A Comparative Study of Biogas Reactor Fluid Rheology—Implications for Mixing Profile and Power Demand

Abstract

Anaerobic digestion (AD) is an established process for integrating waste management with renewable energy and nutrient recovery. Much of the research in this field focuses on the utilisation of new substrates, yet their effects on operational aspects such as fluid behaviour and power requirement for mixing are commonly overlooked, despite their importance for process optimisation. This study analysed rheological characteristics of samples from 21 laboratory-scale continuous stirred-tank biogas reactors (CSTBRs) digesting a range of substrates, in order to evaluate substrate effect on mixing efficiency and power demand through computational fluid dynamics (CFD). The results show that substrate and process parameters, such as solids content and organic loading, all have a significant effect on CSTBR fluid rheology. The correlation levels between rheological and process parameters were different across substrates, while no specific fluid behaviour patterns could be associated with substrate choice. Substrate should thus be considered an equally important rheology effector as process parameters. Additional substrate-related parameters should be identified to explain the differences in correlations between rheological and process parameters across substrate groups. The CFD modelling revealed that the rheology differences among the AD processes have significant implications for mixing efficiency and power demand of the CSTBRs, highlighting the importance of considering the substrate-induced effects on CSTBR rheology before including a new substrate.

Rheology, Micronutrients, and Process Disturbance in Continuous Stirred-Tank Biogas Reactors

“…A shift in substrate composition (e.g., by adding cosubstrates with different physicochemical properties) leads to changes in anaerobic sludge rheology (interaction 1 in Figure ). , The extent of this effect is primarily regulated by the hydraulic retention time (HRT) and organic loading rate (OLR) (interactions 2 and 3 in Figure ). − The substrate composition and loading rate (interaction 32 in Figure ) ,, also define the supply of macro- and micronutrients, potential inhibitory compounds, and the chemical speciation of micronutrients/trace elements (TE) due to complexation and/or precipitation with metal-binding ligands .…”

Identifying targets for increased biogas production through chemical and organic matter characterization of digestate from full-scale biogas plants: what remains and why?

“…The fact that higher viscosity was observed in AW1 and AW2 despite having similar or lower VS contents compared with FW-TD indicates that the composition rather than concentration of the VS is important when comparing digesters operating on different substrates. High viscosity can, apart from increasing the power demand [53], also negatively affect the mixing efficiency of digesters (reviewed by Lindmark et al [54]), causing the formation of dead zones, sedimentation, and floating layers and ultimately leading to reduced degradation efficiency of the biogas process ( [55]). In addition to VS content, a positive correlation between viscosity and the presence of EPS/EPS p and of cations (i.e.…”

Influence of Enzyme Additives on the Rheological Properties of Digester Slurry and on Biomethane Yield

“…The economic effects of reduced viscosity are mainly due to a lower energy demand for stirring processes. Studies [13] have shown that there is a strong correlation between the viscosity and energy consumption in biogas fermenters. A deep discussion of economic aspects would go beyond the scope of this manuscript.…”

“…Other authors studied the influence of origin of digester slurry on rheological parameters. According to these findings, the rheology of digester slurry strongly depends on its origin and several process parameters, such as total solids content (TS), particle size and particle size distribution [13]. Furthermore, the type and configuration of the measurement device are of great importance because digester slurry is a non-Newtonian three-phase fluid containing liquid, gas bubbles and solid particles, partly in the form of longer fibres.…”