Effects of mixing time on methane production from anaerobic co-digestion of food waste and chicken manure: Experimental studies and CFD analysis

Effect of Different Blades on Suspension Characteristics of Anaerobic Digestion Particles and Energy Consumption Optimization Analysis

In this study, a solid-liquid mixing system model was established to simulate the coexistence of floating particles (FP) and sinking particles (SP) in the early stage of anaerobic digestion, and the mixing effect and energy consumption of the system were investigated. Four typical blades were selected to compare the solid phase distribution of straw particles under different blade stirring, and the distribution of FP and SP in the coexistence system was clarified. Then the combination of full-factorial design and numerical simulation was applied to compare the effect of blade diameter and blade width on particle mixing, which was better than that of immersion depth. A comprehensive equation was further established to balance the weight between the particle mixing effect and energy consumption and improve the blade design. It provided theoretical support for the design and amplification of subsequent stirring equipment.

Effect of optimized intermittent mixing during high-solids anaerobic co-digestion of food waste and sewage sludge: Simulation, performance, and mechanisms

Inadequate mixing has been proven to be a major cause of anaerobic digester failure. This study revealed the mechanism of mixing intervals on high-solids anaerobic co-digestion (HS-AcoD) of food waste (FW) and sewage sludge (SS). Optimized intermittent mixing time (15 min/h) was determined through computational fluid dynamics (CFD) simulation. Experimental results indicated that the simulated intermittent mixing could shorten digestion time and increase cumulative methane output (366.8 mL/gVS) compared with continuous mixing and unmixing. Mixing could considerably accelerate substrate solubilization and hydrolysis. Maximum rates of acidogenesis (53.35 %) and methanogenesis (49.41 %) were observed with an optimized intermittent mixing (15 min/h). Vigorous mixing induced apoptosis and disrupted syntrophic metabolism, whereas intermittent mixing promoted the syntrophic metabolism between Syntrophomonas and Methanobacterium, and led to an enrichment of genes involved in acidogenic and methanogenic pathways. These findings have important implications for the development of an optimized intermittent mixing strategy for maximizing HS-AcoD efficiency of FW and SS.

Anaerobic Digestion, Codigestion of Food Waste, and Chicken Dung: Correlation of Kinetic Parameters with Digester Performance and On-Farm Electrical Energy Generation Potential

Valorization of agro-food waste through anaerobic digestion (AD) is gaining prominence as alternative method of waste minimization and renewable energy production. The aim of this study was to identify the key parameters for digester performance subjected to kinetic study and semicontinuous operation. Biochemical methane potential (BMP) tests were conducted in two different operating conditions: without mixing (WM) and continuous mixing (CM). Three different substrates, including food waste (FW), chicken dung (CD), and codigestion of FW and CD (FWCD) were used. Further kinetic evaluation was performed to identify mixing’s effect on kinetic parameters and correlation of the kinetic parameters with digester performance (volatile solid removal (VS%) and specific methane production (SMP)). The four models applied were: modified Gompertz, logistic, first-order, and Monod. It was found that the CM mode revealed higher values of Rm and k as compared to the WM mode, and the trend was consistently observed in the modified Gompertz model. Nonetheless, the logistic model demonstrated good correlation of kinetic parameters with VS% and SMP. In the continuous systems, the optimum OLR was recorded at 4, 5, and 7 g VS/L/d for FW, CD, and FWCD respectively. Therefore, it was deduced that codigestion significantly improved digester performance. Electrical energy generation at the laboratory scale was 0.002, 0.003, and 0.006 kWh for the FW, CD, and FWCD substrates, respectively. Thus, projected electrical energy generation at the on-farm scale was 372 kWh, 382 kWh, and 518 kWh per day, respectively. Hence, the output could be used as a precursor for large-scale digester-system optimization.

A review on anaerobic digestion with focus on the role of biomass co-digestion, modelling and optimisation on biogas production and enhancement

The status, recent trends and future perspectives in modelling and optimisation of anaerobic co-digestion is investigated. Areas that can be focused on and those which need further research towards enhancing biogas production are pointed out. Co-digestion, modelling and optimisation of anaerobic digestion as well as techno-economic aspects are reviewed in this paper. It was noted that co-digestion requires more research into a variety of bio-resources and their specific blend proportions. Modelling and optimisation of co-digestion with substrate seasonal fluctuations has not been addressed in previous studies. Controlling key process factors including temperature, pH, and carbon to nitrogen ratio is critical in improving biogas yield. Biogas hybridisation is yet to be explored in depth. The majority of researches are focused on mono-digestion, feedstock co-digestion, modelling, and optimisation of anaerobic digestion needs significant further investigations. A multi-objective approach taking all technical and economic parameters in the modelling and optimization is essential.

