A Critical Review on Cfd Simulation of Anaerobic Digestion Reactor for Sewage Sludge

Introduction

Biomass is a valuable source of renewable energy with a very low carbon footprint. Biomass tin can be converted to greenish energy through anaerobic digestion, which results in the production of methyl hydride. The anaerobic digestion process has been intensively studied during the last few decades with the aim of optimizing the overall procedure for enhancing methane product [1–iv]. The efficiency of a biogas establish can be calculated in terms of the biogas product rates and power consumption past the constitute itself. Biogas production is a cost-intensive procedure compared to other renewable energy resources due to the high energy input during the entire process, particularly in the course of mixing the slurry inside the digester tank [5]. The efficiency of an anaerobic digester depends on various operational and physical parameters such every bit the type of substrate, carbon/nitrogen ratio, temperature, TS content, mixing, pH value and HRT. From the above stated factors, the mixing of the slurry is a very crucial parameter which has a major impact on the efficiency of a biogas plant in terms of both the quality of the biogas and power consumption. Approximately 44% of the biogas plants failures occurs due to mixing flaws [half dozen]. Adequate mixing is divers qualitatively as the input of sufficient energy to mobilize the reactor content without producing significant regions of inhibitory shear force which can intermission the bacterial/archaeal morphology. In general, mixing helps to achieve homogeneity of the substrate and a uniform distribution of nutrients, pH, and temperature in the digester. The outcome of mixing has been studied by many researchers, merely it is however a topic of debate [7].

In general, the aim of mixing in an anaerobic digester is to:

• Avoid solid accumulation at the base of the vessel;

• Ensure that the supply of substrate is distributed uniformly for the homogeneous distribution of nutrients and micro-organisms;

• Release the entrapped biogas in the slurry;

• Avoid settlings, scum and floating layers.

Mixing in the digester can be conducted out by various methods, such as impeller mixing, slurry recirculation and gas sparging, depending on the digester design and substrate. In the case of mechanical mixing, the impellers are rotated while beingness submerged in the slurry [8,9]. Whereas in slurry recirculation, the slurry is drawn from the centre of the digester by an external centrifugal pump and is diffused in the digester through nozzles [x]. For gas mixing, the biogas is compressed and subsequently forced through the entire volume of the digester to obtain mixing [11]. More often than not, mechanical mixing is preferred over the other mixing modes due to lower ability 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].

The novelty of this article lies in focusing on determining the outcome of shear charge per unit, mixing intensity, mixing time, and viscosity on net biogas production rates. Various factors affecting the mixing in a digester such as OLR, TS, hydraulic retentivity time and temperature have been discussed. A comparison has been drawn between the latest findings of various researchers in terms of methane content, net biogas product and microbial customs composition. This review supports the fact that an evaluation of mixing can help to improve the overall energy efficiency, net energy production and mixing energy input, and thus enhances biogas production rates.

The present written report has been undertaken with the post-obit objectives:

• To understand the influence of mixing regimes and their intensity at the molecular level and to ascertain overall biogas yields;

• To identify the optimum range of mixing intensities and fourth dimension;

• To report the rheology of the slurry in order to determine the optimum design of a digester and its mixing equipment;

• To analyze the effect of mixing on the efficiency of a big-scale biogas found;

• To explore the diverse approaches followed by researchers in evaluating mixing efficiency in anaerobic digesters at both the lab-scale and total-scale.

Diverse effects of mixing on anaerobic digestion processes

Outcome of mixing on the microbial community

The anaerobic digestion process involves dissimilar steps, such as: hydrolysis, acidogenesis, acetogenesis and methanogenesis. Each step involves specific species of bacteria and/or archaea responsible for converting molecules from one form to some other through biochemical reactions. Many researchers have observed that micro organisms at all stages showroom dissimilar behavior at different shear rates [14,xv]. Additionally, excessive mixing and high shear rates have a negative effect on biogas production [15,16]. This results in a lower charge per unit of methanogenesis due to the reduced presence of methanogens and the dissipation of methanogenic centers in the vessel [fourteen].

Recently, many studies have been published which have evaluated the outcome of mixing during hydrolysis, acidification and methanogenic phases. In a recent piece of work by Si-jia et al. [17], the result of mixing was analyzed through the concrete separation of different phases. During hydrolysis and acidification phases, slurry was mixed at different intensities of 30, threescore, 90 and 120 rpm. Whereas, during the methanogenic phase continuous mixing at 120 rpm was practical. The results showed that mixing at ninety and 120 rpm was favorable for hydrolysis and the acidification stage because an affluence of Proteobacteria, Chloroflexi, Firmicutes, Actinobacteria and Bacteroidetes was found.

Ghanimeh et al. [18] carried out microbial analysis at various mixing speeds ranging between eighty and 160 rpm. It has been observed that the agitated digester was dominated by Thermotogae phylum (89% at fourscore rpm, 87% at l rpm and 85% at 160 rpm), which was gradually replaced by Synergistetes. In contrast, under non-mixing atmospheric condition, Synergistetes dominated past 72% and Thermotogae phylum was reduced to equally low as 5%. Further, information technology was observed that Petrotoga genus (phylum of Thermotogae) proliferated under mixing weather condition and was absent in non-mixed digesters.

