Filtration: process and types

Sistemi filtranti - Spadoni

In recent decades, modern plant engineering has acquired ever-increasing importance in innovation in the food sector, implementing automation processes in order to optimize plant productivity and profitability.

The combination of plant engineering companies and food companies has become increasingly consolidated, following the growth path of many production realities, parallel to the need to maintain high quality standards of the products and guarantee a “shelf life” suitable for the expansion of the markets. 

One of the sectors where research was most concentrated was the Filtration sector. The need to “separate and filter” can be defined as a constant in all production and/or packaging processes in the food sector and is performed with a wide range of systems that offer wide-ranging solutions in the most varied conditions. 

What is the filtration process?

The filtration is a physical-mechanical operation with which a moving liquid, under the action of a pressure gradient, separates from the solid particles dispersed in it., due to their retention by the porous filter medium, through which the liquid is passed.      The pressure difference determines the passage of the liquid to be filtered through the filter layer. It can be achieved both by applying a pressure upstream of this layer (ie on the incoming liquid), and by creating a depression downstream.

In these processes, particles with dimensions smaller than 0.1 μm they cannot be separated by means of the aforementioned filtration techniques; in fact, below this limit there are macromolecules or aggregates of molecules which form colloidal dispersions. The physical formation of these particles can be achieved by means of other techniques conceptually similar to filtration, including: microfiltration (MF), ultrafiltration (UF), nano-filtration (NF) and reverse osmosis (OI).

Mainly 3 types of filtration can be distinguished:
  1. of surface;
  2. of depth;
  3. with deposit.
Surface filtration

In this type of filtration, solid particles with a size larger than the pore size of the filter medium are stopped on its surface by simple sieving. The filter medium used can be either a cloth or a membrane. The retention mechanism that is put in place is precisely called “sieving.” Membrane filtration, given the characteristics of the filter medium, is applied to suspensions with a very low percentage of suspended solids and having very small dimensions.

Depth filtration

This technique is adopted for the treatment of fluids with a low concentration of suspended solids and consisting of very fine particles. In this case the solids follow a tortuous path inside the empty interstices of the filter medium, then are retained and removed from the continuous liquid phase.

The substantial difference between these two types of processing is given by the active filtration surface for the retention of solids, which in depth filtration develops throughout the internal area of the capillaries, while in sieving filtration, it consists of the external surface of the filter medium. Therefore the contact surface with the suspended solid particles in depth filters is clearly higher than that of surface filters.

Filtrazione di superficie e di profondità
Principle of action of surface filtration (a) and depth (b).
Filtration with deposit

instead, it relies on the deposition of a filter aid on a supporting structure. In this case there is a simultaneous surface and depth retention action. If the solid particles present in suspension are all of the same size, the prevailing retention mechanism is sieving, otherwise, depth filtration prevails, especially with the presence of very fine particles.

The use of adjuvants allows to apply the deposit accumulation technique and therefore to carry out a clarifying filtration. In these cases we operate with the continuous flooding technique, which implies the addition or rather the “continuous dosage” of filtration aids (such as diatomaceous earth, perlite, cellulose fibres, etc.) in the suspension to be filtered. Usually, together with flooding, another method of using adjuvants is used, which involves the creation of a prepanel which prevents the finest particles from clogging the support. The prepanel is designed to deposit a layer of auxiliary material on the support, making it circulate mixed with water in a cycle that precedes filtration. In these cases the prepanel acts as a real filtering medium, for example in the first roughing filtration of wines.

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1. Assoenologi: “La filtrazione“, L’Enologo, N°4 2020, pp 55-58. 

2. Antonio Zapulla: “Filtrazione tangenziale con membrane ceramiche statiche e dinamiche”. University of Bologna, 2017.

3. Michele Sensidoni: “Ricerca sulle caratteristiche di filtrabilità della birra e ottimizzazione del processo dil filtrazione”. PhD, University of Bologna, 2012