Sedimentation and Thickening

Clarifier Design. Design methods for dilate sedimoitation are based on the prin(%le of providing enough residence time for tiie separation to take place. Estimates can be made using Stokes law or Newton s law as described in Chapter 3, e ecially for well-defiaed homogeneous systems, but due allowance must be nude for deleterious effects due to convection, air-induced surface waves, inlet and outlet turbulence etc. 

Batch Settling Dilute Systems. The residence time required for tire batch settling of a suspension that is con osed of a size distribution of particles of the same material can be calculated easily using the size of the smallest particle to estimate the time it will take to settle the full hei t of the vessel firom surface to base. 

However for many suspensions it is only economic to settle out the larger particles leaving a reduced concentration of snuller ones to be removed by other means such as clarification filtration, hi this case the settled material will comprise two conqtonents in particle size terms material of sizes that have been settled out totalfy and, secondfy, material of sizes that have only been s ed out partially. 

Some of the particles of the smallest aze to be settled out totally must have travelled the full height of the suspension and this condition defines that aze. All tizes greater than this one must be separated. For smaller tize particles the proportion tiut me s arated is related directly to the proportion of the full height that they sediment in the time made available. 

A aqueous efifiuent containing mineral particles is to be treated prior to the discharge of the liquid to a watercourse. The first stage in the process is to pun the suspoiaon into a 5 m deq vessel of area 10 m, and allow solids to s e for half an hour. Calculate 

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Camacho, Carlos. 1975. Smdies in filtration of particulate mixtures. MS thesis. University of Houston, Texas. Chen, W. 1986. Sedimentation and thickening. PhD diss.. University of Houston, Texas. 

Fundamental aspects of the above processes are conadered in detail in Cba ptes 3. Appropriate tetts are required to measure ttie effect of dianges in concentratian aaUo.1t may be obsraved that since the downward flux of solids in a settling suspension equates to the product of Uo and C, the possibility of a minimum fux presents itself Identification of sudi minima is required in the spectficalion ofprocess plant used for sedimentation and thickening. 

The raw potassium manganate(VI) from the secondary roaster or the Hquid-phase oxidizer contains a fair amount of insoluble material such as unreacted MnO and ore gangue. In most continuous processes, these insolubles are removed by sedimentation using thickeners or filtration and are disposed of as waste. 

More detailea descriptions of small-scale sedimentation and filtration tests are presented in other parts of this section. Interpretation of the results and their conversion into preliminary estimates of such quantities as thickener size, centrifuge capacity, filter area, sludge density, cake diyness, and wash requirements also are discussed. Both the tests and the data treatment must be in experienced hands if error is to be avoided. 

Filters generally achieve a lower final moisture content than obtained by gravity sedimentation and are often fed from thickeners, as indicated in the schematic particulate process shown in Figure 9.2. In this chapter the principles of slurry filtration will be described and certain simplified filter design equations derived. For more complex derivations the reader is referred to specialist texts e.g. Coulson and Richardson (1991), Wakeman (1990a) and Purchas (1981). 

The time required to concentrate the sediment after it has reached the critical condition can be determined approximately by allowing a sample of the slurry at its critical composition to settle in a vertical glass tube, and measuring the time taken for the interface between the sediment and the clear liquid to fall to such a level that the concentration is that required in the underflow from the thickener. The use of data so obtained assumes that the average concentration in the sediment in the laboratory test is the same as that which would be obtained in the thickener after the same time. This is not quite so because, in the thickener, the various parts of the sediment have been under compression for different times. Further, it assumes that the time taken for the sediment to increase in concentration by a given amount is independent of its depth. 

In a batch sedimentation experiment, the sediment builds up gradually and the solids which are deposited in the early stages are those which are subjected to the compressive forces for the longest period of time. In the continuous thickener, on the other hand, all of the particles are retained for the same length of time with fresh particles continuously being deposited at the top of the sediment and others being removed at the same rate in the underflow, with the inventory thus remaining constant. Residence time distributions are therefore not the same in batch and continuous systems. Therefore, the value of tR calculated from equation 5.59 will be subject to some inaccuracy because of the mismatch between the models for batch and continuous operation. 

The above-cited studies demonstrate the performance of a particular unit system for the treatment of specific type of waste stream. A particular unit system alone may not be able to treat the wastewater to a level of effluent standard prescribed for its safe disposal. Hence a number of pretreatments, such as screening, sedimentation, equalization, and neutralization, and post-treatment units such as secondary sedimentation, sludge thickening, digestion and disposal, disinfection, and so on, are extremely important for complete treatment. The effluent treatment and disposal facilities adopted by various types of pharmaceutical industries are described in the following sections. 

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