The characteristics of granules from upflow anaerobic sludge blanket reactors

Granules from an industrial UASB reactor were maintained in bench scale (201.) UASB reactors and fed with a synthetic medium based on volatile fatty acids.

After 60 days of reactor operation, three distinct granule types were found to have evolved within the reactors. These types were defined by their colour: black, grey and white. These granules also differed in structure (texture) and elemental composition. The proportion of black, grey and white granules in the bed changed when the organic loading rate was changed. At the low loading rate, a large number of “hollow” granules appeared which were less dense than the liquid and were washed out at the top of the reactor.     

Effects of mixing velocity on anaerobic fixed film reactors

Four laboratory-model upflow anaerobic fixed film reactors (AFFR 1, 2, 3 and 4) treating landfill leachate were subject to identical volumetric organic load (7 kg COD m⁻³ d⁻¹) and hydraulic retention time (3d), but the contents in each unit were continuously recirculated for 10 months at four different velocities, respectively, of 21, 66, 680 and 3063 cm h⁻¹. The objective was to assess the effects of such mixing velocities (ν) on COD removal efficiencies (E), mean cell residence time (MCRT) and substrate utilization rate (U expressed as g COD removed d⁻¹ g⁻¹ VSS). The results showed that the relationships between E and ν and MCRT and ν were inverted U-shaped curves. The two middle reactors (AFFR 2 and 3) had near-optimum velocities (ν₂ and ν₃) with maximum E values of 88–89%. AFFR 4 had a high value of ν scouring biofilm on the biorings, resulting in higher concentrations of SS, VSS and COD in the effluent. All four reactors had nearly similar values of U (1.85–2.14 g COD d⁻¹ g⁻¹ VSS). The value of ν₁ (AFFR 1) was too low to enhance performance and ν₄ was too high to retain the biomass. The optimum recycle velocity, under the test conditions, was in the range of 66–680 cm h⁻¹.     

Accelerated start-up and enhanced granulation in upflow anaerobic sludge blanket reactors

In the present study, the effects of a cationic polymer on reactor start-up and granule development were evaluated. A control reactor R1 was operated without adding polymer, while the other five reactors designated R2, R3, R4, R5 and R6 were operated with different polymer concentrations of 20, 40, 80, 160 and 320 mg/L, respectively. Experimental results demonstrated that adding the polymer at a concentration of 80 mg/L markedly accelerated the start-up time. The time required to reach stable treatment at an organic loading rate (OLR) of 4 g COD/L.d was reduced by approximately 50% in R4 as compared with the control reactor. The same reactor with 80 mg/L polymer was able to achieve an OLR of 12 g COD/L.d after 59 days of operation, while R1, R2, R3, R5 and R6 achieved the same loading rate at much longer period of 104, 80, 69, 63 and 69 days, respectively. Comparing with the control reactor, the start-up time of R4 was shortened markedly by about 43% at this OLR, while other reactors also recorded varying degree of shortening. Monitoring on granule development showed that the granule formation was accelerated by 30% from the use of the appropriate dosage of polymer. Subsequent granules characterization indicated that the granules developed in R4 with 80 mg/L polymer exhibited the best settleability, strength and methanogenic activity at all OLRs. The organic loading capacities of reactors were also increased by the polymer addition. The maximum organic loading of the control reactor was 24 g COD/L.d, while the polymer-assisted reactor added with 80 mg/L polymer attained a markedly increased organic loading of 40 g COD/L.d. The laboratory results obtained demonstrated that adding the cationic polymer could result in shortening of start-up time and enhancement of granulation, which in turn lead to improvement in organics removal efficiency and loading capacity of the UASB system.     

