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March 17 2021 11:18 AM
The author is an associate professor at the University of Idaho.

Centrifuge-separated solids (left) contain finer particles than screen-separated solids (right).

Some dairies prefer to use flush systems for manure handling because of their ease of mechanization and low labor requirements. Flushing manure results in large amounts of lagoon water that are often applied via irrigation systems to adjacent cropland during the growing season.

The main challenges

Solids and nutrients in liquid manure pose challenges to manure handling processes. There are at least three challenges and concerns to consider.

First, liquid manure with a high solids content flows into a storage lagoon where some of the solids settle at the bottom of the lagoon, resulting in reduced lagoon capacity. Eventually, the solids at the bottom of the lagoon need to be removed to increase the storage space, which is a costly financial burden for dairy producers.

Second, liquid manure with high solids and nutrients (especially phosphorus) limits the land application rate, which means larger areas are needed to apply the same amount of liquid manure. This results in transporting large volumes of liquid manure longer distances, which steadily raises costs.

Third, liquid manure with high solids has greater potential to plug pumps, transfer pipes, and sprinkler nozzles. It requires more power to pump the same volume and higher pressure at the pump, thus elevating the risk of ruptured seals and manure spills. Separating solids and nutrients from liquid dairy manure is a big challenge being faced by the dairy industry.

Many dairy producers are familiar with the benefits of solid-liquid separation and have incorporated solid-liquid separation into their manure management systems. Most of them use primary solid-liquid separation technologies such as settling ponds, inclined screens, rotary screens, and screw presses. Even though the inclined screens are simple and effective for capturing larger particles, their removal efficiencies are generally less than 40% of total solids, 10% to 20% of total nitrogen (TN), and 10% to 20% of total phosphorous (TP).

Centrifuges separate solids from liquid using spinning forces that permit removal of fine solids and nutrients more efficiently than inclined screens. In this case study, we intended to measure the separation efficiency of a centrifuge running on a commercial dairy in order to generate science-based information that will help dairy producers make informed decisions regarding the adoption of centrifuges on their farms.

An Idaho case study

A year long manure sampling and sample-analysis were conducted on a commercial dairy. The dairy consisted of about 4,000 milk cows housed in both open lots and freestall barns. Sixteen manure lanes were flushed three times each day. The flushed liquid manure first passed through two inclined screens, working in parallel, and then a centrifuge for removing solids.

Each month, three samples were collected from the screen-separated solids and the centrifuge-separated solids, respectively. These samples were immediately sent to a commercial lab for nutrient analysis.

This is what we learned:

1. Separated solids

Seven loads of solids (one load equaled about 25 cubic yards or 12 to 15 tons) were separated by the two screens; meanwhile, about three loads of solids were separated by the centrifuge every day (see photo). Notice that the screen-separated solids are mostly undigested fibers while the centrifuge-separated solids are finer particles.

2. Nutrients in separated solids

Phosphorus (P, as P2O5), total nitrogen (TN), and potassium (K, as K2O) concentrations in the screen separated and the centrifuge-separated solids are shown in the figures. It came across that P concentration in the centrifuge-separated solids is much higher than that in the screen-separated solids.

Year-long averages of P, TN, and K concentrations in the centrifuge-separated solids were 3.81 pounds per ton, 8.99 pounds per ton, and 4.42 pounds per ton, respectively. This was compared to the average concentrations of 1.96 pounds per ton of P, 7.68 pounds per ton of TN, and 4.20 pounds per ton of K in the screen-separated solids.

Consolidate the nutrients

The centrifuge tested in this study can remove much finer particles and their associated nutrients after the first separation of solids and nutrients by the screens at the dairy. The centrifuge-separated solids had higher N, P, and K concentrations than those from the screen-separated solids. The one-year averaged P concentration in the centrifuge-separated solids was about two times that of the screen-separated solids.

The added centrifuge not only reduced the land area needed for lagoon water applications and reduced frequency and costs of cleaning storage lagoons. It also generated nutrient rich solids that could be composted and marketed as a value-added product. The result was reduced overall manure handling costs and better manure nutrient use.

This article appeared in the February 2021 issue of Management on page 20 and 21.
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