Submerged and pressurized membrane filters for drinking water treatment:
Membrane filtration processes such as ultrafiltration and microfiltration can be operated either under negative (vacuum) pressure as in submerged membrane filter systems, or under positive pressure in pressure membrane filtering systems (EPA 2016).
Submerged membrane filters comprise of hollow membrane strands that are immersed in tanks containing raw water. A pump is used to create vacuum pressure that causes water molecules to be pulled into the hollow cores of the membrane strands thus separating the now purified water from the contaminants. These types of membrane filters are generally used to replace conventional water treatment processes such as chemical coagulation, sedimentation and gravel-sand filtration (Shubert 2008). In most submerged membrane filter systems, the operating pressure ranges between -3 to -12 psi (EPA 2016).
In positive pressure membrane filter systems, raw water is force-fed across the membrane pore using a pressure pump. In this case, the force-fed water goes through the membrane pores and comes out of the other side as purified water since the contaminants are unable to pass through the membrane pores (Martinez and O’Connell 2005). In essence, these systems work just like sieves where particles that cannot penetrate the sieve’s pores are filtered out. Positive pressure membrane filter systems usually operate at pressures ranging from 3 to 40 psi (EPA 2016).
While both pressure and submerged filters have the capability to deliver the inherent benefits associated with membrane technology, pressure based systems do have some advantages over submerged systems. First of all, pressure systems run at higher operating pressure ranges compared to submerged systems that normally operate at atmospheric pressure ranges. In this case, pressure systems have greater pressure reserves if upset conditions do occur in the treatment plant systems. This reserve pressure helps improve filtration system design safety and allows the system to deal easily with changes in the quality of influent water, process upsets or other unexpected changes in the plant’s operations (Martinez and O’Connell 2005). For example, under low temperature (winter) conditions, the pressure and viscosity required to push water through membrane pores increases and since the absolute vacuum is about -14.7 psi while the maximum operating pressure of most submerged systems is -12 psi. In this case, submerged membrane systems are usually designed to operate at significantly lower fluxes compared to pressure systems. Pressure systems have much wider differential pressure range (3-40 psi) that allows them to have consistent water flow even when the feed water has a low temperature. The robust nature of pressure systems i.e. operating at high-pressure ranges and working efficiently in low-temperature conditions makes them safer and more reliable when compared to submerged membrane filter systems. From a cost perspective, the overall costs of installing pressure membrane filter systems are much less compared to submerged membrane filter systems (Martinez and O’Connell 2005).
However, submerged filter systems significantly require less power to operate and produce better quality water when compared to pressure systems (Shubert 2008). Regardless of the type of membrane filtration system used, membrane technology is a huge asset when it comes to creating multi-barrier systems for treating surface water since membrane filters are capable of filtering contaminants as small as bacteria and viruses thus making drinking water very pure and safe for consumption (Gmünder and Vescoli 2009).
References:
EPA,. 2016. "Water Treatability Database | US EPA". Iaspub.Epa.Gov. http://iaspub.epa.gov/tdb/pages/treatment/treatmentOverview.do;jsessionid=Xd7vQHhBQ1cdv26TJmT2GWvpCv1P9jFsYp0bpqp8x6bjWG5cKN2M!989777045?treatmentProcessId=510273414.
Gmünder, A., and D Vescoli. 2009. "Pressure And Submerged Membranes In Multi Barrier Systems For The Treatment Of Surface Water". In The IWA Membrane Technology Conference And Exhibition, 10. Winterthur, Switzerland: WABAG Water Technology Ltd. http://www.wabag.com/wp-content/uploads/2012/04/IWA-Beijing_2009_membranes.pdf.
Martinez, Jesus, and Pete O’Connell. 2005. "Advantages Of Pressure MF Systems Over Vacuum Systems | the Solutions Source of the Water & Wastewater Industry". Water and Wastes Digest Magazine (WWDMAG). http://www.wwdmag.com/membranes-microfiltration/advantages-pressure-mf-systems-over-vacuum-systems.
Shubert,. 2008. Submerged Membrane Water Treatment - Fact Sheet. E-book. 1st ed. San Diego, CA: San Diego County Water Authority. http://sdcwa.org/sites/default/files/files/MembraneFactSheet.pdf.
