Characteristics of Membranes
Membranes for reverse osmosis and ultrafiltration have been the first types available for application in the dairy industry, whereas the NF and MF types have been developed starting from the 1980s. As regards the materials, cellulose acetate membranes were the first to be produced on industrial scale. Their susceptibility to extreme pH conditions was a major issue that caused very short lifetime and limited economic sustainability. Nowadays, two categories of membranes are used for dairy applications: organic (polymeric) and inorganic (ceramic). Polymeric types are made of different materials (i.e., polyvinylidenefluoride, polypropylene, polysulfone, polyacrylonitrile, polyvinyl chloride), and their main point of strength is the wide spectrum of pore size, which allows a number of applications. Ceramic membranes are made of aluminum, titanium, and zirconium oxides, are highly resistant to heat and chemicals, are easily cleanable and have longer shelf life than polymeric. On the other hand, they are more expensive and can be used only for microfiltration and, to a lesser extent, for ultrafiltration.
Membranes are allocated within modules that can have different configuration: plate and frame, tubular, spiral wound, and hollow fiber. The plate and frame is the simplest and oldest configuration: it consists of a stack of plates and flat sheet membranes fixed together as a sandwich. Spiral-wound configuration is the most used in the dairy industry and is the cheapest type: It is mainly used in nanofiltration and reverse osmosis. A spiral-wound module is made of two membranes along with spacers wrapped around a perforate permeate-collecting tube; as a consequence, the filtration process takes place from the outer to the inner part of the module.
In tubular and hollow fiber configurations, by contrast, filtration takes place from the inner to the outer part of the system. In tubular type the membrane is fitted on the inner wall of a tube that serves as a supporting structure, whereas in hollow fiber modules the membrane is self-supporting. Tubular configuration is a characteristic of ceramic membranes (Sincero & Sincero, 2003; Fox et al., 2004). Membrane properties can be resumed into three main aspects: morphology (characteristics of the porous structure), performance (behavior during filtration), and lifetime (susceptibility to fouling and resistance to cleaning). Morphology-related parameters are pore size, shape and distribution, membrane thickness, and charge density. Performance is strictly connected to membrane morphology and can be essentially expressed as permeability and selectivity. Lifetime is mainly determined by the rate at which the membrane surface and pores are clogged by insoluble materials. The phenomenon is called fouling, and in milk processing all compounds can act as foulant agents, depending on the type of filtration considered. However, it is generally accepted that proteins and minerals account for the majority of foulants in dairy membrane operations (Marshall and Daufin, 1995). Fouling prolongs processing times, increases energy and decreases separation efficiency, and, in severe cases, may lead to irreversible clogging of the membrane (Brans et al., 2004). Implementation of strategies for fouling reduction and effectiveness of the cleaning procedures are two decisive aspects that can directly influence the economic management of the whole process.