Sand-laden manure handling and separation in freezing conditions
September 18, 2012 by Andrew W. Wedel P.E
Ask someone from South Dakota at what temperature handling sand-laden manure (SLM) becomes problematic and you’ll get one answer – probably something in the neighborhood of prolonged periods below 0 F. Ask someone from North Carolina the same question and you’ll surely get a different answer – probably, nights with temperatures below freezing.
To be sure, every producer has a varying level of tolerance and preparedness for freezing conditions. No matter where you are located, the laws of nature stipulate water freezes (converts from a liquid to a solid) at 32 F or 0 C. Manure, as excreted, is roughly 85 percent water. With freezing temperatures just a few weeks away in some places, take a few moments to evaluate whether or not you and your manure system are prepared.
Expectations and preparations
First, it’s important to recognize the freestall barn environment is part of a SLM handling and separation system. To maintain an optimal barn environment, it is necessary to maintain some air exchange when cold weather hits. In order to preserve barn air quality, avoid completely closing all air inlets such as curtains and doors, even as temperatures drop.
This is less of an issue with the advent of controlled-environment freestall barns. In any case, remember, it’s about the cows and their environment even if the ideal environment is achieved at the expense of allowing manure to freeze.
Start with freeze-prevention basics. Are above- and below-ground pipes adequately insulated and buried, respectively, and buildings with separation systems equipped with heating systems in good working order? Find the ground-frost depth for your location at http://nsidc.org/cryosphere/frozenground/images/NA_permafrost.jpg. It’s important to set some reasonable expectations before prolonged freezing conditions occur. In the most extreme circumstances, manure will freeze on freestall alleys and be immovable by rubber tire scrapers or vacuum tanks. On these days, options are limited to handling manure as a solid, which means scraping with a steel blade such as a loader bucket. Under these conditions, manure will appear freeze-dried and indeed behave as a solid.
Avoid scraping frozen manure into reception tanks, since once in a reception tank, frozen SLM will remain frozen due to the insulating capability of the earth. Furthermore, when manure freezes solid, sand grains are locked in the solid structure and cannot be readily dispersed, which is why attempting to separate sand from frozen manure can be futile.
Instead of attempting to separate sand from frozen manure, some producers will scrape the manure out of the barn onto a concrete pad where the daytime radiation will liquefy (melt) manure at such time, making it suitable for separation. Of course there are circumstances where there is no end in sight for sub-zero F weather, so provisions should be in place to convey SLM directly to long-term manure storage for future land application.
Viscosity is the key
Don’t wait for manure to freeze solid to begin addressing the effects of temperature change on conveyance and separation systems since manure viscosity increases as temperature drops. Loosely speaking, viscosity is a measure of a fluid’s resistance to flow – the more resistant to flow, like tomato ketchup, the higher the viscosity. As temperature decreases from 80 F to 32 F, water viscosity doubles. This means two things in the world of conveyance and separation. The first is colder water (higher viscosity) requires more power to pump than warm water. This phenomenon will go unnoticed provided pumps are sized to account for higher-viscosity material. Far more noticeable is the effect of viscosity on conveyance and separation systems. When manure is more viscous, it has a reduced capability to entrain and convey solids. To compensate, water flow volume must be increased or fresh water added. Separation systems also rely on water to dilute manure. Dilution reduces the viscosity of as-excreted manure thereby allowing sand to settle. Accordingly, the viscosity of the dilutant (manure storage or other process water) must be sufficiently low to reduce manure viscosity. Cold water makes for a poor dilutant when compared to warm water. Cold or frozen manure can be added to separation systems, but sand recovery is reduced since sand grains are not easily dispersed from the manure mass and thereby unable to settle. Most producers agree the limiting manure consistency for properly performing sand separation is “slushy” – that is, semisolid.
Enter the closed loop
The negative effects of cold weather on conveyance and sand-manure separation can be minimized using closed-loop water-recovery systems that are housed in a structure maintained at just above freezing. Closed-loop systems do not warm water, per se, but do maintain a more consistent dilution water viscosity, and therefore, consistent handling and separating system performance. Some systems acquire dilution water from manure storage, which is subject to changes in viscosity simply due to weather. In a closed-loop system, the dilution water comes from manure liquids as extracted by liquid-solid separators.
The diagram (Figure 1) shows a closed-loop conveyance and separation system. SLM is delivered to a sand-manure separation system either by scrape or by flume. Water, liquids and solids discharged from the sand-manure separator are pumped to a solids separator. Liquids from the liquid-solids separator are used for sand-manure dilution water and perhaps flume water.
As sure as the sun rises, freezing conditions will eventually occur; however, despite this certainty, oftentimes our level of preparedness and planning suggests we didn’t know it was coming. Start by understanding the not-so-obvious effects of cold weather on manure handling – that is, how viscosity effects things like conveyance and separation. Consider closed-loop conveyance and separation systems to minimize the effects of cold weather.
Avoid compromising air quality in freestall barns by completely closing air inlets in an attempt to facilitate manure handling. When all else fails and manure does freeze solid, plan to have an alternative method for handling sand-laden manure. This could mean scraping directly to long-term storage for eventual land application or stockpiling until conditions are suitable for separation. In any case, be prepared.
Andrew W. Wedel is a professional engineer. He is the general manager of the agriculture division of McLanahan Corporation, based in Holidaysburg, Penn.