Manure Processing Technologies 3.2 Separation

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1 Manure Processing Technologies 3.2 Separation Mary Wicks and Harold Keener What Is Separation? Separation is the dividing of materials into various components. For livestock manures, it usually applies to the separation of solids and liquids using physical or chemical methods. This paper will discuss physical separation methods, including gravity settling and mechanical separators; drying; and the use of coagulants and flocculants for chemical separation (Figure 1). P removal via struvite or lime precipitation, a chemical method for capturing phosphorus, is discussed in more depth in a separate fact sheet. Manure SOLID-LQUID SEPARATION Gravity settling Mechanical separation - Screen separators - Belt presses - Screw presses - Centrifuges Drying Chemical separation - Coagulants/flocculants Solids - bedding - compost - off-farm Liquids - flushing - irrigation Figure 1. Solid-liquid separation methods and products. Interest in physical separation is driven by enhancing the value of manure per unit volume and the ability to transport, handle, and process manure streams. Figure 2 shows estimated dry matter weight per cubic yard (cy) as a function of moisture content. Because weight per cy for manure is highly variable below 60% wb due to bedding content, porosity, and any prior processing (separation, composting, grinding, ), these numbers are only an estimate. Figure 2 also shows the value of the manure, assuming $100/ton ds, on a 1000 bushel grain truck (46.3 cy). Note, it is unlikely manure at or above 65% wb would be hauled in a semi-trailer used for grain (stickiness, flow able, density). For this scenario, a truckload at 10, 30, 50, and 70% moisture is valued at $1519, $1400, $1219 and $938, FOB origin, assuming nutrients per unit dry solids is same for all moisture levels. Since truck transportation cost are as high as $3/semi-truck mile (McNeill, 2013), separation or removal of water becomes very critical when considering transporting manure solids long distances. The high water content of livestock manures, especially dairy and swine, not only negatively affect handling and transportation economics but and can increase the risk of runoff. The first step in reducing the water content is to keep excess water out of the manure. Runoff from roofs or hard surfaces and milk house wastewater can be diverted to grass or other treatment areas rather than to manure storage. Water and misting systems should be inspected to identify leaking pipes and practices that may add excess water to the manure. Finally, animal diet may be adjusted to reduce salt levels which can affect water intake and (urine) excretion. To further reduce the water content, mechanical or chemical methods can be used to remove a portion of the liquids from the solid portions of manure, creating two waste streams with different properties. Separation may be used alone or in conjunction with another manure processing technology, such as removing some water to increase the solids content so that it can be composted or separating the solid and liquid portions of effluent remaining after anaerobic digester. Manure Processing Technologies 3.2 Separation Page 1

2 Factors affecting separation Solid content. The dry matter or total solids of manure is the solids left after its water is excluded (Wikipedia. 2013a). It is determined by a standard laboratory procedure of drying samples at o C for 16 hrs or until weight doesn t change (Wisconsin, 2013). For liquid manures, total solids are comprised of total dissolved solids (TDS), total suspended solids (TSS), and settleable solids. TS include ash, the mineral component, and volatile solids (VS) which decompose over time and they impart manure characteristics, such as particle size Figure 2. Effect of moisture content on dry matter density (lb/cy) and value per 1000 bu grain truck load for manure valued at $100/t dry solids. and nutrient and salt concentrations. TDS is comprised of dissolved inorganic and organic substances in solution or suspended form that will pass through a sieve size of 1.5 m (1/1,700 inch) (Hamilton and Zhang, 2013). TSS is solids in the liquid stream that exceed 2 m and will remain suspended in the solution indefinitely (Wikipedia, 2013b). Settleable solids are particulate matter or insoluble molecules which do not remain in suspension or dissolved form in a holding tank not subject to motion. Solids content dramatically affects the overall performance of separation equipment. Particle size. The distribution of particle sizes of solids will vary with the type of manure, feed, bedding, manure handling, and storage time. Zhang and Westerman (1997) reported data on particle size and chemical constituent distributions of dairy, beef, poultry, and swine from numerous sources. They concluded fine particles in the manure decompose faster than the coarse particle and most of the reduced carbon compounds, protein, and nutrient elements (especially nitrogen and phosphorous) are contained in fine particles. Because these compounds are the precursors for odor generation and the carrier of organic nitrogen and phosphorus, solid liquid separation process should be designed to remove fine particles effectively (at least smaller than mm) as well as coarse particles. Figure 3, adapted from Fleming and MacAlpine (2003), shows distribution of particle size for swine and dairy, as found in their studies on separation. Figure 3. Size of common particles found in liquids and slurries (Hamilton and Zhang, 2013) Manure Processing Technologies 3.2 Separation Page 2