Stroot et al. [19] revealed that methanogenic archaea and propionate oxidizing bacteria live in close vicinity in granules with hydrogen and formate as an electron carrier. For a thermodynamically stable reaction, the concentration of the electron carrier should be low, and therefore the high rate of propionate conversion observed tin can only exist explained by the brusk improvidence altitude possible in obligate syntrophic consortia. Excessive mixing distorts the granule construction and results in a pass up rate for the oxidation of fatty acids, which can lead to digester instability. According to Vivilian et al. [xiv] higher mixing intensities inhibit hydrolysis, acidogenesis and methanogenesis due to the fact that higher VFA concentrations result in an instability of the digestion procedure. Supporting this fact, Sindall et al. [9] demonstrated that due to increased turbulence (100 and 200 rpm) in the digester, localized pockets of acetate are disturbed, and this results in a refuse in the ratio of acetoclastic methanogens to hydrogenotrophic methanogens. This resulted in a decline of the biogas production rates.

In the ii-stage process, and acidogenic leaner were grown in an acidogenic reactor with the pH naturally low with a residence time between ane and 4 days. Whereas, methanogenic bacteria are grown in a methanogenic reactor, which has naturally a much college pH and a residence time of 15–20 days [xx]. Mixing by MI gave excellent results for two-stage configurations, efficiency jumped from 22.64% (mono-stage) to 30.24% (two-stage configuration) with a 1.34-fold improvement for the process. Still, AI mixing reduced the efficiency of the AD and the efficiency decreased from over 33–17.five%. Thus, the system should be configured in order that it permits sufficient mixing without the mechanical stresses that can destroy methanogenic leaner [21].

Z. Tian et al. [22] observed unlike microbial community structures for agitated and non-agitated digesters. A higher multifariousness of methanogen species was observed in non-agitated digesters, whereas a high proportion of Petrotoga-related (an anaerobic, thermophilic, xylanolytic, motile rod-shaped bacterium) species with Methanosaeta-related methanogens was observed in agitated digesters. Petrotoga produces H₂ through the fermentation of sugars [23]. Continuing their research in 2014, Z. Tian et al. [24] described the microbial community structure and digestion performance for not-agitated and agitated digesters (180 rpm). Information technology was observed that Methanosarcina (acetoclastic methanogen) was more abundant than Methanoculleus (hydrogenotrophic methanogen) in not-agitated equally compared to agitated digesters, which was probably the reason for college methyl hydride production in non-agitated digesters. A relatively higher amount of Acetanareobacterium, Ruminococcus and Ruminococcaceae was found in the agitated digester. These species produced hydrogen from cellulose, and further hydrogenotrophic methanogens are required to convert hydrogen to methane, just they were not found in sufficient quantity in agitated digesters, and a retardation of methyl hydride production was noted. Similar results were obtained past Ghanimeh et al. [xviii] in which the Petrotoga genus (phylum of Thermotogae) proliferated under mixing weather condition, whereas in not-mixed digesters its absence was noted. It was also stated that the genus Petrotoga played an important part in the degradation of organic affair.

Kaparaju et al. [16] analyzed the microbial community in both continuously mixed and intermittently mixed digesters. Results showed an affluence of small rod-shaped bacteria in continuously stirred digesters, whereas the presence of Methanosarcina and Methanosaeta was noted in intermittently mixed digesters. An appropriate remainder of acetotrophic and hydrogenotrophic methanogenesis was observed in intermittently mixed digesters, which might be the reason for their college biogas production.

Considerable differences in levels of Methanosaeta concilii and Methanosarcina were noted by Hoffman et al. [xv]. For Methanosaeta concilii the relative level of the small subunit rRNA was 2%, and this increased to 4% on the six^thursday twenty-four hours. Furthermore, the level approached nil for mixing at 1500 rpm. For the digester mixing at 500 rpm, the levels of Methanosaeta concilii were between 3.two% and iv.8% for first 75 days and so decreased to i% for the remaining period of the performance. In add-on, it was observed that the level of concilii was higher for 250 and 50 rpm compared to 1500 and 500 rpm from day 117 to the end of functioning. This is considering Methanosaeta concilii cells have long filaments, and hence higher mixing intensities can touch on the formation of filaments. Depression levels of Methanosarcina were observed at depression mixing levels of 50 rpm, whereas it increased for higher mixing intensities every bit Methanosarcina was between 2% and 4.5% at 1500 rpm and betwixt 1% and 5% at 500 rpm for the last 60 days of reaction. Moreover, with intensive mixing, a greater increase in the level of the hydrogenotrophic methanogenic family Methanobacteriaceae was observed. However, they remained abiding at 250 and fifty rpm, and levels of acetoclastic methanogens were also loftier during intensive mixing (1500 rpm). They concluded that intensive and continuous mixing are counter-productive in terms of biogas production. In the batch type anaerobic digester, Mohammadrezaei et al. [25] observed that during the commencement 4 days of hydrolysis biogas product was highest with mixing at 120 rpm as compared to 0, xl and 80 rpm. After solar day 10, the rate of biogas product declined sharply in case of a mixing speed of 120 rpm. Whereas at 40 and eighty rpm biogas production reached to highest levels of 1.1 m³ d ⁻¹. It tin be conspicuously observed that the acetogenesis and methanogenesis are more than sensitive to higher mixing rates as compared to hydrolysis.

