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.     

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.     

Reynolds number-dependent permeability of wastewater sludge flocs

Most theoretical models assume constant permeability of wastewater sludge floc. This work shows that, at creeping flow limit with small intrafloc Reynolds number, the permeability of floc can not only be affected by floc structure, but also by the external flow condition. The three-dimensional structure of flocs using the fluorescence in situ hybridization (FISH) and the confocal laser scanning microscope (CLSM) was firstly probed. Then, the volumetric grid models for sludge flocs were constructed. We noted that the floc permeability could keep unchanged, increased, or decreased at increased Reynolds number (Re). Flow redistribution among channels of various sizes contributes to the noted Re-dependent permeability of flocs.     

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.     

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.     

The Effect of Ozone on the Reversibility of Floc Breakage: Suspensions with High Humic Acid Content

The effects of pre-ozonation and humic acid concentration on the formation, breakup and reformation of flocs generated using aluminium sulphate and polyaluminium chloride was investigated using conventional jar test procedure and by continuous optical monitoring. Using a range of different humic acid and ozone concentrations, it was found that increasing either concentration was detrimental to floc formation, leading to reduced turbidity removal. Once ruptured, only limited regrowth of flocs occurred, indicating irreversible floc breakage. The findings showed that the use of pre-ozonation did not provide beneficial results to improve turbidity removal. Nevertheless, improved dissolved organic carbon removal was achievable.     

Systematic Analysis of the Biochemical Characteristics of Activated Sludge During Ozonation for Lowering of Biomass Production

Properties of activated sludge during ozonation were analyzed. The structure and surface characteristics altered with the increase of ozone dosage. At low ozone dosage, the floc structure was completely dismantled. Floc fragments reformed through reflocculation at an ozone dosage greater than 0.20 g O₃·g^?1 mixed liquor suspended solids (MLSS). Inactivation of microorganisms in the activated sludge mixture was caused by ozonation. Microbial growth decreased by up to 65% compared to the control. Simultaneously, 92.5% of nucleotide and 97.4% of protein in microbial cells of the sludge were released. Organic substance, nitrogen and phosphorus were released from the sludge during the ozonation process. The initial value of soluble chemical oxygen demand (SCOD) was 72 mg·L^?1. When the ozone dosage was 0.12 g O₃·g^?1 MLSS, the value of SCOD rapidly reached 925 mg·L^?1, increased by almost 12-fold. Simultaneously, 54.7% of MLSS was reduced. The composition of MLSS was changed, indicating that the inner water of cells and volatile organic substance decreased during the ozonation process.     

Permeable carbon nanotube-reinforced silicon oxycarbide via freeze casting with enhanced mechanical stability

Reinforcement strategies were applied to solution-based freeze-casting systems to form porous ceramic composites. Multi-walled carbon nanotubes (MWCNT) were used as the reinforcing fillers in a polysiloxane preceramic polymer freeze cast with dimethyl carbonate to produce silicon oxycarbide-based lamellar pore structures. Using a carefully designed dispersion procedure, CNT agglomerates were reduced both in suspension and in pore walls. Electrical conductivity increased by ten orders of magnitude over pure silicon oxycarbide, indicating the preservation of CNTs after pyrolysis. Permeability, compression, and diametral compression (Brazilian disk) tests were performed to demonstrate the reinforcing effect of CNTs without sacrificing the permeability of the final porous structures.     

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.     

Analysis of Gravity Filtration Behaviors of Waterworks Sludge Based upon Sedimentation Tests

Both gravity filtration experiments under constant pressure conditions and gravity sedimentation experiments were conducted using the municipal waterworks sludge. It was clarified from the theoretical analysis that the effect of sedimentation on the filtration rate was noticeable for the sludge used in this study. The local specific flow resistance at various sludge concentrations was determined by the sedimentation velocity method. The local porosity was related to the local solid compressive pressure by the sedimentation equilibrium method. The decline behaviors in the filtration rate in gravity filtration accompanied by sedimentation were well evaluated only from the sedimentation data based upon the sedimentation velocity method and the sedimentation equilibrium method.     

海上低渗油田注CO2开发可行性探索

海上油田低品质、难动用储量所占比例逐渐增加,其中低渗透油藏储量已达20％,如何动用开发低渗透油田储量对海上石油而言是一个艰巨的挑战.针对海上低渗油田W-6开展了注CO2气体开发的可行性研究.通过与二氧化碳驱筛选标准对比,认为W-6油田的油藏地质条件基本符合二氧化碳驱筛选标准.在此基础上对W-6油田的流体相态进行模拟研究,为数值模拟组分模型提供了流体状态方程,之后通过数值模拟方法对注气井位、转注气时机、注气方式及混相可能性进行了研究,最后的优化注气方案表明,W-6油田注CO2后虽然不能达到混相,但仍能提高采收率6.1％.     

Drag force on a floc in a flow field: two-layer model

The behavior of a floc in a flow field is analyzed theoretically; in particular, the force it experienced is estimated. Here, a floc is simulated by an entity having a two-layer type of structure, and its porous nature mimicked by varying the relative magnitudes of the permeability of its inner layer and that of its outer layer. The results of numerical simulation reveal that, for the same volume-averaged permeability, the drag coefficient of a floc with a heterogeneous structure is always much larger than that of a floc with a homogeneous structure. This is true regardless of the relative magnitudes of the permeability of the inner layer and that of the outer layer. The drag coefficient of a floc is mainly determined by the part having a less porous structure. We show that for the same volume-averaged permeability, the more heterogeneous the structure of a floc is, the easier for the relation between the drag coefficient and the Reynolds number to deviate from a Stokes’ law-like correlation.     

