CFD simulation of non-Newtonian fluid flow in anaerobic digesters

A general mathematical model that predicts the flow fields in a mixed-flow anaerobic digester was developed. In this model, the liquid manure was assumed to be a non-Newtonian fluid, and the flow governed by the continuity, momentum, and k-epsilon standard turbulence equations, and non-Newtonian power law model. The commercial computational fluid dynamics (CFD) software, Fluent, was applied to simulate the flow fields of lab-scale, scale-up, and pilot-scale anaerobic digesters. The simulation results were validated against the experimental data from literature. The flow patterns were qualitatively compared for Newtonian and non-Newtonian fluids flow in a lab-scale digester. Numerical simulations were performed to predict the flow fields in scale-up and pilot-scale anaerobic digesters with different water pump power inputs and different total solid concentration (TS) in the liquid manure. The optimal power inputs were determined for the pilot-scale anaerobic digester. Some measures for reducing dead and low velocity zones were proposed based upon the CFD simulation results.     

Anaerobic digestion of animal waste: effect of mixing

Six laboratory scale biogas mixed anaerobic digesters were operated to study the effect of biogas recycling rates and draft tube height on their performance. The digesters produced methane at 0.40-0.45 L per liter of digester volume per day. A higher methane production rate was observed in unmixed digesters, while increased biogas circulation rate reduced methane production. However, different draft tube heights caused no difference in the methane production rate. Air infiltration (up to 15% oxygen in the biogas) was observed in the digesters mixed by biogas recirculation. Slight air permeability of tubing or leakage on the vacuum side of the air pump may have caused the observed air infiltration. The similar performance of the mixed and unmixed digesters might be the result of the low solids concentration (50 g dry solids per liter of slurry) in the fed animal slurry, which could be sufficiently mixed by the naturally produced biogas.     

Effect of sparger design on hydrodynamics of a gas recirculation anaerobic bioreactor

The effects of sparger design and gas flow rate on, gas holdup distribution and liquid (slurry) recirculation velocity have been studied in a surrogate anaerobic bioreactor used for treating bovine waste with a conical bottom mixed by gas recirculation. A single orifice sparger (SOS) and a multi-orifice ring sparger (MORS) with the same orifice open area and gas flow rates (hence the same process power input) are compared in this study. The advanced non-invasive techniques of computer automated tomography (CT) and computer automated radioactive particle tracking (CARPT) were employed to determine gas holdup, liquid recirculation velocity, and the poorly mixed zones. Gas flows (Q(g)) ranging of 0.017 x 10(-3) m(3)/s to 0.083 x 10(-3) m(3)/s were used which correspond to draft tube superficial gas velocities ranging from 1.46 x 10(-2) m/s to 7.35 x 10(-2) m/s (based on draft tube diameter). Air was used for the gas, as the molecular weights of air and biogas (consisting mainly of CH(4) and CO(2)) are in the same range (biogas: 28.32-26.08 kg/kmol and air: 28.58 kg/kmol). When compared to the SOS for a given gas flow rate, the MORS gave better gas holdup distribution in the draft tube, enhanced the liquid (slurry) recirculation, and reduced the fraction of the poorly mixed zones. The improved gas holdup distribution in the draft tube was found to have increased the overall liquid velocity. Hence, for the same process power input the MORS system performed better by enhancing the liquid recirculation and reducing the poorly mixed zones.     

Liquid circulation velocity and overall gas holdup in a modified jet loop bioreactor with low density particles

Effect of low density particles on the apparent liquid circulation velocity and overall gas holdup was studied in a modified reversed flow jet loop bioreactor. Experiments were conducted using polyurethane beads, polystyrene particles which are comparable to bioparticles found in biological applications and glass beads. Influence of gas and liquid flow rates, draft tube to reactor diameter ratio and solids loading on these hydrodynamic properties were studied. The liquid circulation velocity was found to increase with an increase in liquid flow rate but decrease with an increase in gas flow rate or solids loading. The overall gas holdup increased with an increase in gas or liquid flow rate but decreased with an increase in solids loading. The range of optimum draft tube to reactor diameter ratio was found to be 04–0.5. The results obtained with low density particles were comparatively better than those with glass beads. Correlations were proposed to evaluate liquid circulation velocity and overall gas holdup in terms of operational and geometrical variables.     

