ULTRASOUND IN THE SUSPENSION SEPARATION METHODS

Investigations were carried out on the effect of the ultrasonic field and some selected chemical compounds on the process of preparing sludges to be dewatered by mechanical devices.

The tests; were performed in a laboratory on real sludges. The processes occurring in sludges due to presence of ultrasonic field and applied chemical reagents were determined on the basis of a commonly used method of determining the sludge dewatering, in both filtration and sedimentation.

The interdependences of value changes of the above mentioned factors and the possibilities of applying ultrasonic waves to intensifying sludge dewatering process were examined. The obtained results confirm the applicability of ultrasonic waves to the process of sludge treatment and prove that the initial assumptions concerning the use of ultrasonic field in the suspension separation methods to be valid.     

ULTRASOUND IN THE SUSPENSION SEPARATION METHODS

Investigations were carried out on the effect of the ultrasonic field and some selected chemical compounds on the process of preparing sludges to be dewatered by mechanical devices.

The tests; were performed in a laboratory on real sludges. The processes occurring in sludges due to presence of ultrasonic field and applied chemical reagents were determined on the basis of a commonly used method of determining the sludge dewatering, in both filtration and sedimentation.

The interdependences of value changes of the above mentioned factors and the possibilities of applying ultrasonic waves to intensifying sludge dewatering process were examined. The obtained results confirm the applicability of ultrasonic waves to the process of sludge treatment and prove that the initial assumptions concerning the use of ultrasonic field in the suspension separation methods to be valid.     

Performance evaluation of electrodewatering system for sewage sludges

A laboratory-scale electrodewatering system, incorporating an electric field as an additional driving force to conventional pressure dewatering, has been developed to decrease the water content of sludges generated in wastewater treatment. Consisting of a piston-type filter press, a power supply and a data acquisition system, the electrodewatering system’s performance was evaluated as a function of applied pressure, applied voltage, sludge type and filtration time. Experiments were carried out using sewage sludges with the electric field up to 120 V/cm and pressure ranging from 98.1 to 392.4 kPa. Electrodewatering involving a combination of electric field and pressure enhances both the dewatering rate and final dewatered volume. The final water content of sewage sludges in the electrodewatering system can be reduced to 62 wt%, as compared to 78 wt% achieved with the pressure filtration alone. The electrodewatering system shows the potential to be an effective method for reducing the water content in sludges.     

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.     

Effect of Cake Compression on the Performance of Centrifugal Dewatering

The effect of cake compression on the centrifugal dewatering is investigated under various rotating speeds and different kinds of cakes. A numerical method is proposed to estimate the capillary pressure and cake permeability under various cake saturations. The most dewatering in this study are operated at the funicular state of cake. The cake permeability decreases, while the capillary pressure increases rapidly, with the decrease of cake saturation during a centrifugal dewatering. The cake saturation can also be simulated once the relations among capillary pressure, cake permeability, and cake saturation are known. The deviations between simulated saturations and experimental data for compressible talc cakes are less than 2%. The centrifugal dewatering of compressible cake includes two mechanisms, the water squeeze due to cake compression and the centrifugal drainage due to pressure difference. The role of cake compression is dominant at the initial 500 s of the dewatering period, and these two factors are almost equal in magnitude as well as in importance when the cake approaches its equilibrium saturation. The porosity and compressibility of cake play the major roles in determining the equilibrium cake saturation. The equilibrium saturation of the compressible talc cake with a lower porosity is much higher than that of incompressible Al₂O₃ cake under a fixed rotating speed. An increase in rotating speed (centrifugal effect) results in a lower equilibrium cake saturation for either compressible or incompressible cakes; however, the increase in rotating speed is more efficient for an incompressible cake in the centrifugal dewatering.     

Effect of Cake Compression on the Performance of Centrifugal Dewatering

The effect of cake compression on the centrifugal dewatering is investigated under various rotating speeds and different kinds of cakes. A numerical method is proposed to estimate the capillary pressure and cake permeability under various cake saturations. The most dewatering in this study are operated at the funicular state of cake. The cake permeability decreases, while the capillary pressure increases rapidly, with the decrease of cake saturation during a centrifugal dewatering. The cake saturation can also be simulated once the relations among capillary pressure, cake permeability, and cake saturation are known. The deviations between simulated saturations and experimental data for compressible talc cakes are less than 2%. The centrifugal dewatering of compressible cake includes two mechanisms, the water squeeze due to cake compression and the centrifugal drainage due to pressure difference. The role of cake compression is dominant at the initial 500 s of the dewatering period, and these two factors are almost equal in magnitude as well as in importance when the cake approaches its equilibrium saturation. The porosity and compressibility of cake play the major roles in determining the equilibrium cake saturation. The equilibrium saturation of the compressible talc cake with a lower porosity is much higher than that of incompressible Al₂O₃ cake under a fixed rotating speed. An increase in rotating speed (centrifugal effect) results in a lower equilibrium cake saturation for either compressible or incompressible cakes; however, the increase in rotating speed is more efficient for an incompressible cake in the centrifugal dewatering.     

