screw Press.
These methods are effective at removing a portion of the larger fibers and material that will settle with gravity. Essentially, manure is conveyed into a large settling area that creates retention time sufficient for the material to settle. In the case of a weeping wall or variations of them, there are areas in the walls or floors that allow liquids to drain through. Fibrous material tends to blind over the drainage areas rather quickly, rendering them ineffective for dewatering. These systems can be labor-intensive during cleanout.
Weeping walls allow liquid to drain through.
More complex methods include centrifuges, ultrafiltration membranes, disc presses, belt filter presses and chemical dosing systems. These complex methods are generally more effective at removing solids but require higher capital expenditures and operating costs. These higher costs must be weighed against the benefit of greater solids removal.
Adding a manure solid-liquid separation system on your dairy is often a great way to manage your manure nutrients more strategically and allow you to implement methods of manure transfer and application that are more economical. There are multiple methods for achieving the manure separation goals of your dairy, so look carefully before choosing the method that is best suited for your dairy's needs and management style.
For help selecting a manure separation system that's right for your dairy, contact McLanahan.
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