Hydrodynamics and mixing in down flow jet loop bioreactor with a non-Newtonian fluid

Gas holdup and liquid circulation time were measured in a down flow jet loop bioreactor with a non-Newtonian fluid. It was observed that the circulation time decreases with increase in nozzle diameter, draft tube to column diameter ratio and shear thinning of the media. The gas holdup increases with increase in gas and liquid velocities. The optimum draft tube to column diameter ratio was found to be 0.438. Correlations for gas holdup and circulation time involving operational and geometrical variables were presented.     

Influence of H(2) stripping on methane production in conventional digesters

Hydrogen is a central metabolite in the methanization process. In this study the partial pressure of hydrogen in the gas phase of laboratory manure digesters was monitored over extensive periods of time and found to vary between 50 and 100.10(-6) atm. By sparging the gas phase of the digester through an auxiliary reactor, hydrogenotrophic methanogens were allowed to develop at the expense of hydrogen and carbon dioxide present in the biogas, independently of the liquid or cell residence time in the main reactor. By scrubbing ca. 100 volumes of biogas per liter reactor per day through an auxiliary reactor, hydrogen concentration could be decreased maximally 25%. This resulted in an increase in the gas production rate of the main digester of ca. 10% and a concomitant improved removal of volatile fatty acids from the mixed liquor. The results obtained indicate that considerable stripping of hydrogen from the digester could be achieved at acceptable energy expenditure. However, the microbial removal of the hydrogen at these low concentrations is extremely slow and limits the applicability of this approach.     

Mixing in anaerobic digestion

The mixing of the anaerobic digester contents significantly influences the efficiency of this operation; in particular, hydraulic dead zones are extremely detrimental to the reaction kinetics involved in anaerobic digestion. An analysis of the relative importance of thermal fluid movement in the digester to those caused by fluid inflow and outflow is presented. As an example, these principles are applied to a digester at the South Bend Wastewater Treatment Plant. Experimental measurements, which have general applicability for the measurement of digester mixing volume, confirm the theoretical conjectures. Various types of optimizations can be attempted on this mixing operation. One such optimization applied to gas lift mixers, as employed in the South Bend Treatment Plant, is illustrated.     

Flow regimes in a two phase reversed flow jet loop bioreactor

Reversed flow jet loop bioreactors (RFJLB) have been used extensively for 2 or 3 phase biochemical reactions. From visual observations and gas holdup data, 3 distinct flow regimes are identified in RFJLB, namely: (1) Bubble free regime (BFR), where bubbles are observed in the draft tube only; (2) Transition regime (TR), where bubbles are observed in both the draft tube and the annulus, but without circulation; and (3) Complete bubble circulation regime (CBCR), where bubbles circulate in both the draft tube and annulus. CBCR is the most desirable regime, since the reactor operation in this regime gives a higher gas holdup and mass transfer rate than in the other two regimes. In the present study, the hydrodynamic behavior of RFJLB was investigated under various operational and geometrical conditions, such as gas and liquid velocity and nozzle configuration. Factors affecting the critical liquid circulation velocity (CLCV) above which the CBCR is established were identified and evaluated quantitatively.     

A comparison of the performance of mesophilic and thermophilic anaerobic filters treating papermill wastewater

Two anaerobic filters, one mesophilic (35 degrees C) and one thermophilic (55 degrees C), were operated with a papermill wastewater at a series of organic loadings. The hydraulic retention time (HRT) ranged from 6 to 24 h with organic loading rates (OLR) 1.07-12.25 g/l per day. At loading rates up to 8.4 g COD/l d, there was no difference in terms of the removal of soluble COD (SCOD) and gas production. At the higher organic loading rate, the SCOD removal performance of thermophilic digester was slightly better compare to mesophilic digester. Similar trend was also observed in terms of the daily methane production. The stability of thermophilic digester was also better than mesophilic digester particularly for the higher organic loadings. Volatile fatty acid accumulation was observed in the effluent of the mesophilic filter at the higher organic loading rates. The Stover-Kincannon model was applied to both digesters and it was found that model was applicable to both digesters for papermill wastewater. K(B) and U(max) constants from the Stover-Kincannon model were also derived.     