PRACTICAL ASPECTS OF DEWATERING ENHANCED BY ELECTRO-OSMOSIS

Dewatering mechanism due to electro-osmosis is fairly different from that of mechanical dewataine which has been used conventionally. and electro-osmotic dewatering has Several advantages compared with the mechanical dewatering and it can be remarkably effective for hardly dewaterable sludges such as very fine Particles and gelatinous materials. In electro-osmotically enhanced dewatering, however, it would be a great problem that the achievement of complete water removal within the sludge could not be possible in principle. Because. when a liauid state in the sludge becomes to be not continuous gradually with proceeding of dewatering, the sludge does not lead electricity and and then electro-osmosis is no longer caused.

Applying electro-osmosis to sludge dewatering practically, methods for improvement of the electro-osmotic dewatering must be investigated taking account ofapos;such a situation for the performance of high dewatering efficiency. Some practical applications of the dewatering which can be considered to be available are described in this paper.     

Unified determination of relative molecular diffusivity and fluid permeability for partially saturated cement-based materials

From conductivity theory, general models for relative molecular diffusivity and fluid permeability are first derived with unknown tortuosity function and modification coefficient, which can be respectively deduced from hydraulic diffusivity and water retention curve (WRC). Based on empirical laws for hydraulic diffusivity and WRC of cement-based material, unified models for relative molecular diffusivity and fluid permeability are further formulated with only two measurable parameters. Because of practical difficulty for measuring water permeability and pure gaseous molecular diffusivity, only relative gas permeability and relative chloride diffusivity models are verified by the reported data. It is found that the predicted relative gas permeability agrees with measured values and exponential law is a little more preferable than power law for quantifying hydraulic diffusivity. Moreover, relative chloride diffusivity from the unified model also agrees well with experimental data derived via Nernst–Einstein Equation. However, the unified model doesn't capture the possibly overestimated relative chloride diffusivity from Fick's law.     

PRACTICAL ASPECTS OF DEWATERING ENHANCED BY ELECTRO-OSMOSIS

ABSTRACT

Dewatering mechanism due to electro-osmosis is fairly different from that of mechanical dewataine which has been used conventionally. and electro-osmotic dewatering has Several advantages compared with the mechanical dewatering and it can be remarkably effective for hardly dewaterable sludges such as very fine Particles and gelatinous materials. In electro-osmotically enhanced dewatering, however, it would be a great problem that the achievement of complete water removal within the sludge could not be possible in principle. Because. when a liauid state in the sludge becomes to be not continuous gradually with proceeding of dewatering, the sludge does not lead electricity and and then electro-osmosis is no longer caused.

Applying electro-osmosis to sludge dewatering practically, methods for improvement of the electro-osmotic dewatering must be investigated taking account ofapos;such a situation for the performance of high dewatering efficiency. Some practical applications of the dewatering which can be considered to be available are described in this paper.     

复合型无机凝聚剂对剩余污泥减容处理技术

利用复合型无机凝聚剂替代PAM对剩余污泥的处理技术,有利于污泥与水的高效分离,经凝聚减容沉淀后的浓缩污泥具有很好的压缩性和脱水性,产生的泥饼含水率大幅度降低至60%以下,便于后期无害化处置与资源化再利用。     

Three-dimensional CFD modelling of PEM fuel cells: An investigation into the effects of water flooding

In this work, a three-dimensional PEM fuel cell model has been developed and is used to investigate the effects of water flooding on cell performance parameters. The presence of liquid water in the cathode gas diffusion layer (GDL) limits the flow of reactants to the cathode catalyst layer, thereby reducing the overall reaction rate and curtailing the maximum power that can be derived from the cell. To characterize the effects of water flooding on gas diffusion, effective diffusivity models that account for the tortuosity and relative water saturation of the porous fuel cell electrodes have been derived from percolation theory and coupled with the CFD model within a single phase flow skeleton. The governing equations of the overall three-dimensional PEM fuel cell model, which are a representative of the coupled CFD and percolation theory based effective diffusivity models, are then solved using the finite volume method. Parametric studies have been conducted to characterize the effects of GDL permeability, inlet humidity and diffusivity of the reactants on the various cell performance parameters such as concentration of reactants/products and cell current densities. It is determined that the GDL permeability has little or no effect on the current densities due to the diffusion dominated nature of the gas flow. However, through the incorporation of percolation theory based effective diffusivity model; a marked reduction in the cell performance is observed which closely resembles published experimental observations. This is a reasonable approximation for effects of water flooding which has been inherently used for further parametric studies.     

