Five Common Questions About Using Sand Bedding Before Anaerobic Digestion on Dairies

Andrew Wedel
By: Andrew Wedel May 15, 2020
Dairies using sand bedding in their freestalls can utilize anaerobic digestion if they have a sand-manure separation system in place. Here are some common questions about sand-manure separation before anaerobic digestion.

Sand is here to stay as dairy cow bedding in freestalls. Sand keeps cows cleaner, drier and more comfortable than other bedding materials. Clean, dry and comfortable cows are healthier and produce more milk. 

A majority of dairy farms using sand will not consider anaerobic digestion due to the widely propagated myth that manure bedding is the only option for anaerobic digestion systems. Dairies bedding on sand also have valid concerns about filling digester tanks with sand.   

Sand-manure separation can be used to pre-treat sand-laden manure by separating sand prior to anaerobic digestion. Adoption of sand-manure separation has the potential to substantially increase anaerobic digestion utilization, as it gives dairies the ability to recycle bedding sand and produce an effluent suitable to anaerobic digestion.

Without separation beforehand, sand bedding is incompatible with anaerobic digestion systems. However, with sand-manure separation, sand can be recycled for reuse as freestall bedding with milk cows and the manure effluent can be anaerobically digested. 

Here are five common questions about using sand bedding and anaerobic digestion systems.

1. What is the ideal dairy size for sand-manure separation? Are there different system sizes?

Sand-manure separation systems are operating with anaerobic digestion on dairies ranging in size from 100 to 20,000 cows. To cover this size range, there are four different sizes of sand-manure separation systems. Of the farms on the low end of the size range, the 100-cow dairy separates sand and sends the effluent to an anaerobic digester at a local wastewater treatment plant. The 20,000-cow dairy converts biogas to compressed natural gas. 

There is a sand-manure separation system to suit any dairy regardless of size.

The SMS12 is a sand-manure separation system for dairies with fewer than 500 cows. 

2. What is the recommended total solids for anaerobic digester influent?

A lot of this depends on the type of digester — different types of digesters operate at different total solids ranges. Moreover, the question to ask is “What is the total solids of the effluent on my dairy” and selecting an anaerobic digestion system accordingly. 

In today’s big push to capture the monetary benefits of anaerobic digestion, the anaerobic digester design market has been flooded with well-intended, capable firms unfamiliar with practical dairy production systems. Many of them recognize correctly that the total solids of excreted manure is roughly 12%. However, the total solids of your effluent is a result of how you manage your dairy and may vary with the weather and/or season. Adding parlor wash water can decrease total solids to 6% or lower. 

Choose an anaerobic digester designer, supplier, etc. who can design and build a digester to suit the way you manage your dairy, taking into consideration first and foremost what is best for the cows.  

3. Concrete sand is expensive in my area. Are there other sand gradation options I can use as bedding?

ASTM C33 concrete sand is the washed fine aggregate ingredient used in making concrete. Sand possessing this size gradation gives concrete the strength required. The ASTM C33 is simply the particle size range for this sand ingredient. 

If there is competition for the use of concrete sand, the price may be high relative to other sands. There are two things to keep in mind. First, even if the upfront cost of using concrete sand is higher, it is possible to capture 95% of the sand for reuse, which means only small future purchases to replenish sand inventory. 

Second, even if it is more expensive (and sometimes it is less expensive), some producers will pay the price knowing concrete sand minimizes the risk of discharging sand to an anaerobic digestion system. Think of it as an insurance policy.

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Sand bedding is considered the gold standard of freestall bedding because it is comfortable for the cows and prohibits bacteria growth.

Another thing to consider is that because concrete sand simply describes a size gradation, any sand by any name fitting that gradation is acceptable for bedding. We have seen this sand called “construction sand,” “torpedo sand,” “bedding sand” (for pipes), and/or “river sand.”

Perhaps most important, concrete sand is great for cows. It stays loose in the stalls and drains away any liquids from the surface, ensuring maximum cow comfort.

4. How dry is the separated sand, and it is possible to make it drier yet?

An Agricultural Sand Dewatering Screen is integral to any sand-manure separation system. Discharged bedding sand from a Dewatering Screen is approximately 12% moisture. At 12% moisture, the sand is “drip free” and reusable in less than one week. 

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Agricultural Sand Dewatering Screens produce a drip-free sand that can be reused in the freestalls within a week.

It is possible to obtain drier sand using a Bedding Dryer. What’s more, biogas produced from a digester (or natural gas or propane) can be used to fuel the dryer and produce sand at 99%+ total solids (less than 1% moisture). Thermal drying removes all moisture in addition to pathogen kill.

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Sand discharged from a Bedding Dryer can be reused immediately in the freestalls.

5. Does the Hydrocyclone target the removal of finer sand before a digester?

The sand-manure separation process has three steps pre-anaerobic digestion, with each subsequent step capturing finer sand. Hydrocyclones are the second step in the sand-manure separation process. 

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Hydrocyclones help recover fine sand in the sand-manure separation process.

Hydrocyclones drive separation by settling “sideways.” As the manure effluent is pumped in a tight radius, the G-forces are increased, thereby facilitating the settling of particles that might not otherwise readily settle simply by gravitational forces. 

As with any separation process, Hydrocyclones are not 100% efficient at capturing sand. Typically with anaerobic digestion systems, sand lanes are used as the third step in the sand-manure separation process to capture bypassed sand.

McLanahan Sand-Manure Separation Systems can be found throughout the world on dairies of all sizes and in all climates operating with anaerobic digestion plants. Contact us for more information about how sand-manure separation systems can work with anaerobic digesters.

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Tags: Dairy, Agriculture, Sand-Manure Separation, Sand Bedding, Anaerobic Digestion