3 The particle size will affect which separation method is most effective as illustrated in Fig. 3 for various particle types. Based on particle size of swine and dairy (Fig 4 ), many separation process should remove more than 80% of solids. In liquid manure storage ponds, a portion of the solids will settle out creating one or more zones with different properties. In general, deeper zones have larger particles. Type of manure. Liquid-solid separation is primarily used for swine or dairy manures which tend to have a high initial moisture content of around 88-92% (Bulletin 604). With these manures, the concentration of N and P Table 1. Characteristics of fresh manure (Bulletin 604). Manure Type N P TS % dry basis % dry basis #/animal/day Dairy Swine Figure 4. Particle size distribution for swine and dairy manure (adapted from Fleming and MacAlpine, 2003). Feed for swine was ground to 4.7mm (4700 m, 3/16 ). Note: Removal rates will vary with feed and manure handling practices. and the total solids produced vary with diet, animal size and stage of production, i.e. gestating or lactating (Table 1). Manure handling. Manure handling practices, such as using water to wash alleys or open storage systems, can further increase the moisture content. The addition of organic bedding, such as straw or sawdust, can reduce the moisture content and affect particle size. Storage time. Microbial activity occurs during storage, decomposing the carbon-based manure solids and bedding materials into smaller particles. During storage, conversion of N to ammonia (gas) may reduce the N content of the storage; however, covers and a ph of less than 9 can minimize emissions. P precipitates may dissolve at ph of less than 7 (Hjorth et al., 2010). Products of separation Separation produces solid and liquid waste streams. The nutrient and moisture content of each stream will vary depending on the manure and system used. Potential uses for each stream are discussed below. Solid stream. As P tends to form a precipitate or adhere to particles in manure, the concentration of P in the solid stream is often thought to be higher than in the un-separated manure. However, a greater percentage of P is associated with smaller particle sizes which may remain in the liquid stream (Moller et al., 2000). Thus, the transfer of P to the solid stream will vary depending on the particle size distribution in the manure and on the separation method. Moller et al. (2000) indicates DM fractions > 0.5mm will contain 5-7% of the TP. For watersheds with excess P levels, separation can improve transportation economics but the dry matter and nutrient content of the solid stream should be taken into account. Another use for the solid stream is to compost it and then sell it to value-added markets such as nurseries. On dairies, the solids can be used as a bedding, either before or after composting. All of Manure Processing Technologies 3.2 Separation Page 3

4 these uses have the potential to reduce costs or add revenues, thus offsetting the separation costs. Liquid stream. The nutrient concentration in the liquid stream may be reduced by X-Y% for P and Y-Z% for N, depending on the type of manure and separation method. The liquids can be land applied for irrigation; however, the nutrient content should be determined and best practices for applying liquid manure should be followed. In areas where there is not enough land for application, additional options may be needed, such as treatment of the liquid to allow discharge to streams. Separation systems The type of separation system needed depends on the factors discussed above and your goal. Separation systems differ in the amount and particle size of dry matter (DM) that can be removed, which can affect the concentration of N and P in the liquid and solid streams (Ackerman and Cicek, 2010). Although there are numerous companies offering separation systems, they usually fit within one of the five categories discussed below. Gravity settling. A typical settling basin is less than 3 feet deep and designed to allow settling and drainage of liquids from the structure. Figure 5 shows the key features of a concrete settling basin with a perforated pipe for liquid removal. Gravity settling is most effective for manures with TS of less than 6.5%, with larger particles settling more quickly (Ackeman and Cicek, 2010). Increased retention time will increase the settling of smaller particles, resulting in improved P removal. A current Canadian study is evaluating the effectiveness using a relatively inexpensive settling tank Figure 5. Concrete settling basin with screened perforated pipe (Bulletin 604). system (40 x8 x4 in depth) to remove P and solids from swine manure with an initial TS of about 5%. Preliminary data indicated that within 24 hour, P removal was 75% and solids removal was 50-70% in the top 3 feet of manure (Slevinsky et al., 2009). An earlier study on swine manure, in which the liquids were removed from a settling basin after 1 hour, found that about 60% of the solids were removed from the liquid stream. The removal increased to 75% when alum was added to the manure as it entered the settling basin. Analysis of the liquid stream showed that 38% of the phosphorus was removed, increasing to 75% with the addition of alum; whereas, only 20% of the nitrogen and 8% of the potassium were removed (Worley and Das, 2000). Inclined, vibrating, or rotating screen. Screen separators capture manure on one side of the screen while the liquid moves through it. For example, the inclined screen separator shown in Figure 6 captures the liquid stream below the screen while the solids slide down it and are collected at the base. The mesh size of screens vary, ranging from 1/100 to 1/10 in. (0.254 to 2.54 mm), with smaller openings typically used for swine manure and larger for dairy. Screen sizes smaller than 1/50 in. (0.51 mm) are more effective in removing P (Moller et al., 2000), but are more likely to clog and require more cleaning. A study by Powers et al. (1995) found that for dairy manure, a mesh size of 1/12 in. (2.1mm) resulted in 24% of total solids being captured Figure 6. Inclined screen at Fair Oaks Dairy, IN. but only 5% of P and 10% of N remained with the solid stream. The authors concluded that a smaller opening would not be practical. Manure Processing Technologies 3.2 Separation Page 4

