A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Biological membranes include cell membranes (outer coverings of cells or organelles that allow passage of certain constituents); nuclear membranes, which cover a cell nucleus; and tissue membranes, such as mucosae and serosae. Synthetic membranes are made by humans for use in laboratories and industry (such as chemical plants).
This concept of a membrane has been known since the eighteenth century but was used little outside of the laboratory until the end of World War II. Drinking water supplies in Europe had been compromised by The War and membrane filters were used to test for water safety. However, due to the lack of reliability, slow operation, reduced selectivity and elevated costs, membranes were not widely exploited. The first use of membranes on a large scale was with microfiltration and ultrafiltration technologies. Since the 1980s, these separation processes, along with electrodialysis, are employed in large plants and, today, several experienced companies serve the market.
The degree of selectivity of a membrane depends on the membrane pore size. Depending on the pore size, they can be classified as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes.Membrane Classification: Typical Operating Values. Government of Newfoundland and Labrador. www.gov.nl.ca. Membranes can also be of various thickness, with homogeneous or heterogeneous structure. Membranes can be neutral or charged, and particle transport can be active or passive. The latter can be facilitated by pressure, concentration, chemical or electrical gradients of the membrane process.
Membrane processes classifications
Microfiltration (MF)
Microfiltration removes particles higher than 0.08–2 μm and operates within a range of 7–100 kPa. Microfiltration is used to remove residual suspended solids (SS), to remove bacteria in order to condition the water for effective disinfection and as a pre-treatment step for reverse osmosis.
Relatively recent developments are membrane bioreactors (MBR) which combine microfiltration and a bioreactor for biological treatment.
Ultrafiltration (UF)
Ultrafiltration removes particles higher than 0.005–2 μm and operates within a range of 70–700 kPa. Ultrafiltration is used for many of the same applications as microfiltration. Some ultrafiltration membranes have also been used to remove dissolved compounds with high molecular weight, such as proteins and carbohydrates. Also, they can remove viruses and some endotoxins.
Nanofiltration (NF)
Nanofiltration is also known as "loose" RO and can reject particles smaller than 0.002 μm. Nanofiltration is used for the removal of selected dissolved constituents from wastewater. NF is primarily developed as a membrane softening process which offers an alternative to chemical softening.
Likewise, nanofiltration can be used as a pre-treatment before directed reverse osmosis. The main objectives of NF pre-treatment are: (1). minimize particulate and microbial fouling of the RO membranes by removal of turbidity and bacteria, (2) prevent scaling by removal of the hardness ions, (3) lower the operating pressure of the RO process by reducing the feed-water total dissolved solids (TDS) concentration.
Reverse osmosis (RO)
Reverse osmosis is commonly used for desalination. As well, RO is commonly used for the removal of dissolved constituents from wastewater remaining after advanced treatment with microfiltration. RO excludes ions but requires high pressures to produce deionized water (850–7000 kPa). RO is the most widely used desalination technology because of its simplicity of use and relatively low energy costs compared with distillation, which uses technology based on thermal processes. Note that RO membranes remove water constituents at the ionic level. To do so, most current RO systems use a thin-film composite (TFC), mainly consisting of three layers: a polyamide layer, a polysulphone layer and a polyester layer.Alfonsín C, Lebrero R, Estrada JM, et al. (2015) Selection of odour removal technologies in wastewater treatment plants: A guideline based on life cycle assessment. Journal of Environmental Management 149: 77–84.
Nanostructured membranes An emerging class of membranes rely on nanostructure channels to separate materials at the molecular scale. These include carbon nanotube membranes, graphene membranes, membranes made from polymers of intrinsic microporosity (PIMS), and membranes incorporating metal–organic frameworks (MOFs). These membranes can be used for size selective separations such as nanofiltration and reverse osmosis, but also adsorption selective separations such as olefins from paraffins and alcohols from water that traditionally have required expensive and energy intensive distillation.
Membrane configurations In the membrane field, the term module is used to describe a complete unit composed of the membranes, the pressure support structure, the feed inlet, the outlet permeate and retentate streams, and an overall support structure. The principal types of membrane modules are:
Tubular, where membranes are placed inside a support porous tubes, and these tubes are placed together in a cylindrical shell to form the unit module. Tubular devices are primarily used in micro- and ultrafiltration applications because of their ability to handle process streams with high solids and high viscosity properties, as well as for their relative ease of cleaning.
Hollow fiber membrane, consists of a bundle of hundreds to thousands of hollow fibers. The entire assembly is inserted into a pressure vessel. The feed can be applied to the inside of the fiber (inside-out flow) or the outside of the fiber (outside-in flow).
