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Not all manure is conveyed equally

Andrew Wedel Published on 18 May 2012

Not all manure is conveyed equally, so to speak, because not all manure is subjected to the same environmental conditions after being “deposited” on the alley floor. One of those conditions is bedding, especially where sand bedding is concerned.

Whereas non-sand-laden manure can be conveyed using gravity flow in large-diameter, flat pipes or stirred up and pumped long distances using centrifugal pumps, sand-laden manure cannot reliably over the long term.

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Several factors go into selecting a manure conveyance system – for example, site topography, building layout, climate and management style. And there’s more. What will be the ultimate fate of the manure – manure storage, digester, irrigation, etc.?

Can the downstream storage or treatment process tolerate dilution? Selecting conveyance systems for manure containing sand requires, first, an understanding of all of the site-specific factors, in addition to the physical characteristics of sand-laden manure (SLM).

The mining industry routinely conveys heavy, abrasive materials in the production of concrete and other building materials. Also, your local municipality conveys sand, in some cases, for miles in underground sewers relying only on gravity. These industries offer insights into how farms can successfully convey and process SLM.

Sand usage in freestalls averages around 50 pounds per cow per day, give or take. Adding this much sand (a stackable material) to manure (a flowable product), the result is a material that is neither stackable nor flowable.

Being a non-flowable material, gravity flow conveyance of undiluted SLM is not recommended under any circumstances, even in short runs of large-diameter pipes. High friction and low velocity under these circumstances ultimately results in clogging.

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There are surely examples (albeit fleeting) of success with these kinds of systems; however, it is in cases where either producers are cleaning the “gravity” flow gutter with a skid loader-mounted implement and/or sand usage is low – somewhere in the 30 pounds per cow per day range. Under no circumstances is this low level of sand usage recommended for the sake of “fixing” a failed SLM system at the great expense of cow health.

Whereas gravity conveyance of undiluted SLM is a non-starter, gravity conveyance of diluted SLM is a very viable option using flush flumes. Flumes are used in one of two ways. The first is to convey diluted SLM from flush alleys.

The second is where a combination of hydraulic energy derived from pumped lagoon water and pipe slope, also a source of energy, is used to dilute and keep scraped SLM moving, or scoured, in a pipe usually 24 inches in diameter.

As engineers, we are often asked the question: “How many gpm does it take to convey sand-laden manure?” The answer is the typical engineer’s response: “It depends.” Seriously, it depends. It depends on whether or not dilution is present, site elevation, pipe diameter, to name a few factors.

Using commonly accepted sewer design methodologies, it was determined and proven that on-farm flow velocity of five feet per second is adequate to scour most sand grains. The viability of flume systems is most often limited only by site elevation or quality of the water doing the conveying.

Site elevation is critical since a pipe placed on a 1 percent slope (1 foot of fall per 100 feet of run), if conveying 500 feet – taking into account pipe diameter – the flume would discharge 7 feet minimum below grade (2 feet pipe diameter + 5 feet pipe fall), assuming a flat site.

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Flume pipe with sand must always discharge to daylight. What this means is, all usable tank depth begins at depths lower than 7 feet. Be sure to consider site topography before selecting a flume system.

It was mentioned water is used to dilute manure in the flume conveyance process. Flume water must be more like the consistency of water rather than manure – that is, you can’t dilute manure with manure. Field experience has shown flume water should contain less than 2 to 3 percent total solids. This is most often achieved using multi-stage manure storages.

In some situations, whether a site is incompatible with fluming, flume water isn’t available, downstream processes potentially incompatible with dilution, etc., SLM is conveyed without the addition of dilution water, and without relying on gravity, using auger systems. Augers used with SLM rely on design characteristics common to the mining industries.

For long life, materials of construction must be harder than the sand grains being conveyed and rotational speeds are kept to a minimum. Also, the clearance between the moving auger flighting and concrete trough is one inch, minimum, in order to reduce abrasive wear.

Augers are placed at the same locations as flumes – either through the center or at the end of barns – and discharge at a reception tank or manure storage. Auger lengths and power requirements vary. Multiple augers can be placed in series to achieve any length desired.

The challenge, however, becomes (with multiple augers) placement of the drive units so as not to interfere with cow movement and manure scraping. Four augers placed in series to achieve lengths in excess of 700 feet is achievable. With augers, safety is paramount. Top covers and grates must be maintained for the entire auger length.

Vacuum (vac) tanks, as the name implies, are manure tankers equipped with a vacuum arrangement that “suck” manure off the alley floor and into the tank. Manure can then be transported to the desired location. Vac tanks have found favor in situations where flumes and augers are not options due to facility layout and/or site topography.

Vac tanks have proven particularly useful on large facilities where manure must be conveyed from multiple barns over a large area to one location for processing, say, an anaerobic digester.

Vac tanks have also been useful where facilities have converted from non-sand bedding to sand – that is, where an existing manure conveyance system like gravity flow is incompatible with sand or where SLM conveyance systems have failed. Vac tanks are large and heavy implements that are either truck-mounted or trailer-mounted.

Due to high axle loads, concrete placed over inadequate subgrade is subject to cracking. A skilled tractor operator is required to successfully maneuver a vac tank around a dairy.

When purchasing vac tanks, opt for features that reduce or eliminate sand buildup in the tanks. This may be augers, agitators or a combination of the two. Vac tanks are so integral to a manure-handling program that two units should be considered in order to manage around breakdowns, maintenance and freezing.

Remember to use systems specifically designed for sand-laden manure where sand bedding is used. The most common reason for SLM handling system failure is not taking into consideration the fact sand settles out of manure when diluted (some even settles when undiluted) as well as use of equipment not intended or designed for sand.

Sadly, the predominant reason for producers not using sand bedding is fear of manure handling. Industries such as mining and domestic sewage treatment both handle sand. Adopting design methodologies and equipment from these industries can result in favorable outcomes when it comes to sand bedding both from cow comfort and producer comfort (manure handling) standpoints. PD

Click here to email an editor for references which have been omitted due to space.

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  • Andrew Wedel

  • Agricultural Engineer
  • McLanahan Corporation
  • Email Andrew Wedel

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