Mechanical dewatering of peat

The mechanical dewatering of peat by pressing is analysed in terms of the compressibility and the permeability of the peat bed. It is demonstrated that the presence of electrostatically stabilized small particles has a great influence on permeability. Reduction of the electrostatic interactions makes the suspended particles adhere to the peat matrix, thus strongly increasing permeability. The electrostatic interactions can be modified by changing the pH or by adding ionic surfactant, polyelectrolyte or inorganic electrolyte. The combined influence of polycation and reduced pH strongly facilitates mechanical dewatering and has a beneficial influence on the purity of the wastewater, on mineral matter content (and hence ash yield) and on the sensitivity of peat to mechanical pretreatment.     

DRYING OF WASTE SLUDGES IN A FLUIDIZED BED DRYER WITH A MIXER

ABSTRACT

Sludge wastes are a very attractive source of many valuable compounds such as nitrogen, phosphorus and organic matter. One of the interesting methods of sludge utilization is dewatering and drying. In this paper some problems of drying of two types of sludge wastes of different form and origin are presented. One of the sludges—a brewery by-product— contains spent grain suspension of brewer's yeast and diatomaceous earth. The second one is a by-product of a biotechnological process which contains mainly lime, protein, many microelements and some microorganisms. Moisture content of both sludges is about 65–70% w/w. The laboratory experiments of sludge drying in the fluidized bed dryer with a mixer, fluidized bed hydrodynamics, and the effects and advantages of sludge drying are shown.     

Pore Destruction Resulting from Mechanical Thermal Expression

Mechanical thermal expression (MTE) is a dewatering technology ideally suited for the dewatering of internally porous biomaterials. For such materials, the combined application of temperature and compressive force in the MTE process enhances the collapse of the porous structure, resulting in effective water removal. In this article, a comparison of the dewatering of titanium dioxide, which is an ideal incompressible, non-porous material, and lignite, which is a porous plant-based biomaterial, is presented. The comparison is based on the parameters critical to dewatering, namely the material compressibility and the permeability. With the aid of mercury porosimetry results, a detailed discussion of the pore destruction of lignite resulting from MTE processing is presented. It is illustrated that there is a well-defined relationship between the pore size distribution after MTE dewatering and the MTE temperature and pressure. The discussion is extended to an investigation of the effects of MTE processing conditions on the effective and non-effective porosity. The effective porosity is defined as the interconnected porosity, which contributes to flow through the compressed matrix, while the non-effective porosity is the remaining porosity, which does not contribute to flow. It is illustrated that there is a linear relationship in both the effective and non-effective porosity with the total porosity. The linear relationship is independent of the processing conditions. It is also shown that MTE processing collapses the effective and non-effective pores at roughly the same rate.     

Pore Destruction Resulting from Mechanical Thermal Expression

Mechanical thermal expression (MTE) is a dewatering technology ideally suited for the dewatering of internally porous biomaterials. For such materials, the combined application of temperature and compressive force in the MTE process enhances the collapse of the porous structure, resulting in effective water removal. In this article, a comparison of the dewatering of titanium dioxide, which is an ideal incompressible, non-porous material, and lignite, which is a porous plant-based biomaterial, is presented. The comparison is based on the parameters critical to dewatering, namely the material compressibility and the permeability. With the aid of mercury porosimetry results, a detailed discussion of the pore destruction of lignite resulting from MTE processing is presented. It is illustrated that there is a well-defined relationship between the pore size distribution after MTE dewatering and the MTE temperature and pressure. The discussion is extended to an investigation of the effects of MTE processing conditions on the effective and non-effective porosity. The effective porosity is defined as the interconnected porosity, which contributes to flow through the compressed matrix, while the non-effective porosity is the remaining porosity, which does not contribute to flow. It is illustrated that there is a linear relationship in both the effective and non-effective porosity with the total porosity. The linear relationship is independent of the processing conditions. It is also shown that MTE processing collapses the effective and non-effective pores at roughly the same rate.     

