Strength and breakage of activated sludge flocs

The breakage of activated sludge flocs under turbulent shear conditions was investigated as a function of floc size. Municipal activated sludge flocs were fractionated by sieving to narrow size fractions. Shear stress distribution functions for the breakage of activated sludge floc samples were obtained. It was found that by increasing the floc size, this distribution was skewed towards smaller shear stress values and became broader. Results of experiments showed that the median shear stress, τ₅₀, required for floc breakage reduced by about 23% from 3.9 Pa for 45-63 μm sieve fraction to 3 Pa for the 150-180 μm sieve fraction. Under steady conditions, the median shear stress for the breakage of fragments that formed due to the breakage of larger flocs was as much as three times larger than that of the original flocs.     

Sedimentation of Permeable Floc

Sedimentation is an essential step to dewatering of wet materials. The present work utilized the computerized axial tomography scanner (CAT scan) to probe the sedimentation behavior of suspension with different initial solid concentrations. Not as assumed by traditional Kynch theory, the solid flux was not only a function of local solid fraction, but also of the settling time and the initial concentrations. Hence, the interior structure of floc was explored by the fluorescence in situ hybridization (FISH) and confocal laser scanning microscope (CLSM). Subsequently, the three-dimensional floc model was built up and intrafloc flow fields numerically solved. The area-weighted average pressure drop in the floc and mean flow rates estimated the floc permeability, based on Darcy's law, which were noted to vary along different principal axes and for different flocs. Local structure determines local flow field, hence determining the settling behavior of floc. Floc of large pores with low tortuosity produced easy flow-through paths, giving fast settling. Use of the average properties, such as porosity, cannot describe the sedimentation behavior of permeable flocs.     

Sedimentation of Permeable Floc

Sedimentation is an essential step to dewatering of wet materials. The present work utilized the computerized axial tomography scanner (CAT scan) to probe the sedimentation behavior of suspension with different initial solid concentrations. Not as assumed by traditional Kynch theory, the solid flux was not only a function of local solid fraction, but also of the settling time and the initial concentrations. Hence, the interior structure of floc was explored by the fluorescence in situ hybridization (FISH) and confocal laser scanning microscope (CLSM). Subsequently, the three-dimensional floc model was built up and intrafloc flow fields numerically solved. The area-weighted average pressure drop in the floc and mean flow rates estimated the floc permeability, based on Darcy's law, which were noted to vary along different principal axes and for different flocs. Local structure determines local flow field, hence determining the settling behavior of floc. Floc of large pores with low tortuosity produced easy flow-through paths, giving fast settling. Use of the average properties, such as porosity, cannot describe the sedimentation behavior of permeable flocs.     

Estimate of sludge floc permeability

This paper provides an estimate of the waste-activated sludge floc interior permeability on the basis of observing the motion of individual floc moving vertically towards an impermeable flat plate. The fluid flow fields surrounding and inside the floc were modeled numerically, from which the hydrodynamic drag force was calculated. The experimental data correlate with the numerical solutions regardless of the floc Reynolds number. Over the floc size range investigated in this paper, the permeability is noted to be approximately proportional to the square of floc size, ranging from 2.5×10⁻⁹ to 9×10⁻⁶ m² for floc size ranging from 150 to 10 000 μm. The structure of waste-activated sludge floc is proposed as a multi-fractal. Permeability measured in this work is thereby mainly attributed to the global structure level of the flocs.     

Extracellular enzymes in sludge flocs collected at 14 full‐scale wastewater treatment plants

BACKGROUND: Extracellular enzymes and chemical composition of sludge flocs affect the wastewater treatment capability. Here a fractionation protocol is presented describing the distributions of extracellular enzymes, proteins (PN) and polysaccharides (PS) in sludge flocs. Sludge floc samples collected from 14 full‐scale wastwater treatment plants (WWTPs), including those treating sewage, leachate and industrial wastewaters, were fractionated through centrifugation and ultrasound into five fractions: (1) supernatant; (2) slime; (3) loosely bound extracellular polymeric substances (LB‐EPS); (4) tightly bound EPS (TB‐EPS); and (5) pellets. RESULTS: The distributions of extracellular enzymes, PN and PS in different types of sludge flocs were almost identical. Considerable quantities of α‐amylase were bound with the pellet fraction, while the remainder was uniformly dispersed over the sludge matrix. Conversely, alkaline phosphatase, acid phosphatase and protease bound mainly with the pellet and TB‐EPS fractions. CONCLUSION: The enzyme activities distributed in sludge flocs achieved from 14 full‐scale WWTPs revealed a non‐wastewater‐specific manner. This work for the first time demonstrated that the activities of enzymes correlated with the characteristics of the wastewaters treated, and raised the possibility of manipulating the hydrolysis reactions using process parameters. Copyright © 2008 Society of Chemical Industry     

