The decanter centrifuge is used to separate a sludge or a slurry into a solid part and a liquid part. The slurry treatment and disposal plants generate a final residue though the various process stages (equalisation, neutralisation, coagulation, sedimentation, floatation, biological treatment etc.). This residue is generally subjected to a further series of treatments, before final disposal. It is in one of these processes, thickening or de-watering, that the decanter centrifuge is used.

They are processes through which effluent slurry is generated (output slurry), more concentrated than the influent (input slurry).

This name comes from the word DECANTATION, which means the clarification of a liquid, obtained through the separation of suspended particles (sedimentation).

This is a process in which solid particles in a liquid undergo the effect of the force of gravity and deposit on the bottom, separating themselves from the liquid itself. In some cases (biological fluids) the time for spontaneous sedimentation would be too long and some particles would take years for a sedimentation process. In these cases, the centrifugation is used.

With centrifugation it is possible to generate an increase of gravity with respect to the earth’s gravity, thus accelerating the sedimentation process. The container of the product to treat rotates. The solution is thus subjected to an artificial force of gravity (centrifugal force) proportional to the distance from the centre of rotation and to the square of the rotation speed.

The decanter centrifuge is made up from a cylindrical-conical container (bowl) containing a worm screw mechanism for the removal of the sludge. The slurry is fed into the unit through the screw support axle. The centrifugal force acting, generated by the bowl rotation, makes the sludge compact against the walls of the container and the dragging action of the worm screw, which turns at a slightly lower speed, makes the compacted sludge advance along the container and towards the conical section of the same, from where it is extracted.

The yield is affected by the construction of the decanter centrifuge itself and by the operational parameters of the process. The variables due to the construction are the rotation speed, the volume available and the sludge extraction speed. The process variables are the product feeding speed, the characteristics of the solids, the dosing of chemical additives (polyelectrolyte for the ecology sector and fossil flours for the food sector) and the operational temperature.

By increasing the supply flow rate in a certain decanter centrifuge, decreases the retention time (product preservation in the decanter centrifuge) and decreases the recovery of solids.

The polyelectrolyte increases the structural resistance of the solids and causes the formation of flakes even starting from the finest particles. The fossil flours (food sector) allow weighing down the solid. The use of these substances allows facilitating the separation and increasing the recovery of the solids.

It is an electronic appliance that produces a three-phase signal used for start-up of the electric motor.

The decanter centrifuge works in closed circuit and in continuous cycle. The product to treat enters the decanter centrifuge through a feeding tube and the solids and liquid are to the outside through pipes and evacuation screw, keeping the working environment clean and uncontaminated by residues and odors of sludge or slurry. Furthermore, the downtimes of the typical filter press are eliminated, necessary for unloading and cleaning. The decanter centrifuge is very compact and space-saving in relation to the volume of slurry that it can treat and occupies much less space than a filter-press with the same flow rate. To ensure good operation of the decanter centrifuge it only needs cleaning and maintenance at regular intervals (e.g. greasing the bearings), thus saving large amounts of consumables, such as filters and cloths, which a filter-press requires.