煤气化细渣陶瓷膜真空脱水试验与数值模拟

气化细渣是煤炭气化过程的废弃物,高效脱水是其资源化利用、减量化处置的必要前提。本文采用陶瓷膜真空过滤系统开展了脱水实验并对脱水过程进行了数值模拟。气化细渣料浆浓度和液下吸附时间影响滤饼厚度且滤饼厚度增加导致水分运移路径增长,使得有效脱水时间增加;滤饼脱水过程的脱水速率值呈现非线性降低趋势且滤饼水分极限值为40%,这与气化细渣物化性质有关;真空度>0.08MPa时气化细渣滤饼中“通道水”能够在约24s有效脱除。Fluent数值模拟过程选用了欧拉模型并确定了陶瓷膜滤板和气化细渣滤饼的阻力系数,脱水过程的实验值与模拟结果误差小于5%,证实了模型可靠性。模拟过程分析了气化细渣脱水过程中压力场和水分含量分布云图的演变规律,结果表明,增加脱水系统真空度、降低滤饼厚度、提高“通道水”比例以及增大气化细渣颗粒等效当量直径能够提高气化细渣脱水效率。此外,陶瓷膜真空脱水过程所得滤液洁净度高且部分指标达到了工业用水的标准。     

Fundamental salt and water transport properties in directly copolymerized disulfonated poly(arylene ether sulfone) random copolymers

Water and sodium chloride solubility, diffusivity and permeability in disulfonated poly(arylene ether sulfone) (BPS) copolymers were measured for both acid and salt form samples at sulfonation levels from 20 to 40 mol percent. The hydrophilicity of these materials, based on water uptake, increased significantly as sulfonation level increased. The water permeability of BPS materials in both the salt and acid forms increases more than one order of magnitude as sulfonation level increases from 20% to 40%, while NaCl permeability increases by two orders of magnitude. The water and salt diffusivity and permeability were correlated with water uptake, consistent with expectations from free volume theory. In addition, a tradeoff was observed between water/salt solubility, diffusivity, and permeability selectivity and water solubility, diffusivity and permeability, respectively. This finding suggests a water/salt permeability/selectivity tradeoff, similar to that operative in gas separation polymers, in this family of polymers.     

Indirect assessment of hydraulic diffusivity and permeability for unsaturated cement-based material from sorptivity

The hydraulic diffusivity, water permeability and relative gas permeability for cement-based materials are indirectly evaluated from measured sorptivity and water vapor sorption isotherms (WVSIs). The dependence of sorptivity on initial saturation degree is first established to help calculate hydraulic diffusivity and other transport properties. An experimental program with a self-scaled preconditioning strategy is also carefully designed and conducted on three concretes to measure their sorptivity, WVSIs as well as permeability to various fluids. It's found that hydraulic diffusivity of ambiguous physical significance may be not a good durability indicator. The predicted water permeability is larger than measured value but at the same order of magnitude. This overestimation is attributed to the required drying preconditioning. The predicted relative water permeability agrees well with reported data. However, the predicted relative gas permeability agrees with the measured data from classical CEMBUREAU method better than that from tri-axial permeameter with higher inlet gas pressure.     

The effect of shear on gravity thickening: Pilot scale modelling

Mathematical models are potentially a valuable tool for the prediction of continuous gravity thickener operation. However, experience shows that existing mathematical models underestimate dewatering in thickeners for flocculated feed materials when predictions are made of either the underflow solids concentration for a given solids feed flux density or the maximum solids feed flux density achievable for a minimum underflow solids concentration set point. One reason postulated for this discrepancy is shear enhancement of sedimentation and bed dewatering as a result of aggregate densification. This process is not taken into account in conventional 1-D thickener models. A pilot scale column, operated at low bed heights without the addition of mechanical shear, produced results that compared well with 1-D model predictions. The effect of mechanical shear and/or greater bed height was to significantly enhance thickener performance relative to model predictions (as measured by underflow density or maximum solids flux density achievable for a nominated underflow density). An experimental method was developed that enabled shear to be incorporated into the suspension dewatering characterisation. The results suggest an order of magnitude increase in solid flux density can be expected under controlled shear conditions with polymer flocculated aggregates. The results also indicate that mechanical shear is not the only factor that can enhance dewatering, since higher beds, and hence longer residence times, also improve the achievable solids flux density. This is despite the fact that the thickener is operating in a regime that is predicted to be limited by the sediment permeability and not its compressibility. This suggests an additional mechanism must be at play in full scale operation and points a direction for further experimentation.