Rheological and fractal characteristics of granular sludge in an upflow anaerobic reactor

The rheological and fractal characteristics of the granular sludge in an upflow anaerobic sludge blanket (UASB) reactor were investigated in this study. The influences of sludge concentration and temperature on the rheological properties of the granular sludge were evaluated, and the Bingham model was adopted to describe its rheology. In addition, image analysis was used to determine the sludge fractal dimension. The results indicate that the UASB granular sludge showed a shear-thinning behavior. The relationships between the limiting viscosity and the sludge concentration, as well as the limiting viscosity and temperature could be respectively modeled using an exponential equation and Arrhenius equation well. The Bingham model was able to adequately describe the rheology of the granular sludge. The fractal dimension of the granular sludge, 2.79+/-0.03, was larger than that of some other aggregates, suggesting that the granular sludge were more compact and denser. Furthermore, the relationship between rheological and fractal properties of the granular sludge could be properly described with the model proposed by Shih et al. [1990. Scaling behavior of the elastic properties of colloidal. Phys. Rev. A 42, 4772-4779].     

Anaerobic treatment of wastewater containing fatty acids in upflow reactors

An extended summary is presented of a Dr.Sc. Thesis in which the results of a study on anaerobic treatment of wastewater containing fatty acids have been reported. This study, concerning the technological features of this process in upflow reactors, was aimed at the following subjects :

• - the dynamics of the fluid flow in the reactor;

• - the dynamic behaviour of the sludge particles in the reactor;

• - the kinetics of the conversion of the fatty acids and of the formation of the products (mainly biogas and anaerobic sludge); and

• - the separation of the gas and the sludge from the treated water.

From the results obtained in experiments on lab-scale and (semi) technical scale, a quantitative model for the operation of the reactor has been derived. This model can be used for scaling-up purposes and for optimisation of the process performance.     

Effect of settling on performance of the upflow anaerobic sludge bed reactors

The effects of settler volume on the start-up and steady-state performance of 41. laboratory upflow sludge bed reactors treating bean blanching waste of 10,000 mg COD l⁻¹ were determined. The rate of start-up, as well as the maximum loading rate, increased with increased settler volume and performance. A loading rate of 30 kg COD m⁻³ day⁻¹ (based on reactor volume alone) and a COD removal of 95% was obtained with a 21. settling flask and a 4 to 1 recirculation rate. Without a settler, the maximum loading rate was 10 kg COD m⁻³ day⁻¹. The sludge was flocculent rather than granular. Sludge profiles and characteristics in the reactors and settlers were determined.     

Impacts of hydrodynamic shear force on nucleation of flocculent sludge in anaerobic reactor

The Sludge granulation in an anaerobic reactor consists of two steps: nucleation and maturation of nuclei. Nucleation as the starting point is of particular importance. In this paper, the nucleation of flocculent sludge as seed under weak, strong and violent hydrodynamic shear conditions is studied with an original quantitative method, and then the satisfactory linear correlations between the average sludge diameters and the operation time during the nucleation are demonstrated. Nucleation under strong shear conditions with a shear rate of about 8.28 s⁻¹, corresponding to superficial liquid and gas velocities of 2.66 and 0.24 m h⁻¹, develops fastest compared to weak shear conditions with a shear rate of about 0.04 s⁻¹ and violent shear conditions with a shear rate of about 12.42 s⁻¹ with the average augmentation rate of average sludge diameter of 0.57, 0.40 and 0.41 μm day⁻¹ respectively. One of the major mechanisms of the shear force on nucleation is that a high shear force accelerates the extracellular protein secretion of sludge. Although high extracellular protein content benefits nucleation, it is also shown that the extracellular proteins over-produced above around 80.5 mg gVSS⁻¹ leads nuclei to weaken and inhibit nucleation. So the violent shear force would result in disruption and wash-out of nuclei. However, the high extracellular polymers could intensify the shear force by raising the viscosity in the reactor, thus, in practice, it is important to monitor the shear conditions and extracellular protein content of sludge simultaneously in high rate reactors for stable operation.     

