Impact of Swine Sludge Inclusion Rate on the Composting Process and Compost Quality


The purpose of this study was to develop and analyze potential recipes for composting swine lagoon sludge. Composting is a simple treatment; it is widely adopted on farms, generates a stable value-added stackable product, and conserves organic matter and nutrients. All these benefits along with an affordable cost and lower environmental emissions make it a potential candidate for the management of lagoon sludge, a byproduct of swine operations in southeast US.

Sludge accumulation in lagoons can result in increased odor from lagoons, impact animal productivity, increase risk of environmental and social consequences and lead to operation non-compliance. Developing affordable sludge management alternatives is important because current practices (land application post dredging and dewatering using organic polymers and geo-bags) are not widely adoptable, cost-prohibitive and non-sustainable (Owusu-Twum and Sharara, 2020, Soil facts) and current farm nutrient management plans do not consider management of sludge nutrients.

What Did We Do?

We developed two recipes by mixing different sludge amounts with locally available low-cost amendments: poultry litter, Bermuda hay, yard debris and lagoon liquid. We composted these recipes in triplicates using 13-cubic feet in-vessel composters and recorded changes in temperatures, weight loss, volume, moisture, and organic matter. We also recorded greenhouse gases emitted from the piles at regular intervals. Forced, intermittent aeration was maintained during composting for replicates to ensure adequate oxygen supply and avoid prematurely drying mixtures. Finally, we analyzed the final compost to determine its suitability as a soil amendment.

We used the observations from the experiments to evaluate if proposed recipes resulted in successful compost and determine whether sludge inclusion significantly impacts composting process and product quality. We also analyzed which factors influence weight and organic matter losses in the piles and if the proposed recipes have comparable cumulative GHG and NH3 emissions to previous observations.

What Have We Learned?

We learned that sludge can be composted at both 10% and 20% inclusion rates using the above ingredients, as the process met time and temperatures for pathogen reduction (15A NCAC, 13B.1406) and the final product were stable (TMECC, US Composting council). For 100 lbs. of an initial wet mixture (60.8 to 61.4% moisture) both recipes experienced a total weight loss of 33.8-35.2 lbs. with 24.5 to 25.4 lbs. being lost as moisture and 8.8 to 9.7 lbs. lost as organic matter during the active phase of composting (31 days). Post-screening the recipes resulted in 42.3 to 48.6 lbs. of the stable final product (45 to 47% moisture) that can be directly land applied.

We learned that the composting process generated similar GHG, and ammonia emissions as reported in the previous studies however, most of the methane (CH4) and nitrous oxide (N2O) were generated in the later stages of composting, which can be potentially reduced by proper management of the composting process. Another observation was larger losses in ammonia in the earlier stages of composting which on reduction; using certain additives, changes in recipe or management practices, can result in optimal utilization of nitrogen, increase product value, and reduce environmental impacts.

Future Plans

We plan to further analyze the impact of the composting process on total nutrients and water-extractable fractions, this will provide information on land use rate and potential losses in runoffs. This information is critical for swine lagoon sludge-derived products due to the high concentration of P, Zn, and Cu in sludge as losses can lead to eutrophication in surface and marine waters and potential toxicity in soils.

Future work proposed also involves techno-economic evaluation of this process to determine the cost of treatment, and fair price of the final product. We also plan to conduct a cradle to gate life cycle assessment of the process to determine global warming potential, eutrophication, acidification, and particulate matter generation for farm and large-scale systems. These efforts will help guide further research to improve the technology and provide knowledge to stakeholders and producers on alternative sludge management options.

Figure 1. Swine lagoon sludge composting process and products.




Piyush Patil, Ph.D. Candidate, Bio&Ag. Engineering, North Carolina State University

Corresponding author

Mahmoud Sharara, Asst. Professor and Extension Specialist, Bio&Ag. Eng. North Carolina State University

Corresponding author email address

Additional authors

Stephanie Kulesza, Assistant Professor, Crop & Soil Sciences, North Carolina State University

Sanjay Shah, Professor and Extension specialist, Bio&Ag. Eng. North Carolina State University

John Classen, Associate Professor, Bio&Ag. Eng. North Carolina State University

Additional Information

Publication is in progress currently so best resource is the corresponding author.


We would like to acknowledge the support from Joseph Stuckey and Chris Hopkins (Poultry, livestock, and animal waste management facility, NCSU).

