Environmental Engineering Reference
In-Depth Information
the media and are held there by the vacuum as the drum rotates out of
the tank. The vacuum removes additional water from the captured solids.
When solids reach the discharge zone, the vacuum is released and the
dewatered solids are discharged onto a conveyor belt for disposal. The
media are then washed prior to returning to the start of the cycle.
10.7.2.1 Types of Rotary Vacuum Filters
The three principal types of rotary vacuum filters are rotary drum,
coil, and belt. The
rotary drum
filter consists of a cylindrical drum
rotating partially submerged in a vat or pan of conditioned sludge. The
drum is divided lengthwise into a number of sections that are connected
through internal piping to ports in the valve body (plant) at the hub. This
plate rotates in contact with a fixed valve plate with similar parts, which
are connected to a vacuum supply, a compressed air supply, and an
atmosphere vent. As the drum rotates, each section is thus connected
to the appropriate service.
The
coil type
vacuum filter uses two layers of stainless steel coils
arranged in corduroy fashion around the drum. After a dewatering cycle,
the two layers of springs leave the drum bed and are separated from each
other so the cake is lifted off the lower layer and is discharged from the
upper layer. The coils are then washed and reapplied to the drum. The
coil filter is used successfully for all types of sludges; however, sludges
with extremely fine particles or ones that are resistant to flocculation
dewater poorly with this system.
The media on a
belt filter,
leave the drum surface at the end of
the drying zone and pass over a small-diameter discharge roll to aid
cake discharge. Washing of the media occurs next. The media are then
returned to the drum and to the vat for another cycle. This type of filter
normally has a small-diameter curved bar between the point where the
belt leaves the drum and the discharge roll. This bar primarily aids in
maintaining belt dimensional stability.
10.7.2.1.1 filter media
Drum and belt vacuum filters use natural or synthetic fiber materi-
als. On the drum filter, the cloth is stretched and secured to the surface
of the drum. In the belt filter, the cloth is stretched over the drum and
through the pulley system. Installation of a blanket takes several days.
With proper care, the cloth will last several hundred to several thousand
hours. The life of the blanket depends on the cloth selected, conditioning
chemical, backwash frequency, and cleaning (e.g., acid bath) frequency.
10.7.2.1.2 filter drum
The filter drum is a maze of pipe work running from a metal screen
and wooden skeleton and connecting to a rotating valve port at each end
of the drum. The drum is equipped with a variable speed drive to turn
