Sterilization of air: Methods

Most industrial fermentations are operated under vigorous aeration and the air supplied to the fermenter must be sterilized.

To prevent contamination of either the fermentation by airborne.

Microorganisms or the environment by aerosols generated within the fermenter, both air input and air exhaust ports have air filters attached.

Suspended solids may be separated from a fluidduring filtration by the following mechanisms:

(j) Inertial impaction.

(ii) Diffusion.

(iii) Electrostatic attraction.

(iv) Interception.

Advantages of continuous sterilization over batch Sterilization

(i) Superior maintenance of medium quality.(ii) Ease of scale-up - discussed later.(iii) Easier automatic control.(iv) The reduction of surge capacity for steam.(v) The reduction of sterilization cycle time.(vi) Under certain circumstances, the reduction of

fermenter corrosion.

Advantages of batch sterilization over continuous sterilization

(i) Lower capital equipment costs.

(ii) Lower risk of contamination - continuous processes require the aseptic transfer of the sterile broth to the sterile vessel.

(iii) Easier manual control.

(iv) Easier to use with media containing a high proportion of solid matter.

Filters and design of depth filters

• Filtration is one of the most common processes– used at all scales of operation – to separate suspended particles – from a liquid or gas, – using a porous medium called filter – which retains the particles – but allows the liquid or gas when pass through filter.

The theory of depth filters:

Aiba et al. (1973) have given detailed quantitative analysis of these mechanisms but this account will be limited to a description of the overall efficiency of operation of fibrous filters.

It is assumed that if a particle touches a fibre it remains attached to it, and that there is a uniform concentration of particles at any given depth in the filter, then each layer of a unit thickness of the filter should reduce the population entering it by the same proportion; which may be expressed mathematically as:

dN/dx = -KN 1• Where,

• N is the concentration of particles in the air at a depth, x, in the filter and

• K is a constant.

On integrating equation (1) over the length of the filter it become:

N/No = e-kx ------------2where No is the number of particles entering the filter and N is the number of particles leaving the filter.On taking natural logarithms, equation (2) becomes: In (N / No) = -Kx. ----------3Equation (3) is termed the log penetration

relationship.

• The efficiency of the filter is given by the ratio of the number of particles removed to the original number present, thus:

E = (No - N)/No --------4

where E is the efficiency of the filter.

(No - N)/No = 1 - (N / No). ------5

Substituting N/No = e-Kx ,Therefore

(No - N)/No 1 e-Kx -----6

and E = 1 - e-Kx

The log penetration relationship [equation (3)] has been used by Humphrey and Gaden (1955) in filter design, by using the concept X9O ' the depth of filter required to remove 90% of the total number of particles entering the filter; thus:

If No were 10 and x were X 9O ' then N would be 1:

In (1/10) = -KX90

or 2.303 10glO(1/IO) = - KX9O

2.303( -1) = -KX90

therefore,

X 90 = 2.303/K. ------ 7