Different membrane processes are suited for a wide range of applications
Membrane separation processes are used for the treatment of municipal water and wastewater. In these processes, thin layers of semipermeable material (membranes) are implemented to filter out pathogens and contaminants from water as it’s forced through their tiny pores. There are several types of membranes, primarily differentiated by their pore sizes, and each has its own advantages and drawbacks.
Microfiltration (MF) and ultrafiltration (UF) remove suspended solids using low-pressure membranes in aeration tanks with the aid of a vacuum system, or in external pressure-driven membrane units that can supplant secondary clarifiers and tertiary filtration.
MF membranes have a pore size of approximately 0.03 to 10 microns, and can remove:
- Sand, silt, and clay.
- Parasites like giardia lamblia and cryptosporidium cysts.
- Some bacterial species.
Microfiltration typically is used as a pretreatment stage for other pressure-driven membrane processes like reverse osmosis (RO) and nanofiltration (NF) to reduce membrane fouling.
Ultrafiltration, on the other hand, uses dense structure membranes with pore sizes of 0.002 to 0.1 microns. It can remove particles, viruses, and microorganisms, and it is more effective for removal of smaller particles than MF at the submicron and colloidal sizes. It can remove dissolved substances, such as proteins and polysaccharides.
Ultrafiltration membrane processes have a number of advantages over conventional clarification and disinfection (post-chlorination) processes. They only require a small physical plant footprint, and automation is relatively simple. The addition of chemicals (coagulants, flocculants, disinfectants, pH adjustment) are not required, and the process provides a consistent level of particle and microbial removal.
Nanofiltration membranes have a nominal pore size of about 0.001 microns to separate dissolved substances such as ions and small molecules. Frequently, this technology is utilized to soften water and remove color, and organic carbon, however, it is rarely used for municipal wastewater treatment processes.
Reverse osmosis uses a partially permeable membrane that remove dissolved substances, including salts and minerals. It is widely used for brackish and seawater desalination, drinking water purification, and for industrial water treatment applications.
Membrane bioreactor (MBR) technology deserves a mention in a discussion of membrane technologies as it combines membrane processes like microfiltration and ultrafiltration with a suspended-growth bioreactor. The combination can produce a very high-quality effluent suitable for discharge in coastal or surface waterways or for reuse in urban irrigation. MBR also enables an easy retrofit and offers more treatment capacity with lower energy requirements. Typically, MBR is followed by reverse osmosis and disinfection stages.
Municipal Membrane Processes
Microfiltration and ultrafiltration are relatively inexpensive and easy to operate, but they are not as effective for the removal of dissolved substances to the extent of nanofiltration and reverse osmosis. On the other hand, nanofiltration and reverse osmosis are very effective at removing dissolved substances, but they require higher pressures and can be more expensive to operate and maintain.
For municipal water and wastewater treatment, membrane processes are known for their high efficiency and small footprint. Because the pressure-driven membrane separation technologies used in municipal wastewater treatment have such different abilities, two or more of them are generally used in each plant, but, for most wastewater treatment plants, microfiltration and ultrafiltration are the most common membrane option.
Membrane processes can, however, require substantial capital to establish, which has limited their adoption in the United States. But, Seven Seas Water Group has pioneered Water-as-a-Service® (WaaS®) infrastructure delivery, which can help. WaaS® provides in-house financing for projects with bundled long-term operations and maintenance. Municipal utilities with limited access to capital or organizational resources to build, operate, and maintain technically demanding membrane systems can still reap their benefits. Contact Seven Seas to learn about upgrades to our full complement of wastewater treatment technologies.