Coalescence of a Ferrofluid Drop at Its Bulk Surface with or without a Magnetic Field.

The coalescence of a ferrofluid drop at its bulk surface, with or without a magnetic field, was investigated experimentally by a high-speed camera. Shape deformations of both the pendant ferrofluid drop and the bulk surface in the axial direction were observed during the approaching process even in the absence of a magnetic field. The angle of the upper pendant peak at the first contact decreases with the magnetic flux density, while the lower ferrofluid peak displays an opposite trend. The coalescing width of the ferrofluid drop follows a power-law relationship. The exponent of 0.64 under medium and high magnetic fields as well as the case without magnetic field confirms the inertial regime of drop coalescence. Under the low magnetic field, the significant exponent increasing from 0.59 to 3.02 at about 4 ms is in coincidence with the sudden change to a smooth coalescing section according to the visualized images. A high-speed microparticle image velocimetry (micro-PIV) technique was employed with a transparent model fluid to reveal the flow fields during the drop coalescence instead of opaque ferrofluids.

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.

Visualization of mass transfer in mixing processes in high solid anaerobic digestion using Laser Induced Fluorescence (LIF) technique.

High solid anaerobic digestion (HSAD) is a promising technology for the treatment of organic waste. Mixing process in HSAD is quite difficult with long mixing time, poor homogenization, significant short-circuiting and stagnant zones. However, the mass transfer in mixing process in HSAD has not been visualized due to the lack of a proper method. In this study, a novel approach for experimentally quantifying the mass transfer in HSAD's mixing process was proposed in a mixing tank equipped with multistage impellers by means of the Laser Induced Fluorescence (LIF) technique. Flow field was investigated for better illustrating the mass transfer, thus Particle Image Velocimetry (PIV) and Computational Fluid Dynamics (CFD) technique were conducted for flow field measurement. The obtained results revealed that the feedstock tended to accumulate around the impeller in the HSAD system, especially near the 1st stage. The tracer diffused rapidly around the 1st impeller in t = 10 s, but the diffusion around the 2nd impeller was quite tough even after 4 h 08 min 23 s. And prolonging mixing time could not significantly improve the flow pattern along with the mixing. In this study, new insight was thrown into the visualization of the mass transfer in mixing process within a HSAD reactor. The visualization of mass transfer in the mixing process in HSAD could offer reference information to the study of the mixing process of HSAD even in full-scale.

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.

Mineral pollutants removal through immobilized microalgae-bacterial flocs in a multitrophic microreactor.

Microalgae-bacterial flocs (MaB-flocs) immobilization technique using polyvinyl alcohol (PVA) crosslinked with sodium alginate represent a novel approach for sustainable pollutants removal. The present work was performed to evaluate the performance of a multitrophic batch reactor at microscale for treating two synthetic wastewater solutions prepared with two different initial Chemical Oxygen Demand (COD): 200 mg.L-1 and 450 mg.L-1, respectively. Three MaB-flocs concentrations were entrapped into PVA-alginate beads: C1 (2%, v/v), C2 (5%, v/v) and C3 (10%, v/v), without O2 supply, during three periods 2, 4 and 6 days of batch incubation. PVA-alginate beads containing the highest concentration C3 of MaB-flocs improved the performance of the microreactor to remove significantly NH4+ and PO43- of about 61% and 82%, respectively, from wastewater more than two other concentrations used. This result confirms that C3 of MaB-flocs displays not only a good potential for nutrients removals but also the highest MaB-flocs morphological progression after 6 days of treatment with the highest COD of 450 mg.L-1. The feasibility of the PVA-alginate for cells immobilization, investigated through microscopy analysis, reveals that the evolution of multicellularity in MaB-flocs, for all experiments.

Study of an enhanced dry anaerobic digestion of swine manure: Performance and microbial community property.

