Conditioning for Municipal Sludge Dewatering. From Filtration Compression Cell Tests to Belt Press

Belt presses are commonly used to achieve sewage sludge dewatering. As this process essentially works by squeezing, it was interesting to better understand how the sludge conditioning affects cake expression. Experiments were carried out with conditioned municipal activated sludge on both a filtration compression cell (FCC) and a laboratory-scale belt press. Two cationic polyelectrolytes with a high charge density (80%) but different molecular weights were used. A significant impact of the conditioning was observed for FCC tests. The higher molecular weight polyelectrolyte led to much shorter filtration phases than with the medium molecular weight one. However, it produced wetter filter cakes. It also led to more elastic filter cakes but less susceptibility to creep. Higher final dryness was observed for sludge conditioned by the medium molecular weight polymer. Experiments carried out on the belt press device did not exhibit such impact. Consequently, the FCC test is not suitable to analyze the sludge dewatering by belt press, although it enables characterization of the expression phase. Indeed, some additional features involved in belt press, such as shear forces or progressive pressurization of the cake, are not characterized by classical FCC tests.     

Municipal sludge dewatering by belt filter press: effect of operating parameters

A belt filter press is commonly used to dewater sewage sludge but few data concerning this process have been published. A laboratory‐scale investigation was carried out to highlight the main process parameters affecting belt filter press efficiency as measured by the dry solid content of the final cake, percentage of solids recovered, and lateral sludge migration on the belt. Belt speed was determined to be the most important parameter: a low belt speed produces drier cakes with lower lateral sludge migration on the belt and also improves filtrate quality. At low belt speed, pressure is gently applied to the initial sludge, enabling a slow structuring of the cake and leading to better water release. Copyright © 2005 Society of Chemical Industry     

Effect of Operating Parameters on the Dewatering Performance of Press Filters: A Sensitivity Analysis

To optimize the press filters commonly used to dewater residual sludge, the impact of the operating parameters must be better understood. The objectives of this article are (1) to identify the impact of the operating parameters on press filter performances and (2) to classify the operating parameters regarding their impact on dewatering performances. Experiments were performed with conditioned activated sludge in a tangential filtration cell, and a sensitivity analysis of the process was performed. This study emphasizes that cake thickness and filtration pressure have the most significant impacts on dewatering efficiency. These results suggest a promising outlook for press-filter optimization.     

Dewatering of Sewage Sludge

Dewatering of sewage sludges is usually carried out using belt filter presses or centrifuges. The ability to remove water from sludges by mechanical dewatering is a property of both the sludge and the equipment used for dewatering. Newer high-solids centrifuges can remove more water than a belt press but require a higher chemical conditioning dose. The Gt value, the product of the mean velocity gradient and the shear time, has been found to be a useful parameter for characterizing the shear in mechanical dewatering equipment. Dewatering can be considered a two-step process, filtration followed by expression. The expression step is the most important of these and the benefits of various sludge processing techniques and conditioning chemical types remains a fruitful area of research.     

Dewatering of Sewage Sludge

Dewatering of sewage sludges is usually carried out using belt filter presses or centrifuges. The ability to remove water from sludges by mechanical dewatering is a property of both the sludge and the equipment used for dewatering. Newer high-solids centrifuges can remove more water than a belt press but require a higher chemical conditioning dose. The Gt value, the product of the mean velocity gradient and the shear time, has been found to be a useful parameter for characterizing the shear in mechanical dewatering equipment. Dewatering can be considered a two-step process, filtration followed by expression. The expression step is the most important of these and the benefits of various sludge processing techniques and conditioning chemical types remains a fruitful area of research.     