Quantitative hydrodynamic characterization of high solid anaerobic digestion: Correlation of "mixing-fluidity-energy" and scale-up effect

High solid anaerobic digestion (HSAD)'s complex rheological behavior exhibits short-circuiting and dead zone. Mixing optimization is potential to enhance HSAD hydrodynamics. Besides, scale-up effect is quite essential for HSAD's applications, but remains rarely studied yet. Effect of impeller with different width on the correlation of "mixing-fluidity-energy" at different rotating speeds was first investigated at pilot-scale in present work. Then, scale-up effect based on rotating speed and a generalized Reynolds number was revealed from the aspects of fluidity and energy consumption. Results show that impeller width of 100 mm (10 rpm), 200 mm and 300 mm (5 and 10 rpm) are preferred for hydrodynamics and energy economics. Furthermore, Re similarity has better referential significance for the scale-up. In this study, new insight is gained into the correlation of "mixing-fluidity-energy" within a pilot-scale digester. Scale-up effect based Re similarity could potentially offer guidance for HSAD's application in the practical engineering.

Simultaneous evaluation of the effect of mixing efficiency on power consumption and methane production in an anaerobic digester with different wastewaters

This study simultaneously examines the effect of mixing rate on power consumption and methane production in a stirrer anaerobic reactor. The numerical simulation is carried out using the finite volume approach and validated against available experimental data. The methane production rate is determined using governing equations in the anaerobic digestion (AD) process. The results showed that a 60% increase in the mixing rate of the system (from 50% to 80%) in the wastewater with concentration of 14,549 (mgl^-1) increased the methane production rate by about 35% and increased the power consumption of the system by about 13 times. Among the 144 cases studied, the best stirrer model is figured out as the optimal model by applying an index of performance coefficient and also this model is investigated with scale-up criteria in larger sizes. A novel equation for evaluating the power production value is suggested in real digesters.

Localized mixing of anaerobic plug flow reactors

An innovative localized-mixing concept was tested in an Anaerobic Plug Flow Reactor (AnPFR) treating Food Waste (FW) mixed with municipal Wastewater (WW). The proposed concept consists of placing propellers along the shaft of the AnPFR at key points that represent the mid-region of each of the anaerobic digestion stages: hydrolysis, acidogenesis, and methanogenesis. First, the need for and efficiency of localized mixing (the new concept suggested by the authors) were investigated. While the main benefit of localized mixing is the reduction of energy demand associated with (conventional) uniform mixing (i.e., throughout the longitudinal axis), the system can also benefit from synergetic reactions in non-mixed zones. In fact, at a Total Solid (TS) content of 15% (Organic Loading Rate (OLR) of 4.2 g VS.L ^- 1.d ^- 1) and a Hydraulic Retention Time (HRT) of 28 days, the mixing pattern was sufficient to maintain stable operation, with high removal rates (up to 96% of solids) and high biogas generation (1128 ± 55 ml.g VS_fed^-1, of which 68.9% consisted of CH₄); but when mixing was halted, the system's performance deteriorated. Second, the loading capacity of the locally-mixed AnPFR was investigated by subjecting it to different TS content (10%, 15%, 20%, and 22.5%, corresponding to OLRs of 2.8, 4.2, 6.3, and 7.9 g VS.L ^- 1.d ^- 1, respectively) while operating under the same HRT. It was found that the system can adequately sustain a feed with a maximum TS of 20% while achieving removal rates up to 92% for solids and a CH₄ yield of 613 ml.g VS_fed^-1. The digester was simulated using computational fluid dynamics. The outputs revealed: (1) highest radial mixing at the center of the methanogenesis zone where the propeller is located and (2) low longitudinal mixing before and after the propeller of the methanogenesis stage, implying the presence of sedimentation zones that was visually verified. The former is assumed to favor better dispersion of inhibitors and improved stability, while the latter is expected to provide stagnant areas for enhanced biochemical synergies.

Intermittent air mixing system for anaerobic digestion of animal wastewater: Operating conditions and full-scale validation

An air mixing system for anaerobic digestion has been proved to be beneficial for methane production. The aim of the present study was to further investigate the appropriate conditions for air mixing. The effective methane production time (EMPT) was defined to determine the air mixing time in the article. The results indicated that the appropriate aeration intensity was 66.7 mL air per volume of reactor per min and mixing time was 1.5 min. When air mixing time exceeded 3 min on each occasion, total CH₄ production was less than that achieved under the no mixing condition due to a decrease in the EMPT. In addition, the possibility of air mixing was evaluated in an anaerobic full-scale plant comprising a continuous stirred tank reactor. One year of operating data validated the feasibility of air mixing during the anaerobic digestion of swine wastewater.

