North Carolina solar energy powers nationwide distribution of hog manure nutrients
By Mahmoud Sharara, Christopher Hopkins, Sanjay Shah and Joseph Stuckey
Lagoons are critical to swine production in North Carolina as they provide cost-effective manure storage and treatment. The temperate climate in the southeastern United States keeps the microbial communities active and improves manure digestibility. Over years of operation, however, lagoons accumulate a solids residue, known as sludge (Figure 1), which cannot be further digested.
On average, sludge accumulates in the lagoon at an annual rate of 0.36 gallons per pound of live animal weight2. This means that for a 3,000-head finishing farm, around 145,800 gallons of sludge accumulates in the lagoon annually. If left in the lagoon, the sludge reduces the treatment capacity of the lagoon leading to higher odor emissions, and increases risk of lagoon failure under extreme rainfall events.
The North Carolina Department of Environmental Quality (NCDEQ) requires that sludge removal to maintain at least 50% of the lagoon treatment volume sludge-free at all times.
Figure 1. Cross-view of anaerobic manure treatment lagoon.
Sludge, unlike manure, contains a high concentration of minerals relative to nitrogen (Table 1). As a result, sludge requires more acres for land application to avoid over-application of phosphorus (P), zinc (Zn), and copper (Cu).
Today, many swine producers are facing a challenge finding acres that can accept and beneficially utilize sludge nutrients. The high density of hogs and poultry production across North Carolina, and the reliance on feed imports, has led to regional nutrient accumulation. A recent study by ARS researchers3 identified the Carolinas among the largest sources of manure P in the continental United States that can benefit from a wider distribution of nutrients. For North Carolina hog producers, identifying a cost-effective technology to concentrating sludge nutrients and allow export is a priority.
Why dry sludge? To remove sludge, lagoons are typically agitated to suspend sludge solids before the slurry is loaded into spreaders for land application.
More recently, growers resorted to sludge dredging and dewatering using on-site permeable bags (Figure 2).
Figure 2. Dewatering bags with swine lagoon sludge (at 20% solids) on a North Carolina hog farm.
This approach was adopted to concentrate the sludge and make it more transportable. For comparison, the agitation approach removes sludge at 6% solids, compared to 20% solids with dewatering.
Even with after dewatering, finding suitable land base for the dewatered sludge remained a challenge, in addition to the cost involved in the dewatering step. Hence, drying offers a great opportunity to concentrate nutrients and create a product that is easy to store, transport and apply using existing equipment.
Through funding from the NC Department of Agriculture and Consumer Services - Bioenergy Research Initiative (BRI) and Smithfield Foods, we set out to investigate opportunities for sludge removing, drying and utilizing sludge as a renewable nutrients and energy source.
Phosphate (P2O5)
Potash (K2O)
Table 1. Typical nutrient content in fresh swine manure, lagoon liquid, and lagoon sludge in pounds per 1,000 gallon (not-for-use as planning guide).
a: Chastain, J.P.; Camberato, J.J.; Albrecht, J.E.; Adam, J. Swine Manure Production and Nutrient Content; South Carolina Confined Animal Manure Managers Certification Program; Clemson University: Clemson, SC, USA, 1999; Chapter 3; pp. 1–17. b: Manure Nutrient Content, Nutrient Management in North Carolina – NC State University guide. Online at: https://nutrientmanagement.wordpress.ncsu.edu/ c: NCDA&CS Agronomics Laboratory, swine lagoon supernatant samples submitted between 2010 and 2019/ d: Sharara, M and Owusu-Twum, 2020, Sludge Sampling in Anaerobic Treatment Swine Lagoons (Factsheet). NC Extension Factsheet. Online at: https://content.ces.ncsu.edu/sludge-sampling-in-anaerobic-treatment-swine-lagoons
Industrial drying is commonly used to handle sludge in municipal wastewater treatment plants, particularly in large metropolitan districts.
We evaluated four commercial systems currently in use in municipal sludge drying, using technologies such as rotary drums (Option 1), steam moving beds (Option 2), belt drying (Option 3), and high-velocity cyclones (Option 4).
We used a one dry ton of sludge (1 ton at 10% moisture content) as the basis for comparing the cost of the drying technologies, with an annual throughput of 20,000 dry tons for each system. Such a scale is common for these technologies and often provide economy of scale to the drying and processing.
The first barrier we identified is the logistical planning and cost involved in aggregating sludge to satisfy the system capacity. The typical scale for such dryers makes them too large for a single farm or a small cluster of farms. On the other hand, aggregating wet sludge involves significant hauling cost and increases the risk of spill during transportation.
The second barrier we identified is the significant energy cost involved in producing a dry sludge product (Figure 3). The energy inputs, both heat and electricity, represent between 35% and 50% of the cost of drying. Due to these barriers, we began investigating the use of low-input drying technologies such as ambient air-drying and solar drying.
Figure 3. Sludge drying cost using different technology options at a 20,000 ton/year capacity. The cost estimates include share of capital cost (CAPEX), labor, and energy needs (heat, and electricity).
Solar drying opportunities Solar energy is an abundant resource particularly in the south and southeastern United States. In North Carolina, solar radiation provides between 4.5 and 5 kWh of energy per square meter per day (equivalent to 1.4 to 1.6 MMBtu/sq.ft.-day)4. This resource is already being harnessed by the solar energy sector across North Carolina, less so in thermal applications such as drying.
