Solids-Liquid Separation of Manure
Ron Fleming

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Liquid manure typically has a total solids concentration of 3 to 6%. However, some farmers are handling liquid manure with over 10% solids. This range in moisture con-tent is due to such factors as method of storage, type of livestock, feeding program, and type and amount of bed-ding used (if any).

The ratio of solids to liquids can be reduced simply by adding more dilution liquid. However, this increases the volume of material that must be handled. Also, it does nothing to reduce the size of particles that must be han-dled. There are several advantages to reducing the amount of solids in the manure and at the same time reducing the particle size of the solids:

Less possibility of plugging transfer pipes - The larger solid particles in manure tend to build up on rough sur-faces inside a piping system. Sometimes this results in a plug being formed. The effluent from a solid/liquid separator has far less chance of this happening. Less power needed to pump the same volume of material - Pumps are rated on their ability to pump water. As the percentage of solids increases, so does the viscosity (resistance to flow). This makes the manure harder to pump. For the same volume of manure, more power is needed as the viscosity increases. Much easier to irrigate - Based on the previous two reasons, it is obvious that when manure must be pumped through pipe for several hundred metres, the lower the percentage of solids in the manure, the easier it is to pump. Also, there is less chance of plugging the pipes. Problems can arise when irrigating manure with high solids content due to high pressures in the pipe near the pump. The greater the percentage of solids in the manure, the higher this pressure must be in order to achieve the desired pressure at the irrigation gun. These high pressures can result in ruptured seals, leading to manure spills. Agitation time reduced - With separated liquid, very little agitation is needed prior to removal from storage. There is some settling of suspended solids, but these par-ticles are very small and are usually easy to stir up. Other uses for solids - Especially with the mechanical separators, the solids are relatively easy to store and can be handled with conventional solid manure equipment. Sometimes, these solids have other uses. Some farmers recycle the solids as bedding material. Others re-feed some of the solids to livestock, while others compost and sell the solids. Other uses for liquids - The separated liquids are ideal for in-barn flushing systems. Gutters may be flushed regularly using this liquid. It has a greater ability to remove solids than does unseparated manure. Odor control - Farmers with separators are finding that the total level of odor from the solid and liquid manure storages is less than the level of odor would have been with no separation. If the liquid is being treated for odor control (either chemically or by aeration, the job is much easier if the solids content is low.

Along with these advantages of separation, there are several disadvantages:

The cost is relatively high when considered on its own. Some mechanical separators cost around $15,000. To some farmers, this would be one of the most expensive parts of the manure system and it may be hard to justify the purchase. The mechanical systems are working in a material that is hard on machines. Regular maintenance is needed to avoid breakdowns. Extra space is needed to accommodate the system. Some of the mechanical units are compact and require very little extra space. On the other hand, the settling basins occupy about 20% of the area of the long-term storage. Two separate manure handling systems are needed -one for liquid and one for solids. While a farmer may have spreading equipment for the liquids, he may have to hire someone to spread the solids or purchase his own spreading equipment. Some systems have high energy costs to operate. The different systems have widely varying energy needs. Some may require no energy inputs while others may require several kilowatts of power. There is an increased management requirement. An extra step has been added to the manure handling system. Someone has to take time to make sure the system is operating properly. Again, this will vary depending on the type of separator. The separation of solids from liquid is usually, achieved in one of two ways: (a) by using the effects of gravity, or (b) by using a mechanical device.

Gravity Separation

Gravity separation, or sedimentation, is a fairly simple means of removing solids from manure. It involves a settling pond or basin. The flow of the liquid is slowed down to a point where solid particles settle. Larger, heavier solids settle first; floating or suspended solids won't settle. The liquid portion usually spills over or is pumped to a separate storage. A fairly high percentage of solids can be removed from the manure using this system. The settled portion has a high moisture content and is handled as a thick slurry. This system of separation is not common in Ontario but is used extensively in certain areas of the U.S.A. The most common application is with feedlots where large amounts of runoff have to be handled. The system can be used with other types of liquid manure, however. Figure 1 shows a settling basin which is part of a swine manure handling system.

Figure 1. Liquid/solid separation using sedimentation in 2-stage storage.