Extracellular polymeric substances (EPS) in upflow anaerobic sludge blanket (UASB) reactors operated under high salinity conditions

Considering the importance of stable and well-functioning granular sludge in anaerobic high-rate reactors, a series of experiments were conducted to determine the production and composition of EPS in high sodium concentration wastewaters pertaining to anaerobic granule properties. The UASB reactors were fed with either fully acidified substrate (FAS) consisting of an acetate medium (reactor R1) or partly acidified substrate (PAS) consisting of acetate, gelatine and starch medium (reactors R2, R3, and R4). For EPS extraction, the cation exchange resin (CER) method was used. Strength and particle size distribution were determined by assessing the formation of fines sludge under conditions of high shear rate and by laser diffraction, respectively. Batch tests were performed in 0.25 L bottles to study Ca²⁺ leaching from anaerobic granular sludge when incubated in 20 g Na⁺/L in the absence of feeding for 30 days. Results show a steady increase in the bulk liquid Ca²⁺ concentration during the incubation period. UASB reactor results show that the amounts of extracted proteins were higher from reactors R2 and R3, fed with PAS compared to the sludge samples from reactor R1, fed with FAS. Strikingly, the amount of extracted proteins also increased for all reactor sludges, irrespective of the Na⁺ concentration applied in the feed, i.e. 10 or 20 gNa⁺/L. PAS grown granular sludges showed an important increase in particle size during the operation of the UASB reactors. Results also show that, addition of 1 gCa²⁺/L to the high salinity wastewater increases the granules' strength.     

Modeling of the upflow anaerobic sludge bed-filter system: a case with hysteresis

A hybrid upflow sludge bed-filter (UBF) anaerobic reactor was successfully used for treatment of synthetic soluble 1% sugar waste. An hysteresis phenomenon was observed and discussed with respect to changes in acidogenic/acetoclastic specific activities ratio. The Haldane equation was adapted for predicting performance inhibition by undissociated acids as a function of substrate-COD and pH. The Monod and adapted Haldane models were solved simultaneously with substrate mass balance for a UBF reactor. The soluble COD removal efficiency and methane productivity were predicted as a function of the dilution rate. Critical values of solid and hydraulic residence times were also estimated and discussed in detail.     

Impact of shear force on the biofilm structure and performance of a membrane biofilm reactor for tertiary hydrogen-driven denitrification of municipal wastewater

Hydrogen-driven denitrification using a hollow-fiber membrane biofilm reactor (MBfR) was evaluated for operation in tertiary wastewater treatment. Specific objectives were to evaluate the impact of different levels of shearing stress caused by mixing and nitrogen sparging on the biofilm structure and denitrification rates. Applying high shear force proved to be effective in improving denitrification rates by reducing the thickness of the biofilm. With intensive mixing a biofilm thickness of approximately 800 microm was maintained, while additional nitrogen sparging could further reduce the biofilm thickness to approximately 300 microm. The highest denitrification rates of 0.93 gN/m(2)d were obtained when biofilm thickness was lower than 500 microm. Lower extracellular polymeric substances (EPS) accumulation and carbohydrates to protein ratio observed in thinner biofilms allowed for higher nitrate removal in the system. No significant sloughing of biomass or change in total and soluble COD in the final effluent was observed under steady-state conditions.     

Mixing in upflow anaerobic filters and its influence on performance and scale-up

Tracer studies were carried out in laboratory-scale and pilot-scale upflow anaerobic filters to determine the effect of liquid velocity, gas production and media depth on mixing patterns. A computer simulation model was developed to analyse tracer-response curves. In water studies at laboratory scale, gas production was shown to have a significantly greater effect on mixing than liquid upflow velocity. A reduction in the quantity of media also resulted in greater mixing due to the greater void space in which synthetic gas bubbles could cause turbulence. In the presence of sludge during reactor operation, at pilot and laboratory-scale, gas production had a significant influence on mixing. However, liquid velocity played an important role in solids distribution in the filter, in conjunction with media depth. At pilot-scale, at a low solids concentration, a high liquid velocity lifted the sludge “bed”, raising the source of gas production. The absence of gas below the sludge bed resulted in a plug flow regime which the incoming substrate entered. A reduction in the quantity of media increased the degree of mixing for a given liquid velocity and gas surface load. Lower liquid upflow velocities are required at a reduced media depth to prevent excessive biomass loss. Shear rates increase at high liquid and gas velocities, resulting in detachment of solids from the media and biomass washout. A close correlation was established between mixing and process performance which led to the development of a programme for start-up and operation of the filter to maintain optimum biomass/substrate contact. A strategy for scale-up was proposed through the development of correlations obtained from laboratory-scale filter studies which were used to predict pilot-scale mixing characteristics. This research highlighted the important factors influencing mixing patterns and scale-up in anaerobic upflow filters.     

