While most livestock manure is land-applied in one form or another, Dr. Rebecca Larson, University Wisconsin-Madison, said there are opportunities to integrate new technologies for manure management that would benefit producers and, ultimately, the environment.
“A perfect system would be keeping it all contained on a field,” said Larson. “But we know that isn’t achievable, and some losses are actually okay. But the more we keep on the field, the more we benefit from nutrients.”
Every processing option results in a byproduct that must be managed. “Processing means you’re changing some of the characteristics but not really treating to hit a final end goal,” said Larson. “Treatment means you’re trying to obtain some final goal.”
When working with farmers, Larson can easily determine whether they can successfully implement a technology system by good data on how much manure is hauled, the annual cost of managing manure and what the farmer likes or doesn’t like about their system.
“It’s critical to understand your system,” said Larson – identifying goals is important. “A common issue is people see technology and want to try to fit it into their system. I recommend going about it a little differently: What is the problem with the existing system, or what improvement do you want to make?” Larson said a goal of reducing hauling costs is reasonable, but still requires good data including current hauling costs and options to reduce costs.
Economics and other drivers are important factors that may lead to changes in a system. Before choosing technology, consider factors such as phosphorus export requirements, insufficient land base, labor or manure collection that doesn’t match the land application system.
“Data is important,” said Larson. “You can’t do the analysis unless you know the components of your system. Knowing how much manure you’re producing annually, what are you hauling every year, how many tankers did you take out, how long did you run pumps and tracking all of this information over the years.” Analysis should consider major costs within a system, influenced by the layout of the manure handling system and how manure is hauled to the application site. Ideally, the processing technology fits within the existing system unless changes can be made within a reasonable expense frame.
Composting is a low-cost option with benefits that include a stabilized product, reduced odor and volume reduction (about 40%) that reduces hauling costs. Composting destroys weed seeds and pathogens and results in a more consistent fertilizer product. There’s also potential for revenue, but unless the composting is managed by an entity off the farm, farmers should realize they will be responsible for marketing.
Farmers can implement a low-end composting system or purchase equipment to compost in windrows, which can be labor intensive and require a lot of space. Bagging composted material requires specialized equipment, but farms close to population centers can usually recoup such costs.
Digesters are increasing across the nation and are often touted as a means to reduce methane emissions. Large-scale digesters operate continually, can easily handle liquid manure and collect gas which can be converted to electricity or scrubbed to remove contaminants before entering a gas pipeline. Additional benefits of digesters include antibiotic degradation and reduction in pathogens and odors. Small-scale digesters aren’t as economical, and Larson said it can be difficult to determine the payback for small systems.
Solid/liquid separation technology can be combined with other technologies and generally has a lower price point for entry. A basic system separates manure into solid and liquid components, while a more advanced system with several separators can further break down components.
A screw press and centrifuge are the most common solid/liquid separation systems. Larson said a screw press costs less than a centrifuge and is easier to operate, but the only way to know which is best for an existing system is running the numbers and reviewing goals. Each type of separator has different input characteristics and produces different kinds of products, so consider how the system will work on the farm.
Separation efficiency is critical in solid/liquid separation and varies widely among systems. “When you look at your system, understand your goals,” said Larson. “If your goal is to remove phosphorus because you don’t have enough land base, it’s important to think about which separation system can do that.” Separation offers great management options and is also the first step for other technologies, such as treated liquid manure.
Struvite recovery captures and concentrates manure nutrients as struvite and removes total phosphorus. Capital investment and operating costs for struvite recovery are high, but farms with excess phosphorus issues can sell the recovered product.
In pyrolysis systems, manure solids are converted to biochar using pyrolysis. Larson said there aren’t many pyrolysis systems in place on livestock operations but there’s interest in using manure solids to produce biochar. “One benefit is reducing volume by densifying phosphorus,” she said. “You’re losing nitrogen in this system, but future development might negate this.”
Pelleting involves compacting and balling manure solids. Such systems densify manure nutrients for easy transport. Benefits of pelleting include reduced odors and the potential for precise land application of nutrients. Pelleting systems are costly and are in place only on large operations that can justify the cost or farms with restricted manure application.
Treating for clean water involves a series of treatments. Manure is treated to water quality standards for discharge or animal drinking water. Treating for clean water reduces hauling costs but results in numerous byproducts to manage. Such systems reduce greenhouse gas emissions and pathogens and densify nutrients. While clean water systems are becoming more cost effective, farms should be large enough to justify the cost.
When considering manure processing technology, Larson recommended considering the practices implemented according to scale rather than looking at farm size. “You can be a large farm with less impact than a permitted farm,” she said. “It’s all based on management practices and individual decisions.”
When looking at manure processing from a larger viewpoint, such as a state or watershed, Larson said environmental and economic goals can influence policies that lead to improved management. “If we can add a certain policy where there’s a price point per pound of phosphorus, a lot of technologies are more cost effective,” she said. “Credits are available now for digesters and biogas, and I think in the future we’ll see more credits that will help some of these technologies move to a more financially viable situation for the producer.”
by Sally Colby