Critical Analysis of Methods Adopted for Evaluation of Mixing Efficiency in an Anaerobic Digester

Abstract

The effect of slurry mixing in an anaerobic digester on biogas production was intensively studied in the last few years. This subject is still debatable due to fact that this process involves three phases, solid-gas-liquid, along with the involvement of microbes during biochemical reactions, which are highly vulnerable to changes in hydrodynamic shear stresses and mixing conditions. Moreover, the complexity in the direction of optimization of mixing magnifies due to the implication of both fluid mechanics and biochemical engineering to study the effect of mixing in anaerobic digestion (AD). The effect of mixing on AD is explored using recent literature and theoretical analysis, concentrating on the multi-phase and multi-scale aspects of AD. The tools and methods available to experimentally quantify the function of mixing on both the global and local scales are summarized in this study. The major challenge for mixing in an anaerobic digester is to minimize dead zones and maintain uniform distribution of viscosity and shear at low mixing intensities without disrupting the microbial flocs and syntrophic relationships between the bacteria during the AD process. This study is a critical analysis of various techniques and approaches adopted by researchers to evaluate the effectiveness of mixing regimes and mixing equipment. Most studies describe biogas production performance and hydrodynamic characteristics of the digesters separately, but the evaluation of mixing requires interdisciplinary experts, which include mechanical engineers, microbiologists and hydrodynamic experts. Through this review, the readers will be guided through intensive literature regarding agitation, the best possible way to scrutinize the agitation problems and the approach to answering the question “why is the optimization of mixing in an anaerobic digester still a debatable subject?”.

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

“…Regardless of how or why sludge rheology changes, the change itself may affect not only the power needed for mixing and pumping but also the AD process itself, primarily by affecting the mixing efficiency and fluid flow patterns in the reactors (interactions 5, 7, and 28 in Figure ). This aspect has long been studied in CSTBRs, mainly by performing computational fluid dynamics modeling (CFD, reviewed by refs and ). While this approach is helpful for designing reactors and dimensioning their mixing and pumping equipment, it does not necessarily provide information on when the mixing is affected to a high enough extent that effects on process performance may be expected.…”

Evaluation of the efficiency of the mixing system of the laboratory SBR-type reactor

“…This contribution focuses on the performance of spiral ribbon mixers in non-compartmentalized stirred tanks. Singh et al [25] conducted a series of studies to analyze and optimize various forms of mixer and agitator geometries. The results obtained in the mixing process saved 7.3% of the total energy used in the process with the mixers used.…”

“…The main purpose of the mixing operation is to provide the organisms carrying out biochemical processes with the correct working conditions by creating the appropriate degree of mixing of the bioreactor volume and maintaining it for the time specified in the technology [6,23]. The mixing phase is usually implemented using mechanical stirrers so that the sludge at the bottom of the chamber comes into contact with the raw wastewater introduced into the system during the filling phase [24][25][26].…”

Significance of Intermittent Mixing in Mesophilic Anaerobic Digester

“…Previous studies showed how the results of the optimization of mixing in digesters can vary. In addition, the data regarding the rheology of the slurry, the impeller, and the digester design are missing in most studies, which is a very important aspect to evaluate the mixing in any vessel because the mixing operation is a physical process [29]. The present study investigated the effect of increasing the mixing interval time on the efficiency of the AD process and overall BY.…”