Principle Microbiology

PRESSURE LEAF FILTERS

There are a number of intermittent batch filters usually called by their trade names. These filters incorporate a number of leaves, each consisting of a metal framework of grooved plates which is covered with a fine wire mesh, or occasionally a filter cloth and often precoated with a layer of cellulose fibres. The process slurry is fed into the filter which is operated under pressure or by suction with a vacuum pump. Because the filters are totally enclosed it is possible to sterilize them with steam. This type of filter is particularly suitable for ‘polishing’ large volumes of liquids with low solids content or small batch filtrations of valuable solids.

(i) Vertical metal-leaf filter

This filter consist of a number of verticel porous metal leaves mounted on a hollow shaft in a cylindrical pressure vessel. The solids from the slurry gradually build up on the surface of the leaves and the filtrate is removed from the plates via the horizontal hollow shaft. In some designs the hollow shaft can be slowly rotated during filtration. Solids are normally removed at the end of a cycle by blowing air through the shalt and into the filter leaves.

(ii) Horizontal metal—leaf filter
In this filter the metal leaves are mounted on a vertical hollow shaft within a pressure vessel. Often only the upper surfaces of the leaves are porous. Filtration is continued until the cake fills the spacc between the disc-shaped leaves or when the operational pressure has become excessive. At the end of a process cycle, the solid cake can be discharged by releasing the pressure and spinning the shaft with a drive motor.

(iii) Stacked-disc filter

One kind of filter of this type is the Metafilter. This is a very robust device and because there is no filter cloth and the bed is easly replaced, labour cost are low. It Consists of a number of precision-made rings which are stacked on a fluted rod. The rings are assembled on the rods.
The assembled stacks a placed in a pressure vessel which can be sterilized if necessary. The packs are normally coated with a thin layer of kieselguhr which is used as a filter aid. During use, the filtrate passes between the discs and is removed through the grooves of the fluted rods, while solids are deposited on the filter coating. Operation is continued until the resistance becomes too high and the solids are removed from the rings by applying back pressure via the fluted rods. Metafilters are primarily used for ‘polishing’ liquids such as beer.

Filters may be of two types.
(1) Batch Filters (2) Continuous Filters

Batch Filters

PLATE AND FRAME FILTERS
A plate and frame filter is a pressure filter in which the simplest form consists of plates and frames ar ranged alternately. The plates are covered with filter cloths or filter pads. The plates and frames are assembled on a horizontal framework and held together by means of a hand screw or hydraulic tam so that there is no leakage between the plates and frames which form a series of liquid-tight compartments. The slurry is fed to the filter frame through the continuous channel formed by the holes in the corners of the plates and frames. The filtrate passes through the filter cloth or pad, runs down grooves in the filter plates and is then discharged through outlet taps to a channel. Sometimes, if aseptic conditions are required, the outlets may lead directly into a pipe. The solids are retained within the frame and filtration is stopped when the frames are completely filled or when the flow of filtrate becomes uneconomicaly low.
On an industrial scale the plate and frame filter is one of the cheapest filters per unit of filtering space area requires the least floor space, but it is intermittent in operation (a batch process) and there may be considerable wear of filter cloths as a result of frequent dismantling.
This type of filter is most suitable for fermentation broths with a solids content and low resistance to filtration. It is widely used as a ‘polishing’ device in breweries to filter out residual yeast cells. following initial clarification by centrifugation or rotary vacuum filtration.
It may also be used for collecting high value solids that would not justify the use of a continuous filter. Because of high labour costs and the time involved in dismantling, cleaning and reassembly, these filters should not be used when removing large quantities of worthless solids from a broth.

It is common practice to use filter aids when filtering bacteria or other fine or gelatinous suspensions which prove slow to filter or partially block a filter. Kieselguhr (diatomaceous earth) is the most widely used material. It has a voidage of approxitnately 0.85, and when it is mixed with the initial cell suspension, improves the porosity of a resulting filter cake leading to a faster flow rate. Alternatively it may be used as an initial bridging agent in the wider pores of a filter to prevent or reduce blinding. The term ‘blinding’ means the wedging of particles which are not quite large enough to pass through the pores, so that an a fraction of the filter surface becomes inactive. The minimum quantity of filter aid to be used in filtration of a broth should be established experimentally. Kieselguhr is not cheap, and it will also absorb some of the filtrate, which will be lost when the filter cake is disposed.

The main methods of using the filter aid are:

1. A thin layer of kiesetguhr is applied to the filter to form a precoat prior to broth filtration.

2. The appropriate quantity of filter aid is mixed with the harvested broth. Filtration is started, to build up a satisfactory filter bed. The initial raffinate is returned to the remaining broth prior to starting the true filteration.

3. When vacuum drum filters are to be used which are fitted with advancing knife blades, a thick precoat filter is initially built up on the drum.

In some processes such as microbial biomass production, filter aids cannot be used and cell pretreatment by flocculation or heating must be considered. In addition it is not normally practical to use filter aids when the product is intracellular and its removal would present a further stage of purification.