The practice of granulating organic wastes such as manure has seen an influx of attention in recent years, due to the many advantages that the beneficial reuse of organics has to offer.
See the infographic on the benefits of granulating manure here.
And with so many benefits, it’s important to understand the process of transforming raw manure into a value-added, marketable product. This article looks at why pre-conditioning manure is important for both wet and dry feedstock, as well as what options are available to turn a raw manure into an acceptable feedstock for granulation.
Why Pre-Conditioning is Necessary
When working with raw manure, many characteristics make it unusable as a feedstock for granulation. The pre-conditioning process varies greatly from one material to the next, with many options available, and often a combination of methods being desirable. The method(s) chosen depends on the characteristics listed below and what it will take to transform the raw material into a suitable feedstock. More specifically, pre-conditioning serves to get a material to the desired particle size distribution, moisture content, and material composition that will allow the end product to be created. In general, pre-conditioning focuses on:
Particle Size Distribution: In order to granulate a material, large chunks will need to be removed. A particle size distribution for organic feedstock of around 60 mesh is ideal.
Moisture Content: The right moisture content is also critical to the granulation process. The ideal feedstock for poultry manure is under 20%, or dry enough so that the material can go through grinding. Ideal moisture content for hog and dairy manure is around 60% to 80% so the material is in a cake form, instead of a slurry.
Material Composition: The composition of the manure will also need to be addressed during the pre-conditioning stage. The material should be examined to determine if it contains coarse fibers or bedding, which might dilute nutrient content and inhibit granulation. The material should also be analyzed to determine if it contains natural binding agents that will aid in the granulation process, or if an alternative binder will be required.
Aside from the general guidelines above, wet and dry feedstocks often require different methods of pre-conditioning, due to their differences in moisture content, consistency, and makeup.
Pre-Conditioning Wet Feedstock
Wet feedstock is most commonly dairy manure, hog manure, or municipal biosolids. Listed below are some of the common pre-conditioning methods used with wet feedstock.
In dairy applications where sand is used as a bedding, sand removal is critical. Not only is sand highly abrasive, which would degrade equipment over time, but it also reduces the effectiveness of granulation and would be undesirable in an end product. Additionally, sand can fill up digesters if not removed before the anaerobic digestion stage.
Sand removal is carried out using a sand separation system – a combination of steps that washes the material and runs it through an inclined screw to separate the manure from the sand.
Anaerobic digestion is the breakdown of manure through biological processes that occur in the absence of oxygen. This process helps to homogenize and stabilize the manure in order to prepare it for solid/liquid separation. The resulting material is a slurry, reduced in odor and pathogens, and ready for solid/liquid separation.
In addition to the slurry produced, this process also produces biogas, which can be converted into a usable fuel source to make electricity, be used on-farm, or sold to energy producers.
Coarse Fiber Removal
The removal of coarse fibers is also commonly seen with wet feedstock. This step is necessary to remove particles such as bedding or straw that would impede granulation or create a weak pellet product.
When moisture can be freely pressed out, solid/liquid separation (with nutrient capture) is required. This step yields a concentrated cake material, ideal for use as a granular product because of its rich nutrient content.
A variety of options in methods and equipment exist for this purpose, with techniques combining mechanical, gravity, and even chemical action.
The diagram below illustrates a common organics granulation “wet” process. The light gray area on the left illustrates a common pre-conditioning workflow in which the raw manure first goes through sand removal, then anaerobic digestion, coarse fiber removal, and finally solid/liquid separation with nutrient capture in order to recover nutrients and prepare the feedstock for granulation.
Pre-Conditioning Dry Feedstock
Dry feedstock is generally a poultry manure, poultry litter, or a compost type of material. Listed below are some of the common pre-conditioning methods used for a dry feedstock.
Pre-drying is sometimes necessary if the moisture content of the material is too high. When a grinding step is necessary, material with a moisture content that is too high would clog the grinding system. Drying can be carried out using a variety of equipment, or through the practice of back mixing, in which dried product is thoroughly mixed with wet feedstock to bring down the moisture content.
Large particles cannot be present in the feedstock, because they will inhibit effective granulation. For this reason, grinding is necessary when the material contains large particles, or has a particle size distribution that is too large. Grinding is commonly carried out using a hammer mill.
Since anaerobic digestion requires a pumpable slurry, it is not an option with dry materials. Similar to anaerobic digestion, however, composting can be used to homogenize the feedstock for granulation and break down the organic material through micro-organism action.
While composting can be used for both wet and dry feedstocks, it is typically only seen with dry feedstock, because wet feedstock is often too wet to be efficiently composted. Wet feedstock would likely require a lot of bulking agents in order to bring the moisture content down, which would in turn, dilute the nutrient content.
The diagram below illustrates a common organics granulation “dry” process, where the manure first goes through composting and then grinding before granulation begins.
Pre-conditioning is a necessary step to transform a raw manure or organic waste product into a suitable feedstock for creating a premium fertilizer product. Many pre-conditioning methods are available, with a combination of techniques often being desirable.
FEECO has been helping customers transform organic wastes and manures into premium fertilizer products since 1951. For more information on the granulation of manure or other organic wastes, contact us today!