According to the literature survey, in the batch reactor type fermentation, mixing intensity should be adjusted according to the specific stage of anaerobic digestion. Every bit the digestion process approaches the concluding stage of methanogenesis, the shear rate should subtract in correspondence with the lower mixing speed of mixers. However, during the startup college mixing speeds are favorable. Whereas, in the continuous type digester, the mixing speed of impellers should exist in the range of l to 80 rpm because the different stages of the digestion processes are not separated. The overall literature agrees on the minimal mixing of the slurry. An effective distribution of H_two is a very important aspect which tin can be enhanced by optimized mixing.

Issue of mixing on the methane content

In full general, the marsh gas content of biogas depends on both the substrate composition and the operational parameters, which include HRT, OLR, TS and the mixing schemes. This section will focus on the effect of mixing on the methane content and its correlation with mixing intensity and mixing operation time.

In a written report of lab-scale experiments, Lin and Pearce [26] observed that methyl hydride product was higher during intermittent mixing when compared to an unmixed digester. Moreover, the methane production rate decreased when the duration of mixing was reduced from 45 min/h to fifteen min/h. Lebranch et al. [27] observed that during agitation rates of 50 and xc rpm in that location was a decrease in marsh gas content from 64% to 57% compared to a rate of 10 rpm when using a helical ribbon impeller. On the other mitt, it decreased from 64% to 59% at a charge per unit of 110 rpm compared to 22 and 66 rpm when mixing with a Rushton turbine. During research by D.A. Stafford at Cardiff University in Wales, the effect of mixing intensity on biogas production was analyzed at low speed (140 rpm) and loftier speed (1000 rpm) in chief sewage sludge. It was observed that a low mixing speed at nearly 150 rpm maximized biogas production, whereas at a higher speed, i.e. above 700 rpm, gas product was reduced [28]. Similarly, Ghanimeh et al. [29] compared the different mixing schemes in two separate digesters. First, digester "A" was continuously mixed at 100 rpm, while digester "B" was mixed before and later feeding for a few minutes. Digester "A "produced a college methane content compared to digester "B". The methyl hydride yield in digesters "A" and "B" was 0.60 methane l/g VS and 0.45 methane 50/g VS respectively. At an organic loading rate of 1.nine thou VS/fifty/day, the peak methane content was 5.29 l/day and 5.x fifty/24-hour interval for digesters "A" and "B" respectively. In farther studies, Ghanimeh et al. [18] compared mixing intensities from 50 to 160 rpm. 26 − 41% college methane content was obtained at 50 rpm, than at 80-rpm mixing intensity.

The results support the fact that lower mixing intensities can enhance methane content. In a study by Wang et al. [30], unlike mixing intensities were applied along with a combination of unidirectional and bi-directional circulation of slurry. Information technology was observed that the marsh gas product increased significantly when the mixing speed was increased. Methane production increased past 77% and 220% with x rpm unidirectional mixing and 160 rpm bidirectional mixing, respectively. This was because mixing enhanced sludge liquefaction, which was helpful in the transportation of substrates and nutrients, and also in the mass transfer within the reactor. In low TS systems, inhibiting byproducts are diluted, and an adequate mixing can exist achieved past the slightest stirring endeavour, or fifty-fifty past the natural movement of the generated biogas.

According to Z. Tian et al. [22], a not-agitated digester showed uniform methyl hydride yields, marsh gas production rates and SCOD profiles, whereas a continuously mixing, the digester showed lower methyl hydride product rates from Run 4, 5 and 6. For the proceeding runs four, 5 and 6 the unconverted SCOD was used for inoculation, which resulted in excessive accumulation of SCOD, hence decreasing methane content in the final run, but the overall methane production was similar for both agitated and not-agitated digesters. Hashimoto et al. [31] and Kim et al. [32] obtained almost similar results for both continuous and intermittent mixing in terms of the methyl hydride content. According to Rojas et al. [33], a significant stirring effects on the anaerobic digestion was noted only when the seed sludge from a biogas institute was used as a starter.

Finally, it can be deduced that non-mixing and continuous vigorous mixing has a negative affect on the methane content in biogas production, and it is simply a waste of energy to continuously agitate the anaerobic digester. Information technology should be noted that the variation in methane content between different experiments can be due to the properties of substrates and other operational parameters. Moreover, the mixing too depends on the type of impeller and the shape of the digester. Finally, lower mixing intensities should be preferred, but the uniform distribution of velocity and viscosity is a very important attribute which tin can also lead to an increased mixing time. Graph i presents the variation in methane content due to different mixing regimes.