Funding sources

Bioenergy Research Initiative (BRI) – Contract No #17-072-4015, North Carolina Department of Agriculture & Consumer Services

National Institute of Food and Agriculture (NIFA) – Critical Agricultural Research and Extension (CARE) – Award No. 2019-68008-29894, U.S. Department of Agriculture


The authors are solely responsible for the content of these proceedings. The technical information does not necessarily reflect the official position of the sponsoring agencies or institutions represented by planning committee members, and inclusion and distribution herein does not constitute an endorsement of views expressed by the same. Printed materials included herein are not refereed publications. Citations should appear as follows. EXAMPLE: Authors. 2022. Title of presentation. Waste to Worth. Oregon, OH. April 18-22, 2022. URL of this page. Accessed on: today’s date.

Evaluating Full-Scale Greenhouse Systems for Lagoon Sludge Drying and Pathogen Reduction


This study aims to assess the performance of solar greenhouse drying as a management strategy for swine lagoon sludge. In North Carolina, swine lagoons are heavily concentrated in a small geographic area (Figure 1), which overlaps with a high concentration of poultry operations. This results in a challenge for identifying suitable acres for sludge application, particularly for nursery operations with high concentrations of zinc (Zn) in the sludge. Drying the sludge creates opportunity for long-distance transportation, further processing to produce marketable granules/pellets, as well as co-firing to produce energy and concentrate minerals in ash form. While a useful strategy, drying is an energy-intensive process and often requires significant capital investment and trained operators, both of which are a barrier for on-farm adoption. Solar drying using greenhouses is a low-cost technology that requires minimal oversight and management. While successfully adopted in wastewater treatment applications, solar drying has not been fully evaluated in manure management contexts.

Figure 1. Distribution of swine operations (red dots) in Eastern North Carolina (Source: NC Department of Environmental Quality)

What Did We Do?

Figure 2. Novel lagoon sludge drying system showing: (A) entrance and air inlet louvers, (B) exhaust fans, (C) sludge layer at beginning of drying test, and (D) panoramic view of drying bed at end of drying test.

We conducted two sludge drying tests in Summer 2021 using a greenhouse structure we built on NC State University campus (Figure 2). The tests utilized freshly dredged sludge with a total solid concentration of 8%. The sludge was in a pumpable state and was applied to the drying bed in a uniform layer. The drying bed consists of a concrete floor (17m long, 6 m wide) with a sweeping mechanism to level the sludge and facilitate removal at the study termination. Two different loading rates were tested in this study: 13.9 kg-m-2 (low loading density test), and 28.3 kg-m-2 (high loading density test). Only the two middle fans for the greenhouse structure were operated continuously during the testing period. Each fan had a nominal flowrate of 20,000 cubic feet per minute. The sweeping mechanism was operated twice daily to mix the material in the drying bed. Temperature and relative humidity were monitored inside the greenhouse structure near the air inlet, at the middle of the building, and near exhaust fans. Ambient conditions were monitored using an on-site weather station. At the completion of each test, the recovered material was analyzed for moisture, nutrient, ash, and energy content.

What Have We Learned?

The daily weather conditions during the two drying tests are summarized in Table 1. Ambient temperature and relative humidity within the greenhouse structure were greater for the high-loading density test than the low-loading density test by around 3℃ and 17% relative humidity points, respectively. This data suggests favorable drying conditions for low loading density. Wide variability in weather conditions during the spring and summer seasons in North Carolina are common and likely to be a significant factor in the performance of similar drying systems.

Table 1. Daily weather conditions during test periods (Source: Lake Wheeler Road Field Laboratory Weather station)

Test Date Temperature (C) Relative Humidity (%) Rain (mm)
Avg. Solar Radiation (W.m-2) ETo* (mm)
Mean Max. Min. Mean Max. Min.
Low Loading Density 5/19 20.2 28.3 11.2 55.5 91.7 25.1 0.0 334.3 5.2
5/20 22.0 29.8 11.4 54.9 91.9 27.5 0.0 318.5 5.5
5/21 20.9 27.8 14.7 47.8 78.2 22.6 0.0 296.4 5.7
High Loading Density 5/25 22.9 29.6 17.6 76.7 92.5 53.0 0.0 211.3 4.2
5/26 26.3 34.3 20.6 65.4 86.4 34.4 0.0 281.4 6.1
5/27 26.5 33.5 18.9 70.4 86.6 42.4 0.0 280.2 5.8
5/28 25.9 33.0 18.7 62.1 91.3 30.5 1.8 267.7 6.4
5/29 20.8 30.3 12.4 76.0 43.0 43.0 1.8 162.8 4.6

