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Applied Colloids Surfactants
wfc9 - World Filtration Congress 2004 - sludge dewatering
AFS 2005 - American Filtration & Separation Society Annual Conference - tracing the centrifugal separation of fine-particle slurries
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2005 AFS 18th Annual Conference
April 10-13 Hyatt Regency Hotel
Atlanta Georgia
contribution - Tracing the centrifugal separation of fine-particle slurries - effect of centrifugal acceleration, particle interaction and concentration
9th WORLD FILTRATION CONGRESS 2004
American Filtration and Separation Society
18 - 22 April 2004
New Orleans - Louisiana
oral presentation in 414 Centrifuge Filtration
Use of analytical centrifugation for evaluation of
solid-liquid separation in decanter centrifuges
Application for selection of flocculants for sludge dewatering
The operation of high performance dewatering machines such as high pressure filter presses and decanters relies heavily on the use of flocculants for sludge conditioning. Due to the varying composition of wastewater sludges pre-selection of promising flocculants before field tests is a frequent task. To this end, a lot of different laboratory methods have been developed, which are in part very sophisticated or on the other end may be very simple but strongly based on individual judgement and experience.
Measurement of the capillary suction time (CST), of electrokinetic and rheological properties, jar-, filtration and centrifugation experiments were often used, sometimes with good correlation to the performance under practical conditions, sometimes with misleading results.
The efficiency of sludge dewatering by decanter centrifuges is governed by two critical parameters – on the one hand the separation rate of the sludge flocs, on the other hand by the compressibility of the sludge cake. Therefore, a laboratory procedure, able to measure these parameters simultaneously, was developed. Further, the action of high shear stresses during mechanical processing had to be encountered for.
The key to the development of the new method was the application of a multisample analytical centrifuge. This device offers a wide potential for applied research, product control and optimization related to separation stability of dispersions and separation processes. It is also very capable for characterization of sludges regarding their settling behaviour and shear stability. By use of the Lumifuge 114 separation rate and compressibility (dewaterability) can be determined for 8 samples at the same time. The measurements are carried out at a centrifugal acceleration of 1140 x g. The Lumifuge is an analytical centrifuge which traces the separation process during centrifugation over the whole sample length simultaneously.
The initial separation rate gives information whether or not the sludge flocs are separated fast enough from the bulk liquid. The total solids content after centrifugation measures to what extent the separated sludge cake can be compressed. Sample pre-treatment was adapted to reflect the shear forces acting on the polymer conditioned sludges during processing under practical conditions.
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Case study: flocculant selection - anaerobic sludge digested in a sludge reactor
The laboratory procedure developed was used to investigate the efficiency of 4 flocculants. The technique allows to directly compare the performance of maximum 8 flocculants during one measurement. The relative separation degree as a measure of the initial separation speed is the ratio of total solids of the sludge cake obtained after 50 seconds in relation to that obtained after 10 minutes.
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Fig. 1 Comparison of separation properties of polymer conditioned sludges digested in a sludge reactor (total solids 4 percent, ignition loss 36 percent) as function of flocculant applied (3.8 kg polymer / t dry solids).
Results are displayed in Figure 1. Despite the fact that there are only minor differences in total solids in the compressed state (± 0.01 t solids / m³ sludge) addition of DK-65N and DK-65S results only in a substantially lower initial separation rate. Therefore, in this case the decanter cannot operate with satisfying throughput performance if these flocculants are applied. For this sludge on-site DK-65 and DK-65H proved to be efficient. They are alternatively used depending on variations of sludge quality.
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Increasing the initial concentration of total solids of the sludge often results in higher total solids in the sludge cake. Therefore, the influence of the initial sludge concentration on the separation efficiency was investigated in model experiments with an anaerobic sludge. The total solids content of the sludge was varied and initial separation rate and total solids obtained in the sludge cake were measured as a function of flocculant dosage (Fig. 2).
 Fig. 2 Influence of initial total solids content and flocculant dosage (2, 3 and 4 kg polymer / t dry solids) on separation efficiency As expected the increase in the initial total solids leads to a dryer sludge cake, however, it also causes a decrease in the initial separation rate. The second effect can be reduced by increasing the flocculant concentration. The critical separation rate has to be determined for the practical application on-site at which a sufficient throughput performance can be maintained. available application notes see
www.lum-gmbh.com
for detailed publications see
Chemistry Preprint Archive, Volume 2002, Issue 4, April 2002, Pages 265-274
Chemistry Preprint Archive, Volume 2002, Issue 10, October 2002, Pages 1-11
paper presentation in 146 Applications in liquid - Particle separations
Characterization of the sedimentation and consolidation behaviour of concentrated dispersions by analytical centrifugation
D. Lerche, T. Sobisch Solid-liquid separation by gravity or centrifugal sedimentation as well as by filtration is broadly employed in the biotechnological, pharmaceutical, chemical, food, mineral, pulp and paper industry. Sedimentation and filtration have many features in common, particularly the relative flow of particles and liquid as the basic physical principle. Only recently it was comprehensively elaborated that these processes may be described by a unified theory of solid-liquid separation of non-interacting as well as flocculated suspensions. However, this mathematical description requires the knowledge of the characteristic material parameter of the dispersions as function of the volume concentration. Until now the prediction of these parameter is not possible. Their broad variation range for different dispersions requires efficient analytical tools. In this paper the experimental setup of a multisample analytical centrifuge for determination of the characteristic material properties related to the sedimentation and consolidation behaviour of dispersions is described. Accompanied by a brief description of the theory the potential of analytical centrifugation is demonstrated for dispersions of non-interacting rigid and deformable particles and for flocculated systems as well.
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