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

Published online:

30 March 2020

Graph 1. Presents the variation in methyl hydride content (%) due to changes in the mixing intensity, mixing fourth dimension and interval in the lab-scale digesters in literature. The variation among the results can be due to differences in substrate usage and digester geometry.

Graph 1. Presents the variation in methane content (%) due to changes in the mixing intensity, mixing time and interval in the lab-scale digesters in literature. The variation among the results tin can be due to differences in substrate usage and digester geometry.

Effect of mixing on VFA concentration and TVS reduction

During the anaerobic digestion process, the major intermediate products are; acetic acid, propionic acid and butyric acid. For an ideal anaerobic digestion, the pH range is between half dozen.8 and 7.2. Apparently, beneath pH six.6 the growth rate of methanogens is greatly reduced [34] and an excessive subtract in pH can pb to microbial granule disintegration and the failure of the procedure [35,36]. Accordingly, the optimum pH range for hydrolysis and acidogenesis is between 5.5 and 6.5 [37]. Co-ordinate to Drosg [38], the optimum value of the VFAs during methanogenesis should be less than 1 thou/50. Ratanatamskul et al. [10] analyzed the production of various VFA and TVS reduction in a single-stage anaerobic digester at different mixing times of thirty, sixty and ninety min/day by slurry recirculation. The VFA concentration was noted as iii.5 g/l and 2.5 grand/50, which were higher than recommended values. Information technology was also observed that at a mixing fourth dimension of lx min/solar day, propionic acrid and butyric acrid were effectively converted into acetic acid, which led to the effective conversion of acetic acid into methane. Therefore, a mixing time of 60 min/day was observed every bit the optimum mixing fourth dimension. The meaning rise in acerb acrid and propionic acid was noted in the digester with intensive mixing, and this resulted in a fast diffusion from top to bottom. The substrate blazon, mixing intensity and the structure and volume of the reactor affected biogas product efficiency [39]. Supporting this fact, Stroot et al. [19] determined that the level of VFAs which increased sharply under continuous mixing conditions considering of an increase of acetate concentrations, which was due to an imbalance in the digestion process at higher mixing intensities. Instability in the process occurred because of increased hydrolysis and a lower growth rate of methanogens leads to higher VFA concentrations. Similar results were obtained by Kim et al. [forty], equally an intermittently mixed digester and was observed to be more stable nether mesophilic and thermophilic conditions, while a continuously mixed digester yielded an elevated propionate concentration and hence created an imbalance in the digestion process.

Ghanimeh et al. [xviii] noted that the digestion process was most stable at mixing speeds of 80 and 50 rpm equally the VFA concentration was below ii thousand/l. On the other hand, vigorous mixing at 120–160 rpm displayed an instability in the process along with reduction in removal efficiencies as the VFA concentrations were 3.3 and 1.8 times higher than the slower mixing. The increase in VFA levels at 120 rpm and 160 rpm was due to the fact that at vigorous mixing syntrophic microbial flocs were damaged, as noted past Suwannoppadol et al. [41]. Like observations were noted past Latha et al. [42]. During impeller mixing at fifty and 200 rpm, the VFA concentrations were i.2 g/l and 9.28 g/fifty respectively. The average VFA/ALK for 50, 100, 150 and 200 was establish to exist 0.15, 0.xx, 0.19 and 0.28 respectively.

Jiang et al. [43] observed that at a certain level, a higher shear charge per unit increases the convection transfer of glucose in the direction of granules via boundary layer around the granule, creating disequilibrium in yield and utilization rates of volatile fat acids, which gives rising to accumulations inside the granules. This leads to inhibition of acetogenesis and methanogenesis because carbon dioxide and hydrogen are non fully consumed, which results in a reduction of methane content in biogas. By increasing the mixing intensity, the concentration of volatile fatty acids is increased. This study supports the thought of using minimum mixing intensity. Moreover, under mixing conditions lower pH values were observed during the startup catamenia because of an imbalance of hydrolysis, acidogenesis and methanogenesis.

In a study past Ismail et al. [44], the concentration of acetic acid and propionic acrid boosted with an increase in the Re_o from 100, 300 and 500. Minimum mixing can intensify the digestion procedure by ameliorating the concentration of volatile acids in the impregnable range, while at a higher mixing intensity the pH decreases and the digestion is interrupted, resulting in a turn down of biogas production. Similarly, Rebecca et al. [15] analyzed iv different mixing intensities, i.eastward. 1,50,05,00,25,050 rpm. Information technology was observed that at 1500 rpm the concentration of volatile fatty acids was higher compared to the lower mixing intensities and there was a negative effect on the biogas product rates. This fact is supported by Sulaiman et al. [45], who analyzed the VFAs in a vigorously mixed (VM) anaerobic digester. It was observed that during VM, the concentration of VFAs exceeded 3500 mg/l at the end of 13 days incubation, which displayed the negative effect of VM on VFA utilization past methanogens. This agrees with the results of Stroot et al. [19], whose study support the importance of minimal mixing for stability of digestion process. Z. Tian et al. [22] observed that in a continuously agitated digester at 100 rpm, propionic acid accumulation were much college than in the non-agitated digesters considering the latter hindered the digestion procedure and inhibited methanogenesis. This indicated that continuous mixing declines the efficiency of anaerobic digestion. According to Lindmark et al. [46], biogas production decreased and the procedure was destabilized during loftier intensity mixing, but the results showed that VFA accumulation is not the only reason for a pass up in biogas yield.

