Granulated fertiliser from poultry litter
Northwest Arkansas is a region of high density confined animal feeding operations that produce annual surpluses of poultry litter (PL) and municipal biosolids (BS). Approximately 1.2 billion broilers are produced in Arkansas each year, resulting in 1.9 million metric tonnes of PL annually applied to pastures. Continued application of PL has increased environmental problems due to excessive phosphorus (P) runoff that pollutes waterways. Such public concern has also forced municipalities to landfill biosolids and place restrictions on PL application in sensitive Arkansas watersheds.
The overall objective of this study was to develop and test the utility of converting surplus PL and BS into a nitrogen (N) fortified granulated fertiliser that sustains crop production and environmental quality. To pursue this main objective, two subobjectives were required. Primarily, we developed a formulation and process to produce a granulated fertiliser that was approximately 14 percent N, 4 percent phosphate (P2O5), and 4 percent potash (K2O) using a combination of PL, BS, binding agents with different release characteristics, supplemental N, and a nitrification inhibitor [dicyandiamide (DCD)]. After the granular products were produced, quantification of elemental composition and water soluble release characteristics were investigated.
We produced four different formulations of products [PL + urea (PLU), PLU + DCD (PLUDCD), PLU + biosolids (PLUB), and PLUB + DCD (PLUDCD)] through agglomeration using a pin mixer. Three different binding agents [lignosulfonate (LS), urea formaldehyde (UF), and water] were used to give different physical and chemical properties. These 12 granular products were tested for bulk density, physical strength, release of water soluble elements, and total elemental concentration.
Overall, these products were bulkier than commercial fertilisers, but required less volume for shipping than fresh PL, especially if calculated on the nutritional fertiliser value. Compared to urea and triple super phosphate, granules were equally resistant to crumbling during shipment or application. Generally, products ranging from 14.3 to 16.8 percent N were produced, with percentages depending on dry ingredient, formulation or binding agents. Urea formaldehyde products contained the highest N content since UF resin contained N and also produced more resistant granules that may have inhibited ammonia (NH3) volatilization during the high heat agglomeration and drying processes. Total P concentrations ranged from 3.3 to 3.6 percent P2O5 while K2O ranged from 1.8 to 2.8 percent, both lower than anticipated.
Although low, P and K macronutrient concentrations were still acceptable for urban and agricultural markets and were highly dependent on formulation and binding agent. Formulations containing PL had higher concentrations of water soluble inorganic N and soluble reactive phosphorus (SRP) than biosolid formulations. Readily available soluble inorganic N and SRP were lowest in UF formulations but may be released later in the growing season when the plants are readily consuming large amounts of nutrients.
Preliminary data indicated that formulating granular products out of PL and biosolids made a suitable product for use in urban and agricultural markets. Granules can be produced for $0.71 to $1.15 per kilogram of N, similar to commercially available urea ($0.73 per kilogram N). The user would also receive other macro and micronutrients Â¡Â¡Ã£freeÂ¡Â¡Ã€ that they would not receive with a commercial fertiliser. A granulator plant producing 18 metric tonnes per hour, 16 hours per day, 24 days per month could produce 36,000 t fertiliser a year. Significant amounts of PL and BS could be transported out of sensitive nutrient surplus watersheds in Arkansas to areas suffering from nutrient deficiencies, especially P. Mineralization characteristics, production field trials, and environmental impacts still need to be quantified with these formulations and binding agents before they are produced on a commercial scale.
Tommy C. Daniel, Ph.D., Nathan A. Slaton, Ph.D., Morteza Mozaffari, Ph.D., Rick J. Norman, Ph.D., Mark Cochran, Ph.D., Findlay Edwards, Ph.D., and Mark S. Reiter, Crop, Soil, and Environmental Sciences Department, University of Arkansas, Fayetteville, 115 Plant Science Building, Fayetteville, Ark. 72701