The role of formate in the anaerobic baffled reactor

The anaerobic baffled reactor (ABR) contains a mixed anaerobic culture segregated into compartments. During pseudo-steady state runs, formate was detected in the first two or three compartments, thereafter dropping off sharply. Under conditions of shock loading, formate was detected in the reactor effluent, up to peak concentrations of 2500 mg/l. There are indications that formate may play an important role as an intermediate in the anaerobic digestion process, and that its production may contribute significantly to reactor stability.     

Quantification of the shear stresses in a microbial granular sludge reactor

Since a certain level of hydrodynamic shear force is needed in the formation of microbial granules for wastewater treatment, a method for quantifying the shear stresses in a microbial granular sludge reactor is highly desirable. In this work a novel energy-dissipation-based model was established and validated to quantitatively describe the shear stresses in a granular sludge sequencing batch reactor (SBR). With this model, the shear stress at the solid–liquid interface in an SBR was estimated and the relative magnitudes of shear stresses induced by fluid, gas bubble and collision on granules were evaluated. The results demonstrate that the effect of reactor geometry on the global shear stress was significant. Both the shear stress at the microbial granule surface and the biomass-loss rate increased with an increase in biomass concentration in the SBR. The gas bubble and the collision were found to be the main source for the shear stress at the granule surface.     

Comparing the anaerobic digestion of Ascophyllum nodosum,the organic fraction of municipal solid waste and white rice in batch reactors

The biogas potential of marine macroalga Ascophyllum nodosum was compared with the organic fraction of municipal solid waste (OFMSW) and white rice to determine the applicability of the feedstock for anaerobic digestion. For OFMSW three dry matter contents were compared, 3%, 10% and 25%, to determine the effect of dry matter on methane yield. Biogas was evolved in each system, but the rate of evolution of biogas changed with moisture content. The highest total methane yield was obtained from 3% OFMSW but A. nodosum yielded more methane at 176±37.62 L/kg VS than white rice and the drier anaerobic digestion of OFMSW. The substrates were digested using wastewater treatment plant inoculum to determine gas yield and gas quality under batch mesophilic digestion conditions.     

Effect of shear force on the initial stiffness of top and seat angle connections with double web angles

This study focused on the moment-rotation behavior of bolted top and seat angles with double web angle connections, especially the initial stiffness of this type of connection under the combination of shear force and moment. Several 3D parametric finite element models are presented in this regard, with the geometrical and mechanical properties of connections are as parameters. In the models, all the connection components such as beam, column, angles and bolts are modeled using eight node brick elements. The effects of all component interactions, such as slippage of bolts and frictional forces, are modeled using a surface contact algorithm, and to evaluate the connection behavior more precisely, bolt pre-tensioning is applied on the bolts shanks as the first load case. The results of numerical modeling are compared with test results of experiments that have been done by researchers, and show good agreement with them. To evaluate the effect of shear force on the behavior of such connections, several models were analyzed under different magnitudes of shear force, and the results of the analyses showed that the shear force has a reducing effect on the initial stiffness of bolted angle connections. Therefore, an equation is proposed to determine the reduction factor of a connection’s initial stiffness in terms of the connection’s initial stiffness and yield moment in the case of no shear force, and any expected shear force that might be applied.     

Low temperature treatment of municipal sewage in anaerobic fluidized bed reactors

The anaerobic fluidized bed reactor (AFBR) appears to be most promising for the treatment of low strength wastes, such as municipal sewage, at low temperature, since the process is able to maintain a large mass of active microorganisms and provides effective removal of TSS. The study is divided in three parts. The objective of the first part is to characterize the effect of decreasing temperature on the performance of two mature AFBR reactors. The second part presents the data from 220 days of operation at 10°C; and in the third part two start-ups, with and without inoculum at 15°C, are evaluated. A gradual temperature decrease from 20 to 5°C, allowing the microorganisms to acclimate to the new lower temperature, did not have a great effect on effluent quality. However a great accumulation of TSS was observed in the top of the fluidized bed. At 10°C, and a hydraulic retention time of 1.5 h, 70% of T_COD removal was achieved. It is possible to start-up the AFBR at 15°C without inoculation; however, at least 4 months is required to get good quality effluents.     