Overall volumetric mass transfer coefficient in a modified reversed flow jet loop bioreactor with low density particles

Experiments were conducted using glass beads and low-density particles such as polyurethane and polystyrene which are comparable to bioparticles found in biological applications to evaluate the overall volumetric mass transfer coefficient (K _L a) in a modified reversed flow jet loop bioreactor having the liquid outlet at the top section of the reactor. The influence of the gas and liquid flow rates, draft tube to reactor diameter ratio, solids loading and physical properties of solids onK _L a were studied. TheK _L a was found to increase with the increased gas and liquid flow rates. TheK _L a values were found to be higher in the bubbly flow region i.e., at the lower range of energy dissipation rates. The optimum draft tube to reactor diameter ratio and solids loading with respect to maximumK _L a were found to be 0.4 and 0.9×10^?3 m³ (? _s =0.025) respectively. Dimensionless correlations were presented to predict the experimental values in terms of operational and geometrical variables.     

Integration of mixing, heat transfer, and biochemical reaction kinetics in anaerobic methane fermentation

An extensive investigation of anaerobic methane fermentation requires identifying the relationship between the physical environment and biological process. In this study, a computational fluid dynamics (CFD) technique was used to characterize bacterial fermentation mechanisms intertwined with mixing and heat transfer in anaerobic digesters. The results demonstrate that the methane yield remains almost unchanged while the energy efficiency decreases with increasing mixing power in a complete‐mix digester, and that the energy output increases nonlinearly with the increase in heating energy in a plug‐flow digester. The CFD method can be applied to other bioreactors to gain valuable insights into their behavior as well. Integrating flow and temperature with kinetic behavior for anaerobic digestion not only solves the controversy about how mixing influences the digestive process, but also assists in optimizing the digester design and increasing the efficiency of energy conversion, and additionally, provides a reference for improving the mixing guidelines recommended by the U.S. Environmental Protection Agency. Biotechnol. Bioeng. 2012; 109: 2864–2874. © 2012 Wiley Periodicals, Inc.     

Macromixing hydrodynamic study in draft-tube airlift reactors using electrical resistance tomography

The present study summarizes results of mixing characteristics in a draft tube airlift bioreactor using ERT. This technique offers the possibility for noninvasive and nonintrusive visualization of flow fields in the bioreactor and has rarely been utilized previously to analyze operating parameters and mixing characteristics in this type of bioreactors. Several operating parameters and geometric characteristics were examined. In general, results showed that the increase in superficial gas velocity corresponds to an increase in energy applied and thus, to a decrease in mixing time. This generally corresponded to an increase in liquid circulation velocity and shear rate values. Bottom clearances and draft tube diameters affected flow resistance and frictional losses. The influence of sparger configurations on mixing time and liquid circulation velocity was significant due to their effect on gas distribution. However, the effect of sparger configuration on shear rate was not significant, with 20% reduction in shear rates using the cross-shaped sparger. Fluid viscosity showed a marked influence on both mixing times and circulation velocity especially in the coalescing media of sugar and xanthan gum (XG) solutions. Results from this work will help to develop a clear pattern for operation and mixing that can help to improve several industrial processes, especially the ones related to emerging fields of technology such as the biotechnology industry.     

Selective desorption of carbon dioxide from sewage sludge for in situ methane enrichment--part I: pilot-plant experiments

In situ methane enrichment in anaerobic digestion of sewage sludge has been investigated by experiments and by modeling. In this first part, the experimental work on the desorption of carbon dioxide and methane from sewage sludge is reported. The bubble column, had a diameter of 0.3 m and a variable height up to 1.8 m. At operation the dispersion height in the column was between 1 and 1.3 m. Outdoor air was used. The column was placed close to a full-scale sewage sludge digester, at a municipal wastewater treatment plant. The digester was operated at mesophilic conditions with a hydraulic retention time of about 20 days. The bubble column was operated to steady-state, at which carbon dioxide concentration and alkalinity were determined on the liquid side, and the concentration of carbon dioxide and methane on the gas side. Thirty-eight experiments were performed at various liquid and gas flow rates. The experimental results show that the desorption rates achieved for carbon dioxide ranges from 0.07 to 0.25 m(3) CO(2)/m(3) sludge per day, which is comparable to the rate of generation by the anaerobic digestion. With increasing liquid flow rate and decreasing gas flow rate the amount of methane desorbed per amount of carbon dioxide desorbed increases. The lowest methane loss achieved is approximately 2% of the estimated methane production in the digestion process.     