Open-air drying is the lowest cost option but not suitable for the Southeastern United States due to high rainfall; year-round rainfall that averages 48 inches in Eastern North Carolina where hog production is clustered. As a result, we adopted greenhouse designs to both utilize solar energy and avoid rainfall interference. We conducted preliminary studies to quantify the rate of sludge drying for swine lagoon sludge in greenhouse structures.
For the first tests, we utilized a greenhouse system on NC State University campus (Figure 4) to dry freshly dredged lagoon sludge. The sludge was dredged from the on-campus swine unit and were introduced to the greenhouse at an initial solid content (TS) of 7.9%.
Two tests were conducted during summer 2021 utilizing two loading rates: 2.85 lb. and 5.80 lb. of sludge per square foot.
During both tests, we observed a daily drying rate between 1.1 and 1.2 pounds of water removed per square foot.
Under the high loading rate, the sludge reached 91% total solid content after 102 hours. The dried product N: P2O5: K2O equivalent to 5: 15: 1, with 72 pounds of N, 229 pounds of P2O5, and 14.5 pounds of K2O per dry ton.
Figure 4. Greenhouse drying structure on NC State University campus (left) and drying a thin-layer (0.5-inch) of dredged sludge in the greenhouse (right).
In summer 2021, Smithfield Foods built two greenhouse-drying units in Duplin County, North Carolina (Figure 5). The long axis of these greenhouses are east-west. These units (5,000-sq.ft area each) will be used to dry on-site dewatered sludge currently in geobags as well as off-site material.
Our team was funded by the Virginia Pork Council to collect yearlong data on the performance of this technology and identify opportunities to improve its performance.
Figure 5. Solar drying greenhouses for lagoon sludge in Duplin County, North Carolina.
The greenhouses are operated as deep bed drying units with a loading rate of 34 pounds of sludge per square foot. The sludge is loaded into greenhouses at a total solid of 20%.
The material is regularly turned using a tiller to avoid crusting which was observed to slow drying.
The greenhouse ventilation is managed using a controller equipped with temperature and humidity sensors inside and outside both greenhouses.
A key priority is avoiding operating the ventilation system during high humidity conditions in the summer and during rainfall events. Our team started the data collection/monitoring during winter 2021.
We observed significant heat gain during winter due to the greenhouse effect (Figure 6). This thermal gain has a dual benefit of increasing the rate of drying as well as deactivating any remaining fecal communities in the sludge to ensure safe handling and use.
During December-January 2022, the daily rate of drying was between 0.5 and 0.6 pounds of water removed per square foot, requiring 40 days to reach desired total solid concentration.
The seasonality of solar radiation and temperatures play a large role in the observed rate of drying in these systems. We plan to continue comparative evaluation of solar drying on thin-layer and deep-bed sludge under different weather conditions to develop management recommendations.
Figure 6. IR image of solar drying greenhouse showing temperatures between 9 and 25.7°C (48 to 78°F) with outside air temperatures at 5.2°C (41.4°F).
A key question is how the economics of solar drying compare to industrial dryers discussed earlier. To answer this question, we utilized data collected so far from solar greenhouse drying (during winter 2022) and compared it to commercial-scale systems reviewed earlier.
Using one ton dried sludge as a comparison basis, the energy consumption associated with industrial drying ranged from $49.3 and $69.2 per ton. By comparison, the energy expenditure for solar drying ranged between $9.4 and $11.9 per ton of dried sludge.
Energy consumption for solar drying is primarily associated with ventilation power consumption. We are currently working to complete a year-round performance assessment of solar greenhouse sludge drying and compare it to the systems reviewed earlier.
Using winter drying performance as basis for comparison shows that solar drying energy cost is 18% to 22% of the cost associated with the most energy-efficient industrial dryer. In addition, reducing energy consumption by adopting solar drying also lowers greenhouse gas (GHG) emissions, which aligns with the priorities of the hog industry nationwide.
The team is continuing to study these systems to improve drying performance, reduce energy use and identify value-added uses for dried products using post-processing technologies such as pelleting and formulating the dried product with other nutrients. These technologies could be centralized once sufficient farms adopt the concept of sludge drying.
Interest by industry groups in this technology and the entrance of commercial developers that are already installing sludge drying systems5 indicate the need for such technology and its fit with North Carolina hog production context.
This move is projected to greatly improve manure nutrient use and increase the sustainability of hog production in the United States.
References 1. US Department of Agriculture, Natural Resources Conservation Service (USDA-NRCS), Agricultural Waste Management System Component (Chapter 10), in Part 651 – Agricultural Waste Management Field Handbook. Available online here. 2. Bicudo, J. R., Safley Jr, L. M., & Westerman, P. W. (1999). Nutrient content and sludge volumes in single-cell recycle anaerobic swine lagoons in North Carolina. Transactions of the ASAE, 42(4), 1087. 3. Spiegal, S., Kleinman, P. J., Endale, D. M., Bryant, R. B., Dell, C., Goslee, S., ... & Yang, Q. (2020). Manuresheds: Advancing nutrient recycling in US agriculture. Agricultural Systems, 182, 102813. 4. National Renewable Energy Laboratory (NREL), Department of Energy (DOE), Solar Resource Maps and Data: Global Horizontal Irradiance. Available online here. 5. [Press Release] Could NC Export Poop For Profit? Crop and Soil Sciences News - February 9, 2022.
Sharara is an assistant professor and Extension specialist and Shah is a professor and Extension specialist, both in the Biological and Agricultural Engineering Department; Hopkins is a research associate in the Department of Forest Biomaterials, College of Natural Resources; and Stuckey is a research operations manager at the Animal Poultry Waste Management Processing Facility, Prestage Department of Poultry Science; all with North Carolina State University.
