Continuous Thickener Research Articles

Settling velocities of particulate systems: 9. Phenomenological theory of sedimentation processes: numerical simulation of the transient behaviour of flocculated suspensions in an ideal batch or continuous thickener

The transient behaviour of flocculated suspensions in an ideal thickener is simulated by numerical solution of the parabolic–hyperbolic equation of the phenomenological theory of sedimentation. The numerical results yield the expected sedimentation and consolidation behaviour for batch sedimentation and for the most important operations of continuous thickening: filling up, transition between steady states and emptying of a continuous thickener.

Pilot-plant studies of non-steady-state continuous sedimentation: hydrodynamics and solids distribution profiles

A sedimentation—flocculation pilot plant has been designed and built, capable of operating under continuous and discontinuous conditions, incorporating a system to detect solids concentration profiles throughout the settler using electrical conductivity, and an automatic control device, supported by suitable computer equipment with its own software. The hydrodynamic behaviour of the continuous thickener has been studied through transient-response techniques. Results of experiments carried out with mineral suspensions in continuous operation are presented here.

Design method of continuous thickeners from semi-batch tests of sedimentation

An extension of a previous method for designing continuous thickeners is presented. The previous method was based on data obtained for batch testing, where the discontinuity heights corresponding to the supernatant-suspension and to the top of the sediment are known at different values of time. From the data batch testing, the area per unit of volumetric flow and the sludge height can be calculated for different underflow solids concentrations in continuous thickeners at a specific interval. This interval can be increased allowing the sediment to rise almost to the top of the cylinder, where the testing is carried out. The method of design has been tested with thickeners at laboratory scale working semicontinuously, with periodic withdrawals of sludge and supernatant and feeding of suspension.

Continuous Consolidation of Sludge in Large Scale Gravity Thickeners

Improved design and performance of continuous thickeners should be achieved through a better understanding of the sludge transport processes involved and the variability of sludge thickenability. A software package called PHOENICS was used to model the three-dimensional flow of sludge in the transport zone of a 20m diameter thickener. A mass transport efficiency was evaluated to test the efficacy of the ploughing system, and tracer simulations were performed to study the degree of back-mixing induced. Interpretation of the results suggests that:○Optimum orientation of the ploughs to the radial arm is 60° to 70°.○Transport of sludge to a central outlet occurs principally in the wake of the advancing ploughs, and that ploughs are less efficient than conventional theories dictate.○Sludge in the ploughing zone is mixed, thus potentially impairing thickener performance. The thickenability of mixed raw sludge arising at a large activated sludge works was monitored. Pronounced seasonal variations were observed, with a marked deterioration in thickenability in late summer and autumn. These were confirmed in trials of a 20m diameter continuous thickener. Consequently the proposed thickening strategy for this works has been revised.

Simulation of batch and continuous thickeners

The design of continuous thickeners from batch tests is considered on the basis of numerical simulations corresponding to both operations. The method proposed in other papers has been corroborated. The simulation has been carried out using dimensionless equations to generalize the discussion. The performance of the continuous thickener at non-steady state is studied following the evolution of the supernatant—suspension interface, the sediment interface and the lines of constant concentration. The utility of the batch test for designing continuous thickeners is discussed. The behaviour of continuous thickeners is analysed on the basis of the simulated results.

Unified Analysis of Compressive Packed Beds, Filter Cakes, and Thickeners

Abstract Fluid—particle systems are widely used throughout industry. Using an ad hoc approach to model a particular fluid—solid process lacks generality. A unified approach using continuum theory for multiphase systems is applied here to evaluate a packed bed, filter cake, and continuous gravity thickener. The unified approach has the advantage that the governing equations are obtained by simplifying the generalized continuum equations. The simplifying assumptions are obvious from the equations and do not require intuitive understanding as does the ad hoc approach. Also, for similar localized conditions the same constitutive relations can be applied when the same material is used in several processes. In the packed bed, filter cake, and thickener modeled here, a compressible solid matrix behavior is represented by a truncated Taylor series expansion for the solid phase stress. The packed bed and filter cake behaviors are evaluated for a range of pump powers and bed masses or cake heights. The thickener be...

