USAGE OF THE CENTRIFUGAL TECHNOLOGY IN DEHYDRATING SLUDGE

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1 USAGE OF THE CENTRIFUGAL TECHNOLOGY IN DEHYDRATING SLUDGE John Kissandrakis Responsible for the Industrial Sector PIERALISI HELLAS A.E. Industrial centrifugal machines 14 Km National Road Athens Lamia, Kifissia Hellas Tel: : i.kissandrakis@pieralisi.gr SUMMARY The by-products of the wastewater and industrial treatment plants are mostly known by the general name sludge and its right management is a crucial factor for the better performance of a biological treatment plant. A very efficient way to manage sludge is dehydration by centrifugation. The centrifugal technology is based upon the separation of the liquid and the solid phase by the influence of the centrifugal force. The centrifugal unit is consisted of a rotating drum that obliges the mass of the sludge to rotate and finally separate the two phases. The solid phase is been expelled by the use of a screw which is rotating inside the drum. The efficiency of the described unit is not only influenced by individual adjustments but also by the adding of polymer, which helps to the further embodying of the solid particles of the sludge and consequently in a faster and better separation. Further to the centrifugal extractor the dehydrating plant includes of individual systems such as pumps, preparing and feeding units of polymer, several automatisms, e.t.c.,. The planning and correct installation of all the above mentioned machines have a great impact in the final efficiency of the plant. The technology of the centrifugal treatment / dehydration of sludge is offering many advantages in comparison to all the other techniques such as high percentage of dehydration with maximum capacity of separation, automatic and continuous function, easy cleaning system that renders it as the leading solution all over the world. 1.INTRODUCTION In the sludge treatment plans are been conducting complicated functions using as row material the unprocessed waste, and with final products are the processed sludge, and clean waste water. As the sludge is being discharged from the several stages of process has great content of water and bacterial. Basic role in the treatment of sludge has the unit of dehydration, and has as purpose the suspension of water content in the sludge. The techniques used in dehydration are the followings : 1

2 Centrifuges Drying beds Belt filter press Filter press In this article we will try to describe the technology of centrifugal extractors in the sludge dehydration. 2. CHARACTERISTICS AND FUNCTION OF THE CENTRIFUGAL EXTRACTORS. The centrifugation is a method for the separation of the solid from the drain, that is based in the difference of the density that appears in the ingredients of sludge and to the effect of the centrifugal force to them. By this way the physical process of sedimentation, with other words the separation of solid from drains is accelerated. The separation is being made in centrifugal extractors (decanter), which consist of a drum cylindrical cone shaped, that spins around a horizontal rotating scroll. The scroll rotates into the drum with the same direction but with slightly less spead. The feeding is being done through the cone shaped part of the bowl, through a special stationery feed tube that leads the sludge in the center of the bowl. With the spinning of the bowl, the sludge is being affected by the centrifugal force and as a result the parts consisting the sludge are separated according to their density. The solids which have setteled along the wall of the bowl are conveyed toward the conical end of the bowl while the liquids form weir plates. The screw conveyor pushes the dewatered sludge forwarded, where are the exits from the bowl, while the drains exit from the back of the bowl passing through the weir plates 2

3 3. DEWATERING SLUDGE UNIT A complete line of dewatering plant, apart from the centrifugal extractor, is consisted of the following machines: Sludge pumping system Chemical conditioning system Unit for the preparation of the solution polymer Feeding mono pump for the solution of the polymer Sludge Conveyor Electrical panel for the unit of dewatering The size of the machines is selected according to the size of the centrifugal extractor. 4. CAPACITY OF THE CENTRIFUGAL EXTRACTOR The capacity of the centrifugal extractor depends on the geometrical and functional characteristics of the extractor. 4.1 Geometrical characteristics : Length of the drum Diameter of the drum Rotation speed of the drum Differential speed of the scroll conveyor Weir plates Ratio drum lengh/diameter Is a useful comparison indicator for the estimation of real capacity of the processed sludge between several different centrifugal extractors. Frequently the comparison is being made taking in to consideration the hydraulic flow which results to unrealistic conclusions. 3

4 Rotation Speed has a direct impact in the centrifugal force that is being acting upon the particles of sludge. Higher rotating speed is increasing the particle sedimentation, keeping at the same time stable the feeding amount of sludge and the dimensions of the drum. The differential speed is the difference between the rotation speed of the drum and the scroll conveyor, which is responsible for the continuous outcome of the dewatered sludge. If we reduce the rotating speed of the screw, keeping all the other parameters stable, we will have sludge with higher percentage of dewatering, as the remaining time is the inner side of the centrifugal extractor is prolungated, resulting in the most efficient effect of the centrifugal force. Weir plates are holes for the exit of the drains (water face), with adjustable distance from the center of the drum. The more close is the hole exit to the shaft, the cleaner drains we obtain. 4.2 Functional Characteristics Feeding amount of sludge Retentions time Amount of solids in the sludge Feeding amount of polymer Percentage of concentration in the solution of polymer The feeding amount of the sludge has a significant role in the percentage of dehydration. If the feeding of the machine is done with smaller amount of sludge, in comparison with the real capacity of process of the centrifugal extractor, keeping all the other parameters stable, we have longer retention time of the sludge in the inner part of the machine, which results in a more satisfying separation of the solids and the drains and lower consumption of polymer. Retention time, the period the sludge remains in the inner side of the separator depends from the following parameters : the differential speed, the diaphragms, the feeding amount and the diameter, the length of the rotating drum. The percentage of the solids contained in the sludge is a crucial factor for the selection of the right size of the centrifugal extractor. Small percentage of solids means that sludge to be proceed has high content of water. If thickening is done and the quantity of solids is doubled, the amount of sludge to be processed is halved and as a result we can choose a smaller centrifugal extractor. The feeding quantity in polymers depends on the percentage of the solids contained in the sludge from the dewatering percentage, that we wish to achieve, and from the type and quality of the polymer. 4

5 The amount of polymer concentration of the solution from polymer to polymer depends on the dewatering percentage we want to achieve. By increasing the polymer we can increase the feeding quantity of the sludge having the same dewatering or having even more higher deisidration keeping stable the quantity of the sludge to be processed. The increase on the concentration amount of the polymer abore a certain point brings negative results. By combining the functional and geometrical parameters we obtain best functional efficiency of the centrifugal extractor. The functions of the centrifugal extractors are fully automated. Human interference is limited only to the starting up of the machine and the feeding polymer unit. CONCLUSIONS The centrifuge dewatering plants with centrifugal extractors have the following advantages. Offer a low total operational and maintenance cost in comparison to the usual belt filter presses. Have automatic and continuous operation Provide high percentage of dewatering with even higher separation ability Have zero consumption of water Need only a smaller area installation space in comparison with their capacity Need minimum care from their operator when functions are stable Cleaner environmental conditions for the operators Are easily cleaned The maintenance of the centrifuge is easy and fast. 5