The Drum Filter Operational Sequence

The entire filtration cycle on a rotary drum filter must be completed within a geometry of 360 degrees. Let us follow the cycle sequence of a single sector assuming that the drum rotates in a clockwise direction while viewing the valve end:

Cake Formation

With the overflow weir set to a maximum the "apparent submergence" is normally 33-35% so the slurry levels between 0400 and 0800 hrs. Once a sector enters submergence vacuum is applied and a cake starts to form up to a point where the sector emerges from the slurry. The portion of the cycle available for formation is the "effective submergence" and its duration depends on the number of sectors, the slurry level in the tank and the bridge setting which controls the form to dry ratio

Cake Washing and Drying

After emerging from submergence the drying portion of the cycle commences and for non-wash applications continues to about 0130 hrs where the vacuum is cut-off. If cake washing is required the wash manifolds will be located from about 1030 to 1130 hrs and the remaining time to vacuum cut-off at 0130 is the portion allocated to final cake drying.

Cake Discharge

After vacuum for the entire sector is cut-off air blow commences at about 0200 hrs in order to facilitate cake discharge. The blow, depending on the position of the tip of the scraper blade, will cut-off at approximately 0300 hrs. Drum filters are normally operated with a low pressure blow but on certain applications a snap blow is applied and to avoid the snapping out of the caulking bars or ropes wire winding of the cloth is recommended . Blow is used on scraper and roll discharge mechanisms but on belt discharge filters vacuum cuts-off when the filter media leaves the drum.

Dead Zone

Once the blow is cut-off the sector passes through a zone blocked with bridges so that no air is drawn through the exposed filter media which might cause the loss of vacuum on the entire drum surface.

 Slurries with solids that do not tend to settle rapidly and will remain in a uniform suspension under gentle agitation.  Cakes when a single washing stage is sufficient to remove residual contaminants from the cake or yield maximum recovery of filtrate.  Cakes which do not require long drying times to reach asymptotic moisture values.

 Filtrates that generally do not require a sharp separation between the mother and wash filtrates. Some complex valves, however, enable atmospheric purging of the sectors and internal piping to facilitate a sharp separation of filtrates.

 Filtrates that are acceptable with a low quantity of fines that pass trough the filter cloth in the first few seconds of cake formation. Broadly, and depending on particle size and cloth permeability, the filtrate may contain 1000 to 5000 ppm insolubles.  For very corrosive applications plastic drum filters are available with up to 10-15 m2 filtration area.

 The strip liner of the trunnion bearing at the valve end will normally wear at the lower half. However, in cases when the slurry has a high specific gravity, the drum may become buoyant causing a wear to the upper half. At this point it should be mentioned that one way to remove the lower half of the liner, when hoisting facilities are not available or operational, is to float the drum by filling the tank with a sufficiently concentrated solution.

 The stuffing boxes on high submergence filters should be inspected for leakage and, if necessary, the stud nuts should be tightened. It should be noted that excess tightening can increase substantially the load on the drum drive so the use of a torque wrench is recommended.

 The face of the wear plate should be checked periodically and remachined if necessary. A whistling noise during operation is an indication the wear plate is worn out or the valve spring requires tensioning.

 The drum has a bailer tube that protrudes from the drive end shaft and must be kept open to atmosphere at all times since its blockage may cause the collapse of the drum. The bailer tube is a tell-tale indication to the following:

 If a lighter flame is drawn through the bailer tube to the inside of the drum it indicates that a vacuum leak exists in the drum shell or the internal piping. It should be noted that in certain instances there is a possibility that explosive gases build-up inside the drum and may pose a safety hazard. In such cases the use of aerosol type smokes or a light tissue paper should be used instead of an open flame to identify a vacuum leak.

 If liquid leakage is observed from the bailer tube it indicates that a hole exists in the drum head causing penetration of slurry from the tank into the drum.

 The on-line filter on the wash headers manifold should be checked periodically for pressure build-up due to progressive blockage. Likewise, the nozzles on the wash headers should be kept clean in order to ensure overlapping for full coverage of the washed cake.

To learn more about what this system might look like for your facility, request an Drum Filter quote today. You can also contact a representative who would be happy to answer your questions concerning Drum Filter and other wastewater treatment methods. Aquasust is a company you can trust for the very best in modern wastewater treatment plant equipment.