Most people understand that anaerobic digesters produce a biogas that can be used to create power (usually in the form of electricity). But few realize that farmers seldom install digesters primarily for this reason – if at all. More often than not, farmers install a digester to enhance their nutrient management plan, as well as control odors, destroy pathogens, kill weed seeds, etc. For these farmers the ability to produce power is a secondary benefit. The revenue from power sales simply helps pay for the digester system that gives these other benefits to their farming operation.

Doug VanOrnum

But how does a digester add value to a farm’s nutrients? And how does one affect a farm’s nutrient management plan?

Mineral nutrients such as nitrogen, phosphorus and potassium (N, P and K) are brought onto a farm in the form of animal feedstocks ... which, in a dairy example, are fed to cattle as a part of their total mixed rations (TMR). Nutrients that are not absorbed by the animals “exit” (of course) in their solid and liquid waste streams.

However, in “raw” animal wastes the N, P and K are tied up in an organic form and are not yet available to plants. Once raw wastes are land-applied it normally takes a long time for bacteria in the soil to re-mineralize these nutrients, putting them in an inorganic form so that crops can readily accept them (for this reason purchased fertilizers are also typically inorganic).

In addition, if raw wastes are applied to an already growing crop there is a risk of damage to the crop. For both reasons above, farmers tend to land-apply their wastewater slurries in large batches, either just before planting or after harvest.

But doing so means a farmer does not have much flexibility as to when and where he can distribute his nutrient fertilizers. The risk of nutrient runoff is also heightened.

In short, anaerobic digester systems provide more nutrient flexibility and lessen the risk of (or impact from) nutrient runoff, in several ways:

• Faster plant uptake: An anaerobic digester performs the natural process of mineralization inside the digester instead of deep in the soil. Plants can immediately accept these fertilizers when applied.

• The ability to apply anaerobically digested nutrients directly onto a growing crop gives a farmer a larger window of opportunity to distribute their fertilizers. For example, in a hot, dry summer their crops may well benefit from a half-inch of water and fertilizer.

• Digesters reduces the COD and BOD in animal wastes (both are measurements of the potential impact a wastewater can have on rivers, lakes and streams)

• Pathogens such as E-coli and other aerobic pathogens are greatly reduced in modern digesters – often to the point of undetectability. One direct benefit can be reduced number of cases of mastitis. An indirect benefit can be a reduced risk of harmful pathogens inadvertently coming into contact with our own food chain.

Clover Hill Dairy in Campbellsport, Wisconsin, milks more than 1,400 Holsteins. According to farmer and proprietor Joe Bonlender, “The biggest advantage our digester gives us is the flexibility it offers for timing manure applications to our fields.”

He adds, “One thing that we do every summer is apply manure onto growing hayfields. It has been a dry summer so far this year. Right after we finished our second cutting last week, we surface-applied digested manure slurry. Our crops received extra nutrients to draw upon, and the extra moisture was also needed.”

In so doing, many other digester owners have reported significant crop yield increases – in some cases by as much as 100 percent during a dry season.

An anaerobic digester also produces a high-quality biosolid that contains 30 to 40 percent of the farm’s total phosphorus output.

Because anaerobic digesters consume the starches that make raw manure wastes “sticky” and wet, these biosolids are light and “fluffy” and can be sold as fertilizer – or, more economically, be transported to the more distant farm fields that need more P.

Anaerobic digestion can also replace more fuel and labor-intensive composting practices. Bacteria do not have legs or tails or fins and so cannot move by themselves. They must be brought to their “food” or the food to them.

Allowing natural bacteria to circulate freely in a (temperature-controlled) liquid slurry is a more efficient means of doing so than periodically turning over a pile and adding water. The end product is typically more completely processed as well. These digested biosolids also make an excellent peat moss replacement, for which there is a growing demand.

Most digester owners with dairy and cattle operations employ their biosolids as bedding for their livestock. As long as the wastes are thoroughly digested, the available “food” for microbials has already been largely exhausted.

This is a meaningful cost offset for materials they had used previously (including sand) – while also often improving their milk quality due to lowered somatic cell counts and increased cow comfort.

Interestingly, when used as bedding the nutrients contained in these biosolids are not lost to the farmer (a digester does not destroy nutrients). As cattle kick their bedding back into the waste collection lanes, the bedding solids eventually get returned to the digester system when combined with fresh manure additions.

Their value as fertilizer is recovered when biosolids and liquids are removed from the digester processing “loop” and are land-applied or sold offsite (yet another revenue stream for the farmer).

Digested biosolids sold “off the farm” carry with them N, P and K (primarily P) and effectively reduce the amount of N, P and K that must be accounted for on a farm’s PDES plan.

One cumulative effect of a more flexible nutrient management system is the amount of farmland needed to support a dairy cow (for example) on a per-cow basis is either reduced or the same amount of farmland can support a larger number of animals.

Mostly it’s about giving the farmer more options. As a result, nutrient fertilizers are more efficiently distributed to where they are really needed – and crops benefit by spreading the manure application out over time (with reduced volumes per application) and/or applying them during times of greater need. PD

Doug VanOrnum
Business Development/R&D
DVO, Inc.