At high agitation rates, i.east. fifty and 90 rpm with a helical ribbon impeller and 110 rpm with a Rushton turbine, a high pH increase was noted during peak production [27]. For Re of 100 and 300 rpm, the pH was observed to be stable (between six.8 and seven.v), but at Re of 500 the pH started to subtract gradually from 6.8 to 4.7–v.3 in iv days. It was concluded that the mixing effects the pH values in an anaerobic digestion procedure, and it is necessary to control the mixing intensities during the agitation. Mixing is to exist optimized and so that a homogeneity of the mixture is maintained within the prescribed limits of the mixing intensity in club to control the pH levels [44]. Kaparaju et al. [16] observed low and delayed marsh gas production for vigorous and continuous mixing in batch experiments because the methanogenesis was inhibited due to a homogenous distribution of volatile fatty acids in the digester. Propionate was consistently produced during vigorous mixing only was not consumed at the same rate. Consequently, when the mixing scheme was moved from vigorous to gentle mixing weather condition, propionate was chop-chop consumed, and the digester attained a stable condition. According to Hashimoto et al. [31], there was a meaning difference in biogas yields during continuous and intermittent mixing for HRT for 4 days. Nevertheless, at an HRT of 6 days the performance of both mixing regimes was the same. Moreover, mixing has a significant issue on digesters with a shorter HRT because by mixing the fresh substrate is introduced to microorganisms at a faster rate and reaction time decreases. Mixing does non bear on the operation of digesters operated nether longer HRT [47,48].

From the above discussion it can be ended that vigorous and continuous mixing should exist avoided equally it showed negative effects on biogas production in every experiment. Intermittent mixing at low intensities is favorable and energy efficient for an anaerobic digestion procedure. Graph 2 demonstrates the results from diverse studies for the TVAF concentration.

Impact of mixing intensity and elapsing on biogas product in an anaerobic digester: a review

Published online:

30 March 2020

Graph 2. Presents the effect of different mixing strategies on accumulation of TVFAs in the anaerobic digestion process. Information technology can exist clearly noted that at higher mixing intensities the concentration of VFAs increases speedily.

Graph 2. Presents the event of different mixing strategies on accumulation of TVFAs in the anaerobic digestion process. It tin be clearly noted that at higher mixing intensities the concentration of VFAs increases rapidly.

Factors affecting mixing in an anaerobic digester

Effect of viscosity, shear stress and TS content

Rheological beliefs and bioreactor hydrodynamics are the central parameters that determine the efficiency of any mixing equipment [27]. The rheological properties of slurry at various temperatures, TS content, and the type of substrate have been extensively studied past many researchers [49–58]. According to literature results, cattle manure and waste water sludges are non-Newtonian fluids considering there is no linear relation between their shear rates and shear stresses [59]. Among all the factors which influence slurry rheology, the TS content is closely associated with credible viscosity. In comparison, a high TS content results in a high viscosity of the liquid, which requires greater stirring efforts to accomplish the same level of mixing. The touch on of mixing appears also to depend on the type of substrate fed into the system, as different substrate composition can atomic number 82 to different microbial setups with varied tolerance, and differences in the abundance of toxins and inhibitors. It has been observed that the impact of mixing on biogas generation is perceptible simply at college TS concentrations (<10%) [60].

Achkari-Begdouri [fifty] demonstrated the rheological properties of cattle manure at a TS content in the range of 2.v–12% TS and a temperature range of xx °C − 60 °C. It was observed that lowering TS content and a higher temperature makes cattle manure slurries bear more like Newtonian fluids. This fact is supported past Baroutian et al. [61], who demonstrated that for waste product h2o sludges temperature and solid concentration are disquisitional parameters which influence slurry rheology. It was noted that shear stress increases non-linearly with shear charge per unit and decreases with an increase in temperature because at higher temperatures cohesive forces betwixt molecules are reduced. Furthermore, apparent viscosity decreases with an increment in temperature. The effect of temperature on apparent viscosity can be evaluated with the Arrhenius model [62]. (one) $$\mu =A{\mathrm{}\text{exp}}^{\left(\frac{Ea}{{RT}_{\mathit{abs}}}\right)}$$ (1)

Grinding or size reduction also plays a vital office in determining the viscosity of the slurry. It has been noted that the fine particles dramatically reduce the slurry's apparent viscosity. Accordingly, grinding of solid manure before feeding it into the digester will decrease the mixing toll and enhance mixing and digestion efficiency [53]. Another method to decrease viscosity is filtration, which results in an comeback of the mixing efficiency of a Rushton turbine. These methods are beneficial from a rheological point of view, merely on the other paw they also atomic number 82 to an increase in the overall operational cost [21].