Mixing effects on decentralized high-solid digester for horticultural waste: Startup, operation and sensitive microorganisms

This work studied the use of a horizontal high-solid digester for the decentralized anaerobic treatment of horticultural waste (fallen leaves), where the effect of intermittent mixing by a modified double helical ribbon impeller was investigated. Before experimental verification, the flow pattern and theoretical mixing time were first characterized by CFD simulation. Subsequently, three mixing time intervals (i.e., 3 min/3 hr; 18 min/3 hr; 108 min/3 hr) and one control setup (i.e., without mixing) were compared for their performance during start-up and semi-continuous operation. It was found that minimal mixing was necessary for an efficient digester's start-up but increased mixing intensity for semi-continuous operation. The results were further interpreted by correlating the digester performance and microbial communities. Those microorganisms sensitive to increased mixing intensity were highlighted and analysed.

On the effect of the inlet configuration for anaerobic digester mixing

Sludge recirculation mixing in anaerobic digesters is essential for the stable operation of the digestion process. While often neglected, the configuration of the sludge inlet has a substantial influence on the efficiency of the mixing process. The fluid is either injected directly into the enclosed fluid domain or splashes onto the free surface of the slurry flow. In this paper, the aim was to investigate the effect of the inlet configuration by means of computational fluid dynamics-using ANSYS Fluent. Single-phase and multi-phase models are applied for a submerged and splashing inlet, respectively. To reduce the high computational demand, we also develop surrogate single-phase models for the splashing inlet. The digester mixing is analyzed by comparing velocity contours, velocity profiles, mixing time and dead volume. The non-Newtonian characteristics of the sludge is considered, and a [Formula: see text] model is employed for obtaining turbulence closure. Our method is validated by means of a previous study on the same geometry. Applying a submerged inlet configuration, the resulting dead volume in the tank is estimated around 80 times lower than for the case of a splashing inlet. Additionally, by emulating the multi-phase model for splashing inlet configurations with a single-phase one, the simulation clock time reduced to 15%.

Investigation of hydrodynamics in high solid anaerobic digestion by particle image velocimetry and computational fluid dynamics: Role of mixing on flow field and dead zone reduction

High solid anaerobic digestion (HSAD) was a potential organic waste treatment. Compared with low solid anaerobic digestion, it had the advantages of small footprint, less digestate, and low heating energy. However, HSAD's methane production is poor, mainly due to the complex hydrodynamics. In this study, computational fluid dynamics were utilized for HSAD's hydrodynamics investigation at 14.3% solid content and compared to the particle image velocimetry measurement. Then, effects of mixing on hydrodynamics were investigated. The results indicated that the diameter of impeller was critical for the radial mixing, and the distance between the impellers dictated the axial mixing. Besides, rotating speed affected flow velocities significantly, but displayed less effect on expanding the mixing range. Furthermore, HSAD's treating capacity could be increased at large extent by optimizing mixing. The visualization of the hydrodynamics in this study could potentially offer conceptual basis for HSAD's design in practical engineering.

Using air instead of biogas for mixing and its effect on anaerobic digestion of animal wastewater with high suspended solids

This study proposed a new mixing method for anaerobic digestion treating animal wastewater using air as gas source of agitation (named "air mixing") and demonstrated its feasibility by comparing with other mixing modes. The results indicated that the methane production for air mixing was increased by 6.4%, 11.9% and 19.6% compared with biogas mixing, mechanical mixing and no mixing. Air mixing improved the mass transfer and the homogeneous mixing time was shortened from 10 min of mechanical mixing to 1.5 min at the same power input. A transient microaerobic environment was created by air mixing, which increased the hydrolysis efficiency by 1.7-11.4% compared with biogas mixing and facilitated VFAs generation and consumption, as well as promoted the syntrophic relationship between facultative bacteria and hydrogenotrophic methanogens. The relative contribution of the improvement of mass transfer and the reaction of microaerobic environment to methane production was 62.9% and 37.1%, respectively.

Mixing strategies - Activated carbon nexus: Rapid start-up of thermophilic anaerobic digestion with the mesophilic anaerobic sludge as inoculum

This study evaluated the mixing - activate carbon nexus in anaerobic digestion with the aim of accelerating start-up of thermophilic anaerobic co-digestion of food waste and chicken manure using mesophilic anaerobic sludge as inoculum. Results showed that the methane yield in the continuous stirred reactor is 71.3% higher than that of intermittent agitated reactor, and the addition of activated carbon can further improve the yield of methane by 18.2%. Continuous mixing mode followed by intermittent mixing was proved to be an alternative strategy to accelerate start-up of thermophilic anaerobic digestion. The optimum mixing time of 120 s/hour were obtained using computational fluid dynamics modeling. Analysis of genomic annotation metabolism indicated that the addition of activated carbon enhanced the dominant metabolism pathways of amino acid, methane and energy. Results of enzymes gene expression suggested that carbohydrates esterases, glycoside hydrolases and glycosyl transferases were dominant, respectively.