5 Belt or screw press. Press separators use pressure to squeeze the water out of the manure (Figure 7). While the screen size can vary, the pressure may force the smaller particles through it and into the liquid stream, reducing the P concentration in the solid stream. One study found that as the TS content of dairy manure and bedding increased from 1% to 8%, the flow rate of the liquid stream decreased from 30 gpm to 6 gpm, respectively, during operation of a screw press. In contrast, the solids capture efficiency increased from 18% at 1% TS to 53% at 8% TS (Burns and Moody, 2003). Press separators may be used after other mechanical or chemical methods to further reduce the moisture content of the solid stream. Centrifuge. This method uses centrifugal force (G force) in which the manure is spun in a vessel, forcing the denser solids to move outward and leaving the liquids in the center portion. The decanter centrifuge is the most common (Figure 8). Compared to other mechanical methods, centrifuges can remove smaller particle sizes and, as a result, more P. However, the DM and P removed will vary with the type of manure and centrifuge. Increasing the rotational speed, which increases the G force, and the retention time can also Figure 8. Diagram of decanting centrifuge (Moller et al., 2000). increase the removal of TS and P, but it also increases the moisture content of the solid stream as well as operating costs (Hjorth et al., 2009; Moller et al., 2007). Drying. All or a portion of the liquids can be driven off as water vapor via evaporation from open ponds or heating of the manure. Evaporation from open ponds is most effective in a hot, dry climate and thus is not used on Ohio farms. Heating the manure to remove water can be effective in any climate; however, this method is usually not economically feasible due the energy costs for heating the manure. Table 2 gives cost to dry material to 0 and 10% moisture per ton of product produced and the weight loss per ton of wet material. Table 2. Energy in gallons propane to dry material per ton of product and weight loss per wet ton. Material Moisture % wb Dry 0% (60% efficiency) Dry 10% (60% efficiency) 1 gal/ton 2 weight $/ton 1 gal/ton 2 $/ton loss, % weight loss, % Evaporate water ~1108 Btu/lb w, Propane ~90,000 Btu/gallon; 2 Propane price = $2.30/gallon Figure 7. Solids expelled from a screw press at a NY dairy. Coagulants and flocculants. The smallest particles in manure slurry are suspended and negatively charged, thus repelling each other. Coagulants change the chemistry of the slurry so that these colloidal particles can stick together to form larger particles, or flocs, which can then improve the effectiveness of sedimentation or mechanical separation methods for removing DM and P (Hjorth et al., 2010). There are two categories of coagulants: (1) metallic salts, such as alum sulfate, ferric chloride, and lime; and (2) Manure Processing Technologies 3.2 Separation Page 5

