Composting Turns Swine Lagoon Sludge to Landscaping Products
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- The process provided here is only an example of an approach to create value-added products from swine lagoon sludge.
Swine lagoon sludge is challenging to use as a soil amendment because of its high phosphorus (P), zinc (Zn), and copper (Cu). These minerals can stress growing crops and increase nutrient losses. One way to overcome this challenge is through composting. Sludge composting relies on bacteria to break down sludge after mixing with carbon-rich material (such as wood chips and peanut hulls). The product (Compost) is a stable soil amendment that can be used in various applications. This process eliminates odor and pathogens and reduces the concentration of P, Zn, and Cu. Many North Carolina swine producers already rely on composting to manage on-farm mortality. Few, however, have used composting to manage swine lagoon sludge and create a valuable product. In the following article, we briefly review a commercial composting facility in Eastern North Carolina (Mirimichi Green) that apply the composting process to convert swine lagoon sludge to a commercial landscaping product used on turf, golf courses, and ornamental plants to promote growth.
Lagoon Sludge Composting
The composting facility receives sludge that has been bagged and dewatered using a chemical polymer. The dewatering process results in a sludge that contains around 20% solids. After the hauling trucks unload the sludge (Figure 1), it gets mixed with cotton gin waste, wood and leaf residue, and peanut hulls to achieve moisture content between 50% to 60%. The mixture is then formed into large static piles to initiate the composting process.
Within two days, the piles heat up to temperatures over 130°F. According to permitting requirement by the North Carolina Department of Environmental Quality (DEQ), this facility type needs to maintain the temperature in the pile at or above 131°F for at least 15 days to reduce pathogens and eliminate flies and other vectors. The permit also requires keeping a daily record of temperature at different points in the pile during composting. Compost thermometers, with a 3ft or 4ft long stem, are typically used to measure temperatures inside the piles. The composting pile is turned every 1 to 2 weeks or when core temperature starts to drop. Turning the pile is critical to introduce oxygen to aerobic bacteria and to make sure it is uniformly composted. Afterwards, the mixture is left to cure for months to ensure it is completely stabilized.
Compost Stabilization and Processing
After curing, this facility operators form the compost into windrows. This step stabilizes the mixture further and drives off residual moisture before processing. A composting row turner is used to turn and aerate each row (Figure 2). The windrow temperatures are slightly above ambient temperature which is a sign of a stabilized compost. With frequent turning, the mixture gets progressively dryer which helps during the screening process.
This particular facility creates a commercial product that requires more processing steps. These include biochar addition, prilling, screening, and commercial bagging. These steps ensure the product has a uniform particle size (Figure 3) and can be easily applied using common applicators. These processes require expensive equipment and facilities which can be a barrier to individual operators. Aggregating sludge from several farm to a central facility helps spread out the cost for establishing a similar composting facility.
The company indicated the finished product is currently marketed across the U.S. through more than 1,000 distributor locations. The prilled compost is used for landscaping and lawn-care, including on golf courses and football and baseball fields (NFL, MLB).
Swine lagoon sludge is a challenge to manage due to its high content of minerals and metals. Composting helps overcome this issue through mixing it with a low-minerals and carbon-rich residue. An added benefit here is the low-pathogen and odor properties which allow a wider product distribution beyond the farm. Establishing similar facilities requires developing a process that align with permit requirements, and establishing distribution arrangements to market the finished product to end-users.