Estimating the Efficiency and Cost of an On-Farm Centrifuge Separator

Solids and nutrient components in liquid manure complicate manure-handling processes. They present at least four concerns:

1. Reduced lagoon capacity. Some of the solids in liquid manure that flow into a storage lagoon settle on the lagoon floor. Eventually, they need to be removed to increase lagoon capacity. However, their removal is costly, an added financial burden for operators.
2. Limited land application rate. Liquid manure with high solids and nutrients (especially phosphorus [P]) limits the land application rate, which means that larger areas are needed to apply the same amount of liquid manure. Consequently, operators must transport large volumes of liquid manure longer distances, steadily increasing costs.
3. Clogged equipment. Liquid manure with high solids will more likely plug pumps, transfer pipes, and sprinkler nozzles. Their presence requires more power to pump the same volume and creates higher pressure at the pump, thus increasing the risk of ruptured seals and manure spills.
4. Impermeable cropland. Liquid manure with high solids seals the surface of croplands, preventing water from penetrating the soil.

Creating effective solid-liquid separation (removing a substantial amount of organic solids from fresh liquid) thus poses an ongoing challenge for Idaho dairy producers, but once solved, its benefits can be enormous: the production of nutrient-rich organic solids, reduction of odors in liquid-manure storage lagoons, and improved treatment-process economics due to reduced organic loading rates each year. Furthermore, it diversifies the uses of separated solids—operators can apply them on farms near animal operations or export them to other areas as fertilizer or other soil-conditioning products.

Many dairy producers in Idaho, familiar with these benefits, have incorporated solid-liquid separation processes in their manure management systems. Most of them use primary solid-liquid separation technologies such as settling ponds, inclined screens, rotary screens, and screw presses. Inclined screens are simple and effective for capturing larger particles, but their removal efficiencies are generally less than 40% of total solids—10%–20% of total nitrogen (TN) and 10%–20% of total phosphorous (TP) (Powers et al. 1995).

Centrifuges, however, offer a more efficient system. Thus, we’ve taken a closer look at the centrifuge separator by measuring the efficiency and cost of running one on a commercial Idaho dairy. Hopefully, the data we’ve gathered will encourage dairy producers to use centrifuges to improve manure management on their farms.

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The Study

The yearlong manure sampling and sample analysis we conducted in 2017 involved the collection of a wealth of other vital data too, including financial investment information; maintenance, routine operation, and labor expenditures; added nutrient values; and other centrifuge-related costs, to determine the annual capital and operating costs associated with a centrifuge application.

The dairy host site, located in the Magic Valley of southern Idaho, consisted of about 4,000 milk cows in both open lots and free stall barns. Operators flushed sixteen manure lanes three times each day. See Figure 1 for a schematic flowchart of the flushed manure path. It is worth mentioning that the centrifuge monitored in this case study was the first and only one used on Idaho dairies at the time of this study.

Manure sampling. We started the manure sampling from January 9, 2017, until November 14, 2017. (We were unable to collect samples in December 2017 because foam covered the flushing receive pit.) Each month, we collected three samples from the flushing receiving pit (location #1 in Figure 1), the before-centrifuge location (#3), and the after-centrifuge location (#5 ) and three samples from the two screen–separated solids location (#2) and the centrifuge-separated solids location (#4), respectively. The samples were immediately sent to a commercial lab for nutrient analysis based on standard methods of practice. Information regarding how many tons of solids from the two screen separators and the centrifuge separator, as well as costs associated with the centrifuge, were obtained directly from the dairy owner. The collected information along with other local costs were used to estimate expenditures associated with the centrifuge’s use.

Cost analysis. Costs associated with the purchase and use of the centrifuge were partitioned into annual capital costs and annual operating costs, following Møller et al. (2000). Capital costs are calculated based on the following equation:

where C ($) indicates the capital costs in annual terms, I ($) the purchase cost of the centrifuge, r (%) the interest rate considered, and n (years) the depreciation period. Table 1 shows the values of each variable and the resulting annual capital costs from comparing a centrifuge with two inclined screens. Results indicate a relative higher annual capital cost of $8,827.85 for the centrifuge when considering seven years of depreciation at an annual interest rate of 3%.