A study by Jiang et al. [43] revealed a close relationship betwixt biogas product rates and mean methyl hydride content on the one manus, and the hydrodynamic shear rate in a digester on the other. The experiment was conducted in a 450 ml CSTR mixed past helical ribbon at rotational speeds of 12, 18, 24, 36 and 60 rpm. Initially, the mean biogas production rate and methyl hydride content increased to their highest values with an increase in shear charge per unit, but later decreased continuously. The maximum methyl hydride product was noted when a shear rate of 6.8 s⁻¹ was applied past Montgomery et al. [63] who studied the rheological beliefs of a slurry consisting of agricultural residues. Various rheological models were adult, such as the Casson model, the power police force and the Bingham plastic model. A non-Newtonian fluid graphic symbol of slurry does not possess a constant viscosity but rather has an apparent viscosity (η_a ), which tin be calculated using Equation 2. (two) $${\eta }_a=\frac{\tau }{\gamma }$$ (ii)

Studies have revealed that apparent viscosity increases at very low shear rates, resulting in more ability consumption. At a shear charge per unit of 18 southward⁻¹ the credible viscosity was 12.1 Pas which decreased to six Pas in the post-obit 5 h. At a shear charge per unit of 36 s⁻¹ the apparent viscosity decreased from eight.0 Pas to four.7 Pas, which proved that credible viscosity and non-Newtonian behavior is time dependent.

Chen [49] observed that at TS 2.84% slurry behaved like Newtonian fluid, whereas for TS > ii.viii% the beliefs was non-Newtonian. Information technology was ended that the values of limiting viscosity ( $\eta$ _o) and the consistency index (m) increases as TS increases and on other hand, decreases equally temperature increases. Similarly, Kumar et al. [64] demonstrated the fluid properties of animal waste slurries. The Power law was used to predict the 1000 at a abiding shear rate of 30 s⁻¹ with respect to TS.

Wang et al. [30] demonstrated the rheological properties of anaerobic inoculum and dewatered sludge, which was collected from a sewage plant in Sweden. The viscosity and the flow curves of substrate showed non-Newtonian shear thinning beliefs and yield stress. The minimum viscosity was observed at the top mixing speed (160) rpm.

Various numerical models accept been developed by researchers to evaluate dissimilar aspects of the characteristics of the slurry. The Power law model [52,64,65] was developed to make up one's mind the rheological characteristics of the medium based upon the local shear, dry thing content, and mean cobweb length. (three) $$\tau =\mathrm{thousand}·{\gamma }^n$$ (3)

In a study by Wang et al., the Herschel Bulkley model (Equation iv) was used to change rheogram data to the rheological behavior of fluid [xxx]. (iv) $$\tau ={\tau }_{o\mathrm{}}+\mathrm{}k{·\gamma }^{\mathrm{due\; north}}$$ (4)

Co-ordinate to Doran et al., due to the non-Newtonian behavior of the slurry, the turbulence during mixing is reduced and stagnant zones inside the digester are formed. Information technology is necessary for the period to be turbulent to ensure effective mixing and interchange of textile betwixt different locations, but on other hand, turbulent mixing can pause the bacterial/archea morphology.

Ability consumption for agitation depends directly on the fluid properties. Ability consumption past an impeller can be related to the viscosity of the slurry every bit proposed by Chen et al. [66]. According to the written report, the higher the viscosity, the higher the power dissipation to achieve homogeneous mixing. Co-ordinate to Keanoi et al. [67], higher biogas product was observed nether mixing at college levels of TS. Reduction in particle velocities in the vessel was observed with increasing solid content due to increased viscosity. Consequently, additional energy is required for mixing nether higher gluey conditions. For case, doubling the TS increased the dead volume by ix-fold. At lower TS content, there is no impact of mixing on biogas yield. In a written report, the mixing speeds were altered from unmixed to 100 rpm at TDS of 2.5, which resulted in no variation of biogas production, but at a TS of 5.4%, dead book increased dramatically [68]. College mixing results in a waste material of free energy when the digester is fed with a lower TS content.

Clarke and Greenwood [69] developed a formula to calculate the rpm of an impeller in social club to generate specific shear rates. Accordingly, if the optimum shear rate values are known, the mixing speed of an impeller can be adapted, which will depend on the geometry of the digester and the impeller. Offset, the rotational velocity has to exist adamant as per shear charge per unit using Equation (v). (5) $$\omega =\frac{\mathrm{\gamma }(r^d-r^i)}{r^i}$$ (v)

Furthermore, the rpm tin be calculated as per Equation (6) (vi) $$\mathrm{north}=\mathrm{}\frac{\omega ·60}{\mathrm{two}\pi }$$ (half-dozen)

The ability of particle motion is reduced when solid particles increase and hence particles mix within the flow field [70]. Co-ordinate to Karim et al. [60], mixing is valuable only when the full solid content is greater than 10%. Results demonstrated that the mixed digesters produced 10-30% more biogas than unmixed digesters at higher TS content values.

