Hydrodynamic analysis of an anaerobic sequencing batch biofilm reactor with liquid-phase external circulation

Eduardo F. M. Camargo¹, Catarina S. A. Canto², Suzana M. Ratusznei³, José A. D. Rodrigues⁴, Marcelo Zaiat⁵ and Walter Borzani⁶

¹ MSc in Civil Engineering, Universidade de São Paulo (USP), Brazil. Researcher, Escola de Engenharia Mauá, Instituto Mauá de Tecnologia (EEM-IMT), São Caetano do Sul, Brazil.

² Doctor in Chemical Engineering, Universidade Federal de São Carlos (UFSCar), Brazil. Researcher, EEM-IMT, São Caetano do Sul, Brazil.

³ Doctor in Chemical Engineering, UFSCar, Brazil. Professor and Researcher, EEM-IMT, São Caetano do Sul, Brazil.

⁴ Doctor in Chemical Engineering, Universidade Estadual de Campinas (UNICAMP), Brazil. Professor and Researcher, EEM-IMT. Address: Praça Mauá 1, CEP 09.580-900, São Caetano do Sul, Brazil. e-mail: rodrigues@maua.br.

⁵ Doctor in Civil Engineering, USP, Brazil. Professor and Researcher, Escola de Engenharia de São Carlos, Universidade de São Paulo (EESC-USP). Address: Av. Trabalhador São-Carlense 400, CEP 13.566-590, São Carlos, Brazil.

⁶ Doctor in Chemical Engineering, USP, Brazil. Researcher, EEM-IMT, São Caetano do Sul, Brazil.

 

Resumen

    Este trabajo evaluó el comportamiento hidrodinámico y la remoción de materia orgánica en un reactor anaerobio de lecho fijo rellenado con espuma de poliuretano (0,5cm) y operado por lotes secuenciales con recirculación de la fase líquida. El estudio hidrodinámico fue realizado sin microorganismos y con velocidad superficial de líquido entre 0,16 y 0,80cm/s. El reactor fue operado de forma continua para evaluar el patrón de flujo en el lecho y por lotes para estimar el tiempo de mezcla de la fase fluida. Bajo operación continua, el flujo pudo ser caracterizado como pistón y, en el caso de operación discontinua, el tiempo de mezcla fue considerado despreciable cuando fue comparado al tiempo total de reacción a ser utilizado en la operación por lotes para remoción de la materia orgánica. El reactor, rellenado con biomasa anaerobia inmovilizada sobre espuma de poliuretano, fue operado en lotes de 8h con temperatura de 30 ±1ºC y alimentado con agua residual elaborada con glucosa y con demanda química de oxígeno de 500mg/L aproximadamente. El desempeño del proceso fue evaluado sin recirculación de la fase líquida y con recirculación, con aplicación de velocidad superficial de 0,19cm/s. Los mejores resultados de estabilidad del proceso anaerobio y de remoción de materia orgánica fueron alcanzados con la recirculación de la fase liquida, con valores hasta 95%.

Summary

    An assessment was made of the hydrodynamic behavior and organic matter removal of a fixed bed anaerobic sequencing batch reactor with liquid-phase circulation, containing 0.5cm polyurethane foam cubes. The hydrodynamic study was performed without biomass and with superficial rates ranging from 0.16 to 0.80cm/s, employing continuous operation to quantify the flow behavior in the bed and discontinuous operation to quantify mixing time. In the continuous mode the bed showed a piston flow behavior. In the discontinuous operation mode the mixing time obtained in all conditions may be considered negligible when compared to the total cycle time to be employed in the assay with biomass. In the assay regarding organic matter removal, 8h cycles were employed in sequencing batch mode using anaerobic biomass. The reactor was maintained at 30 ±1ºC, treated glucose-based synthetic wastewater with a concentration of approximately 500mgCOD/L. Reactor performance was assessed for the condition without liquid circulation as well as at a superficial circulation rate of 0.19cm/s. The reactor presented better performance and stability for the circulation condition, achieving efficiencies as high as 95% organic matter removal.

