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Introduction, Literature Review and Objectives

1.2 Pressure driven membrane technology

Pressure-driven membrane separation processes are those where pressure is applied across the membrane to allow fluid along with the desired solutes to pass through the membrane, undesired ones being retained on the membrane surface to different extents, completely dependent on the structure of the membrane (Mulder, 1996).

Fig. 1.1 Cross flow and dead-end filtration mode

The application of external pressure creates a steady permeate flow across the membrane.

Pressure-driven membrane separation processes can be operated in two different modes,

namely dead-end filtration and cross flow filtration (Cui et al., 2010). In the dead-end filtration mode, the feed is allowed to enter perpendicular to the membrane surface, while in cross flow filtration mode, the feed entry is done tangentially (Fig. 1.1). Application of feed perpendicular to the membrane surface is, however, detrimental to the membrane’s long-term performance as it leads to pronounced effect of concentration polarization, thus reducing the permeate flux across the membrane. In case of cross flow filtration, the application of shear force due to tangential movement of feed reduces the concentration polarization to a significant level (Van der Bruggen, 2018). Besides the mode of application, pressure-driven membrane processes can also be categorized differently into the following categories based on their ability to retain solute particles (Fig. 1.2).

Fig. 1.2 Pressure driven membrane separation processes

Microfiltration: In this process, external pressure is applied for separating macromolecules of sizes greater than 0.1 µm(Koros et al., 1996).

Ultrafiltration: This process is applied for the separation of particles as well as dissolved macromolecules within the size range of 0.1 µm to 2 nm (Koros et al., 1996).

Nanofiltration: Nanofiltration is performed the same way as the above two categories;

however, the size range of particles to be separated here is lesser than 2 nm (Koros et al., 1996).

Reverse osmosis: Reverse osmosis corresponds to those membrane separation processes, where an applied transmembrane pressure causes selective movement of solvent across the membrane against the osmotic pressure difference (Koros et al., 1996). This is the most sophisticated pressure-driven membrane filtration technology as it can block the passage of all suspended solids, colloid, dissolved solids and organic matter, having a molecular weight greater than 100 (Youcai, 2018).

It should be kept in mind that the growing importance for the pressure-driven membrane processes is because of the inherent advantages associated with these processes. The benefits of pressure-driven membrane processes can be summarized in the following points:

Longer lifetime: Pressure-driven membrane separation processes offer great durability with a service life of up to several years, if maintained properly. Durability of such processes makes it suitable for industrial use (Díez and Rosal, 2020).

Easy handling: These processes provide a simpler and easier way of installation as well as handling of the filtration apparatus, with minimal manual intervention.

Moreover, these processes can easily and efficiently be scaled up as and when needed.

Operational ease makes the pressure-driven membrane separation technologies one of the most sought-after technologies among people (Díez and Rosal, 2020).

Higher recovery: Aforementioned processes are known to possess high productivity and selectivity towards both organic as well as inorganic contaminants (Suwaileh, 2020). Based on the inherent size characteristics of the contaminant to be removed, the selection of membrane with optimum pore size can be made for a particular separation

process, which not only helps in achieving higher yield, but also results in a reduction in energy consumption during the process (Gwak and Hong, 2018).

Owing to the number of benefits offered by the pressure-driven membrane technology, it has been widely implemented in various separation applications in different industries. Some of the potential users of this membrane technology are being mentioned in the following text.

Food and dairy industry: The widescale application of membrane technology in food industries can be seen in clarification of fruit and vegetable juice using microfiltration and ultrafiltration membranes. Similarly, in beer industry, membranes are used for recovering the maturation and fermentation tank bottoms. Starch industry also uses membranes for treating the large quantities of wastewater generated during starch processes along with subsequent recovery of starch (Ikonić et al., 2010). In dairy industries, membrane technology has found immense application in the removal of bacteria and vegetative spores from milk. Moreover, membranes are also being extensively used in processes such as whey protein concentration and milk protein standardization in the dairy industry (Daufin et al., 2001).

Textile industry: The wastewater generated from different operations carried out in textile industries contains huge quantities of harmful constituents such as dyes, different salts, heavy metals, surfactants, just to name a few. For instance, the wastewater generated during the scouring process contains oil and grease in emulsified form, the removal of which can be challenging using conventional techniques. In such a situation, membrane filtration can serve the purpose very efficiently. Similarly, the utilization of membrane technology can also be seen in other textile industry processes such as latex recovery, dye recovery, recovery of salt from dyestuffs and dye baths, and so on (Giwa and Ogunribido, 2012).

Pharmaceutical industry: The increased concentration of pharmaceutically active compounds as well as endocrine disrupting agents in wastewater generated in different stages of the pharmaceutical industry has become a matter of concern now-a-days and membrane technology is seen to have successfully addressed this issue. Besides, membranes are also being used in recovering antibiotics and isolation as well as purification of different biologically active compounds such as enzymes and viruses from pharmaceutical industry wastewater (Samaei et al., 2018).

Petroleum industry: Petroleum industries can also be considered as one of the significant implementors of membrane technology. The wastewater generated in petroleum industries contains very high oil concentrations and microfiltration operations play a great role in treating such wastewater. Besides, membranes have also found their application in the treatment of produced water obtained during the process of drilling (Padaki et al., 2015; Alzahrani and Mohammad, 2014).

Implementation of pressure driven membrane technology in above-mentioned processes is primarily dependent on the size of the solute to be separated from its solvent. Membranes with pore size lower than the size of the solute are quite effective in achieving satisfactory separation efficiency. However, it should be kept in mind that among the aforementioned four pressure- driven membrane separation processes, microfiltration is being practiced to a much greater extent than the rest three processes. It is worth to mention that microfiltration has the largest industrial market within the field of membrane technology itself and is responsible for almost 40% of total sales in Europe and USA (Huisman, 2000). Low pressure requirement, lesser energy consumption, cost-effectiveness, simpler operation and the ability to separate contaminants possessing a wide range of molecular diameter have definitely fuelled in the tremendous development of microfiltration membranes since 1960 (Huisman, 2000; Díez and Rosal, 2020).