In situ methane enrichment in anaerobic digestion

A major cost consideration in the use of anaerobic digestion to convert biomass and waste to utility-grade gas is the expense of separating CO(2) from the product gas. Anaerobic digestion has a number of inherent properties that can be exploited to increase the methane content of the gas directly produced by the digester, the most important of which is the high solubility of CO(2)(40-60 times that of methane) in water under digestion conditions. The methane enrichment concept examined in this study involved the recirculation of a liquid stream from the digester through a CO(2) desorption process and the return of the liquid stream back to the digester for absorption of additional CO(2) produced by the conversion of organic materials. A steady-state equilibrium model predicted that a digester gas methane content exceeding 94% could be achieved with this scheme using modest recirculation rates provided a desorption process could be designed to achieve a 60+% CO(2) removal efficiency in the degassing of the liquid recycle stream. Using fixed-film laboratory digesters operated on synthetic feedstocks, the technique of methane enrichment was tested under pressurized and unpressurized conditions. A 93 + 2% methane gas stream was produced from a volatile-acid-fed bench-scale digester simulating the methanogenic stage of two-phase digestion under conditions of (1) a pH swing achieved without caustic addition that allowed digestion at pH 7. 5 and air stripping at pH 6. 5-7. 0, (2) digester pressurization to 30 psig, and (3) a recycle rate of 0. 33 L/L reactor/day. Significant but lower levels of methane enrichment were achieved with the single-stage digester at the low experimental recycle rate. However, the narrow range among all experiments of CO(2) desorption efficiencies achieved in air stripping the recycle stream (35-60% CO(2) removal) suggests that comparable methane enrichment-may be achieved with unpressurized single-stage digestion using greater recycle rates. A materials balance analysis of data from an unpressurized, single-stage digester employing no chemical addition and using laboratory degassing efficiencies indicated that 94% methane could be produced at recycle rates of less than 1. 4 L/L reactor/day with a methane loss of less than 2%.     

Novel membrane bioreactor: Able to cope with fluctuating loads, poorly water soluble VOCs, and biomass accumulation

We determined the effect of different mixing intensities on the performance, methanogenic population dynamics, and juxtaposition of syntrophic microbes in anaerobic digesters treating cow manure from a dairy farm. Computer automated radioactive particle tracking in conjunction with computational fluid dynamics was performed to quantify the shear levels locally. Four continuously stirred anaerobic digesters were operated at different mixing intensities of 1,500, 500, 250, and 50 revolutions per min (RPM) over a 260-day period at a temperature of 34 +/- 1 degrees C. Animal manure at a volatile solids (VS) concentration of 50 g/L was fed into the digesters daily at five different organic loading rates between 0.6 and 3.5 g VS/L day. The different mixing intensities had no effect on the biogas production rates and yields at steady-state conditions. A methane yield of 0.241 +/- 0.007 L CH(4)/g VS fed was obtained by pooling the data of all four digesters during steady-state periods. However, digester performance was affected negatively by mixing intensity during startup of the digesters, with lower biogas production rates and higher volatile fatty acids concentrations observed for the 1,500-RPM digester. Despite similar methane production yields and rates, the acetoclastic methanogenic populations were different for the high- and low-intensity mixed digesters with Methanosarcina spp. and Methanosaeta concilii as the predominant methanogens, respectively. For all four digesters, epifluorescence microscopy revealed decreasing microbial floc sizes beginning at week 4 and continuing through week 26 after which no microbial flocs remained. This decrease in size, and subsequent loss of microbial flocs did not, however, produce any long-term upsets in digester performance.     

Methanogenic population dynamics during start-up of anaerobic digesters treating municipal solid waste and biosolids