6 Separation process Manure Handling cationic polymers, such as polyacrylamide. Metallic salts can be effective in removing phosphorus but form a gelatinous cake which can clog separation screens. Some are caustic, which can be harmful to animals, plants, and equipment. Cationic polymers are not toxic to animals or plants but are not as effective in removing phosphorus (Hamilton, 2010). Daily management of separation systems A review of six different separators found that all systems required an operator, but all could be fully automated to reduce labor needs (Fleming and MacAlpine, 2003). However, labor would be required for cleaning filters and screens and for routine equipment maintenance. Monitoring of pumps delivering manure and of systems for removal of separated solids and liquids would also be needed. What are the Components for Separation? The components of a composting system will vary depending on the type of system used. Table 3 lists the key components. Table 3. Separation system components Process Step Component or Purpose Utilization 1 Manure collection at farm Manure to separation system Separation equipment Separator housing Material handling Separated solids Separated liquids Solids for bedding Solids for compost Liquid stream Collect and transport manure to storage. May require a pump to move liquid manures. System should consider type and volume of manure and characteristics required for solid and liquid streams May need cover and/or impervious surface. Truck or manure spreader and bucket loader for transporting manure and for building windrows or piles. Covered storage preferred. Truck or manure spreader and bucket loader for transport. Storage needed. Pump or gravity feed for transport May want to compost to reduce moisture and pathogens. Consistent particle size of finished compost, depends on feedstock and markets. Bagging system may be required, depending on market. Irrigation of cropland and/or re-use for flush water. What are the Benefits and Limitations of Separation? As discussed, the primary benefit of liquid-solid separation is that two streams are created which have different properties and can be handled individually. Table 4 outlines other the potential benefits and limitation of liquid/solid separation. For all criteria, it is assumed that the liquid stream is stored in the manure storage pond and the solid stream is used for bedding, composted, or moved off farm. Note that anaerobic decomposition of organic matter in manure is the primary source of odors. Manure Processing Technologies 3.2 Separation Page 6

7 Table 4. Benefits and Limitations of Separation CRITERIA BENEFITS LIMITATIONS Odors Liquid stream has less organic material thus less material to decompose anaerobically. Solids can be composted, which can reduce odor. Lower separation efficiencies will result in more solids in the liquid stream which create odor as they decompose. For the solid stream, high moisture levels or compaction may lead to increased odors. Storage pond Energy required Increases capacity and time between cleanouts as volume of material and accumulation of solids is reduced. Electricity for pumps and separation equipment. May require separate structure for storage or composting of solids. Vehicle fuel for transporting and turning materials. Time required Typically can use existing labor pool. Varies depending on system and volume of material. Additional time for composting or marketing solids may be needed. System reliability Operator training? Finished materials Solids Liquids Gravity settling only needs a pump to decant liquids and periodic removal of solids. Nutrients concentrated in solid stream can be more economically moved offfarm. May be used for bedding Composting increases potential markets. Reduced nutrient content. Use for nutrient amendment and/or irrigation. Maintenance and repairs needed on separation equipment. Strongly recommended. Additional time may be needed for management. Nutrients in will vary with manure and separation system used. Economics Capital costs EQIP cost share or grant funding may be available. Operating costs None Additional labor and maintenance costs. What is the Income Potential of Separation? It is important to consider both capital and operating costs as well as revenue streams for different separation systems. For example, a study by Moller et al. (1999) that compared a screw press and centrifuge found the total yearly cost of the centrifuge was 5 times higher, but the TP in solid fraction from the centirufge was also higher. Thus, if the value of the TP in the solid stream is considered, the total yearly cost of the centrigue systems is only 25% higher. Capital costs: The capital costs for separation will vary depending on the type of system. The key components of separation systems are described in Table 4. Below is information that should be considered when evaluating the feasibility of a separation system. Type of system. In choosing a separation system, manure characteristics, total volume to be processed, throughput capacity, and separation efficiency should be considered. The characteristics required for Manure Processing Technologies 3.2 Separation Page 7