Probing Heterogeneous Structure of Aggregates

Naturally occurring aggregates are considered of fractal or fractal-of-fractals interior structure. Recent studies utilizing dye staining and confocal laser scanning microscopy (CLSM) techniques have revealed the extremely complex, interconnected pore networks for floc interior. Detailed structural analysis on the real floc architecture is determined for building up a comprehensive floc model. This work interprets structural characteristics of flocs using multifractal analysis. A direct method by Chhabra and Jensen properly evaluated the singularity strength and singularity spectrum on architectures of two artificial fractal systems. However, direct evaluation of multifractal characteristics of wastewater flocs has limitations, alternatively acquiring analytical results to be misleading. Particularly, the singularity spectrum presents excessively large values in the limit as q → +∞ and exhibits very large error bars in the limit as q → ?∞. This is attributable to the uneven distribution of mass in flocs and insufficient spatial resolutions provided by CLSM techniques. The flaws in applying multifractal analysis on naturally occurring objects are discussed.     

Floc model and intrafloc flow

The three-dimensional pore structures in waste activated sludge floc were identified using the fluorescence in situ hybridization (FISH) and confocal laser scanning microscope (CLSM) images. The meshes of the three-dimensional porous configuration of a sludge floc were constructed from the CLSM series images. The intrafloc flow field was simulated for the constructed floc model when it was subjected to a uniform flow field, based on which the Darcy's permeability was estimated. The permeability (k_DL) of original floc was estimated as . Flocculated flocs had higher k_DL due to their large pore size, while the corresponding values of k_DL of the freeze/thawed flocs were lower. The calculated results indicate that a few large pores in the floc determine the permeability. The fractal dimension and compactness, however, are not correlated with the permeability of the flocs.     

A model of the coagulation process with solid particles and flocs in a turbulent flow

A mathematical model was developed on the basis of population balances to predict the floc size distribution in a coagulating suspension. The coagulation process is performed in a stirred tank reactor with a turbulent flow field. In the population model the influence of the different local energy charges inside the reactor is taken into account. Moreover two sorts of particles are distinguished, i.e. the originally present and completely dispersed primary particles and the flocs. Contrary to the primary particles the flocs can be disrupted due to pressure and shear forces as they are mechanically not very stable. This different behaviour requires separate population balances for the two sorts of particles. The model parameters that are necessary are adapted to one single experiment.For the steady state the results represent different floc size distributions dependent on the solid concentration and the energy charge. Moreover it is shown that the assumption of an ideally mixed reactor that is often used cannot be maintained to be always true for the prediction of the resulting floc size distribution. The calculation results achieved are validated by image processing measurements of coagulating quartz particles in an aqueous suspension.     

好氧颗粒污泥与絮体活性污泥的竞争试验研究

采用在序批式反应器中培养50 d的好氧颗粒污泥与同期反应器排出的絮体污泥,在高有机负荷下曝气4 h,颗粒污泥和絮体污泥MLSS较初始时分别增加了67.8%和58.5%,单位体积混合液中颗粒污泥和絮体污泥的个数分别增加了15.4%和4.8%。颗粒污泥和絮体污泥的粒径分布在4 h试验过程中处于动态稳定状态,基本保持不变。曝气4 h后颗粒污泥和絮体污泥平均粒重分别增加45.4%和51.4%,可以看出在对有机底物的竞争中,与絮体污泥相比较,颗粒污泥由于具有较高的活性而占有优势,颗粒污泥的增长速率大于絮体污泥,絮体污泥和颗粒污泥的密度均有明显增长,但颗粒污泥平均粒重增加比例小于絮体污泥。     

Study of saline wastewater influence on activated sludge flocs through automated image analysis

BACKGROUND: In activated sludge systems, sludge settling ability is considered a critical step in effluent quality and determinant of solid–liquid separation processes. However, few studies have reported the influence of saline wastewater on activated sludge. This work aims the evaluation of settling ability properties of microbial aggregates in a sequencing batch reactor treating saline wastewaters of up to 60 g L^?1 NaCl, by image analysis procedures. RESULTS: It was found that the sludge volume index (SVI) decreased with salt content up to 20 g L^?1, remaining somewhat stable above this value. Furthermore, it was found that between the first salt concentration (5 g L^?1) and 20 g L^?1 aggregates suffered a strong deflocculation phenomenon, leading to a heavy loss of aggregated biomass. Regarding SVI prediction ability, a good correlation coefficient of 0.991 between observed and predicted SVI values was attained. CONCLUSION: From this work the deflocculation of aggregated biomass with salt addition due to pinpoint floc formation, dispersed bacteria growth and protozoa absence could be established. With respect to SVI estimation, and despite the good correlation obtained, caution is advisable given the low number of SVI data points. Copyright © 2008 Society of Chemical Industry     

