Solids & Soil Nutrients

The two products created by anaerobic digestion of manure are methane (biogas) and effluent, a slurry mixture that remains after all methane has been released. This digested manure is generally not wasted by farmers, as it is rich in phosphate and potash. However, those planning to use the manure as fertilizer for corn crops may have to supplement it with nitrogen to make it a more complete fertilizer.

If the effluent is not used in its basic form, it may be further divided into liquids for land application and solids. This section will discuss the solids portion, their makeup and uses.

To separate the solids and soil nutrients, a mechanical separator is needed. This is a device with screens or screw compressors which "squeeze" the liquid fraction out of the material. Depending upon the type of separator used, the consistency of the resulting material may range from a damp, clumped material, to a nearly dry substance that is sometimes reused for animal bedding.

Equipment you'll need and why:

• For covered-lagoon digesters, you'll need a secondary, storage lagoon to receive overflow liquid from the primary digester lagoon. This lagoon may be used to provide recycled water for the farm's flush manure removal system.
• For mechanical digesters, you'll need a concrete holding tank where the solid manure - now significantly lower in odor - will be discharged and held for later uses such as land spreading. In some operations, the solid manure is discharged to the tank by spilling over a dam located at the opposite end of the slurry injection site, draining by gravity into the tank.
• Mechanical digesters also require separators with screens to separate solids from liquid. Recent research suggests that chemical treatment may improve the performance of the separators.
• For farmers involved in composting - to produce high quality soil amendment products - using separators that can effectively handle wet solids is vital. Correct composting requires an educated operator to make sure the right proposions of carbon and nitrogen are present, and correct procedures are used for feedstock resources.

Solids can sometimes be sold as potting soil to local greenhouses, or bagged and sold regionally. If the solids contain significant amounts of fiber, and are separated to a nearly dry form, they can be sold as bedding. Generally, manure is only sold as bedding in states that sustain seasonal climates where dairy cows are housed indoors.

The digester effluent on a swine farm with floor flushing systems may go to a second-cell storage lagoon and be later recycled to flush the swine houses on a continuous cycle. Excess liquids from the second cell can also be land-applied or land-spread on crops as fertilizer.

As an example, BTI Gonzales R&D and Production Plant in Fontana, CA, operates a 1,600 dairy makes use of separated solids. According to BTI, after leaving the digester, the slurry is separated into liquid and fiber fractions by using a revolving squeezing sieve separator. The liquid fraction undergoes additional centrifuge separation and generates a liquid "centrate" and a solid "cake." The centrate is used as an organic liquid fertilizer for organic farm practices or as an addition to conventional fertilizers. The solids portion contains a wide spectrum of microflora, fine organic matter and hormone-like substances, and its use has given very encouraging results in field trials. Other organic liquid fertilizer products are different formulations of bridge-organic liquid fertilizers obtained by stabilizing and enriching the centrate with traditional chemical products.

The cake is processed with other organic materials to produce dry pellets of various sizes. These are reportedly very valuable as specialty organic fertilizers. The fiber fraction is further combined with other selected agricultural byproducts and undergoes a very controlled aerobic composting process. According to the specific formulas used, different specialized products are obtained. These include value-added organic potting soils and partial replacement for peat moss used by the horticultural and ornamental markets and as a soil amendment and fertilizer.

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