8 the solid and liquid stream, such as TS and nutrient content, will also help determine which system is needed.. Screen separators are generally considered the least expensive with centrifuges having the highest cost. Feeding pumps. Pumps may be needed to feed manure into the separator and to move separated liquids to a storage system. If liquids are to be used for flush water, additional pumps and conduits may also be needed. Storage facilities. Additional storage will be needed for the solid and liquid streams. Land application equipment. If solids are to be land applied, a solid manure spreader will be needed. For the liquid stream irrigation or liquid application equipment will be needed. Operating costs and revenue streams: Typical operating costs are discussed below. Electricity. Electicity will be needed for pumps and the separator. Screen separators are generally considered to require the least amount of electricity as it has no or few moving parts; whereas, centrifuges required the most electricity Labor. Labor will be needed for operating and maintaining the system. Depending on the previous manure handling practices, this many not require an increase in staff or time. Maintenance. Time and materials will be needed for cleaning and repairing separation and related equipment. Off-farm sales. Revenue can be generated from the sale of raw or composted solids for land application or as a soil amendment. Although additional costs for producing composting, it should have a higher value. Offset bedding costs. Re-ue of separated solids for bedding in dairies will reduce the costs of purchased bedding. How can Revenues be increased? The separated solids can be composted for value-added sales to markets such as nurseries, greenhouses, landscapers, or golf courses. However, it is important the produce be high quality and consistent. Additional Resources extension: Acknowledgement This project was funded by the USDA-NRCS Conservation Innovation Grant program with additional financial support from the Ohio Soybean Council. Disclaimer Any specific company or process mentioned in these documents is for informational purposes only and Manure Processing Technologies 3.2 Separation Page 8

9 should not be considered an endorsement. References: Ackerman, J., N. Cicek Evaluation of the opportunity for manure treatment/processing technologies to achieve manure phosphorus balance. University of Manitoba, Canada. (Online at Accessed 2/7/13.) Burns, R.T. and L.B. Moody Development of a Standard Method for Testing Mechanical Manure Solids Separators. Paper presentation no , 2003 ASAE Anunal International Meeting, Las Vegas, NV. July 27-30, Fleming, R. and M. MacAlpine Evaluation of Mechanical Liquid/solid Manure Separators. Prepared for Ontario Pork, Project No. 02/39. Ridegetown College and University of Guelph, July (Online at Accessed 3/18/2013.) Hamilton, D Solids Separation in Swine Manure Handling Systems. extension publication, April 26, (Online at Accessed 9/27/2013.) Hamilton, D., H. Zhang. Solids Content of Wastewater and Manure. Oklahoma Cooperative Extension Service. Publication BAE (Online at Accessed 2/7/13.) Hjorth, M., K.V. Christensen, M.L. Christensen, S.G. Sommer Solid-liquied separation of animal slurry in theory and practice. A review. Agron. Sustain. Dev. 30 (2010) Hjorth, M., K.V. Christensen, M.L. Christensen, S.G. Sommer Solid-liquid separation of animal slurry in theory and practice: A review. Agron. Sustain. Dev. 30(2010) Langer, J.M., E.L. Bilbeau Properties of Naturally Settled Hog Manure in Outdoor Lagoons. Transactions of the ASABE. Vol 52(6): McNeill, S. Grain Hauling Cost. Calculator Accessed 3/11/2013 Moller, H.B., J.D. Hansen, C.A.G. Swensen Nutrient recovery by solid-liquid separation and methane productivity of solids. Transactions of the ASABE, vol 50 (1): Moller, H.B., L. Lund, S.G. Sommer Solid-liquid separation of livestock slurry efficiency and cost. Bioresource Technology, 74 (2000) Powers, W.J., R.E. Montoya, H.H. Van Horn, R.A. Nordstedt, R.A. Bucklin Separation of Manure Solids from Simulated Flushed Manures by Screening or Sedimentation. Applied Engineering in Agriculture, Vol. 11 (3): Manure Processing Technologies 3.2 Separation Page 9

10 Sheffield,, R.E Separating Manure Solids. Online at Accessed 5/2/2013. Slevinsky, L., G. Plohman, G. Racz Solid/Liquid Separation of Hog Manure Utilizing a Settling Tank. Report presented to the Manitoba Rural Adaptation Council Inc. (Online at %20Final%20Version%20Settling%20Tank%20Report%20%20-%20Feb%2013%20edit%202.pdf. Accessed 3/5/13.) Texas A&M Wikipedia. 2013a. accessed 3/11/2013 Wikipedia. 2013b. Wisconsin accessed 3/11/2013 Worley, J.W., K.C. Das Swine Manure Solids Separation and Composting Using Alum. Applied Engineering in Agriculture Vol. 16(5): Manure Processing Technologies 3.2 Separation Page 10