The average velocity gradient helps to better sympathise mixing operations in the slurries. Ratanatamskul et al. [x] introduced a new parameter chosen the mixing intensity number. This parameter characterizes the velocity gradient and the mixing fourth dimension using the following Equation (7). (7) $$G·T_m=\mathit{mixing}\mathrm{}\mathit{intensity}\mathrm{}\mathit{number}$$ (7)

According to Sindall et al.[9], the threshold for an average velocity gradient lies between 7.2 to 14.three s⁻¹ to produce the maximum biogas yield[71]. Rivard et al. [72] observed that at that place was no significant difference in biogas production between the mixing intensities of ane and 25 rpm for a high organic loading of up to 9.5 1000 VS/day and TS content of 5–36% in a 20 l digester. A higher mixing intensity was just a waste of energy. At a concentration of x% TS w/west, the fluid possessed a viscosity of 0.fourteen–0.xviii Pas, which resulted in inadequate mixing by RT, although the results were better at a TS concentration of six%, in which viscosity was 0.06–0.08 Pas [21]. In research by Wu [73], six different values of TS content were examined (TS = 0, 2.5%, 5.4%, vii.5%, 9.7% and 12.one%) for various mixing speeds (N = 4,00,45,05,00,55,06,01,00,00,00,000 rpm). It was observed that with an increase of propeller speed, mixing intensity increases, but on other hand, poor mixing was observed at higher TS values. Furthermore, at a TS of 12.1%, expressionless cell book was measured to be 87%, which demonstrates poor mixing at higher viscosity. The average velocity of fluids in a tank increases linearly, while the mixing energy level increases exponentially with an increase of rotation speed at a abiding level of TSC. The effect of increasing the solid content is significant on the mixing characteristics and can exist demonstrated in terms of the velocity magnitude and book of brackish zones. Before optimization of mixing strategies, the value of the solid content should be considered equally information technology will affect the design of both the impeller and the digester.

The literature shows that high ORL tin lead to the destabilization of a digester, which results in an increase of VFA and a decrease of biogas yield. The failure of a digester due to loftier OLR can be avoided by optimized mixing and intermittent mixing is enough to increment the efficiency of anaerobic digestion, whereas natural mixing involves evaluating the gas during digestion, and it can exist controlled past feeding. When OLR is constant, natural mixing occurs at 6.4 kgm⁻³d^−ane [74].

Information technology is concluded that the study of rheology is a very important attribute in designing an anaerobic digester. Slurry with college viscosity faces a trouble of uneven velocity distribution inside the digester. Higher mixing intensities and the resultant shear stress have a negative effect on flock formation and can reduce gas product. In a mechanically mixed digester, the shear rate well-nigh the impeller blades is very loftier but at distances away from blades the shear rate is relatively low. This results in loftier apparent liquid viscosity and poor mixing. The optimum pattern of a digester and impeller according to the rheological properties of slurry can help in a more than uniform distribution of shear stress and viscosity, requiring less mixing time and minimum power consumption by agitating equipment. Hence, it can lead to lower capital investment and operational costs. Here, it should likewise be noted that the rheological data given in the literature (Tabular array ane) tin vary due to animal diets, manure treatment and handling, and measurement inaccuracies. Slurry possesses shear thinning beliefs because viscous forces are very sensitive to the shear charge per unit distribution during mixing. Moreover, to predict the rheology and behavior of slurry, the limerick of the slurry must be well understood.

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

Published online:

30 March 2020

Table 1. Rheological parameters of diverse substrates used for anaerobic digestion.

Effect of digester and impeller pattern on mixing

Bio-digester design and mixing equipment blueprint is a fundamental feature in the techno-economic feasibility of biogas production [thirteen]. Modifications in digester design can issue in lower operating costs and enhance mixing efficiency. Digester and impeller shapes determine the menses patterns, velocity and viscosity distribution within the agile volume of a digester. In a study by Karim et al. [71], digesters with a working volume 3.37 fifty and a lesser slope bending of 60° and twenty° mixed with a 62 mm centric flow impeller were analyzed. A digester with a lx° lesser hopper showed better settling of solids, which resulted in significantly lower VS reduction than other digesters. Similarly, Vesvikar et al. [83] compared ii configurations of conical lesser digesters. The beginning digester had a slope of 25° and the other a slope of sixty°. The flow pattern was almost like to that in a apartment bottom digester, merely the dead zones were reduced in a conical lesser digester with a slope of 25°. They concluded that the lower slope angle tin can assistance to achieve homogeneity in less fourth dimension and improve mixing efficiency.

Wu et al. [73] compared the flow pattern in egg-shaped and cylindrical digesters, both with a working volume of 4888 one thousandⁱⁱⁱ mixed with a dual helical blade propeller rotating from 400 rpm to 750 rpm. The egg-shaped digester showed more uniform mixing and was more effective in maintaining homogeneity in the digester compared to the cylindrical digester. For a cylindrical digester, mixing energy levels were 6.95 West/m³ and 7.forty Due west/thousand³ for TS of ii.5% and five.4% respectively, whereas for the egg-shaped 1 it was noted to be three.25 W/thousand³ and iii.99 West/m^three for the aforementioned TS levels. Egg-shaped digesters are more adaptable when processing upsets and reducing dead zones. The lesser of a digester helps to reduce sedimentation and maintain the agile volume in the vessel, which reduces operational and maintenance costs. Due to the larger surface surface area of cylindrical digesters, a larger volume of gas can be stored, which enhances the aggregating of foam and scum. Moreover, inadequate mixing was observed in cylindrical digesters, which resulted in dead zones and brusque circuiting of slurry. On other hand, egg-shaped digesters have a less surface expanse above the majority phase, which reduces the accumulation potential of scum and foam, and diminishes the dead zones [84].

