Principle Microbiology

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.