Creep effects in activated sludge filter cakes

Constant-pressure and stepped-pressure filtrations have been used to study the creep behaviour of filter cakes formed of activated sludge, anatase, or poly(styrene-co-acrylic acid) particles, respectively. No creep is observed in the case of the anatase filter cake, whereas the poly(styrene-co-acrylic acid) and activated sludge filter cakes exhibit pronounced creep behaviour. The consolidation stage for the poly(styrene-co-acrylic acid) filter cake can be divided into a primary consolidation stage controlled by hydrodynamic effects and a secondary consolidation stage controlled by creep effects. Up to 70% of the consolidation stage was secondary consolidation. Furthermore, the retardation time due to creep deformation was found to be much longer than the filtration time, which suggests that creep does not influence the filtration stage. The creep behaviour observed in activated sludge is pronounced, however no clear transition between primary and secondary consolidation was found. The retardation time is of the same magnitude as the filtration time. This suggests that activated sludge filtration is strongly influenced by creep, and may explain the difficulties in modelling activated sludge filtration and dewatering.     

Cake formation and growth in cake filtration

In this study we develop an experimental method to help understand the formation and growth of dust cakes. An on-line pressure-displacement measurement system is developed to measure the thickness of the dust cakes. A higher filtration superficial velocity resulted in a higher degree of compaction in the dust cakes and thus a higher specific cake resistance, hence a better collection efficiency could be achieved. The empirical equations for cake solidosity and specific cake resistance as a function of the filtration superficial velocity are derived. We find that the cake filter is influenced by the cake thickness and the filtration superficial velocity. The results of this study can be applied to granular bed filters for the removal of dust particulates in advanced coal-fired power systems.     

An experimental and theoretical study on the initial period of cake filtration

Experimental measurements were made of the average specific cake resistance during the initial period of cake filtration, and the theoretical calculations about the period were also performed. The “filtration-permeation method” in the filtration cell of small area was used to measure the flow rate during the initial period of filtration, which is essentially characterized by the large flux due to fast flow rate and the rapid change of flow rate within a relatively short time interval. The measured average specific cake resistances of thin cakes which represent the cakes of initial period had very large values compared to the overall average specific cake resistance. This experimental result was contrary to the conventional theory about the initial period. Applying the “unified theory on solid-liquid separation” to the initial period, the average specific cake resistances at the initial period can have the large values--more than two times greater than that of the overall value.     

A Study for Development of Real-Scale Thermal Filter Press Dewatering (TFPD)

Real-scale thermal filter press dewatering equipment (plate size: 630 mm × 630 mm) was installed and operated at a waterworks for one year in an attempt to achieve sludge reduction. During the period, the dewaterability was evaluated according to the seasonal sludge properties in order to compare the dewaterability of thermal dewatering and mechanical dewatering, as well as to determine the economics of thermal dewatering. According to the results, the winter season sludge showed a 36% decrease in water content and a two-thirds reduction in dewatering velocity compared to the summertime sludge. In addition, the dewatered cakes of the thermal filter press dewatering equipment showed a lower specific cake resistance and water content in the dewatered cakes than the mechanical filter press dewatering equipment, indicating superior dewaterability. This was attributed to the easier removal of the filtrate remaining in the capillary tubes due to thermal dewatering. The energy consumption for thermal dewatering was 300 kJ/dry solids (DS) kg. A comparison of the sludge dryers indicated that it is possible to produce dewatered cakes that consume less energy and can be recycled. According to the performance evaluation results, the real-scale thermal filter press dewatering equipment had high adaptability to the changes in seasonal sludge, showing excellent dewaterability compared to the mechanical filter press dewatering equipment, and was economical due to the lower energy consumption.     

Decreasing Filter Cake Resistance by Using Packing Structures

Packed beds used in absorption columns are evaluated to determine whether they can also be beneficial for cake‐forming filtrations. To assess this, model systems are characterized and separated by using a dead‐end filter cell. Filtrations are conducted with different packings; the filtrate amount over time and resulting turbidity are evaluated. Packings increase the filter cake resistance and the separation time of the cakes formed with approximately incompressible solids. However, they exhibit a positive effect on the filtration of a more complex, compressible substance; the process is not only accelerated, but also the quality of the obtained filtrate is not compromised. These results demonstrate potential in the use of packed beds for the filtration of complex biogenic suspensions.     