Resumo

    O presente trabalho constou da avaliação dos comportamentos hidrodinâmico e de remoção de matéria orgânica de um reator anaeróbio de leito fixo, contendo cubos de espuma de poliuretano de 0,5cm de lado, operado em batelada seqüencial e recirculação da fase líquida. O estudo hidrodinâmico foi realizado sem biomassa e com velocidade superficial entre 0,16 e 0,80cm/s operando o reator nos modos contínuo, no intuito de quantificar o comportamento de escoamento no leito, e descontínuo, no intuito de quantificar o tempo de mistura. O comportamento do escoamento no leito foi como o de um reator de fluxo pistonado, no caso da operação de modo contínuo, e o tempo de mistura obtido em todas as condições pode ser considerado desprezível quando comparado ao tempo total do ciclo a ser utilizado no ensaio com biomassa, no caso da operação de modo descontínuo. No ensaio para remoção de matéria orgânica, o reator foi operado com biomassa anaeróbia, no modo batelada seqüencial com ciclos de 8h e mantido a 30 ±1ºC, tratando água residuária sintética elaborada à base de glicose, com concentração de aproximadamente 500mgDQO/L. O desempenho do reator foi avaliado para a condição sem circulação do líquido e também para velocidade superficial de recirculação de 0,19cm/s, sendo que para a condição com recirculação o reator apresentou melhores desempenho e estabilidade, apresentando eficiências de até 95% de remoção da matéria orgânica.

Keywords / Anaerobic Reactor / Batch Reactor / Immobilized Biomass / Liquid Phase Circulation / Wastewater Treatment /

Received: 07/01/2004. Modified: 03/03/2005. Accepted: 03/14/2005.

 

    Anaerobic batch processes designed for wastewater treatment offer advantages over conventional anaerobic techniques, such as the nonexistence of hydraulic short-circuit, enhanced effluent quality control, no need for primary and secondary settlers, nonexistence of biomass circulation and operation simplicity. Moreover, some kinetic advantages have also been reported, such as high methanogenic activity. On the other hand, this kind of system requires well established and defined operation techniques and methodologies since efficiency may be affected by the following shortcomings: occurrence of dead zones, long settling times, solids washout, long start-up periods, inhibition due to high organic load and lack of knowledge regarding agitation and feed strategies. Yet, many of these shortcomings may be solved through research on fundamental technological aspects of mass transfer, kinetics and reactor hydrodynamics (Zaiat et al., 2001).

    Within this context, anaerobic sequencing batch reactors (ASBR) have shown to be promising in the treatment of low strength industrial as well as domestic wastewaters (Brito et al., 1997; Ndon and Dague, 1997a, 1997b). A typical ASBR cycle comprises 4 stages: feeding of the reactor, biological reaction for organic matter removal, biomass settling and discharge of the treated effluent (Sung and Dague, 1995). The batch operation allows good control of effluent quality, since discharge can be performed only when the permitted emission standards are reached (Zaiat et al., 2001).

    Performance of this type of system may be optimized by a more effective biomass-substrate contact, which may be improved by immobilizing the biomass on inert supports. Immobilization avoids washout of solids and makes the settling step unnecessary. However, mass transfer in the system may be limited, especially when treating low strength wastewater, where biogas production is also low and, consequently, agitation is insufficient. One way of dealing with this limitation is by implementing either mechanical agitation (Ratusznei et al., 2000, 2001) or circulation of the liquid phase by means of a pump (Hirl and Irvine, 1996; Camargo et al., 2002; Ramos et al., 2003). The use of a support for biomass immobilization brings about new issues related to reactor performance, since mass transfer resistance may occur, which is inherent to processes involving two distinct phases, in this case solid and liquid phases. This way, efficiency of reactors containing immobilized cells is also related to the mass flows between the liquid and solid phases, which may be limiting factors and cause considerable decrease in reaction rate. Increasing the superficial liquid velocity or agitation speed in heterogeneous systems may minimize mass transfer resistance in the liquid phase and favor reactor performance.

