Liquid mixing and solid segregation in high-solid anaerobic digesters
Introduction
The generation of agricultural, industrial and municipal waste is a direct consequence of human activity, and it increases along with the population growth and the economic status of the society. In addition to recycling management, different technologies have been developed to treat and make value out of solid organic waste: incineration, landfilling, biological processes (composting and anaerobic digestion). Anaerobic digestion (AD) is one of the most attractive technologies for turning solid organic waste into energy (Mata-Alvarez et al., 2000), because of its economical and energy recovery benefits. It consists in a complex decomposition process of organic matter by a variety of anaerobic microorganisms under oxygen-free conditions. The end products of AD include biogas, a renewable source of energy, and an organic residue (digested material), often named digestate, which can be used as an organic fertilizer in agriculture. For the treatment of organic solid waste, high-solid AD, which is operated at a total solid (TS) content >20%, presents a good alternative to wet systems (TS < 10%), because of the reduction of the amount of water to be added to the raw waste (Jha et al., 2011, Li et al., 2011, Karthikeyan and Visvanathan, 2012). The main advantages of this technology are: less energy needed and smaller digesters for the same organic loading rate, simpler phase-separation step for the digestate, simpler steps for pretreating feed materials.
These technical and operational simplifications contributed to the commercial success of high-solid systems in the last decades. As a consequence, high-solid systems represented about 60% of the total treatment capacity in Europe in 2010 (De Baere et al., 2010). Three main industrial processes (Valorga, Dranco and Kompogas) share the market of continuous AD systems (20% ⩽ TS ⩽ 35%), which mainly differ by the mixing technique (gas injections, mechanical mixing with paddles, or an external pumping device).
In such systems, both physical (mass transfers) and biological (microbial kinetics) processes are interconnected. For the specific case of high-solid anaerobic digestion, the limitation of the global potential by physical phenomena is exacerbated because of the low water content. Due to the presence of solids, the properties of water in high-solid anaerobic digesters differ from those of free water: a substantial amount of water is bound to the solids (Garcia-Bernet et al., 2011a). This strongly affects the global AD performances (Le Hyaric et al., 2012a, Le Hyaric et al., 2012b). The rheological behavior of anaerobically-digested solid waste is also strongly affected by the low water content (Battistoni, 1997, Battistoni et al., 1993). High-solid digestates are now known to be visco-elastic materials characterized by high yield stress over 200 Pa (Garcia-Bernet et al., 2011b). The yield stress corresponds to the force required to make the medium flow. In digestion media, it increases with the TS content according to an exponential law, and its magnitude also depends on the physico-chemical characteristics of the solid matter (e.g., granulometry, waste origin, etc.). Because of such characteristics, mixing a high-solid AD systems is difficult to operate (Terashima et al., 2009, Wu, 2012), and pilot plants are usually mixed sequentially with heavy (and energy consuming) equipments.
Mass transport is governed by two main mechanisms: diffusion and convection (related to the mixing efficiency). Diffusive transport is strongly related to the porosity and the viscosity of the medium (i.e., to the global water content). Based on an innovative experimental method, Bollon et al. (2013) recently determined the diffusion coefficients of high-solid digested media, and showed that the ratio between the diffusion coefficient in the digestate and water was very small, respectively, 1.8 × 10−2 and 0.54 × 10−2 at 8% TS and 25% TS. In unmixed or sequentially mixed conditions where mass transport is mainly governed by diffusive transport, it is now well admitted that transports of soluble substrates and metabolites (dissolved gases, volatile fatty acids, etc.) plays a crucial role in a good operation of high-solid AD (Abbassi-Guendouz et al., 2012, Bollon, 2012). As a consequence, mixing a high-solid anaerobic digester is needed to generate an efficient convective mass transport and to provide a good homogenization of soluble compounds. In addition, an efficient mixing process is required to enhance the contact between the micro-organisms and the organic solid matter, but also to prevent the sedimentation (or the formation of a floating layer) of solid particles having different densities than the digested media. To our knowledge, such behavior of high-solid digesters has never been experimentally identified even at the laboratory-scale.
In the present paper, an experimental procedure was first proposed to identify the macro-mixing of both liquid and solid phases. Indeed, the Residence Time Distribution (RTD) techniques are often used to investigate the hydrodynamic behavior of one single liquid phase (Levenspiel, 1972, Martin, 2000, Escudie et al., 2005). The main challenge of the present work was to define adequate tracers to follow both liquid and solid phases. Based on this experimental approach, the second objective was to analyse the effect of the TS content and waste origin on the macro-mixing of both the liquid phase and the solid phase in order to investigate the segregation of solid particles.
Section snippets
Reactor design
The laboratory-scale dry anaerobic digester used in this study was designed and constructed based on the Valorga® technology. It consists of a 58 L cylindrical vessel (operational volume = 43 L), with a height of 0.49 m and a diameter of 0.39 m (Fig. 1). The main feature of this process was the complete absence of any mechanical equipment inside the reactor. In this lab-scale reactor, the digested media was mixed by 2-s sequential injections of biogas every 30 min through 14 injectors located at the
Macro-mixing characterization of the liquid phase
The macro-mixing of the liquid phase was monitored under three experimental conditions corresponding to the digester fed with MSW at three TS content (MSW 22% TS, MSW 26% TS and MSW 30% TS). For the 3 experiments, the effective (experimental) mean residence time was lower than the theoretical mean residence time, indicating that the effective working volume was lower than expected. As shown in Table 2, a “dead” zone ratio β was calculated from these two values (Eq. (10)). Regarding the results
Conclusion
The Residence Time Distribution technique was used to investigate the hydrodynamic behavior of a high-solid anaerobic digester. The main conclusions of this work are:
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TS content was found to have a small impact on the macro-mixing of the liquid.
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The segregation potential increased with the particle density.
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The yield stress was a key parameter in the mechanism of segregation. At high yield stress (e.g., high TS content), dense particles were more stable and thereby less subjected to settling.
As a
Acknowledgements
This research was supported by the French National Research Agency ANR (Project “ANAMIX”) through the BIOENERGIES 2008 program (n° ANR-08-BIOE-009-03). The authors acknowledge Valorga International for their assistance in supplying the anaerobic inoculum. We are very grateful to Guillaume Guizard, Richard Poncet and Hervé Périer-Camby for technical assistance in reactor designing and constructing.
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