Unified model for the prediction of consecutive solid–liquid filtration and expression at the constant pressure

Unified nonlinear model is proposed for the prediction of consecutive solid–liquid filtration and expression at the constant pressure. This model is based on the Darcy–Terzaghi filtration-consolidation equations modified to consider power-law pressure dependence of the specific cake resistance, and transforming Darcy law to the linear form. The model considers nonuniform structure of compressible filter cakes obtained by filtration and following expression. The profiles of local compressive pressure and local cake characteristics are simulated and compared for different moderately and highly compressible filter cakes (H.K. kaolin, CaCO₃, silica, activated sludge) based on the analytical and numerical solutions of the model. It is shown that the behavior of solid–liquid expression depends from the initial structure of compressed materials. Consolidation ratio U of the filter cakes with initially nonuniform structure formed by filtration differs from that of semi-solid materials with initially uniform structure. Different methods of determination of consolidation coefficient are analyzed and compared for nonuniformly structured filter cakes.     

Pressure Slip Casting of Graded Coarse Grained Oxide Ceramics for Refractory Applications

In the present study the fabrication of graded coarse grained ceramic bodies for refractory applications was investigated using a pressure filtration cell,which uses pressurized air as the pressure medium. The pressure filtration cell was used to examine the influence of different ceramic raw materials,two dispersants,and of the particle size distribution on the filtration behavior and on the filter cake surface quality. Pressure slip casting of spinel based slips resulted in crack-free filter cakes,while alumina based slips resulted in filter cakes,which always stuck on the used PMMA filter medium. Furthermore,citric acid as an electrostatic dispersant resulted in a much lower filtration cake resistance and hence shorter filtration times than an electrosteric dispersant. A broader particle size distribution with a higher addition of the finest particle fraction caused an increase in the filter cake resistance but also yielded much better surface qualities. Finally,graded filter cakes with three layers of alumina rich magnesium aluminate spinel with a maximum grain size of 3 mm,1 mm,and 0. 5 mm,respectively,were fabricated. The computed X-ray analyzes indicated a perfect bonding between the three layers,which is a significant improvement to previous studies. However,the graded filter cakes exhibited frequently cracks at the bottom,which was probably caused by tensile stresses from the friction of the filter cakes with the pressure filtration cell. This will be further investigated in subsequent studies. The achieved results demonstrate thefeasibility to produce coarse grained oxide ceramics for refractory applications by pressure slip casting,which eventually allows the production of composites with tailored compositions,microstructure and functionality.     

A theoretical and experimental study on cake filtration with sedimentation

Sedimentation occurs in almost all cake filtration. To study the role of sedimentation during filtration, filtration-permeations were carried out for suspensions of various concentrations. The average specific cake resistances calculated by the traditional method using the filtration data and initial concentration of suspension give different values according to the suspension concentrations. But the average specific cake resistances from the permeation period show almost the same values in spite of the various suspension concentrations. To exclude the influence of sedimentation, a complete sedimentation was performed before beginning filtration and then filtration-permeation of the sediment was performed. The average specific cake resistance from the filtration period calculated with the mass fraction of sediment and that from the permeation period coincide very well. These values also coincide well with the former average specific resistances during permeation within the experimental error limits. It can be concluded that the average specific cake resistances by permeation operation give almost the same values for various concentrations of suspension. A new definition of a cake and a new concept of the filter medium Rm are proposed based on the analysis of experimental results.     