Effects of the sludge reduction system in MBR on the membrane permeability

For the sludge volume reduction, gravity thickening and mechanical thickening processes have been mainly applied. However, these processes usually cause several problems such as large footprint, low thickening efficiency, etc. To solve these problems, a sludge reduction system using submerged membrane in a reactor with excess sludge from a biological nutrient removal (BNR) system was investigated. Both lab and pilot scale experiments for sludge reduction were carried out. The hydraulic retention time (HRT) was in a range of 4.5-6.7 days during the experimental period. A flat-sheet membrane with a mean pore size of 0.08 µm was used and operated at a flux range of 7-20 L/m²·h.During the experimental period, concentrations of BOD and SS were maintained less than 8 mg/L, 3 mg/L, respectively, which indicated the high quality of treated wastewater. While extracellular polymeric substances (EPS) were lowest at a coagulant concentration of 15 ppm, soluble microbial product (SMP) concentrations did not have any correlation with coagulant concentrations. The critical flux was observed in a range of 18-24 L/m²·h.These results suggested that the sludge reduction system using membrane bioreactor (MBR) could be applied as one of suitable processes to overcome problems of conventional processes such as gravity and mechanical thickening.     

Evaluation of flocs resistance and reflocculation capacity using the LDS technique

In a previous paper we have shown the added value of using LDS to monitor flocculation. It can supply, simultaneously, information on flocs size and structure and enlighten flocculation kinetics and mechanisms. In this paper, LDS is applied to study deflocculation and reflocculation processes of precipitated calcium carbonate (PCC) induced by cationic polyacrylamides, when different types of shear forces are applied. LDS can detect the influence of polymer characteristics and concentration as well as of the type of shearing, on flocs resistance and reflocculation degree, which depend on flocs structure and on the type of bonds between particles. As expected, flocs formed by bridging mechanism reflocculate with difficulty while flocs formed by patching reflocculate to a higher degree. Flocs resulting from reflocculation are more compact than the original ones, as assessed by the mass fractal dimension. Reflocculation is also lower when the flocs are submitted to superficial shearing than when they are submitted to sonication. Shearing induced by sonication is sufficient to break down the flocs in many fragments while the increase of pump speed only detaches particles by erosion, at the flocs surface, where bonds are weaker. Results prove that LDS is useful to monitor deflocculation and reflocculation processes and to predict floc resistance under different conditions. Moreover, the whole study demonstrates the benefit of using LDS for a complete evaluation of flocculants performance in the different stages of flocculation: aggregation, stabilization, deflocculation and reflocculation.     

Comparative study of the effects of experimental variables on growth rates of aluminum and iron hydroxide flocs during coagulation and their structural characteristics

In this study, the dimensions of over six thousand flocs were analyzed to quantitatively and comparatively investigate the effects of several experimental variables on the growth rate of aluminum (Al) and ferric (Fe) hydroxide flocs. Results show that Fe hydroxide flocs have faster growth rate than Al hydroxide flocs; and the average size of the former is larger than that of the latter. Increasing the concentration of the bivalent sulfate ion (SO₄²⁻), initial turbidity, or slow mixing rate, was able to increase the growth rate of both kinds of flocs. On the other hand, steady floc sizes were found to decrease with the increase in SO₄²⁻ concentration, initial turbidity, or shear rate. Fe hydroxide flocs are more prone to be influenced by the changes in the variables than Al hydroxide flocs. While the steady floc sizes became smaller when initial turbidity or slow mixing speed increased, the roundness and smoothness of flocs were found to increase, indicating that higher initial turbidity or larger slow mixing rate produces flocs with more regular and round shape. Furthermore, at a fixed shear rate, Fe hydroxide flocs are stronger than Al hydroxide flocs. However, Fe hydroxide floc sizes are much easier to decrease with the increase in slow mixing intensity.     