Mechanical Separation

Separation of solids and liquids by mechanical means has some advantages over the sedimentation method:

The separated solids portion has a lower moisture content and can usually be handled as solid manure. Most systems are better equipped to remove large floating or suspended particles. Less space is needed. There are basically two methods of mechanical separa-tion: (a) size separation by screening; and (b) size separa-tion by centrifugation. Because each of these has its own advantages and disadvantages, we often see equipment which incorporates the best of both systems. The perfor-mance of these units is governed by many factors and, as such, it is hard to generalize about the capabilities of mechanical separators. However, it is reasonable to expect the amount of solids removed to vary from about 40 to 80% . The moisture content in the separated solids portion will likely be between 70 and 90%. Through put rate will vary greatly, but on most farms will likely be in the range of 3 to 12 cubic metres per hour (approx. 700 to 3000 gal./hr.).

Following is an individual discussion of several systems available in Ontario.

Stationary Rundown Screen: Liquid manure flows onto an inclined screen. The fluids run through the holes, leav-ing the solids trapped on the surface. As the solids ac-cumulate, they move downward due to fluid pressure and gravity forces. The solids fall off the end of the screen and are handled as solid manure. One of the problems with the screen is that a biological slime builds up and plugs the openings. Frequent brushing is needed to keep the holes unplugged. The stationary rundown screen is the least ex-pensive mechanical separator.

Figure 2. Stationary run-down screen.

Vibrating Screen: This has a similar function to the sta-tionary screen. The difference is that a short, rapid reciprocating motion is employed to move the solids to the edge of the screen. The power requirement is higher with this system. Like the stationary screen, this system needs frequent cleaning to remove the slime build up.

Figure 3. Vibrating Screen Separator

Centrifuge: By spinning the manure, the centrifugal forces generated help to speed up the gravitational separa-tion of solid particles. Centrifuge systems can be very ef-fective at separating solids and can leave the solids with relatively low moisture levels. Initial cost is high, however and the energy requirement is quite high compared to most other systems.

Brushed screen with press-rolls: Figure 4 shows a system which is being used successfully on several Ontario farms. In the first stage, manure is separated using a screen. The screen is kept clean by a rotating brush which moves the solids on to the next stage. Here, a roller presses more li-quid out of the solids. The concentrated solids are then brushed out of the separator and are transferred to storage.

Figure 4. Brushed screen with press-rolls

Submerged screen: Figure 5 shows an example of a "batch" system of separation. The tank is filled with li-quid manure. Solid particles are trapped on the screen as the liquid passes through. These solid particles then act as an additional filter to capture even smaller particles. Even-tually, the tank is drained, the solids are removed, and the process starts over. The amount of solids removed, as with most systems, is highly dependent on the rate that manure runs through the system.

Figure 5. Batch separator with scraped submerged screen.

Other designs: Several other separator designs are on the market. Some of these include the vibrating screen with screw press, cyclone, roller press, brushed screen, rotary screw press, and the porous belt press. Each of these has its own merits which should be individually assessed when choosing a system.

Nutrient Analysis

To make the best use of nutrients in the manure, a farmer must know their concentration. Tables of values are found in several reference books. However, these have been found to vary considerably from one farm to the next. Also, these values have been determined for liquid manure and for solid manure, but not the separated li-quids and solids. The best way to find out the nutrient con-tent is to get the manure analyzed. This can be done for a nominal fee through the Agri-Food Laboratory.

Buying a Separator

Those selling separation equipment encourage farmers to picture the overall system of manure handling. It's easiest to justify installing a separator when a system can be set up which makes use of the strong points of the unit. For example, a typical system consists of a separator near the barn with the liquid effluent pumped into an earthen manure storage located in the middle of the farm. The storage is central and irrigation of the liquid onto the near-by fields is quite convenient.

A separator may be installed as part of an existing system to help solve a particular problem. If plugged pipes are a problem, for example, a separator may help to eliminate the problem.

Because of the many advantages and disadvantages of separator systems, and the variety of mechanical separators on the market, whether or not to install a separator must be a very individual decision.

This Factsheet was authored by: Ron Fleming, Agricultural Engineering Service.
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