Principle Microbiology

(1) String discharge :- Fungal mycelia produce a fibrous filter cake which can easily be separated from the drum by string discharge.
Long lengths of string 1.5 cm apart are threaded over the drum and round two rollers.
The cake is lifted free from the upper part of the drum when the vacuum pressure is released and carried to the small rollers where it falls free.

(2) Scraper dircharge :- Yeast cells can be collected on a filter drum with a knife blade for scraper disc.
The filter cake which builds up on the drum is removed by an accurately positioned knife blade.
Because the knife is close to the drum, there may be gradual wearing of the filter cloth on the drum.

(3) Scraper discharge with precoating of the drum :- The filter cloth on the drum can be blocked by bacterial cells or mycelia of actinomycetes.
This problem is overcome by precoating the drum with a layer of filter-aid 2-10 cm thick.
The cake which builds up on the drum during operation is cut away by the knife blade.
Which mechanically advances towards the drum at a controlled slow rate.
Alternatively, the blade may be operated manually when there is an indication of ‘blinding’ which may be apparent from a reduction in the filtration rate.
In either case the cake is removed together with a very thin layer of precoat.

ROTARY VACUUM-FILTERS

Large rotary vacuum filters are commonly used by industries which produce large volumes of liquid which need continuous processing.
The filter consists of a rotating, hollow, segmented drum covered with a fabric or metal filter which is partially immersed in a trough containing the broth to be filtered.
The slurry is fed on to the outside of the revolving drum and vacuum pressure is applied internally so that the filtrate is drawn through the filter, into the drum and finally to a collecting vessel.
The interior of the drum is divided into a series of compartments, to which the vacuum pressure is normally applied for most of each revolution as the drum slowly revolves (~ 1 rpm).
How ever, just before discharge of the filter cake, air pressure may be applied internally to help ease the filter cake off the drum.
A number of spray jets may he carefully positioned so that water can be applied to rinse the cake. This washing is carefully controlled so that dilutions of the filtrate is minimal.
It should be noted that the driving force for filtration (pressure differential across the filter) is limited to one atmosphere (100 kN per meter square) and in practice it is significantly less than this.
In contrast, pressure filter can be operated at many atmospheres pressure. A number of rotary vacuum drum filters are manufactured.
Which differ in the mechanism of cake discharge from the drum.

(1)String discharge.
(2)Scraper discharge.
(3)Scraper discharge with precoating of the drum.