On the shear deformation modes in the framework of Generalized Beam Theory

This paper extends previous work concerning the determination of cross-section deformation modes in thin-walled members with arbitrary polygonal cross-section, in the framework of Generalized Beam Theory (Gonçalves et al., 2010 [1]). In particular, the paper addresses the so-called “natural shear deformation modes” (i.e. the deformation modes that involve non-null membrane shear strains and are independent of the cross-section discretization employed), which are relevant for capturing the behaviour of thin-walled members with complex multi-cell cross-sections undergoing torsion and/or distortion. The contributions of the paper are (i) the derivation of fundamental properties of the shear modes, (ii) the proposal of an efficient mode extraction procedure and (iii) the development of analytical results for several particular cases. In order to illustrate the application of the proposed mode extraction procedure and demonstrate the validity of the derived properties, several cross-sections are analyzed, including complex multi-cell tubes.     

Behavior of bolted angle connections subjected to combined shear force and moment

In this paper, the effect of web angle dimensions on moment-rotation behavior of bolted top and seat angle connections, with double web angles is studied. Several 3D parametric finite element (FE) models are presented in this study whose geometrical and mechanical properties are used as parameters. In these models, all of the connection components, such as beam, column, angles and bolts are modeled using solid elements. The effect of interactions between components, such as slippage of bolts and frictional forces, are modeled using a surface contact algorithm. To evaluate the behavior of connection more precisely, bolt pretensioning force is applied on bolt shanks as the first load case. The results of this numerical modeling are compared with the results of experimental works done by other researchers and good agreement was observed. To study the influence of shear force on behavior of these connections, several models were analyzed using different values of shear force. The effect of important parameters, especially the effect of web angle dimension, is studied then. An equation is proposed to determine the reduction factor for initial rotational stiffness of connection using connection initial rotational stiffness, yield moment, the expected shear force and web angle dimension. The proposed equation is compared with other existing formulations and it was observed that the proposed model is a better estimator of connection behavior.     

Buckling of T-strut subject to compressive force on its shear center

T-struts have the peculiarity of providing larger resistance to buckling about their axis of symmetry when the compression is applied at the shear center in lieu of the centroid of the section. But this feature is not well known to designers of constructional steel. This article introduces critical load formulae, discusses the various factors affecting the capacity increase such as sectional dimensions, buckling mode, fabrication error, and decrease of in-plane capacity due to load eccentricity, and recommends taking advantage of the capacity increase in truss design by shifting the working line of a T-section compression chord to the shear center.     

Anaerobic biodegradability and treatment of grey water in upflow anaerobic sludge blanket (UASB) reactor

Feasibility of grey water treatment in an upflow anaerobic sludge blanket (UASB) reactor operated at different hydraulic retention time (HRT) of 16, 10 and 6h and controlled temperature of 30 degrees C was investigated. Moreover, the maximum anaerobic biodegradability without inoculum addition and maximum removal of chemical oxygen demand (COD) fractions in grey water were determined in batch experiments. High values of maximum anaerobic biodegradability (76%) and maximum COD removal in the UASB reactor (84%) were achieved. The results showed that the colloidal COD had the highest maximum anaerobic biodegradability (86%) and the suspended and dissolved COD had similar maximum anaerobic biodegradability of 70%. Furthermore, the results of the UASB reactor demonstrated that a total COD removal of 52-64% was obtained at HRT between 6 and 16 h. The UASB reactor removed 22-30% and 15-21% of total nitrogen and total phosphorous in the grey water, respectively, mainly due to the removal of particulate nutrients. The characteristics of the sludge in the UASB reactor confirmed that the reactor had a stable performance. The minimum sludge residence time and the maximum specific methanogenic activity of the sludge ranged between 27 and 93 days and 0.18 and 0.28 kg COD/(kg VS d).