Liquid-State Poultry Litter Digestion System under Development to Make Organic Fertilizer, Biogas

Nov. 2021

Jun Zhu_Yuanhang Zhan

Jun Zhu, director of the Center for Agricultural and Rural Sustainability and professor of biological and agricultural engineering for the University of Arkansas System Division of Agriculture, stands with Ph.D. student Yuanhang Zhan in the Biological and Agricultural Engineering Lab with a liquid-state poultry litter digester prototype.

Jun Zhu

Jun Zhu

Professor of Biological and Agricultural Engineering

Contact Jun Zhu

 

The Problem

Poultry production in Arkansas generates about 1.3 million metric tons of litter a year, most of it being concentrated in the northwest region of the state, according to the Center for Agricultural and Rural Sustainability (CARS).

Continuous application of poultry litter as a fertilizer has proven to lead to excessive amounts of plant-extractable phosphorus. Although shipment of poultry litter to neighboring states for use on farms through LitterLink.com has helped mitigate the surplus phosphorus from northwest Arkansas watersheds, the arrangement only “temporarily alleviates the immediate burden of the excessive litter nutrients on water pollution in Arkansas,” the CARS Waste Management program states.

 

The Work

The Arkansas Agricultural Experiment Station is leading the effort in the development of a fertilizer and fuel production system that turns chicken litter into both biogas and struvite. Most of the water used in the anaerobic digestion process is recycled to the first phase of the machine.

Two other universities — the University of Idaho and Virginia Tech University — are working in tandem with the experiment station on the three-piece system that is funded by USDA/NIFA/AFRI Foundational and Applied Science Program.

“Our part is to treat poultry litter using anaerobic digestion, which produces an effluent containing increased magnesium ions that can be used for struvite production in the next step,” explains Jun Zhu, director of the CARS, and professor of biological and agricultural engineering for the Division of Agriculture.

By the end of 2022, the two other pieces of the system are expected to be delivered to the experiment station to complete the system for further testing. The team from Idaho is making the device that creates struvite. The Virginia Tech team is developing the water recovery system. Zhu said 80 to 90 percent of the water used in the poultry digestion process will be recycled to save water used to make struvite. He also expects an upscaled system to be used at an experiment station broiler house for further tests. To his knowledge, this is the only system under development that creates biogas and struvite from liquid-state poultry litter digestion in the nation.

Yuanhang Zhan, a Ph.D. student in Zhu’s lab, is working to optimize the carbon-nitrogen ratios with wheat straw in the poultry litter digestion phase to produce an expected amount of methane and enrich magnesium ions in the effluent. Wheat straw was chosen because of its common use as a winter crop in Arkansas. The project was put on hold during the COVID-19 pandemic and resumed in September with Zhan.

In 2018, Zhu conducted liquid-state digestion experiments with his post-doctoral associate Jiacheng Shen and Sarah Wu, assistant professor at the University of Idaho, to find optimal total solid content for anaerobic digestion of poultry litter with wheat straw. Tests showed it to be around 4.15 percent to achieve the best biogas yield. Similarly, for digesting poultry litter without the addition of wheat straw, the total solid level content should be kept at around 6.8 percent, the test results state.

The 2018 study tested the batch operation of the digester. Based on those results, the current study is evaluating a continuous operating scheme.

Yuanhang Zhan places samples from a liquid-state poultry litter digester to measure ammonia and total nitrogen levels as part of an ongoing study on a poultry litter and wheat straw mixture that will be used to make struvite in a system that recycles most of the water used in the digester.

The Results

Zhu said preliminary data collected in 2021 on the anaerobic digestion portion shows the carbon/nitrogen ratio, total solids content, and hydraulic retention time have the most impact on the digestion process.

The results, however, also showed that adjusting the substrate carbon-nitrogen ratio alone could not always result in high methane yields and biogas volumes. Zhan is trying to determine why there was a total loss of methane production when the free ammonia nitrogen concentration reached 1,000 milligrams per liter.

Zhu hopes to implement a scaled-up system at an experiment station broiler house to further develop a cost-effective technique for individual poultry farms to solve any potential poultry litter issues with an ecological transformation method.

 

The Value

Struvite, one of the primary outputs from the digester, is a granular fertilizer made up of phosphorus, nitrogen and magnesium that offers a slow release of nutrients to plants, according to the University of Arkansas System Division of Agriculture Research & Extension publication. About 0.02 percent of struvite is water-soluble, so there is less phosphorus runoff into water sources to create algal bloom.

Biogas, largely in the form of methane, can also be captured in the liquid-state digestion process to use as a fuel source for heating the poultry house or the farmhouse, Zhu said. The gas could be used for heating water and cooking on individual farms, he added.

One of the primary benefits of using anaerobic digestion on poultry litter, Zhu explains, is that it creates a more stable product, removes nuisance odors, maintains the fertilizer value of the manure, and produces a renewable fuel.

“Another great benefit of anaerobic digestion is the reduction in pathogens in the digested effluent,” Zhu writes. “Salmonellae, fecal coliforms, oocysts (Eimeria tenella), and fungal spores are all either significantly destroyed or inactivated in the anaerobic process.”

To learn more about the Division of Agriculture research, visit the Arkansas Agricultural Experiment Station website: https://aaes.uada.edu/. Follow us on Twitter at @ArkAgResearch and Instagram at @ArkAgResearch.

 

About the Researcher

Jun Zhu

Professor of Biological and Agricultural Engineering

  • B.S. in civil engineering from Zhejiang University, Hangzhou, China; M.S. in civil engineering from Zhejiang University; Ph.D. in agricultural engineering from the University of Illinois at Urbana-Champaign.
  • As director of the Center for Agricultural and Rural Sustainability, Zhu specializes in research that focuses on waste management technologies that protect air and water quality related to animal production. He is also focused on renewable energy production technologies and bioconversion of wastes into value-added products.
Yuanhang Zhan

Yuanhang Zhan

  • B.S. in Biosystems Engineering from Zhejiang University, Hangzhou, China; M.S. in Environmental Engineering, Chinese Academy of Agricultural Sciences, Beijing, China
  • Ph.D. student in the Biological and Agricultural Engineering Department, University of Arkansas at Fayetteville