Purpose

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.

What Did We Do?

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)

*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.

Authors

Mahmoud Sharara, Assistant Professor and Extension Specialist, North Carolina State University
msharar@ncsu.edu

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

https://animalwaste.ces.ncsu.edu/2021/08/drying-sludge-critical-to-improving-nutrient-distribution-and-utilization/

Acknowledgements

• 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.

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