Application: used to remove solids from liquid and slurry manures, reducing their strength, improving their handling characteristics, and the manure’s odor generation potential
Pros
- Makes the liquid waste stream easier to handle and increases treatability.
- Reduces nutrient content of the liquid fraction, reducing transport distance.
- Increases life of manure storage by decreasing sludge build-up.
Cons
- Creates two manure streams, solid and liquid, that need to be managed and land applied.
- High investment in equipment and need frequent maintenance because of the wet, corrosive, and abrasive environment.
Description
Slurry manure handling is often used on confinement animal operations because of the ease of mechanization of these systems. However, choosing slurry manure management leads to greater odor generation potential than other options. Solid-liquid separation is the partial removal of organic and inorganic solids from animal manures. Solids separation creates two manure streams, a solids rich fraction that can be composted or anaerobically digested, and a liquid rich fraction that is easier to pump and handle. Solids separation reduces the amount of organic material remaining in the liquid fraction, thus reducing odor potential.
| Figure 1. Solid settling basins rely on gravity for separation to occur. |
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To be effective as an odor mitigation technique, the solid-liquid separation technique used must be capable of removing a substantial amount of organic solids from the manure. Research has indicated that beef manure has about 30 to 50% solids present,in comparison to swine manure that has about 10 to 15% solidspresent. One of the limiting factors of solid separation as a method of odor control is that the fine particles (smaller than 0.075 mm) contribute the most to the odor of the manure. These particles are some of the hardest to separate from the manure. Solids separation of swine manure tends to be less effective than in cattle manure because of the lower fiber content, and the finer particle size. To be most effective, separation of solids from the swine manure should be done as soon as possible after excretion.
| Figure 2. A mechanical inclined screen solids separator on a dairy farm. |
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One of the primary drawbacks of solid separation is that it results in two manure streams that need to be managed. The newly separated solids are generally still quite wet after separation, often with moisture contents close to 70%. If left this way, the solids will still undergo anaerobic decomposition and become odorous. Composting or immediate drying of these solids will reduce the odor generation potential. The odor generation potential of the liquid fraction is mostly dependent on the amount of organic substances remaining in it. In most instances, treatment of the liquid fraction with additional odor control methods becomes more effective or easier to implement after the removal of solids. For example, removing organic material from the manure slurry reduces its biological oxygen demand, making aeration easier to accomplish. Similarly, removal of this material reduces the buffering capacity of the manure, making acid treatments more effective at lowering the pH.
| Figure 3. Influent free falls over the inclined screen, separating the solids from the liquids. |
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There are numerous solid-liquid separation techniques that have been proposed. In concept they all try to physically separate and remove solids from manure. Techniques that have been attempted include sedimentation (settling by gravity), mechanical separation, and flocculants and polymers. Mechanical separations include inclined screens, vibrating screens, belt presses, and screw presses. Separators with few moving parts (inclined and vibrating-scares) are more effective when large amounts of water are moved through the devices at once. Centrifuge separators separate manure solids from liquids based on the differences in density between the materials. Centrifuge separators function best when treating slurries in the 5 to 8% solids range. Gravity separation systems are typically less costly to contrast and are capable of removing 50% of the solids from liquid manures. These systems generally are shallow basins with concrete floors and walls; settling basins should be designed to facilitate clean-out of the solids with a front-end loader.
| Figure 4. A centrifuge mechanical separator. |
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In some cases, it may be advantageous to use flocculants and polymers to increase the effectiveness of different separation techniques. The flocculent or polymer is added to alter the physical state of dissolved and suspended manure solids by bonding them together to form larger particles or to precipitate dissolved particles. Decisions about whether or not to use a polymer should be based on the cost of the polymer, the additional equipment requirements, and the level of solids removal desired.
Effectiveness
| Component | Reduction | Notes |
|---|---|---|
| NH3 | 0 to 10% | generally little nitrogen separation achieved |
| H2S | 0 to 20% | some sulfur is separate to solid fraction, but usually low separation efficiency |
| Odor | 20 to 30% | dependent on how effective organics removal was |
| Particulate Matter | -- | -- |
| Volatile Organic Compounds (VOC) | -- | -- |
| Cost | $ to $$$ | -- |
Cost Considerations
Solid-liquid separation requires an initial capital investment for purchase and installation of the separation systems as well as annual expenses for labor, maintenance, and system repair. Typical retail prices on mechanical separators vary from $10,000 to in excess of $50,000 depending on the type of separator chosen and the throughput capacity of the separator.
Snealth(1988) evaluated different combinations of aerobic treatment and separation for slurries containing 1.5 to 8% solids and farm sizes from 2000 to 8000 pigs, and storages of either 5 or 30 days. He found that using a brushed screen/roller press separator to remove solids before aeration produced the cheapest odor control. When longer storage intervals were required or farm size increased solids separation became more economical even for manures with low initial solids content.
| Figure 5. When storing separated solids it is critical to make sure that solids are contained and they aren't washed from the farmstead with heavy rainfalls. |
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References
Pain, B.F., Phillips, V.R., Clarkson, C.R., Misselbrook, T.H., Rees, Y.J., &Farrent, J.W. 1990. Odour and ammonia emissions following the spreading of anaerobically treated pig slurry on grassland. Biological Wastes 34(2): 149-160.
Sneath, R.W. 1988. Centrifugation for separating piggery slurry, 3. Economic effects on aerobic methods of odour control. J. Agric. Eng. Res. 39(3): 199-208.
Zhang, R.H. &Westerman, P.W. 1997. Solid-liquid separation of animal manure for odor control and nutrient management. Applied Engineering in Agriculture 13(5): 657-664.