Article pubs.acs.org/EF
Positive Operations on Wastewater Sludge Drying: Comparison of Back-Mixing and Sawdust Addition Jie Li,*,†,‡ Erwan Plougonven,‡ Laurent Fraikin,‡ Thierry Salmon,‡ Dominique Toye,‡ Emmanuel Nistajakis,§ and Angélique Léonard‡ †
Department of Thermal Engineering, School of Energy and Power Engineering, Wuhan University of Technology, Wuhan, Hubei 430063, People’s Republic of China ‡ Laboratory of Chemical Engineering, Department of Applied Chemistry, University of Liège, 4000 Liège, Belgium § Industrie du Bois Vielsalm & Cie SA, Zoning Industriel de Burtonville, 6690 Vielsalm, Belgium ABSTRACT: This work compares two positive operations, back-mixing and sawdust addition, on the drying of wastewater sludges. The influences of back-mixing (with mass percentages of dried sludge of 40, 60, and 70% on a dry basis) and sawdust addition (40% on a dry basis) operations on the drying rates of sludge in a pilot-scale fixed bed were studied. X-ray tomography was used to monitor the evolution of structural characteristics of the samples during drying. When using the same mass percentage of dried materials (40%), the sawdust−sludge mixture has a higher drying rate than the back-mixed sludge as a result of higher initial and final bed volumes together with higher total exchange surface areas. To reach a similar rate increase with a back-mixing operation, 60% dried sludge must be back-mixed.
1. INTRODUCTION The valorization of wastewater sludge has become a critical issue as a result of the continuously increasing quantity of sludge generated from wastewater treatment plants (WWTPs).1,2 Several operations, e.g., land application, gasification, and incineration, are used for this. However, a critical parameter that can hinder the efficiency of these management routes is the high moisture content,3 and previous studies have shown the importance of the thermal drying operation to deal with this.4−14 Thermal drying not only stabilizes the sludge and substantially reduces its mass and volume, it also raises its calorific value, making it better adapted to a thermochemical conversion process.15 The efficiency of the valorization will be optimal if as little energy as possible is used to prepare it and the operation requiring the most amount of energy is the drying step. Therefore, the purpose of this study is to determine the most efficient method to dry sludges and sludge mixtures. In many cases, the thermal drying of sludge can be very difficult as a result of its softness and cohesiveness. The developed exchange surface of the sludge sample is small because of its low stiffness, which leads to a low drying rate; therefore, the addition of a dried product to the sludge to enhance its stiffness can be helpful. The back-mixing operation, in which the recycled dried sludge is mixed back into the raw sludge, has been investigated in a pilot-scale fixed bed by Léonard et al.16 Two sewage sludges were studied, and it was found that the drying rate increased with the ratio of the dried sludge. In addition, the shrinkage of the bed, measured by X-ray tomography, also reduced with more back-mixing. Sawdust is another type of dry product that can be added to improve the drying rates. Drying experiments of sawdust−sludge mixtures in a pilot-scale fixed bed have been reported previously14,17,18 and showed again the increased drying rates and lower shrinkage, when the mass percentage (on a dry basis) of sawdust exceeded © XXXX American Chemical Society
20%. These two operations are beneficial for the drying process, but there is still a lack of information regarding the amount of benefit of each. The present work aims to refine the investigation of the evolution of structural characteristics of the sludge beds after back-mixing and after addition of sawdust and more precisely quantify how these two operations influence the process of sludge drying. X-ray tomography is used to measure the structural characteristics of the samples in a non-destructive way. Three-dimensional (3D) tomographic reconstruction of the interior geometry of the samples is a powerful method for obtaining accurate quantitative information on the bed structural characteristics.19
2. MATERIALS AND METHODS 2.1. Materials. The sludge was collected at the Grosses Battes WWTP located near the University of Liège in Belgium. Its physical and chemical characteristics are reported in Table 1. The moisture content was measured using standard methods,20 and the sludge was stored at 4 °C to conserve drying properties during the storage period.21 The incorporation of sawdust is performed with the initial fresh sludge. The sawdust was gathered in a nearby wood pellet factory (“Industrie du bois”, Vielsalm, Belgium). The particle size is under 5 mm, and 96.5% is common spruce (Picea abies), while the rest is Douglas fir (Pseudotsuga menziesii). Table 2 shows the dry basis elemental and thermal analyses of the sawdust. It is first oven-dried to reach a zero moisture content, to accurately measure the mass percentage of sawdust on a dry basis in the mixture. In this work, it was set at 40% because it was shown in previous studies14,17,18 that, for the sludge from the Grosses Battes WWTP, the best drying rates were Received: December 19, 2015 Revised: March 3, 2016
A
DOI: 10.1021/acs.energyfuels.5b02960 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
with a water/dry air ratio of 0.004 kg of water kg−1 of air during the study. Weight measurements were made in situ every 10 s throughout the drying process. 2.3. X-ray Tomography. The X-ray tomographic setup used was a high-energy X-ray tomograph, first used by Toye et al.24 The detailed description of this X-ray tomograph is shown in previous studies.14,17−19,24 The X-ray source was set at 420 kV and 3.5 mA. The detector in this system is linear; therefore, one acquisition allows for the reconstruction of one cross-section, and the vertical position of the source/detector must be changed before another one-dimensional (1D) acquisition is performed. A distance of 2.2 mm between two such acquisitions was chosen as a compromise between the experimental duration and spatial accuracy. 2.4. Image Analysis. The gray levels in the tomographic reconstructions correspond to X-ray attenuations of the two phases that comprise the material: the solid phase, i.e., the sludge or sawdust− sludge mixture, with a high gray level (i.e., high X-ray attenuation), and the void or pore space, with a low gray level. A circular mask representing the interior diameter of the sample holder defines the region of interest in the two-dimensional (2D) cross-sections, which are then binarized, assigning the value of 0 to pixels belonging to the void and the value of 1 to pixels pertaining to the solid, using Otsu’s method.25 The 3D images of the sample bed are then built by stacking these cross-sections. The calculation methods for the volumes (bed volume and object volume) and total exchange surface area are presented in previous studies.14,16,26 The image analysis toolbox version 6.0 of the MATLAB software (Mathworks, Natick, MA) was used to complete the operations. The detailed approach of the image analysis is described in ref 19.
Table 1. Sludge Characteristics
product sludge
origin
equivalent population
effluent
dewatering
initial moisture content on a wet basis (%)
59040
domestic
belt filter
85.5
Grosses Battes WWTP, Liège, Belgium
Table 2. Dry Basis Elemental and Thermal Analyses of the Sawdust calorific value (MJ/kg)
elemental mass percentage (%) C
H
N
S
Cl
gross
net
50.5
6.10
