State of the art on mixing in an anaerobic digester: A review

Singh, Buta; Szamosi, Zoltán; Siménfalvi, Zoltán

doi:10.1016/j.renene.2019.04.072

Cited by 80 publications

(46 citation statements)

References 60 publications

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“…Among the various available technological methods to increase the efficiency of biogas production via anaerobic digestion, the optimization of the mixing system represents one of the most promising approaches [1]. Cost-benefit analyses indicate that mixing is the highest contributor to the total energy consumption in biogas plants [2]- [4]. Considering the total efficiency of the biogas plants, this parasitic contributor needs to be reduced and testing of different mixing regimes (i.e.…”

Section: Introductionmentioning

confidence: 99%

CFD Modelling of Biomass Mixing in Anaerobic Digesters of Biogas Plants

Conti

Saidi

Goldbrunner

2019

Environmental and Climate Technologies

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Cut in greenhouse gas emissions, increment of energy from renewables and improvement in energy efficiency represent the three key targets for future energy systems. Among the available bioenergy technologies, biogas production via biodegradation and anaerobic digestion is a widely applied approach, not only to produce biofuels but also to manage industrial and domestic organic waste. Within the biogas production, a sufficient mixing of the organic mass is a crucial step to ensure high biogas yields by bacteria and enzymes. Measurements of the electric power consumption of biogas plants revealed that the electrical energy demand of the stirrer system has a high share of the total electricity consumption of a biogas plant. Investigations on real biogas digesters to optimize the mixing process are cost and time intensive. Therefore, laboratory prototypes and computational simulations represent promising alternatives to analyse and improve the efficiency of mixing systems. In this paper, a computational fluid dynamics (CFD) model is presented, which is applied to commercial stirring systems. The case of two propeller stirrers, located in diametrically opposite positions in a tank filled with ca. 1400 m3 of substrate is described in detail. For the simulation, the rheology of the fluid is adapted to a biomass with 12 wt % dry matter content and obeying the non-Newtonian generalized Ostwald-de Waele power law. The developed simulation procedure considers the rotation angle of each propeller and its height. A total of 441 mixing configurations are calculated and evaluated in terms of the technical benefit. The investigation reveals that locations of the rotors far away from the bottom and high rotational angles cause advantageous fluid dynamics.

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Section: Introductionmentioning

confidence: 99%

CFD Modelling of Biomass Mixing in Anaerobic Digesters of Biogas Plants

Conti

Saidi

Goldbrunner

2019

Environmental and Climate Technologies

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“…The acquired knowledge can also be applied to the sludge management in WWTPs. Experimentally, it has been shown that gas mixing is not very suitable from the foaming point of view and, in comparison with mechanical mixing, it also shows a lower biogas production [62]. The negative effect of gas mixing on biogas production was confirmed by another study [63].…”

Section: Mixing Of Anaerobic Digestermentioning

confidence: 95%

A Novel Check-List Strategy to Evaluate the Potential of Operational Improvements in Wastewater Treatment Plants

et al. 2020

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With increasing demands for cleaning and purification of water, wastewater treatment plants (WWTP) require their most efficient operation. The operators are thus obliged to constantly review the efficiency of the processing units and technological equipment of WWTPs and seek opportunities for improvements. To increase the efficiency of particular equipment, the important parameters to be used for the intensification must be correctly selected. A common WWTP consists of different types of processing units, where the basic parameters can be changed to achieve the highest efficiency (i.e., maximum output with minimum energy consumption) in the WWTP. However, due to many possible technologies in the wastewater treatment process, the combinations of processing units can be complex. In such cases, the efficiency assessment can be misleading if only basic parameters were accessed. Moreover, single-unit intensification can potentially improve the efficiency of the unit itself but cannot guarantee full process improvement. This can be due to negative causal effects in the downstream due to that unit intensification. This work reviews of key parameters at five selected pieces of WWTP equipment (inlet pump station, airlift pump, primary sedimentation tank, aeration chamber, and mixing of anaerobic digester) to demonstrate the correct selection of all affected parameters for the efficiency assessment. In the context of the whole WWTP process, it is necessary to take into account several other parameters to evaluate the efficiency of the equipment. Finally, a methodology for assessing the significance of the identified parameters is proposed. This methodology is effectively applied and demonstrated in the WWTP case study.

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“…Bio-digester design and mixing equipment design is a key feature in the techno-economic feasibility of biogas production [13]. Modifications in digester design can result in lower operating costs and enhance mixing efficiency.…”

Section: Effect Of Digester and Impeller Design On Mixingmentioning

confidence: 99%

“…Generally, mechanical mixing is preferred over the other mixing modes due to lower power consumption and the mixing time [12]. Several different types of impellers are reviewed here that have been used by researchers in order to optimize the mixing in terms of power consumption and increasing biogas production rates [13].…”

Section: Introductionmentioning

confidence: 99%

Impact of mixing intensity and duration on biogas production in an anaerobic digester: a review

Singh

Szamosi

Siménfalvi

2020

Critical Reviews in Biotechnology

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This paper is a critical evaluation of the effect of mixing on biogas production rates in an anaerobic digester. Mixing plays a prominent role in determining the efficiency of the anaerobic digestion process. This review analyzes the miscellaneous effects of mixing (on the microbial community, methane content and volatile fatty acids) at various mixing intensities and during different stages of the digestion process. Intermittent mixing (mixing at intervals) seems preferable in terms of the quality and quantity of biogas produced, and results in lower power consumption and maintenance costs associated with large-scale biogas production. Preferable mixing time (the length at intervals) and the intensity depends on the geometry of the digester and impeller. The conclusion is drawn that the study of the slurry rheology is very crucial in the designing of the mixing equipment, the shape and size of the digester, and the pipe transport system which can assist in minimizing the initial investment and operational costs. Accordingly, this paper focuses on the parameters which determine the potency of mixing, such as viscosity, total solid content and digester design. Empirical data demonstrated by various researchers regarding rheological characteristics is compared and reviewed. Consequently, close attention should be paid toward the optimization of mixing in terms of its speed, mixing time and impeller geometry, especially during different stages of the digestion process (hydrolysis, acidogenesis, acetogenesis and methanogenesis). Finally, readers will be guided to the extensive publications regarding optimization, directions of future research, and troubleshooting of the mixing operation in an anaerobic digester. This investigation will help to improve mixing efficiency with biogas plants. HIGHLIGHTSEffect of shear rate on different stages of an anaerobic digestion process. Methane content varies with the variation in mixing speeds. Mixing effect is significant when the total solid content is higher. Intermittent mixing is favorable when compared to continuous mixing. The geometry of the digester and mixer is essential to evaluate digester mixing. ARTICLE HISTORY

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State of the art on mixing in an anaerobic digester: A review

Cited by 80 publications

References 60 publications

CFD Modelling of Biomass Mixing in Anaerobic Digesters of Biogas Plants

CFD Modelling of Biomass Mixing in Anaerobic Digesters of Biogas Plants

A Novel Check-List Strategy to Evaluate the Potential of Operational Improvements in Wastewater Treatment Plants

Impact of mixing intensity and duration on biogas production in an anaerobic digester: a review

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