The CFD model for egg-shaped digesters was developed, and it has been observed that near the tank bottom the density of path lines was denser than the other regions, which provides evidence that particles have to travel a longer distance before settling. This indicates the advantage of the egg-shaped digester over the cylindrical-shaped digester in checking for dead zones, as well every bit reducing scum and grit accumulation [73]. Oloko-Obo et al. [85] studied the effect of digester shape on biogas production. Three unlike shapes of digester (conical, cubical and cylindrical) with a book of 15 l were studied, operating with substrate consisting of poultry droppings, cow manure and pig waste in a steady-country condition. The digesters were not mechanically stirred but shaken manually. It was observed that for a conical-shaped digester the biogas product started on the 6th day, while for the cylindrical and the cubical it started on day seven. The difference in the lag period was due to the difference in time needed by various microbial flora to conform to changed environmental conditions. The cylindrical digester produced the highest book of biogas (23.four l), whereas the cubical and the conical produced xviii.ii and xiv.4 dm³ respectively. Furthermore, it was noted that the cylindrical digester exhibits better mixing and higher biogas production compared to other digesters.

It is observed that mixing equipment should be selected according to the shape and size of the digester. Digesters with the aforementioned design and operational specifications tin perform completely differently due to variations in mixing mode and the rheological backdrop of the slurry.

Conclusion

Later on the literature review, it is ended that, although the importance of mixing to enhance functioning in anaerobic digestion has been noted by many researchers, the optimum mixing strategy is even so unclear. The uncertainty and dissimilarities in the results from the different studies is due to the evaluation approach adopted by the researchers. The optimization of mixing in anaerobic digestion requires a multidisciplinary approach which should include fluid dynamics and microbiology. For instance, many studies lack a focus on the geometry of the impeller and the digester, which makes it very hard to derive the relationship between the hydrodynamic shear and the microbial community in an anaerobic digester during mixing. Moreover, mixing depends on a big number of other factors which cause variation in results for dissimilar experiments. The literature in this field emphasizes the importance of acceptable mixing to reach the appropriate distribution of enzymes and microorganisms in the digester. The detrimental impacts of inadequate mixing are observed, such equally lower methane yield, defective stabilization of raw slurry, loss of digester volumes and an increase in operational expenses. It can also lead to sedimentation at the bottom of the digester, scum germination, short circuiting, uneven distribution of temperature and substrate and dead zones. Most of the studies acknowledge that excessive mixing in an anaerobic digester can result in deteriorating methane production and unnecessary utilization of power. Mixing has not been optimized for the large-scale anaerobic digesters. The following are the conclusions of this review:

• Continuously mixed digesters should not be encouraged anymore equally they accept negative impacts in terms of both biogas product and college power consumption. Intermittent mixing is every bit prominent as nonmixing and continuous mixing, simply the time to mix requires more attention because it depends on the geometry of both the digester and the impeller.

• The blueprint of the impeller plays a meaning office in determining mixing efficiency. The optimum design of an impeller should atomic number 82 to a uniform distribution of viscosity and velocity in the entire volume of the digester at slower mixing speeds. Use of paddle impellers should be increased, in spite of the pitched blade and propellers. Slow-moving impellers with longer agitating wings tin perform better in pilot-calibration digesters. The impeller properties, similar pitch ratio, power number and centric period number, are closely related to attaining homogeneity in the digester. These impellers should be modified to have compatible shear distribution so that the microorganisms remain unaffected and aim to reduce power consumption and improve the period blueprint of the slurry in the digester. Subsequently, the impeller to exist used for mixing slurry in an anaerobic digester should accept a nearly constant pitch as it will provide uniform velocity distribution at low shear rates.

• Performance of mixing equipment depends on the geometry of both the impeller and the digester, every bit well as the rheological properties of the slurry. Rheological backdrop of slurry depends on various physical properties, such as TS, particle size, particle density, majority density, and particle size distribution. Digesters operated at lower TS content and longer HRT seem to be unaffected by mixing, but at higher TS levels, mixing has a significant event.

• The mixing intensity should exist decreased as the procedure approaches methanogenesis only tin can be run high during the startup.

• Slurries showroom non-Newtonian shear-thinning behavior as the viscosity decreases with an increase in shear rate. Uniform distribution of viscosity and velocity should be the foremost aim during mixing. At higher temperatures, the mixing tin can be minimized to reduce power consumption and enhance overall efficiency.

• The scaleup of lab-calibration digesters should be focused on because the geometrical constraints.

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Source: https://www.tandfonline.com/doi/full/10.1080/07388551.2020.1731413

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