Experimental study of cake formation on heat treated and membrane coated needle felts in a pilot scale pulse jet bag filter using optical in-situ cake height measurement

Pulse-jet bag filters are frequently employed for particle removal from off gases. Separated solids form a layer on the permeable filter media called filter cake. The cake is responsible for increasing pressure drop. Therefore, the cake has to be detached at a predefined upper pressure drop limit or at predefined time intervals. Thus the process is intrinsically semi-continuous. The cake formation and cake detachment are interdependent and may influence the performance of the filter. Therefore, understanding formation and detachment of filter cake is important. In this regard, the filter media is the key component in the system. Needle felts are the most commonly used media in bag filters. Cake formation studies with heat treated and membrane coated needle felts in pilot scale pulse jet bag filter were carried out. The data is processed according to the procedures that were published already [Powder Technology, Volume 173, Issue 2, 19 April 2007, Pages 93-106]. Pressure drop evolution, cake height distribution evolution, cake patches area distribution and their characterization using fractal analysis on different needle felts are presented here. It is observed that concavity of pressure drop curve for membrane coated needle felt is principally caused by presence of inhomogeneous cake area load whereas it is inherent for heat treated media. Presence of residual cake enhances the concavity of pressure drop at the start of filtration cycle. Patchy cleaning is observed only when jet pulse pressure is too low and unable to provide the necessary force to detach the cake. The border line is very sharp. Based on experiments with limestone dust and three types of needle felts, for the jet pulse pressure above 4 bar and filtration velocity below 50 mm/s, cake is detached completely except a thin residual layer (100-200 μm). Uniformity and smoothness of residual cake depends on the surface characteristics of the filter media. Cake height distribution of residual cake and newly formed cake during filtration prevails. The patch size analysis and fractal analysis reveal that residual cake grow in size (latterly) following regeneration initially on the base with edges smearing out, however, the cake heights are not leveled off. Fractal dimension of cake patches boundary falls in the range of 1-1.4 and depends on vertical position as well as time of filtration. Cake height measurements with Polyimide (PI) needle felts were hampered on account of its photosensitive nature.     

滤饼结构的分形研究

概括地介绍了分形理论在滤饼结构研究中的应用 ,并采用电子扫描与计算机图象处理技术 ,并根据分形理论对以平板陶瓷膜为过滤介质的十字流微滤稳态阶段的滤饼结构进行了理论及实验研究。     

Filtration properties of two Tunisian clays suspensions: effect of the nature of clay

The knowledge of the filtration properties are very important and can ascertain if a clay is suitable or not for use as a lining material for waste disposal. The general objective of our work is to determine a relation between structure and microtexture of two Tunisian clays (C1 and C2) and macroscopic properties of cakes formed by filtration. The result of permeability by water and waste solution (pH = 3) shows that the clay C2 is more impermeable than C1. In addition, the cake obtained after filtration test by filter-press for both clays present many interesting observations. However, for a ratio of (40%) of clay–water, the thickness of cake (e) in the case of C2 is more important (e = 10mm) than for C1 (e = 7mm). Similarly, the water retention (WR) is of order 34% for C2 and 26% for C1. This is due to the mineralogical composition of sample C2 which is essentially the smectite (swelling clay). Although, (WR) and (e) are slightly changed using the waste solution, it exist a lowering of plasticity of clay filter cake. This is illustrated by the SEM images that showed an interconnected micro-crack for the cake of sample C2, due to the degradation by acidic attack.     

Involvement of Gypsum (CaSO4 · 2H2O) in Water Treatment Sludge Dewatering: A Potential Benefit in Disposal and Reuse

Abstract

This research assessed the use of gypsum (CaSO₄ · 2H₂O) as a skeleton builder for sludge dewatering since polymer conditioning of sludge affected only the rate of water release, not the extent of dewatering. The use of gypsum as a physical conditioner, in association with a polymer, could improve sludge filterability. More significantly, gypsum serves as a skeleton builder, forming a permeable and rigid lattice structure that can remain porous under high positive pressure during the compression step after the cake growth of the filtration, thereby maintaining the size of the micro‐passages through which water is expressed. Experiments using a high pressure cell apparatus showed that a further decrease of two to seven percent of the equilibrium moisture content of the sludge cake was achieved, for sludge thicknesses for dewatering of 1 to 10 cm, by the addition of gypsum with 60% of the original sludge solids when compared to the single polymer conditioning. The importance of the addition of gypsum in alum sludge dewatering is not only the improvement in the extent of dewatering, but also the potential application of transforming dewatered alum sludge from “waste” for landfill to useful “fertilizer” or to be used as a filter medium/adsorbent for wastewater treatment engineering.     

