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The rotary vacuum drum filters described so far utilize a filtrate flow from outside the drum to the inside, where the vacuum is applied. Rotary drum filters also exist with filtrate flow from inside the drum. A simple version is little more than a rotating drum strainer, used to clarify raw water, as filters for the recovery of fibres in pulp and paper mills, and as polishers after final effluent treatment. The filter works on the hydrostatic pressure of the head of suspension, and there is no need for a vacuum pump. The filter medium is pleated so as to increase the total filtration area, and the drive is usually by means of a variable speed motor.

In operation the slurry is fed into the rotating drum through an outer feed box through the open end of the drum. The level inside the drum is controlled from the outer feet box. By maintaining a lower level on the outside of the drum than on the
inside using an overflow device, the water is pressed through the filter medium. The first filtrate may contain some unretained fines that can be returned to the filter feed for reprocessing. The retained filter cake is held along the inside of the drum by baffles until it reaches the top position of the drum where it is then flushed away by an air blast into a discharge funnel.

A more important version, which does employ vacuum to drive the filtration, has the filter medium mounted around the inside of the drum, which is of open construction with annular walls that create a tank between them at the lowest part of
the rotation. Vacuum is applied to the appropriate parts of the outside of the drum, creating a filter cake that is carried to the top of the drum ’s rotation, and is then air blown into a receiving chute. In this filter there is no problem of maintaining a constant concentration in the feed tank and so no need for an agitator.

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