    Camargo et al. (2002) and Ramos et al. (2003) investigated the influence of liquid circulation superficial rate on the treatment efficiency of different synthetic wastewaters as well as on cycle time, using an ASBR containing immobilized biomass on 0.5cm polyurethane foam cubes. Cell immobilization considerably improved biomass retention in the system, allowing suppression of the sedimentation step. Liquid phase circulation reduced mass transfer resistance between biomass and wastewater, increasing process efficiency and reducing cycle time.

    Based on the above, the main objectives of this work were to assess the hydrodynamic and organic matter removal behavior of an ASBR with liquid phase circulation containing polyurethane foam cubes.

Materials and Methods

    The hydrodynamic study, performed without biomass, consisted of determining the residence time distribution (RTD) in the reactor operated in continuous mode, using two flow models (axial dispersion and tank series), and determining the mixing time in the reactor operated in batch mode with circulation, using a first order model. Investigation of the reactor in continuous mode aimed at quantification of the hydrodynamic behavior of the fixed bed only, isolated from and not connected to the system, since in the batch mode wastewater treatment occurs with piston flow hydrodynamics.

    In addition, tests were carried out to assess reactor performance in treating low-strength wastewater in batch mode with anaerobic biomass, with and without circulation.

Bioreactor

    The experimental setup is shown in Figures 1 to 3. The reactor consisted of a cylindrical flask with an external diameter of 60mm, 460mm of height and 3.5mm of wall thickness, resulting in a work volume of approximately 1.0L. The fixed-bed with 410mm of height, consisted of 5mm polyurethane foam cubes (apparent density of 23kg/m3) confined between two punctured stainless steel disks, resulting in a bed porosity of 40% (Zaiat et al., 1997).

    The complete experimental setup is shown in Figure 1, whereas Figure 2 shows the configuration used in the hydrodynamic study in the cylindrical flask containing the biomass immobilization support. Figure 3 shows the setup used in the tests to determine mixing time in the entire system.

    Hydrodynamic behavior assessment of the bioreactor in continuous feed mode

    The real behavior of a fluid inside a reactor can be known through the time each fluid element remains inside the equipment, i.e., the residence time distribution (RTD) of each fluid element.

    Assuming constant fluid density, the nominal residence time ( ) for all fluid elements may be given by = V/Q, where Q is the flow rate and V is the fluid volume. However, each fluid element travels a different way across the reactor and the distribution of these times for the effluent fluid may be represented by a J-curve, which can be considered a dimensionless     measurement of the fluid elements and which in its normalized form is given by J dt= 1. The curve of this variable J can be determined by using a tracer, which may be reactive or not, and generates pulse or step-wise disturbance in the reactor feed line.

    The pulse perturbation is obtained by instantaneously introducing a certain amount of tracer in the reactor feed and registering concentration and the time the tracer takes to exit the reactor. To encounter the J-curve, which is a function of trace concentration (C), Eq. 1 is used, where the variable J’ is the time derivative of the variable

    J [J'(t)=J(t)/dt and J(t)= J'(t)dt], Ci is the trace concentration as a function of time and Dti is the time interval between samples (i).

    In the step perturbation experiment the tracer is fed to the reactor in steady state at initial time (t= 0). As in the previous case, trace concentration at the reactor exit is measured as a function of time. The shape of the J-curve, in this case, is given by Eq. 2, where Ci is trace concentration as a function of time (t) between samples (i) and C0 is the initial trace concentration.

    The variables defining RTD for the pulse and step perturbations are the average residence time of the fluid elements in the reactor (; Eq. 3) and the variance or dispersion of the individual residence time of the respective volume elements in relation to the average time (s2; Eq. 4) of the curve.

    Assays for determining RTD and flow mode through the polyurethane foam bed were performed by using: a) pulse perturbation, and b) step perturbation.