*Penman-Monteith Estimated Evapotranspiration at 2-meter height

Cross-greenhouse variability in air temperature and relative humidity was greatest between sunrise and sunset (7AM and 6PM) indicating active drying. Mechanical mixing of the sludge (Figure 3, denoted by vertical yellow lines) appears to have had a positive impact on the drying process as evidenced by the increase in average air temperature and relative humidity after mixing. This effect can be attributed to exposing more water-saturated sludge to drying air, which increases the moisture gradient, thus boosting convective drying. In addition, wet sludge is darker in color, which increases its radiation heat absorption.

Figure 3. Average greenhouse temperature (upper) and relative humidity (lower) during high-loading test. Vertical yellow lines indicate drying bed turning using the mechanical sweeping mechanism.

For the high-loading rate test, observing temperature and relative humidity on Day 4 (Figure 3) during daytime (hours 77 to 80) indicates the drying process has slowed down considerably with average air temperature and relative humidity inside the greenhouse closely matching inlet air. These observations suggest that accessible water in the sludge has been effectively removed.

Moisture content of sludge in the low-loading rate test decreased by 88% over 46 h while the high-loading rate treatment yielded a 91% reduction in moisture over 101 h. Using the starting total solids content of 7.9%, Using the drying bed dimensions and test duration, the average drying rates observed were 0.26 and 0.25 kgH2O.m-2.h-1 for low and high loading density tests, respectively. Over the drying duration, the average evapotranspiration, estimated using Penman-Monteith equation, was 0.23 kgH2O. m-2.h-1 (with a standard deviation of 0.03 kgH2O. m-2.h-1) which is 9% and 13% lower than observed evaporation rates for low and high loading density tests. These observations indicate the depth of material addition had minimal effect on the drying rate. In addition, these observations suggest the combined effect of mechanical ventilation and energy gain due to the greenhouse effect, increased the evaporation rate beyond average evaporation rate estimates. The air use efficiency was estimated as the amount of water removed during the test to the maximum amount of water removable (i.e., resulting in drying air saturation). Air quality at inlet and exhaust were used to estimate moisture ratio at both points. The air use efficiency for low and high sludge loading density were 21.1% and 21.0%, respectively.

Future Plans

These findings suggest opportunities to improve the drying efficiency. We are currently assessing different controllers in terms of their ability to manage mechanical ventilation to minimize energy use in low-drying conditions. Similarly, we are planning a series of tests to capture seasonal weather conditions. Gaseous emissions (ammonia and greenhouse gases (GHG) will be evaluated on full scale operation). Pathogen count for the starting and ending material will be quantified and reported to assess any risk associated with wider distribution of the dewatered material. Currently, a full-scale sludge drying compound (929square meters) has been built and operated on a swine operation in Eastern North Carolina. Our team is collecting data on the system operation and will be reporting system performance and energy use in upcoming meetings.


Mahmoud Sharara, Assistant Professor and Extension Specialist, North Carolina State University

Christopher Hopkins, Research Associate, Department of Forest Biomaterials, College of Natural Resources, NC State University
Joseph Stuckey, Research Operations Manager, Animal Poultry Waste Management Processing Facility, Prestage Department of Poultry Science, NC State University

Additional Information


  • NC Department of Agriculture and Consumer Services, Bioenergy Research Initiative (BRI) – Contract No #17-072-4015, Potential for Integrating Swine Lagoon Sludge into N.C. Bioenergy Sector
  • Virginia Pork Council, Optimizing Greenhouse Drying of Swine Lagoon Sludge to Support Implementation in NC and VA.


The authors are solely responsible for the content of these proceedings. The technical information does not necessarily reflect the official position of the sponsoring agencies or institutions represented by planning committee members, and inclusion and distribution herein does not constitute an endorsement of views expressed by the same. Printed materials included herein are not refereed publications. Citations should appear as follows. EXAMPLE: Authors. 2022. Title of presentation. Waste to Worth. Oregon, OH. April 18-22, 2022. URL of this page. Accessed on: today’s date.