Settling velocities of particulate systems, 7. Kynch sedimentation processes: continuous thickening

Continuous sedimentation is presented as a logical extension of a batch Kynch sedimentation process. All results are presented in graphical form as settling plots. Transient solutions are obtained to continuous Kynch sedimentation processes for an ideal suspension having a flux density function with one inflection point and two constant states as initial conditions. It is shown that there are only two steady-state solutions to the continuous settling of an ideal suspension in an ideal thickener. The use of Kynch's theory for compressible industrial pulps introduces confusions in the interpretation of unit areas. The traditional equation to calculate unit areas for continuous thickeners based on Kynch's theory is not correctly interpreted in the literature. A different explanation is given here.

Control of a surface of discontinuity in continuous thickness

AbstractThis paper examines the control of an interface between a suspension of sedimenting particles in liquid and a bed of dense-packed particles at the bottom of the suspension. The problem arises in the operation of continuous thickeners (e.g. in mineral processing) and is here mathematically described by a first order inhomogeneous partial differential equation for the concentration C(x, t) of particles. The controlled variable is the height H* of the bed, and the control variables are the volume fluxes injected at the feed level and removed at the bed. A strategy to control the interface is devised, and control is confirmed and demonstrated by a series of numerical experiments.

Optimization of thickener performance

The objective of this investigation was to determine the sensitivity of three selected input parameters in the operational control of continuous bottom-fed thickeners used for the treatment of coal refuse slurries. The three variable parameters were “Feed Flow Rate,” “FeedpH,” and “Feed Solids Concentration.” These three input parameters were studied under three cationic-anionic polymeric conditions. For each polymeric condition, two mathematical models were developed, one for overflow response and the other for underflow response. The combination polymers produced the best quality of overflow and underflow. For this polymeric condition, change in the feed solids concentration was the most sensitive factor in the maintenance of the overflow quality, and the feed pH was the most sensitive factor for the maintenance of underflow quality. The mathematical models generated a set of plots that would allow an operator to control and/or adjust any of the three variables to achieve optimum conditions.

A two‐dimensional model for the free‐settling regime in continuous thickening

AbstractAll classical models for thickening assume one‐dimensional continuity. However, the free‐settling domain in continuous thickeners is not one‐dimensional. Therefore, a two‐dimensional model is investigated. The two‐dimensional model gives the same values for thickener area demands as the one‐dimensional model, but the relationships between batch and steady‐state thickening are not the same. Free‐settling Kynch characteristics can arise in the continuous operation that do not arise in batch tests. Therefore, design procedures that rely on Kynch theory, such as that of Talmage and Fitch, are not completely valid.A method is developed, based on extrapolating the Kynch or free‐settling segment of a batch settling, curve, that yields an improved prediction for thickener area demand.

Global weak solutions to the problem of continuous sedimentation of an ideal suspension

AbstractWe present the sedimentation of an ideal suspension in a continuous thickener as an initial value problem and study the transient states by constructing the corresponding global weak solutions. Finally, we obtain a way to control the operation of the thickener.

Calculation of the compression zone height in continuous thickeners

AbstractA method to calculate the compression zone height in continuous thickeners is presented. With this method, it is necessary to know the variations of the pulp‐supernatant interface and the sediment height vs. time in a batch test. This method is discussed considering the following aspects: relationship between the settling rate of solids and their concentration in the settling zone; and the compression of solids due to the squeeze transmitted by the upper layers—unbuoyed weight of particles minus force of friction due to the Darcian flow. When the variation of the sediment height vs. time becomes linear, it is possible to calculate the maximum solids concentration which can be reached by sedimentation. The change of the solids matrix permeability and its influence on the method proposed are also analyzed.

Operating Experiences of Sludge Disinfection and Stabilization at Colburn Sewage Treatment Works, Yorkshire

ABSTRACTA NEW integrated sludge treatment plant was installed at Colburn sewage treatment works (STW) in 1986. The plant comprises a number of novel features including a continuous gravity thickener features including a continuous gravity thickener, a pasteurization unit using submerged combustion of digester gas for sludge heating, a peat‐bed odour control system, and a ‘pump‐our/pump‐in’prefabricated anaerobic digester. Plant operation is controlled automatically by a computer system The performance of the thickener has been in accordance with predictions and has achieved at least a halving of raw sludge volume. The submerged combustion plant has consistently heated the sludge to 70°C with high efficiency of gas utilization before the anaerobic digestion stage. Some problems were experienced with the continuity of supply of the raw sludge but overall the plant has performed very satisfactorily with the production of a high quality disinfected and stabilized sludge for use by local farmers.