    The pulse perturbation assay was carried out by continuously feeding the reactor with distilled water. The velocities were calculated by vs= Q/jA, where Q is the liquid flow rate, A is the internal cross-sectional area of the reactor and j is the fixed-bed porosity (40%).Ten minutes after feeding started, i.e., sufficient time for complete soaking of the polyurethane foam bed, the system was calibrated by measuring the feed flow rate employing a graduated measuring cylinder and a digital stopwatch. Measuring was performed at the reactor outlet, since the system operated in the continuous mode. The system was then perturbed with a pulse stimulus by quickly adding 5.0ml of a 0.1N H2SO4 solution at the base of the reactor, using a syringe of the same volume. As soon as the stimulus was applied, system pH monitoring was initiated, using a pH meter connected to a microcomputer allowing data acquisition in time intervals of approximately 2sec. The pH electrode was attached at the top of the reactor, above the foam bed. Monitoring was performed up to pH stabilization. Liquid circulation flow rates used were 5, 15 and 25L/h, and the respective superficial velocities were 0.16, 0.48 and 0.80cm/s. After completion of each operational condition, the reactor was rinsed with tap water for 10min and then with distilled water until pH stabilization.

    The step perturbation assay was performed similarly to the previous one. However, perturbation was conducted replacing the distilled feed water with the 0.1N H2SO4 solution. The operational conditions were the same as those used in the pulse assay and pH was monitored until pH stabilization.

    In both assays the output signal measured was the pH variation, i.e., C= [H+]-[Hi+], where [H+] is the measured concentration of H+, and [Hi+] is its initial concentration, making it necessary to transform this into a concentration ([H+]= 10-pH) in order to determine RTD.

    Determination of RTD allowed fitting of a model that best represented flow in the cylindrical flask containing the support material. The theoretical models are used to represent the flow behavior in real reactors and for scale up (Smith, 1981; Levenspiel, 1999). The compartment models are the simplest ones and take on the extremes: plug flow and perfect mixture flow. Comparison of the real J curve with those of the theoretical model allows to encounter the model which best fits the real behavior of the fluid. Two models may be used to treat deviations from ideal flow: the dispersion model and the tank series model.

    Eqs. 5, 6 and 7 represent the axial dispersion model for step perturbation (Eq. 5), pulse perturbations and small dispersions (Eq. 6) and pulse perturbations and large dispersions (Eq. 7), respectively. Therefore, for , dispersion is negligible and flow is plug flow and for dispersion is considerable and flow is complete mixing flow.

    In the serial reactor model, the real reactor is simulated by N perfect mixing series tanks. The total volume of the tanks and the total average residence time are equal to those of a real reactor. The relation between J(t) and N, and between J’(t) and N, for step and pulse stimulus, are given by Eqs. 8 and 9, respectively. For N=1 the reactor presented a complete mixing behavior and for N= the reactor showed plug flow behavior and N is calculated by .

    Assessment of mixing time in the bioreactor operated in batch mode

    This hydrodynamic assay was carried out with the complete equipment (Figure 3) operating in batch mode to determine mixing time. The reactor was fed through the base with distilled water and operated in batch mode. Before perturbation of the system, the liquid was left to circulate for about ten minutes to allow soaking of the foam cubes. After this period, liquid flow was measured with the aid of a measuring cylinder attached to the system and a digital stopwatch.

    As soon as the reactor and side reservoir reached maximum capacity (950 ±10mL) and with the circulation system turned on and calibrated, the pulse perturbation was applied by injecting 5.0mL of a 0.1N H2SO4 solution. Perturbation was effected at two different places: at the top of the side reservoir and at the base of the reactor, in order to determine the influence of perturbation site on system behavior. After perturbation pH variation was monitored by a pH meter connected to a computer, with the electrode located right above the bed. Values of pH were recorded every 2sec approximately and the experiment concluded as soon as the pH was stable. At the end of each experiment the reactor was rinsed with tap water for 10min and subsequently with distilled water until pH attained values near neutrality. Liquid circulation flow rates were the same of the continuous assays. To determine the influence of liquid volume in the side reservoir on system behavior, an assay was performed with the reservoir at minimum capacity. In this case, the pulse was effected at the top of the reservoir, circulation flow rate was 25L/h and the system volume 850 ±10mL.

