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Membrane units for low and high tech separation of liquid manure

Animal manure, such as pig and cattle slurry, is a dilute solution of a complex mixture of organic constituents, macro- and micro-nutrients, and various micro-organisms. However, the BioFuel Technology ApS membrane technology and refinement processes can refine the slurry, whether raw or anaerobically digested, to concentrated fertilizes and pure water at competitive costs.

BioFuel Technology offers a number of possible variations of the separation system, depending on the circumstances of specific projects. In some cases, a “medium” separation, often referred to as “low tech” separation may be an appropriate step or first step for separating animal slurry. By “low tech” separation is understood, that most of the organic matter and most of the total phosphorus is separated from the slurry. It results in a solid or semi solid organic fertilizer and “urine” like liquid containing the bulk of the inorganic nitrogen and potassium. By a complete or “high tech” separation is understood, that the slurry is completely refined to concentrated fertilizers and pure water.

A low tech separation may be preferred by some farmers and agricultural enterprises. For instance, in the vicinity of a common plant for anaerobic digestion (AD) of animal manure several farmers may supply raw manure to the AD plant. In a somewhat longer distance form the AD plant, farmers may separate the slurry and allocate the solid fraction only to the common AD plant. The solids contain the bulk of the biomethane (biogas) potential, and the biogas production per unit solids (100 m3 or more per tons) is much higher in comparison to raw slurry (about 25 m3 per tons). The AD plant can thus process a much larger volume of slurry if part of the slurry is “low tech” separated before added to the plant. The remaining “urine” like liquid can be used as fertilizer in higher rates, and with substantially reduced odour and volatile losses of ammonia.

The solids contain the bulk of the phosphorus (P), i.e., 80-90 % or more depending on specific circumstances, and 10-30% of the total nitrogen (N), again depending on specific circumstances. The low tech separation and export of the solid fraction thus removes most of the P and a substantial fraction of the total N from the farm. The separation of P and N allows for the solid fraction to be managed separately and perhaps transported and applied to agricultural fields in need of P, while the remaining “urine”, containing inorganic and plant available N, can be used in the vicinity of the farm. Here, it may be applied at much higher rates than raw slurry without exceeding the crop N demand and the crop P demand.

In technical terms, the low tech separation includes a first separation of course fibres (solids) by simple conventional means such as screw presses, filter or band presses. Following the first solids separation, the liquid is separated by the CoMeTas A/S silicon carbide micro- or ultra-filtration membranes to remove the bulk of the remaining suspended solids, including particulate P and N.

The high tech separation also includes a pre-separation of course fibres, but the separation is then continued by a process hierarchy including conditioning, ceramic ultra- and nano filtration, and reverse osmosis. The process may include ammonia stripping. A patent application for the process is pending. The resulting fertilizers may be inorganic NKS-fertilizers or ammonia concentrates (e.g. 25%), as well as solid P-fertilizers. The process is outlined in the figure.

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