Compression zone effect in batch sedimentation

AbstractTheories of batch sedimentation, taking into account the compression zone effect, are reinterpreted and modified. According to the discussion presented and the experimental results shown, characteristics—which are the loci of points representative of layers with the same settling rate and the same solids concentration in the settling zone—rise tangentially to the sediment curve on a height vs. time plot. Mathematical expressions that relate the solids concentration to the variations of the pulp‐supernatant interface height and of the sediment height are presented. In addition, the model presented is proposed for use when it is not possible to measure the sediment height. A procedure for calculating the concentration at the top of the sediment is also indicated.Calcium carbonate suspensions in water were selected for testing the equations deduced. The relation between the continuous thickener unit area and the underflow solids concentration was obtained from the batch tests, and is compared with that calculated by the graphic methods of Talmadge and Fitch, and Fitch.

Limiting operating conditions for continuous thickeners

Parameters encountered in idealized thickeners include underflow concentration (ε su ), flow rate of solids in the underflow ( q s ), and the sediment height ( L). Commonly used design procedures based upon the Coe and Clevenger (CC) (1916 Trans. Am. Inst. Min. Engrs 55, 356–384) method only involve the solids rate q s and ε su and do not include L. Although valid for freely settling suspensions, the CC method fails when a compression zone is formed. The permeability, porosity, and unbuoyed sediment weight must then be considered. Equations relating q s and L to ε su are developed in this paper on the basis of Darcy's law. For a given underflow concentration, it is shown that there is a minimum required sediment height and a maximum underflow solids rate that cannot be exceeded. At the maximum flow rate, a saddle point is encountered in the relation between the solids concentration (ε s ) and the height ( x). Placing dε s /d x=0 and d 2ε s /d x 2=0 yields two simultaneous equations which lead to analytical equations relating q smax to ε su . The resulting equations bear a similarity to the standard CC formulas. Examples are presented for a number of constitutive equations appearing in the literature.

Revision of kynch sedimentation theory

AbstractMuch of the theory of gravity sedimentation has been based on the work of Coe and Clevenger (1916) and Kynch (1952). They provided methods for obtaining rates of sedimentation in batch, static tests which are presently used for design of continuous thickeners. Kynch assumed that a first order partial differential equation controlled the entire sedimentation process. His equation was based on: (1) continuity balance; and (2) sedimentation velocity being a unique function of solid particulate concentration. A general solution was presented in the form of volume fraction of solids ϕs = f(x − vt). During the constant rate fall of the upper interface, the boundary condition of uniform initial concentration combines with the Kynch equations to adequately describe the sedimentation phenomena. Kynch ignored the sediment rising from the bottom of the settling chamber, and assumed that the characteristics y = x − vt originated at the origin of coordinates (height, time) during the first falling rate period. The characteristics actually originate at the surface of the rising sediment where the upward liquid velocity affects the rate of fall of the particulates. New equations have been derived based upon the assumption that the characteristics emanate from the rising sediment.

Thickener dynamic analysis, accounting for compression effects

The dynamic behaviour of the thickening zone of a continuous thickener has been analysed, and numerical solutions of the resulting differential equations obtained for a range of conditions. For high throughput, low underflow concentration operation, mechanical (compressive) support of the solids can be very small, providing that sludge funnelling is taken into account. Neglecting mechanical support in such cases gives results which are fairly accurate, but limited by the presence at the top of the sediment of a shallow layer in which mechanical support is significant. For low throughput, high underflow concentration operation hydrodynamic support of the solids can be small, and its neglect gives again fairly accurate results. In general, both hydrodynamic and mechanical support of solids have to be taken into account.

Analysis of Continuous Thickener in Non-Steady State Using a Batch Settling Curve

Analysis of Continuous Thickener in Non-Steady State Using a Batch Settling Curve

Experimental Studies of Analysis of Inclined Continuous Thickener Operation by Use of Vertical and Inclined Solid Flux Curves

Experimental Studies of Analysis of Inclined Continuous Thickener Operation by Use of Vertical and Inclined Solid Flux Curves

Analysis of Continuous Thickener at Steady State Using a Batch Settling Curve

Analysis of Continuous Thickener at Steady State Using a Batch Settling Curve