Advanced Membrane Based Water Treatment Specialists
Membrane treatment is one of the main technologies incorporated into our high performance water treatment solutions. With decades of experience and thousands of systems in place, our designers draw upon the very latest in membrane treatment technologies to create the best water treatment system for your application.
Membrane Filtration Overview
A membrane is a thin layer of semi-permeable material that separates substances when a driving force is applied across the membrane. Membrane processes are increasingly used in water treatment for removal of bacteria, microorganisms, particulates, and natural organic material, all of which can impart color, tastes, and odors to water and react with disinfectants to form disinfection by-products (DBPs). The four main membrane processes discussed here are:
- Microfiltration (MF)
- Ultrafiltration (UF)
- Nanofiltration (NF)
- Reverse Osmosis (RO)
The following graphic illustrates the ability of each type of membrane filtration process to remove various types of contaminants based on their size and molecular weight. While all of these membrane processes function as effective filtration barriers to a wide range of organic and inorganic contaminants, how they each physically operate and function varies greatly as is discussed below in more detail.
By way of historical background, osmotic type membrane filters have been used for several decades in North America with widespread use of reverse osmosis (RO) membrane filters emerging in the early 1960s in seawater desalination applications, followed by nanofiltration (NF) membrane applications in the 1980s centered on water softening and the removal of total organic carbon (TOC). Use of the more common hollow fiber microfiltration (MF) and ultrafiltration (UF) membrane filters for removal of suspended solids ( turbidity ) and microorganisms has been going on since first introduced the early 1900s. Widespread use of MF and UF membranes for wastewater treatment really took off in the 1980s with the advent and application of submersible membranes in membrane bioreactors (MBRs), which greatly improved the overall efficiency and economics of MBRs. The conventional approach to treating drinking water in North America involves a series of processes that may include physical and chemical removal of contaminants by coagulation, flocculation, sedimentation and filtration processes, followed by some form of disinfection to neutralize microbial pathogens. Granular media bed filters are by far the most widely used filtration process for the treatment of potable water sources and have evolved over their continued use for more than 100 years. While these conventional filtration technologies can produce high quality water, they are less effective than membrane filtration systems in removing certain pathogenic organisms, dissolved solids (TDS), suspended solids (TSS) and dissolved organic carbon (DOC) contaminants. Recent regulatory concerns over chlorine resistant microbes, such as Cryptosporidium, and the role certain types of dissolved solids play in the formation of disinfection by-products (DBPs) in drinking water , have resulted in new regulations that limit maximum contaminant levels (MCLs) for DBPs in many jurisdictions. Similarly, concern over the quality of wastewater effluent and its impact on the environment has resulted in tighter regulations enforced on key effluent parameters, such as biological oxygen demand (BOD), phosphorus, ammonia and TSS. Due to the stricter regulations and ongoing advancements in membrane design and operating efficiencies, the adoption of membrane filtration technology is rapidly increasing within the water treatment industry at large.
Basic Principals of Membrane Filtration
The four main membrane filtration processes employed in water treatment systems are reverse osmosis (RO), ultrafiltration (UF), nanofiltration (NF), and microfiltration (MF). In addition, cartridge type filters, termed membrane cartridge filtration (MCF) devices, are often used to pre-filter suspended solids ahead of tighter RO and UF membrane filtration units. Each of these technologies utilizes a membrane barrier that allows the passage of water but rejects contaminants. The membrane material is manufactured as flat sheets or as hollow fibers and then arranged into membrane modules. The most common types of membrane modules are:
- Hollow-fiber: multiple tubular fibers used in MF and UF membranes
- Spiral-wound membranes: a flat sheet membrane material wrapped around a central collection tube and used in RO and NF membranes
- Cartridges: employs a flat sheet pleated membrane material
Ultrafiltration and Microfiltration
UF and MF membrane filters have been in use since the early 1900s. They are widely used in the removal of colloidal and other suspended solids (TSS) from raw water sources. Characterised by membrane pore size measured in microns (µm) and/or by molecular weight cut off (MWCO) measured in Daltons, a unit based on the atomic weight of a contaminant, membranes with a MWCO rating reject compounds with a molecular weight greater than the MWCO rating. The particles rejected by MF membrane filters typically range from 0.1 to 10 microns (µm) with a MWCO of greater than 100,000 daltons. This means that these membranes prevent relatively large, molecularly heavy particles such as sediment, algae, protozoa or bacteria from passing through the specially designed filter membrane. Other microscopic materials, including ionic monovalent species, dissolved organic matter, small colloids and viruses, will still be able to pass through the MF membrane because they are smaller than the membrane’s pore size. These systems operate at relatively low pressure of 15 to 60 psi and come in a variety of configurations. UF membranes on the other hand are much tighter than MF membranes and have the ability to reject smaller organic macromolecules. As such, UF membranes are often characterized by MWCO rather than by pore size. Typical MWCOs for UF membranes range from 10,000 to 500,000 Daltons. By way of comparison, pore sizes for UF membranes generally range from .01 to .05 µm. While excellent at removing suspended and colloidal particles, UF and MF membrane filtration modules will still allow many dissolved solids, such as monovalent and polyvalent ions or dissolved organic carbon contaminants, to pass through the filter.
Reverse Osmosis and Nanofiltration
As the tightest of all classes of membrane filtration modules, the typical MWCO range for RO membranes is less than 100 Daltons. For NF, the MWCO ranges between 200 and 1000 Daltons. Both NF and RO utilize semipermeable membranes as opposed to porous membranes and remove contaminants through the process of reverse osmosis. Osmosis is the natural flow of water across a semipermeable membrane, moving from a solution of low concentration through the membrane into a more concentrated solution. The flow will continue until the concentrations on each side of the semipermeable membrane reach a state of equilibrium. Osmotic pressure is the amount of pressure that must be applied to the concentrated side of the membrane in order to stop the natural flow of water. Reverse osmosis, as the name implies, involves reversing the natural osmotic flow by applying enough pressure to the concentrated solution to overcome the osmotic pressure and to force water to flow in reverse from the more concentrated side to the less concentrated side of the membrane. The amount of pressure required to reverse the flow varies with the Total Dissolved Solids (TDS) concentration of the solution. The required pressure can range from less than 100 psi for some NF applications, to over 1000 psi in RO desalination processes.
Figure 4 Figure 5
The filtered water that defuses through the semipermeable membrane is termed permeate or filtrate. Water remaining on concentrated side of the membrane is termed concentrate. The performance of RO and NF systems is measured by the recovery rate, which is the percentage of permeate generated from the feed water being treated. Recover rates can range from 50% up to 90% with the typical municipal drinking water RO facility producing in the 75% recovery rate range. Apart from operating at lower pressures, NF membranes differ from RO membranes because of their inability to remove certain types of TDS such as monovalent ions. NF membranes are commonly used for the removal of hardness ions, which are cations with a charge of +2 (i.e. Ca2+ and Mg2+). In essence, membrane filtration is an effective means of treating, potable water, wastewater and industrial process water. Each type of membrane filtration process has unique characteristics. The type and configuration of membrane filtration systems varies widely and is based on the feed water composition as well as the required water quality standard specified.
In 2012, global demand for membrane modules was estimated to be close to $15.6 billion USD and is projected to surge to 25 billion by 2018. Industry growth continues to be driven by ongoing advancements in the design and efficiency of membrane modules, stricter water quality regulations and the urgent need to upgrade water treatment infrastructure in many parts of the world.