Distributions of age, thickness and gas velocity in the cake of jet pulsed filters—application and validation of a generations filter model

Different imperfections are observed with jet pulsed filters. They manifest themselves most obviously in the curve of the pressure drop versus time. A convex pressure drop curve indicates cake compaction. But jet pulsed filters frequently show a concave rise of the pressure drop curve. This phenomenon is due to a strongly nonuniform cake area load on the filter and it is generally attributed to incomplete cake removal. Incomplete cake removal takes place when only a fraction of the total filter area is cleaned at the end of a filter cycle or when patchy cleaning prevails. Patchy cleaning means that a jet pulse removes the entire filter cake of only a fraction of the exposed filter area except for a thin adhesive dust layer.In this paper a filter model is proposed in which the different classes of cake thickness are understood to result from different cake generations. A cake becomes one generation older when it survives the jet pulse cleaning at the end of a filtration cycle, although the area that is occupied by the cake on the filter medium is diminished by the jet pulse.This generations filter model can be used to find the distributions of age, thickness and gas velocities in the cake from steady-state operational data. The steady-state, periodic model provides a complete basis for the simulation of heterogeneous gas/solid reactions in the cake of jet pulsed filters.In the model intermediate cake build up during the cleaning procedure is considered. There redeposition of removed filter cake also takes place, and its extent is estimated. The model can also serve to determine from macroscopic process data, if the cleaning system of a filter installation operates in the undesirable mode of patchy cleaning.Experiments from a pilot plant for dry flue gas cleaning are presented and the generations filter model is validated with the experimental data.     

Thermal Dewatering (TDW) to Reduce the Water Content of Sludge

Thermal dewatering describes the process whereby a heating plate and heat supply unit are incorporated into a filter press system to improve separation of water from sludge. The performance of our thermal dewatering system for both wastewater and waterworks sludge was measured and compared with mechanical dewatering in terms of water content, dewatering velocity, cake specific resistance, and energy consumption. Dewatering velocity was improved by a factor of two, cake water content was lower, and specific cake resistance was improved. However, energy consumption was higher. Thermal dewatering systems may be economical and have practical application to improving dewaterability.     

The preparation and filtration characteristics of Dextran-MnO2 gel particles

Different kinds of Dextran-MnO₂ gel particles are prepared in different conditions, depending on variables such as the molecular weight of dextran and formation temperature. Some physical properties and filtration characteristics of these gel particles are measured and discussed. Although the mean sizes of these gel particles are very close to each other, their filtration characteristics are far different due to their mechanical strength and compressibility. A typical filtration curve of gel particles can be divided into three regions, and a retardation cake compression during the filtration can be observed from the curve. The particle formation temperature has a trivial effect on their mechanical strength. An increase in formation temperature leads to only a slight decrease in particle size. On the other hand, gel particles are formed by using dextrans with three different molecular weights - 70,000 (sample A), 500,000 (sample B) and 2,000,000 Da (sample C) - and are used in filtration experiments. The results show that the molecular weight of dextran plays a major role in determining particle mechanical strength; the sequence of particle hardness is sample B > sample A > sample C. The dynamic analysis method proposed by Hwang and Hseuh [K.J. Hwang, C.L. Hseuh, J. Membr. Sci. 214 (2003) 259] is employed to estimate the local cake properties in a filter cake, e.g., solid compressive pressure, porosity and specific filtration resistance. Because sample C has the highest compressibility, it constructs a cake with the lowest porosity and the highest specific filtration resistance; and consequently, the lowest filtration rate. Although the mechanical strength of sample B is higher than that of sample A, its wider particle size distribution results in lower cake porosity and higher filtration resistance, as well as a lower filtration rate. It could be said that the filtration rate decreases with an increase in the molecular weight of dextran.