Spiral wound, where a flexible permeate spacer is placed between two flat membranes sheet. A flexible feed spacer is added and the flat sheets are rolled into a circular configuration. In recent developments, surface patterning techniques have allowed for the integration of permeable feed spacers directly into the membrane, giving rise to the concept of an integrated membrane
Plate and frame consist of a series of flat membrane sheets and support plates. The water to be treated passes between the membranes of two adjacent membrane assemblies. The plate supports the membranes and provides a channel for the permeate to flow out of the unit module.
Ceramic and polymeric flat sheet membranes and modules. Flat sheet membranes are typically built-into submerged vacuum-driven filtration systems which consist of stacks of modules each with several sheets. Filtration mode is outside-in where the water passes through the membrane and is collected in permeate channels. Cleaning can be performed by aeration, backwash and CIP.
Membrane process operation The key elements of any membrane process relate to the influence of the following parameters on the overall permeate flux are: The membrane permeability (k) The operational driving force per unit membrane area (Trans Membrane Pressure, TMP) The fouling and subsequent cleaning of the membrane surface.
Flux, pressure, permeability The total permeate flow from a membrane system is given by following equation:
Where Qp is the permeate stream flowrate [kg·s−1], Fw is the water flux rate [kg·m−2·s−1] and A is the membrane area [m2]
The permeability (k) [m·s−2·bar−1] of a membrane is given by the next equation:
The trans-membrane pressure (TMP) is given by the following expression:
where PTMP is the trans-membrane pressure [kPa], Pf the inlet pressure of feed stream [kPa]; Pc the pressure of concentrate stream [kPa]; Pp the pressure if permeate stream [kPa].
The rejection (r) could be defined as the number of particles that have been removed from the feedwater.
The corresponding mass balance equations are:
To control the operation of a membrane process, two modes, concerning the flux and the TMP, can be used. These modes are (1) constant TMP, and (2) constant flux.
The operation modes will be affected when the rejected materials and particles in the retentate tend to accumulate in the membrane. At a given TMP, the flux of water through the membrane will decrease and at a given flux, the TMP will increase, reducing the permeability (k). This phenomenon is known as fouling, and it is the main limitation to membrane process operation.
Dead-end and cross-flow operation modes Two operation modes for membranes can be used. These modes are:
Dead-end filtration where all the feed applied to the membrane passes through it, obtaining a permeate. Since there is no concentrate stream, all the particles are retained in the membrane. Raw feed-water is sometimes used to flush the accumulated material from the membrane surface.Metcalf and Eddy (2004) Wastewater Engineering, Treatment and Reuse, McGraw-Hill Book Company, New York. Fourth Edition. Cross-flow filtration where the feed water is pumped with a cross-flow tangential to the membrane and concentrate and permeate streams are obtained. This model implies that for a flow of feed-water across the membrane, only a fraction is converted to permeate product. This parameter is termed "conversion" or "recovery" (S). The recovery will be reduced if the permeate is further used for maintaining processes operation, usually for membrane cleaning.
Filtration leads to an increase in the resistance against the flow. In the case of the dead-end filtration process, the resistance increases according to the thickness of the cake formed on the membrane. As a consequence, the permeability (k) and the flux rapidly decrease, proportionally to the solids concentration and, thus, requiring periodic cleaning.
For cross-flow processes, the deposition of material will continue until the forces of the binding cake to the membrane will be balanced by the forces of the fluid. At this point, cross-flow filtration will reach a steady-state condition , and thus, the flux will remain constant with time. Therefore, this configuration will demand less periodic cleaning.
Fouling
Fouling can be defined as the potential deposition and accumulation of constituents in the feed stream on the membrane. The loss of RO performance can result from irreversible organic and/or inorganic fouling and chemical degradation of the active membrane layer. Microbiological fouling, generally defined as the consequence of irreversible attachment and growth of bacterial cells on the membrane, is also a common reason for discarding old membranes. A variety of oxidative solutions, cleaning and anti-fouling agents is widely used in desalination plants, and their repetitive and incidental exposure can adversely affect the membranes, generally through the decrease of their rejection efficiencies.
Fouling can take place through several physicochemical and biological mechanisms which are related to the increased deposition of solid material onto the membrane surface. The main mechanisms by which fouling can occur, are:
Build-up of constituents of the feedwater on the membrane which causes a resistance to flow. This build-up can be divided into different types: Pore narrowing, which consists of solid material that it has been attached to the interior surface of the pores. Pore blocking occurs when the particles of the feed-water become stuck in the pores of the membrane. Gel/cake layer formation takes places when the solid matter in the feed is larger than the pore sizes of the membrane.
Formation of chemical precipitates known as scaling Colonization of the membrane or biofouling takes place when microorganisms grow on the membrane surface.
Fouling control and mitigation