Numerical investigation on the flow characteristics and permeability of three-dimensional reticulated foam materials

In this paper, a three-dimensional (3D) model was proposed for predicting the flow characteristics and permeability of three-dimensional reticulated foam materials, which were prepared by replication process. Parameters, such as permeability, inertia coefficient, and friction factor, were obtained in order to describe the fluid flow characteristics of porous media. The influence of foam structure on the fluid flow characteristics was elucidated. Three flow regimes in porous media, including Darcy's regime, Forchheimer's regime and Froude's regime, were visualized and discussed. The flow transition from linear (Darcy's regime) to nonlinear (Forchheimer's regime) behavior, which is typical of experiments, was founded in the simulation. The data presented revealed the fact that the numerical results are in agreement with the experimental ones published previously.     

活性污泥法污水处理厂的运行模拟

结合ASM1和二沉池模型组成活性污泥模拟系统,将某污水处理厂的常规检测数据转化为活性污泥系统模型所需的组分数据,然后调整模型参数,使污水处理厂出水指标最接近于污水处理厂实际数据,从而确定适合该污水处理厂的模型参数,最后对该污水处理厂的工艺参数进行了模拟改造。     

Fluidization of dried wastewater sludge

Experimental measurements were conducted of the minimum fluidization velocities and the entrainment (terminal) velocities for beds of dry, digested wastewater sludge particles. With the use of air, experiments were performed in an 80 mm i.d. glass column with carefully sieved, very narrow fractions of particles in a wide range of 0.65-5.3 mm. Physical examinations of the dried sludge particles revealed their irregular, mostly isometric shapes, their porous character, and their fibrous nature. New correlations, which cover the transitional flow region, have been developed to estimate the conditions of incipient fluidization and those of entrainment for such peculiar materials. Their explicit forms also enable the direct computation of the particle size corresponding to a chosen terminal velocity and the straightforward calculation of the terminal velocity of a given, irregularly shaped particle.     

Impact of Aggregate Size and Structure on Biosolids Settleability

Settling is an important solid–liquid separation process in wastewater treatment. While solids concentration is an important factor influencing settling velocity of particles, other solids characteristics including particle size, shape, and structure also play an important role in settling rate. The “compactness” of bacterial aggregates in particular is recognized to exert a great influence on solid phase dynamic behavior since it has a substantial effect on fluid flow through the aggregate, which, in turn, affects the particles buoyancy. With the recognition that biosolids structure can be described by fractal methods, we now have a convenient means of parameterizing aggregate “compactness.” In this article, we examine the fractal nature of bacterial aggregates (which are the main component of the solids in wastewater treatment processes) using small angle light-scattering methods and assess the impact of aggregate compactness (as described by fractal dimension) on settling velocity of both single aggregates and large aggregate clusters. The results indicate that settling velocity is strongly dependent on both size and aggregate structure, with the larger and less compact flocs settling more quickly as a result of the significant extent of flow through the bacterial assemblages.     

Impact of Aggregate Size and Structure on Biosolids Settleability

Settling is an important solid-liquid separation process in wastewater treatment. While solids concentration is an important factor influencing settling velocity of particles, other solids characteristics including particle size, shape, and structure also play an important role in settling rate. The “compactness” of bacterial aggregates in particular is recognized to exert a great influence on solid phase dynamic behavior since it has a substantial effect on fluid flow through the aggregate, which, in turn, affects the particles buoyancy. With the recognition that biosolids structure can be described by fractal methods, we now have a convenient means of parameterizing aggregate “compactness.” In this article, we examine the fractal nature of bacterial aggregates (which are the main component of the solids in wastewater treatment processes) using small angle light-scattering methods and assess the impact of aggregate compactness (as described by fractal dimension) on settling velocity of both single aggregates and large aggregate clusters. The results indicate that settling velocity is strongly dependent on both size and aggregate structure, with the larger and less compact flocs settling more quickly as a result of the significant extent of flow through the bacterial assemblages.     

Numerical study on the hydrodynamics of agglomerates at intermediate Reynolds numbers

The flow pattern and hydrodynamics of a heterogeneous permeable agglomerate in a uniform upward flow at intermediate Reynolds numbers(1–40) are analyzed from three-dimensional(3 D) computational fluid dynamics simulations. Different from the homogeneous or stepwise-varying permeability models used in previous papers, a continuously radially varying permeability model is used in the present study. The effects of two dimensionless parameters, the Reynolds number and the permeability ratio, on the flow field and the hydrodynamics were investigated in detail. The results reveal that unlike the solid sphere, a small recirculating wake initially forms inside the agglomerate. The critical Reynolds number for the formation of the recirculating wake is lower than that of the solid sphere and it decreases with the increase of permeability ratio. A correlation of drag coefficient as a function of the Reynolds number and permeability ratio is proposed. Comparisons of drag coefficients obtained by different permeability models show that at intermediate Reynolds numbers(1–40),the effect of radially varying permeability on the drag coefficient must be considered.