Principle Microbiology

Cross-flow filtration (tangential filtration)

In the filtration processes previously described, the flow of broth was perpendicular to the filtration membrane.
Consequently, blockage of the membrane led to lower rates of productivity and/or the need for filter aids to be added, and these were serious disadvantages.
In contrast, an alternative which is rapidly gaining prominence both in the processing of whole fermentation broths and cell lysates is cross-flow filtration.
Here, the flow of medium to be filtered is tangential to the membrane, and no filter cake builds up on the membrane.

The benefits of cross-flow filtration are:

(a) Efficient separation : 99.9% cell retention.
(b) Closed system : For the containment of organisms with no aerosol formation.
(c) Separation is independent of cell and media densities, in contrast to centrifugation.
(d) No addition of filter aid.

(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.

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.

The simple filtration apparatus is illustrated which consists of a support covered with a porous filter cloth. A filter cake gradually builds up as filtrate passes through the filter cloth. As the filter cake increases in thickness the resistance to flow will gradually increase. Thus, if the pressure applied to the surface of slurry is kept constant the rate of flow will gradually diminish. Alternatively, if the flow rate is to be kept constant the pressure will gradually have to be increased. The flow rate may also be reduced by blocking of holes in the filter cloth and closure of voids between particles, if the particles are soft and compressible. When particles are compressible it may not feasible to apply increased pressure.