    The results obtained in these experiments were used to estimate mixing time, considered as a parameter that characterizes the state of mixing of a reactor operating in discontinuous mode. Considering a first order model system behavior, increased with a lag time, it may be represented by Eq. 10, where C, Co and Cf are the variable values measured at any time t, initial time and final time, respectively; t is time; to is lag time and t first order time constant.

    Assuming mixing time (tmix) as the time necessary for the value (C-Cf) to reach 0.1% of (Co-Cf), Eq. 11 can be obtained.

    Another way to estimate the total mixing time may be performed by quantifying the time necessary for the experimental curve to lie within an interval (e.g. 2.5%) near the value of stable condition (tmix%).

    Assay for assessing performance of the anaerobic reactor

    The reactor was inoculated with granular anaerobic sludge with total volatile solids and total solids of 51 and 62 g/L, respectively, from a UASB reactor treating poultry slaughterhouse wastewater. The sludge was immobilized on 20g of foam as described by Zaiat et al. (1994), attaining total solids concentrations of 0.66 g/g-foam and total volatile solids concentration of 0.53 g/g-foam, thus resulting in an initial volatile solids content of 10.6g in the reactor volume. Batch cycles of eight hours were carried out at a temperature of 30 ±1ºC, treating approximately 500mL of a glucose-based synthetic wastewater with approximately 500mgCOD/L, prepared with 500.0mg/L glucose, 125.0mg/L urea, 1.0mg/L NiSO4.6H2O, 5.0mg/L FeSO4.7H2O, 0.5mg/L FeCl3.6H2O, 47.0mg/L CaCl2.H2O, 0.08mg/L CoCl2.6H2O, 0.07mg/L Na2SeO3, 85.0mg/L KH2PO4, 22.0mg/L K2HPO4, 33.0mg/L Na2HPO4, and 500.0mg/L NaHCO3.

    The reactor was operated without liquid circulation (158 cycles) and with liquid circulation flow rate of 6L/h, thus resulting in liquid superficial velocity of 0.19cm/s (58 cycles). The concentrations of chemical oxygen demand (COD) for non-filtered and filtered samples (CET and CES), total volatile acid (TVA), bicarbonate alkalinity (BA), volatile suspended solids (VSS), as well as the discharged volume (V) and the pH were monitored in both the influent and effluent in different cycles for all conditions according to SMEWW (1995) procedures, and the biogas composition was evaluated through gas chromatography.

Results and Discussion

    Figure 4 shows typical experimental and modeling profiles of the hydrodynamic assays in continuous mode for the condition with superficial velocity of 0.48cm/s and step, pulse and inverted pulse perturbations. For model fitting H+ concentrations were used, calculated from the measured pH. Analysis of pH profiles showed tail formation likely due to retention and posterior release of the tracer in the foam bed. Nardi et al. (1999) mentioned that these tails may cause inaccuracy in parameter determination of the proposed model. Hence, to verify the effect of these tails on RTD determination and on model fitting, the pH profiles were inverted so that pH increase rate equaled the decrease rate. Table I shows a summary of the operation conditions, theoretical model parameters, average experimental and theoretical values (,) and their variances (σ²).

 

    Comparison between the results obtained for the pulse and step perturbation allow to conclude that, despite similar residence times obtained for the same condition, the proposed model parameters (N and D/uL) showed considerable difference. In this way the values obtained in the pulse assay were considered to be more representative than those obtained in the step assay, as tracer diffusion in the liquid medium caused greater anomalies in the step assay results than in the pulse assays. In the pulse perturbation assays tail formation was observed with pH increase, whereas with pH decrease no anomalies were observed. It was hence assumed that only the increase was appreciably impaired by diffusion, whereas in the step assay diffusion was assumed to cause alteration in the entire profile.