Anaerobic digestion could treat organic wastes and recovery energy. Dry anaerobic digestion had advantages of low heating energy, small digester and less digestate, but its methane production was poor. In this study, an enhanced dry anaerobic digestion of swine manure (thermal treatment + dry anaerobic digestion) was proposed, and its feasibility was investigated via semi-continuous experiment. Results showed that methane production rates were 314.6, 416.0, 298.0 and 69.9 mL CH4/g VS at solid retention time (SRT) of 41 d, 35 d, 29 d and 23 d. Volatile solids (VS) removal rate and methane production rate could reached 71.4% and 416.0 mL CH4/g VS respectively at SRT of 35 d. Methane production rate of the enhanced dry anaerobic digestion was 390% higher than that of dry anaerobic digestion. Microbial study indicated that hydrogenotrophic methanogens predominated with the abundance of 90.2%, while acetoclastic methanogens were not detected. This process was feasible, and was of great practical importance.

Novel insight into high solid anaerobic digestion of swine manure after thermal treatment: Kinetics and microbial community properties.

Compared to traditional anaerobic digestion (AD), high solid anaerobic digestion (HSAD) had the advantages of small digester, low heating energy and less digestate. However, the methane production was poor. In our previous study, thermal treatment (70 ±â€¯1 °C, 3 days) without any dilution could satisfactorily enhance the methane production rate of HSAD by up to 39.5%. However, effects of solid content on HSAD after thermal treatment were not yet studied. In this study, HSAD was conducted at 11.7-17.6% solid content, and the control experiment was conducted at low solid content (4.4% solid content). Results showed that HSAD's methane production rate was the highest at 11.7% solid content (158 mL CH4/g VS), and could reach up to 89.2% of that at 4.4% solid content. The utilization of organics was revealed by kinetics analysis that the readily biodegradable organics could be utilized at increasing solid content with decreasing hydrolysis rate. Furthermore, it was notable that methylotrophic methanogens predominated in HSAD with the abundance of 82.6%. This was quite unique from the general belief that AD system was usually dominated by acetoclastic or hydrogenotrophic methanogenic pathways. In this study, the microbial community structure of HSAD after thermal treatment was firstly studied, its unique specific methanogenic pathways was firstly revealed.

Flow field investigation of high solid anaerobic digestion by Particle Image Velocimetry (PIV).

High solid anaerobic digestion (HSAD) is a promising anaerobic digestion technology. Homogenization and mixing mechanism are essential for HSAD's performance, but relative knowledge still remains poor. In order to investigate HSAD's mixing behavior, a novel flow field measuring approach was proposed as following. Firstly, laponite suspension was selected as the model fluid of HSAD digestate, because the rheological properties and material structure they displayed were highly similar. Then, water and polyacrylamide (PAAm) solution were chosen as basic reference fluid and another non-Newtonian fluid respectively. Flow fields of the three fluids under different rotation speeds were measured via Particle Image Velocimetry (PIV). The evolution of working fluids did induce consecutively the significant flow and mixing behavior of HSAD, because their rheological properties and complexity were getting progressively closer to the real HSAD digestate. Results indicated that the flow field of simulated HSAD fluid was quite different from those of water and PAAm solution, i.e. only the fluid around the impeller could be mixed in HSAD. Besides, increasing rotation speed could not significantly enhance the mixing area of HSAD. Thus, multilayer impellers arranged abreast were recommended for HSAD's mixing. Considering that HSAD's flow field had never been measured before, this study proposed a novel flow field measuring method for such opaque non-Newtonian fluid for the first time. The visualization of HSAD's complex hydrodynamic conditions was also firstly achieved in this study, and thus could further help improve the homogenization of HSAD.

Rheological characterization of digested sludge by solid sphere impact.

