Hog Manure Revisited
With funding from a MnDRIVE Environment Demonstration Grant, UMN researcher Roger Ruan makes the most of manure.
by Reed Grumann
Where others see waste, Bioproducts and Biosystems Engineering professor Roger Ruan sees opportunity—in this case, repurposing wastewater from livestock operations to extract economic value and address a potential environmental threat from one of Minnesota’s largest industries: hog farming.
Minnesota is the second largest pork producer in the country, with a headcount of nearly 9 million pigs and hogs. Like any livestock industry, pork producers need solutions for treating manure. There’s inherent value in animal waste as a source of biogas, bedding, and even building materials. But before it is reused, it must be properly treated. One popular treatment choice, and the focus of Ruan’s research, is anaerobic digestion.
Anaerobic digestion involves the controlled growth of microbes in an oxygen-free environment. Large tanks similar to those you might see at your local brewpub can convert food and agricultural waste into valuable byproducts that are safe for the environment. Typical anaerobic digestion systems, however, are unsuitable for hog manure, which contains higher levels of nitrogen and organic materials when compared to other animal waste. “Even after traditional anaerobic digestion, hog waste still contains high levels of nitrogen and other nutrients,” says Ruan, which can leach into water systems and cause eutrophication and kill fish, prompt unwanted and overwhelming vegetative growth, and even pollute our drinking water.
From an environmental perspective, such high concentrations of protein and nutrients are a problem. But from a biosystems lens, they’re an opportunity. “We developed a systematic approach for manure wastewater from pigs while treating it at the same time,” says Ruan. His system expands the scope of anaerobic digestion from treatment to resource recovery, aiming not only to prepare hog manure for proper disposal but also to extract and process the excess nutrients into valuable byproducts.
The first issues Ruan’s system addresses are low carbon levels and excess ammonia. When cycled through the digestion process, the protein-rich contents of hog waste are converted into ammonia, which inhibits certain microbes from further digesting the manure. “We add cellulosic biomass, hydrolyze and apply a small vacuum to strip away the ammonia gas,” says Ruan, which is then absorbed with sulfuric acid to produce ammonium sulfate, a valuable fertilizer.
Adding biomass and removing ammonia from the digestion system also balance the carbon-nitrogen ratio, making the manure slurry an ideal growth medium for methane biogas production, and the remaining wastewater can be used for algae cultivation. Algae extract CO2 and organic carbon, nitrogen, and phosphorus as it grows, further reducing the concentration of nutrients in the hog manure. Algae can also effectively remove heavy metals and other hazardous materials. “So, the algae can be grown, harvested, and used as animal feed because it often has very high levels of lipids and protein,” says Ruan, “If levels of heavy metals are high, we can then prioritize the algae to produce biofuel.”
After the algae is removed, the remaining manure wastewater can further be used to grow hydroponic vegetables like tomatoes, lettuce, and microgreens. These vegetables absorb much of the remaining nutrients from the wastewater while effectively reusing manure wastewater at yet another step of Ruan’s system. Ruan also conditions the wastewater in between different processes and removes any leftover particulate matter using biochar, a carbon-dense material similar to charcoal. After this, the wastewater is safe to be disposed of.
“These kinds of processes are actually very successful in treating the wastewater,” says Ruan. “We produce ammonium sulfate fertilizer and methane biogas. We can grow algae for animal feed or biofuel production. We can grow vegetables. We can process solids to use for agricultural byproducts or biochar. And, we can use biogas and biochar as fuels to generate electricity.” There is clear economic value in bioproducts generated by Ruan’s system. The only remaining challenge is scaling his design for mainstream use.
The current system has only been showcased on a small scale in laboratory and greenhouse settings. Using the Demonstration Grant and other funding supports, Ruan hopes to expand his system, showing that it can be used on a larger scale and identifying areas where automation could reduce costs. “When it’s small scale, you still need human labor to monitor, control, and harvest… In the future, we could use robotics, sensors, and big data control systems.” By increasing the capacity and reducing labor costs, Ruan envisions a design viable in rural and urban agricultural operations within the next decade.
With MnDRIVE Environment’s support, says Ruan, “We were able to optimize each step of the process. Now it seems to function quite well.” He hopes that the results of his Demonstration Grant will inspire new partnerships with private sector companies ranging from urban hydroponic centers to rural agricultural settings.