The comparison of annual operating costs are shown in Table 2, which separately takes into account the cost of maintenance and repair, fuel and lubrication, removal of solids, cleaning the lagoon, electricity, transportation of liquid manure, and cleaning the “settling cell.” The settling cell is replaced by the use of a centrifuge.

^a uses 50 amps or 24 kWatt-hr at $0.10/kW-hr for $57.60/day. Inclined screens use 4 kW-hr at $0.10/kW-hr for $9.60/day.
^b Assuming 1.5 million gal at $0.01/gal; centrifuge assumes transport every six years (conservative estimate, which has not currently been reached) and for inclined screens.

As noted from Table 2, the resulting annual operating costs for the centrifuge are $41,949. Moreover, the alternative inclined screens indicate two results. The first considers the annual operating costs solely for two inclined screens at $86,454; while the second accounts for the annual operating costs of the two inclined screens at $43,154, assuming the existence of the operating centrifuge after these, as seen in points 3 and 4 from Figure 1.

In summary and observing Figure 1, the resulting annual operating costs for the inclined screens only (at point #2) is $86,454 while the resulting total annual operating costs for the inclined screens and the centrifuge (at point #5) is $85,103 ($43,154 + $41,949). Thus, using a centrifuge results in annual operating net savings of $1,351 ($86,454 − $85,103).

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2. Additional (Potential) Revenues

Considering the plausibility that separating liquid manure into solids can be done opens the door to potential commercial use. Separated solids can be converted into compost, biochar, hydrochar, or manure pellets, further concentrating the nutrients as marketable products. Yet further studies of their production and marketability as manure products are merited. In addition, prices for the N, P, and K nutrients may vary broadly: as an example, the United States Department of Agriculture (USDA) reported in 2020 that the average price in Illinois (early July) for urea (46% N) was $357/ton, diammonium phosphate (18% N, 46% P) $483/ton, and potash (potassium) $354/ton (USDA 2020).

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Summary of Findings

The centrifuge separator tested in this study removes much finer particles and nutrients than the screens at a dairy:

• Centrifuge-separated solids have higher N, P, K concentrations than those from the screen-separated solids.
• The one-year average of P concentration in the centrifuge-separated solids is about two times that of the screen-separated solids.
• The added centrifuge not only reduces the land areas needed for lagoon water applications and the frequency and costs of cleaning storage lagoons, it also generates nutrient-rich solids, whose composting could be marketed as a value-added product, resulting in reduced overall manure-handling costs and better use of manure nutrient uses.

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References

Idaho Dairymen’s Association. 2019. 2019 Industry Profile. Twin Falls, ID: Idaho Dairymen’s Association. https://static1.squarespace.com/static/5980acf9197aea5e857a81eb/t/5e8cfaa2717d2e316287dff2/1586297563288/2019+Industry+Profile_WEB.pdf. Accessed 16 November 2020.

Møller H.B., I. Lund, and S.G. Sommer. 2000. Solid-liquid separation of livestock slurry: Efficiency and cost. Bioresource Technology 74(3): 223–29.

Powers, W.J., R.E. Montoya, H.H. Van Horn, R.A. Nordstedt, and R.A. Bucklin. 1995. Separation of manure solids from simulated flushed manures by screening or sedimentation. Applied Engineering in Agriculture 11(3):431–36.

United States Department of Agriculture-Illinois (Department of Ag Market News Service). 2020. Illinois Production Cost Report, GX-GR210. Springfield, IL. https://www.ams.usda.gov/mnreports/gx_gr210.txt. Accessed 24 July 2020.

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About the Authors

Lide Chen—Associate Professor and Waste Management Engineer, University of Idaho (UI) Department of Soil and Water Systems, Twin Falls Research and Extension Center

Hernan Tejeda—Assistant Professor and Extension Specialist, UI Department of Agricultural Economics and Rural Sociology, Twin Falls Research and Extension Center

BUL 991 | Published December 2020 | © 2022 by the University of Idaho

Issued in furtherance of cooperative extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Barbara Petty, Director of University of Idaho Extension, University of Idaho, Moscow, Idaho 83844. The University of Idaho has a policy of nondiscrimination on the basis of race, color, religion, national origin, sex, sexual orientation, gender identity/expression, age, disability or status as a Vietnam-era veteran.