Manure Manager

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Gotta keep ‘em separated

Capitalizing on innovations in separation.

July 11, 2023  by  Bree Rody

Stored manure can be separated at a farm level through various means, starting with a method as simple as a screen or filter. Photo: © Lagui / adobe stock

If you’re a regular reader of Manure Manager – whether it’s because you’re a custom applicator for hire or because you manage and apply your own manure on your farm – you don’t have to be told about the value of manure. That’s why it’s practically an industry curse word to refer to manure as a “waste” product.

However, a new study from the USDA’s Economic Research Service (ERS) has found that there is nevertheless still plenty of value to be extracted from manure – and that, whether or not “waste” is a dirty word in the livestock world, plenty of nutrients from manure are, indeed, going to waste. For the year 2021, manure was applied to only about eight percent of the 240.9 million acres planted to seven major field crops in the U.S. There’s potential for much more activity, particularly considering the rising demand for organic food in the U.S.

Unfortunately, the problem is not quite as simple as convincing more growers to fertilize using manure.

Ray Massey, extension professor of agricultural economics at the University of Missouri, and one of the research study’s co-authors, says it’s not simply a matter of looking at the land and assuming you will get a certain dollar value per acre – because not all cropland needs the same nutrients. When manure-supplied nutrients exceed crop needs, crop producers will not demand the excess nutrients, lowering their value. According to the study, 371 counties in the U.S. have been identified as having more manure-supplied nutrients than crop needs. And therein lies the logistical challenge.


“It really is all about capturing the value of the manure nutrient,” says Massey. “Take phosphorus, for example. If you’re able to take phosphorus and label it at 50 cents per pound, but you’re putting it on land that doesn’t really need phosphorus, then there’s no value to that phosphorus.”

But in challenge often lies opportunity.

“If you’re able to haul [the manure] an extra two miles to some land that needs phosphorus, then you’re capturing that value.”

The study found that producers who make better use of existing and emerging technologies including solid-liquid separation, composting and anaerobic digestion technologies, will obtain even more value from their manure.

Many are already taking advantage of existing and emerging innovations – Manure Manager has profiled its fair share of farms implementing anaerobic digesters, and our in-house podcasts have explored the time, money and emissions that can be saved using precision technology in the field. Nevertheless, according to the report, there is still plenty of money to be saved.

One of the first steps in realizing the value of manure is in separation – the separation of solid and liquid matter can allow producers and haulers to transport nutrients to where they’re needed more efficiently, and for overall more efficient nutrient uptake.

It’s ‘not a money loser’
When economic studies find that money is wasted, that generally does not mean farmers are writing cheques on wasteful purchases. Instead, that money is generally wasted through the expenditure of too much time and effort on a task, which could otherwise be spent elsewhere – such as equipment that breaks down, practices that don’t net any returns.

Massey adds: “It really depends on what the return to assets or to equity that they’re seeking.” Some of the study conducted in Missouri found that there is indeed a return, but not one so high that one might invest in growing a livestock operation.

However, solid-liquid separation, says Massey, is “not a money loser.”

There are a number of different methods for solid-liquid separation, which all come with different advantages and barriers.

Some of these methods, say Massey, are pricey enough that they would have to be installed in a new build. “Ideally, they would do it in a new facility, as opposed to retrofitting an old facility.”

However, one of the most accessible methods for separation is through a screen. Of all the various separation technologies listed in the USDA study, screens were the only method that did not have cost listed as a barrier. “That’s the easiest to manage,” says Massey. “A screen system is pretty much a passive system.”

There are different forms of screens; farmers may choose to implement mechanical screens such as rotary drums or sloped screens; they might also opt for non-mechanical screens such as weeping walls, baffled sedimentation basins. Both types function by ultimately minimizing the amount of solid material entering lagoons. This gives the lagoon greater capacity to hold liquid and reduces related costs for farms to remove solids from the lagoon. As an added benefit, screens remove some nutrients – estimated to be five percent of nitrogen and phosphorus for mechanical screens versus 10 percent of N and 18 percent of P for a two-stage weeping wall.