An impact method was applied to investigate the rheological characteristics of digested sludge and reveal its transient dynamics. A high-speed camera allowed visualizing the dynamic impact process and observing interaction between impacting sphere and targeted sludge. A damping oscillation was observed after the impact. The crater diameter followed an exponential function, while the crater depth varied as a logarithmic function of both sphere diameter and free fall height. Furthermore, the viscosity and elasticity of digested sludge were evaluated by establishing a simplified impact drag force model. The impact elastic modulus was consistent with the Young's modulus measured by a penetrometer. The impact viscosity was reasonable as the estimated impact shear stress was greater than the yield stress of digested sludge resulting in the formation of crater. The impact method offers an alternative way to reveal the viscoelasticity of digested sludge through a dynamic process.

Size effect of anaerobic granular sludge on biogas production: A micro scale study.

This study investigated the influence of anaerobic granular sludge size on its bioactivity at COD concentration of 1000, 3000 and 6000 mg/L. Based on size, granules were categorized as large (3-3.5 mm), medium (1.5-2 mm) and small (0.5-1 mm). A positive relationship was obtained between granule size and biogas production rate. For instance, at COD 6000 mg/L, large granules had highest biogas production rate of 0.031 m(3)/kgVSS/d while medium and small granules had 0.016 and 0.006 m(3)/kgVSS/d respectively. The results were reaffirmed by applying modified Fick's law of diffusion. Diffusion rates of substrate for large, medium and small granules were 1.67×10(-3), 6.1×10(-4)and 1.8×10(-4) mg/s respectively at that COD. Large granules were highly bio-active due to their internal structure, i.e. big pore size, high porosity and short diffusion distance as compared to medium and small granules, thus large granules could improve the performance of reactor.

Multiscale hydrodynamic investigation to intensify the biogas production in upflow anaerobic reactors.

Hydrodynamics plays a main role for the performance of an anaerobic reactor involving three phases: wastewater, sludge granules and biogas bubbles. The present work was focused on an original approach to investigate the hydrodynamics at different scales and then to intensify the performance of such complex reactors. The experiments were carried out respectively in a 3D reactor at macroscale, a 2D reactor at mesoscale and a 1D anaerobic reactor at microscale. A Particle Image Velocimetry (PIV), a micro-PIV and a high-speed camera were employed to quantify the liquid flow fields and the relative motion between sludge granules and bubbles. Shear rates exerted on sludge granules were quantified from liquid flow fields. The optimal biogas production is obtained at mean shear rate varying from 28 to 48s(-1), which is controlled by two antagonistic mechanisms. The multiscale approach demonstrates pertinent mechanisms proper to each scale and allows a better understanding of such reactors.

Effects of increase modes of shear force on granule disruption in upflow anaerobic reactors.

Sludge washout is listed among the top practical problems of the high rate upflow anaerobic reactors. This study investigated quantitatively two sludge washout processes operated under different hydrodynamic shear increase modes with the intervals of 1 and 10 days respectively. The results reveal that the sludge washout accompanying with large-scale granule disruption could lead to performance failure with heavy sludge loss ratio of about 46.1% at sludge loss rate about 0.35 gVSS L(-1) d(-1) during the process with shear increase interval of 1 day, while the highest sludge loss rate was only 0.12 gVSS L(-1) d(-1) during the process with 10-day interval. The intensified shear conditions could weaken the granules through inhibiting the extracellular polymers production and bioactivity. As consequences, an outbreak of large-scale granule disruption would raise and then significantly accelerate the sludge washout. Since long interval could provide the granules the opportunity to recover from these negative effects to some extent, the shear increase strategy of long interval over 10 days is favorably recommended to operate full-scale reactors during the start-up and shock load periods. The pioneer use of the micro particle image velocimetry in this study offers the possibility to discover the real hydrodynamic conditions around granules at microscale for the first time and reveals that the shear force exerts directly on the granular surface as a mechanical disruption force and big granules undergo high disruption force. The granule disruption is a result of the competition between the granule and the ambient hydrodynamic shear conditions rather than a process with shear force as a sole dominant factor. These could facilitate the understanding of the complicated interactions between the hydrodynamics and reactor performance and favor then a better control of the full-scale reactors.