Sand Is Great for Cows, but Other Challenges Exist
William Bickert [retired] and Dana Kirk
Sand in manure is abrasive to handling equipment and tends to settle in trenches, pipes, tanks, and storages. Experience has taught us that you should not put sand-laden manure in pits, pipes, trenches, or tanks which you cannot access easily in order to clean out the sand or grit that settles.
Managing sand-laden manure with daily haul or long-term concrete storages operated as skim-and-haul are time-proven methods. Additional water such as rain or milking center wastewater is usually excluded from the sand-laden manure to avoid dilution and unintentional settling of the sand.
"Rule Number One: Don’t allow manure that contains sand or grit to enter places where, if it separates and settles, it could not be removed easily."
Why We Separate Sand
Actually, this list of reasons mirrors the progression of our past research on sand separation. We started with the goal of removing sand from the manure stream to reduce damage to equipment and sand settling in storage. Then we discovered that we could reclaim sand without manure solids being included. The fact that small quantities of sand remained in the manure stream was tolerated. More recently, we are seeing interest in manure treatment technologies that demand a “sand-free” manure stream. No sand should remain in the manure stream, and no manure solids should be left in the reclaimed sand.
Achieving the last goal presents a complex challenge. The ideal conditions for removing sand from manure are not necessarily the same as the conditions most ideal for reclaiming clean sand for reuse. Subsequently, systems in the third category must provide a high degree of control and likely will require more than one stage of separation. Sand-manure separation is not as simple as it first appears.
Here’s How It Works
Simply put, this difference in settling rates is the basis for all methods of separation currently in use — mechanical separators, cyclones, sand lanes, settling aprons and basins.
The degree of success depends on high-quality dilution water containing few manure solids and a velocity component for buoyancy to keep manure solids in suspension. In addition, choice of sand is important to the success of separation. Because sand grains vary in size and shape, they settle at different rates — coarse sand settles faster than fine sand. Unfortunately, fine-grained sands tend to settle at a similar rate as some of the manure solids which presents a challenge that can range from moderate, if some fine sand left in the manure stream is tolerable, to complex, if the goal is to reclaim sand for bedding and a sand-free manure stream.
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