Most screens come with a relatively affordable price tags, at approximately USD $40,000 for a 120-head dairy farm. The study identifies these as a viable “first step” toward better manure management and better management of nutrients.

Bigger investments
Centrifuges and presses are other examples of separation technology that makes for more efficient management of manure nutrients. However, Massey says these would be more typically installed in new builds – although he adds that his fellow Missouri professor and study co-author Teng Lim has more experience with farms that retrofit on various separation technology. Centrifuges, like screens, are ideal for high-moisture manure such as hog and dairy manure and can work on medium-to-large farms. The practice of a centrifuge is nearly a century old.

In a conventional centrifuge system, a liquid-solids slurry, typically with three to 10 percent solids content, is introduced into a tube, which then spins at high speeds to separate solids from liquids. The separated solids are discharged from the centrifuge with solids content of 18 to 26 percent.

Centrifuges have also been noted to remove particles that conventional cloth filtration and microfiltration would screen.

There are, of course, barriers to centrifuges, with cost being the biggest. Additionally, polymers – chemicals added to increase aggregates and enhance separation – might be needed. Additionally, centrifuges require a high degree of power. The centrifuge requires 10 times more power than a screw or belt press, and maintenance costs and management costs are another factor to consider.

Because of the cost barrier associated, Massey says it’s highly unlikely one would retrofit an operation they are already running to add a centrifuge. However, he says it might be a smart idea for someone installing a new system. “If you’re going to spend [the money] on a lagoon or other form of separation, it’s not going to double the cost, it’s going to add a percentage onto the cost,” says Massey.  “They can make a positive return on investment, [although] not a high return.”

Centrifuges, along with filtration and dissolved air flotation technologies, separate fine solids from liquid. Those, too, are cited as having high costs, with filtration being identified as “high to very high” costs. Some advancements have occurred in filtration, including metal membranes and vibrating membranes. At least two companies have begun to offer titanium and stainless-steel membranes to extend operational life. However, the study notes that these technologies are new to market, and the effectiveness remains unclear.

Presses, which dewater coarse fiber and fine solids, are often a core component of recycling fiber for animal bedding. The three main forms of presses are belt presses, which conveys the solids on belts with small perforations and through a series of rollers with increasing pressure; filter presses, which uses a series of filter bags on a rack or bar; and screw presses and moving ring/disc presses, which moves the solids through a cylinder using an auger. Press systems can begin to process slurry at three percent or higher solid content, and they produce an end product called “cake” with a solids content of 30 to 50 percent.

Managing separated manure
Once manure has been separated into solid and liquid, even through something as simple as a screen, Massey says an operation needs to then ensure that it’s able to distribute the manure effectively. “You need liquid distribution equipment and solid distribution equipment,” he says. “The benefit we found here in Missouri is that… if nearby land is already high in nutrients, the liquid can be distributed nearby [and] the separated solid, because there’s more nutrients per ton, can be hauled a greater distance. If you do have a system where you’re trying to move it further away to a land that needs more nutrients, the solid-liquid separation can change that.” In general, with separation, Massey says liquid can be applied more closely, whereas solid can be applied at a greater distance. He points to programs such as the National Resource Conservation Service’s Environmental Quality Incentives Program (EQIP), which can provide grants or subsidizations. “EQIP is what most people would rely upon. There are also some local soil and water conservation district programs that… subsidize it in some way through a cost share. But those would be very specific. Even Missouri, there would be a couple counties that might not [subsidize].”

In the end, says Massey, the goal is to capture the value of the manure nutrient – and not just nutrients like nitrogen and phosphorus. “Is there sulfur that you can value? That’s becoming a more important nutrient, and [it’s important to] actually understand if your manure contains sulfur.” •


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