Design and optimization of ceramic membrane structure: From the perspective of flux matching between support and membrane

The support flux was first investigated as a separate influencing factor for its effect on performances of ceramic filtration membranes. Three pre-membranes were prepared by tape-casting and then transfer-coated to supports to form dual-layer ceramic membranes after sintering. Experiments demonstrated that membrane layers with almost the same properties were obtained despite the huge difference in support flux. When the support flux increases from 3.120 to 97.53 m³m^?2h^?1, the flux of these three membrane series have increased by 75%, 186% and 228%, respectively. Experimental rules can provide structural design and evaluation from the perspective of permeability. The limit membrane flux of a certain system was derived according to the resistance distribution law of internal membrane structure and the Darcy's theorem. On this basis, a method for designing support flux was proposed. Furthermore, we present a criterion to quickly and easily evaluate the match between the support and the top layer, which is the ratio of membrane resistance to total resistance. Finally, the filtration resistance of penetration caused by suction of membrane particles into the support was measured for the first time, taking the advantage of the transfer-coating method that inherently free of penetration. Our works are expected to deepen the understanding of the ceramic membrane structure and provided guidance for its rational design and optimization.     

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.     

Stereological estimation of structural change in floc matrix following weak ultrasonication

Ultrasonication is an energy intensive process used to enhance organics’ solubilisation and used in promoting methane yield from wastewater sludge. Applying limited ultrasound without full disruption of floc structure or cell walls can significantly enhance methane yield from sludge at reduced energy cost. This work probed the spatial distribution of extracellular polymeric substances, cells in the original wastewater flocs, and the yielded fragments following weak ultrasonication using a process of multiple staining and a confocal laser scanning microscope technique. The stereological information of the three-dimensional architecture of the original wastewater floc and ultrasonicated fragments was extracted and compared. The local structural changes following weak ultrasonication were presented for the first time, which should also correlate with the enhanced methanogenesis thereby produced.     

Relationship Between Methanol Permeability and Structure of Different Radiation‐Grafted Membranes

Styrene grafted and sulfonated poly(vinylidene fluoride) and poly(vinylidene fluoride‐co‐hexafluoropropylene) films are candidates as electrolytes in direct methanol fuel cells. Their behaviour in water, 1 and 3 mol dm^–3 aqueous methanol, and pure methanol were studied. According to SAXS results, water and methanol‐water solutions have similar effects on the membranes, i.e., the lamellar period increases and the ionic domains enlarge. Furthermore, differences in the ionic domain structures in pure methanol and water were observed. These structural changes together with dissimilar liquid uptakes in water and in methanol are reflected as changes in the conductivities. An increase in the SAXS intensity and changes in the Bragg distance of the ionic peak were observed in methanol compared to aqueous solutions. This may be related to the hydrophobicity of the CH₃ group on methanol. Dissimilarities in methanol permeability through the radiation‐grafted membrane can be related to structural differences in membranes observed with SAXS. Permeabilities were observed to be lower for the radiation‐grafted membranes compared to Nafion® 115, which compensates for the higher area resistance of the experimental membranes and thus improves their performance in a fuel cell.     

改良型平流式沉淀池流态的实验模拟

沉淀池流体流态对沉降效果有较大影响,通过相似定理制作沉淀池物理模型,以进水挡板位置高度和进水流量为考量因子,进行清水实验与活性污泥实验。结果表明,进水挡板位置高度过低时,停留时间较长,且人流区部分产生堆积现象,影响沉淀池去除效果,进水挡板位置高度过高时,挡板导流作用不明显,流态较差,沉淀池去除率较低。低流量时,人流区产生堆积现象,影响沉降效果,高流量时,产生沿挡板向上的上升流,部分悬浮物被带到清水区,降低沉淀池去除率。在设计进水流量1．39L／min下,清水实验进水挡板位置高度为60mm时流态最佳,污泥实验进水挡板位置高度为40mm时流态最佳。     

中原油田文东沙三中储层孔隙结构特征评价

中原油田文东沙三中为深层低渗油藏 ,通过应用普通薄片、铸体薄片、扫描电镜、岩石物性、压汞试验等资料 ,对孔隙和喉道的类型及特征研究和评价 ,提出了研究区具三种孔隙结构类型 ,并作了应用数学地质方法进行孔隙结构分类的探索 ,对相同类型油藏的储层微观结构研究具有重要指导作用。     

Impact of Sludge Floc Size and Water Composition on Dewaterability

In order to observe the impact of different water compositions on sludge dewaterability, assessments of floc sizes using a particle size analyzer and of sludge dewaterability based on the capillary suction time (CST) test were carried out. Synthetic raw water had small floc sizes, and synthetic domestic wastewater had both larger median floc sizes and a better correlation between sludge dewaterability and median floc sizes. The floc size distribution results showed that synthetic raw water is associated with a narrow particle size distribution. In comparison, synthetic domestic wastewater produced a wider distribution. However, the CST values were similar for both waters. Compared to synthetic wastewater, natural wastewater had the largest distribution with generally larger particle sizes.