Chapter 5 Microfiltration A.S. GRANDISON, Department of Food Science and Technology, The University ot Reading, Reading RG6 6AP and T J. A FINNIGAN, Marlow Foods, Middlesbrough 5.1 INTRODUCTION Microfiltration (MF) is the oldest membrane technology, having been used several decades before the first industrial use of reverse osmosis(Glimenius, 1985). However, subsequent development of the technology has been slow. Until recently microfilters were operated in the dead-end mode and were exclusively of the depth-filter type which particles become trapped within the filter structure, but recent developments have led to membrane-type microfilters, with a narrow pore size distribution, which can be operated in the cross-flow mode. This has led to an increase in possible applications, ncluding clarification of fluids in the food and beverage industries, recovery of cells and cell debris in the biotechnology industries, and the treatment of wastes 5.2 THEORY, MATERIALS AND EQUIPMENT Like ultrafiltration (UF), MF is a pressure-driven process employing pressures consider- ably lower than reverse osmosis. In fact the distinction between U MF is Somewhe arbitrary and there is no distinction on purely theoretical grounds. The distinction lies in the size ranges of the materials which are separated. UF is considered to involve the processing of dissolved macromolecules, while MF involves separation of dispersed articles such as colloids, fat globules or cells. MF can be considered to fall between UF and conventional filtration, although there is overlap at both ends of the spectrum. A guide to the pore sizes used for MF could be 0.01-10 um 2. For many years, MF has been applied as a dead-end operation using highly microporous symmetric membranes of the depth-filter type. Such membranes retain particles and consequently result in the build-up of a filter cake. This reduces flow, and when the pressure drop has reached a certain level the membrane must be replaced or removed and regenerated. In addition, the presence of a filter cake radically alters the filtering characteristics, effectively acting as a prefilter which removes particles whichChapter 5 Microfiltration A. S. GRANDISON, Departrnent of Food Science and Technology, The University of Reading, Reading RG6 6AP and T. J. A. FINNIGAN, Marlow Foods, Middlesbrough. 5.1 INTRODUCTION Microfiltration (MF) is the oldest membrane technology, having been used several decades before the first industrial use of reverse osmosis (Glimenius, 1985). However, subsequent development of the technology has been slow. Until recently microfilters were operated in the dead-end mode and were exclusively of the depth-filter type in which particles become trapped within the filter structure, but recent developments have led to membrane-type microfilters, with a narrow pore size distribution, which can be operated in the cross-flow mode. This has led to an increase in possible applications, including clarification of fluids in the food and beverage industries, recovery of cells and cell debris in the biotechnology industries, and the treatment of wastes. 5.2 THEORY, MATERIALS AND EQUIPMENT Like ultrafiltration (UF), MF is a pressure-driven process employing pressures consider￾ably lower than reverse osmosis. In fact the distinction between UF and MF is somewhat arbitrary and there is no distinction on purely theoretical grounds. The distinction lies in the size ranges of the materials which are separated. UF is considered to involve the processing of dissolved macromolecules, while MF involves separation of dispersed particles such as colloids, fat globules or cells. MF can be considered to fall between UF and conventional filtration, although there is overlap at both ends of the spectrum. A guide to the pore sizes used for MF could be 0.01-10 pm. For many years, MF has been applied as a dead-end operation using highly microporous symmetric membranes of the depth-filter type. Such membranes retain particles and consequently result in the build-up of a filter cake. This reduces flow, and when the pressure drop has reached a certain level the membrane must be replaced or removed and regenerated. In addition, the presence of a filter cake radically alters the filtering characteristics, effectively acting as a prefilter which removes particles which