Membrane filtration usage in waste water treatment
About membrane filtration
Membrane filtration is a process used to remove microorganisms or particles from suspension. Plainly speaking membranes form a barrier for some substances and allow others to pass through. The process is pressure or vacuum-driven and uses a layer of porous material to remove particles from suspension. Membrane separation operates without heating which saves energy compared to other methods. The separation process is purely physical and both fractions (the permeate and the retentate) can be used for further processes.
Overall, there are four main membrane types: Microfiltration, Ultrafiltration, Nanofiltration, and Reverse osmosis. The choice of membrane, and thus the pore size, depends on the filtration purpose, which can be disinfection, desalination, organic removal, or softening of water and wastewater. The membrane types can be divided into two subcategories:
1. Low pressure operations: Microfiltration (0,1µm pores) and Ultrafiltration (0,01µm pores)
2. High pressure operations: Nanofiltration (0,001µm pores) and Reverse osmosis (<0,001µm)
Treatment of industrial wastewater
Membrane technology is widely used in many industries, from the food industry to pharmaceutical industry and the metal finishing industry.
Membranes are very flexible and can be used as both a pre-treatment or as the main source of treatment. Furthermore, both the permeate and the retentate can be re-used for process purposes.
As an example membrane filtration is used as a pretreatment for perkolat in one of Aquardens projects, as showed in the picture below. In this example the nanofiltration is used to remove particles from the stream before the stream is treated with ion exchange and SCWO. Reverse osmosis is used to create soft water, or in combination with ion exchange to get demineralised water.
Advantage of membrane filtration:
1. Flexibility – Membrane filtration covers a wide range and applications. Can be used both as a pre- and main-treatment for suspensions.
2. Low energy requirements compared to other technologies, which results in a lower cost per cubic meter treated
3. Non-chemically, meaning that the process is free of additives. It can be necessary to clean in place (CIP) using chemicals, but it’s a small amount of waste.
Size exclusion is the dominant filtration mechanism in membrane filtration. Particles larger than the retention rating of the membrane collect at the surface, while water, a smaller particle passes through. Therefore, it is necessary to choose the right filtration for the task at hand:
Membrane filtration in action where larger particles are physically retained due to the pore size
Adsorption, which occurs when material small enough enter the pores and adsorbs to the walls of the pores
Cake formation, which occurs as the membrane filters over time. A cake layer is formed from larger particles blocking the pores. At a point even the particles small enough to pass through are retained by the cake layer.