Liquid mixing and solid segregation in high-solid anaerobic digesters

Highlights

• Macro-mixing of high-solid anaerobic digester was analyzed through RTD technique.

• Liquid macro-mixing is characterized by a small dead-zone volume.

• Digester is closer to a Plug Flow Reactor by increasing TS content.

• Yield stress plays a key role in sedimentation of denser particles.

• Sedimentation potential is reduced by increasing TS content.

Abstract

An experimental procedure (Residence Time Distribution technique) was used to characterize the macro-mixing of both liquid and solid phases of a laboratory-scale dry anaerobic digester using appropriate tracers. The effects of the waste origin and total solid content were studied. An increase in TS content from 22% to 30% TS (w/w) induced a macro-mixing mode closer to a theoretical Plug Flow Reactor. The segregation of particles having different densities was investigated regarding the RTD of the solid phase. Segregation of dense particles occurred at low TS content. By using different TS content and waste origins, it was also determined that the yield stress was a key parameter in the mechanism of segregation. At high yield stress, dense particles were more stable and thus less subjected to settling. As a consequence, operating at high TS content may permit to prevent the sedimentation of the denser particles.

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.