    In order to offset the anomalies observed in pH increase, pH profiles were inverted (Nardi et al., 1999). This inversion was done assuming that the profile of decreasing pH followed exactly the same values of increasing pH, but in the reverse direction, i.e., inversion was carried out in a way that a symmetry axis can be assumed at the maximum pH of the profile. This way, the right-hand side of the profile (increasing range) is symmetrical to the left-hand side (decreasing range).

    The N values virtually doubled when the hydrodynamic analysis was performed with the inverted curve and the D/uL values were reduced in approximately 50%. The results obtained allowed to conclude that the flow in the polyurethane foam bed resembles plug flow. This conclusion is based on the N values exceeding 30 for the conditions with higher superficial velocities and near 20, when superficial velocity of 0.16cm/s was used. Furthermore, comparison of the experimental residence time with the calculated one allows to conclude that in the reactor bed no considerable preferential ways, dead zones nor short circuits exist that might exert significant influence on the flow, since the values obtained are relatively similar in all operation conditions explored.

    Figure 5 shows typical experimental and modeling profiles of the hydrodynamic assays in discontinuous mode for the condition with superficial velocity of 0.32 and 0.48cm/s. Table II summarizes operation conditions of the assays, fitting parameters, lag and total mixing time obtained by the first order model (tmix) and by the total mixing time (tmix%), considering a variation interval of ±2.5% in pH at stable condition. Analysis of the results allows to conclude that despite the fact that the first order model does not adequately represent the oscillatory behavior of the system, characteristic of circulation, it is adequate to estimate mixing time since the system behavior, except for oscillation, approaches the proposed model, meeting in this way the desired objectives. Although the analysis regarding variation interval results in greater mixing times than those obtained by the first order model fit, the trend observed as liquid superficial velocity increases remains the same.

 

    Comparison of the assays with circulation superficial velocity of 0.16cm/s allows to conclude that pulse application site does not interfere in mixing time determination, as the output signal does not differ for the two cases investigated. This is also true for the fluid volume in the reservoir, after comparison of the two assays performed with circulation superficial velocity of 0.80cm/s. The results indicate that the longer total mixing times obtained (tmix%= 17.5min) may be considered negligible when compared to the total cycle time (480min). Figure 6 shows the behavior of lag time and total mixing times with fluid superficial velocity, obtained in the mixing time assays. Lag and total mixing times tend to decrease with increasing superficial velocity. However, this decrease is not linear and tends to stabilize at superficial velocities above 0.5cm/s. Hence, using liquid superficial velocities (vS) higher than 0.5cm/s do not result in advantage in terms of mixture homogeneity. Yet, mass transfer conditions in the liquid phase might be improved with vS increase above 0.5cm/s.

    Figure 7 and Table III present the values of the monitored parameters in the assays with no circulation and with circulation using superficial velocity of 0.19cm/s. Analysis of the results obtained indicate that the reactor presented high efficiency and stability. Comparison of the results obtained with and with no circulation indicates better substrate removal performance when circulation is employed, since the efficiency achieved in the assay with circulation was significantly higher than that with no circulation. Moreover, reactor stability was maintained when circulation was employed, indicating that this did not cause biomass wash-out.

 

Conclusions

    The stimulus and response method used to perform the hydrodynamic study showed to be appropriate and simple. The mixing times obtained may be considered negligible in relation to the total cycle time, whereas the bed showed a plug flow reactor behavior.

    The reactor setup proposed showed to be efficient in treating glucose-based synthetic wastewater at a laboratory scale, guaranteeing stable operation and organic matter removal efficiency. Employing circulation allowed significant improvement in reactor performance, increasing organic matter removal from 86% to as high as 95%, indicating that mass transfer resistance in the liquid phase limited reactor performance.

ACKNOWLEDGMENTS

    This study was supported by FAPESP. The authors gratefully acknowledge Baltus C. Bonse for the revision of this paper.

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