An aggressive start-up strategy was used to initiate codigestion in two anaerobic, continuously mixed bench-top reactors at mesophilic (37 degrees C) and thermophilic (55 degrees C) conditions. The digesters were inoculated with mesophilic anaerobic sewage sludge and cattle manure and were fed a mixture of simulated municipal solid waste and biosolids in proportions that reflect U.S. production rates. The design organic loading rate was 3.1 kg volatile solids/m3/day and the retention time was 20 days. Ribosomal RNA-targeted oligonucleotide probes were used to determine the methanogenic community structure in the inocula and the digesters. Chemical analyses were performed to evaluate digester performance. The aggressive start-up strategy was successful for the thermophilic reactor, despite the use of a mesophilic inoculum. After a short start-up period (20 days), stable performance was observed with high gas production rates (1.52 m3/m3/day), high levels of methane in the biogas (59%), and substantial volatile solids (54%) and cellulose (58%) removals. In contrast, the mesophilic digester did not respond favorably to the start-up method. The concentrations of volatile fatty acids increased dramatically and pH control was difficult. After several weeks of operation, the mesophilic digester became more stable, but propionate levels remained very high. Methanogenic population dynamics correlated well with performance measures. Large fluctuations were observed in methanogenic population levels during the start-up period as volatile fatty acids accumulated and were subsequently consumed. Methanosaeta species were the most abundant methanogens in the inoculum, but their levels decreased rapidly as acetate built up. The increase in acetate levels was paralleled by an increase in Methanosarcina species abundance (up to 11.6 and 4.8% of total ribosomal RNA consisted of Methanosarcina species ribosomal RNA in mesophilic and thermophilic digesters, respectively). Methanobacteriaceae were the most abundant hydrogenotrophic methanogens in both digesters, but their levels were higher in the thermophilic digester.     

Characterization of a pilot plant airlift tower loop bioreactor: II. Evaluation of global mixing properties of the gas phase during yeast cultivation

Saccharomyces cerevisiae was cultivated in a 4-m(3) pilot plant airlift tower loop reactor with a draft tube in batch and continuous operations and for comparison in a laboratory airlift tower loop reactor of 0.08 m(3) volume. The reactors were characterized during and after the cultivation by measuring the distributions of the residence times of the gas phase with pseudostochastic tracer signals and mass spectrometer and by evaluating the mixing in the liquid phase with a pulse-shaped volatile tracer signal and mass spectrometer as a detector. The mean residence times and the intensities of the axial mixing in the riser and downcomer, the circulation times of the gas phase, and the fraction of the recirculated gas phase were evaluated and compared.     

Mass transfer in an airlift reactor with a net draft tube.

Gas-liquid mass transfer in an airlift reactor with net draft tube is investigated. The effects of both the ratio of draft tube to reactor diameter and the reactor pressure on oxygen transfer are considered. The value of the volumetric mass transfer coefficient, kLa, increases with a decreasing diameter ratio at higher air flow rates. The correlation of volumetric mass transfer coefficient with respect to the true superficial air velocity under different reactor pressures is determined. The kLa value decreases with increasing reactor pressure.     

Microbiological aspects of anaerobic digestion of swine slurry in upflow fixed-bed digesters with different packing materials

Samples taken from liquid influents, liquid effluents, bottom sediments and biofilms attached to the supports of three laboratory anaerobic, fixed-bed, upflow digesters, filled with wood chips, PVC or expanded-clay support media and fed with swine slurry, were tested microbiologically. The numbers of anaerobic heterotrophic, anaerobic cellulolytic, acidogenic-peptone-glucose fermenting and methanogenic bacteria were determined. For each digester the biogas production was monitored.The highest biogas production, referred to the volatile solids concentration in the feed, was obtained in the digester with wood chips, while production was almost nil in the digester with expanded clay.     

Inactivation of Salmonella typhi by high levels of volatile fatty acids during anaerobic digestion

Survival of Salmonella typhi was investigated in an anaerobic digester for cattle dung with volatile fatty acid (VFA) levels of 5000 mg l(-1) and pH 6.0. The organism was added to the digester only once in the first experiment and daily in the other. Survival was monitored on alternate days. In the single dose experiment, the counts of Salm. typhi declined rapidly and the pathogen was completely eliminated within 12 d in the experimental digester (VFA ca 5000 mg l(-1) and pH 6.0), whereas 26 d were required in the control digester (VFA ca 100 mg l(-1) and pH 6.8). T90 values for the experimental and control digesters were 2.44 d and 4.80 d, respectively. In the daily dose experiment, a four log reduction in the pathogen count was observed in the experimental digester, but only a two log reduction in the control digester at the end of the experimental period. The mean T90 values for the experimental and the control digester were 4.22 d and 18.63 d, respectively. In both the experiments, statistical analysis of the data showed significant differences in the survival pattern of Salm. typhi in the two digesters.