A NATIONAL DRINKING WATER CLEARINGHOUSE FACT SHEET
Membrane Filtration: Alternative toConventional Filtration
Membrane filtration systems’ capital costs, on abasis of dollars per volume of installed treat-ment capacity, do not escalate rapidly as plantsize decreases. This factor makes membranesquite attractive for small systems. In addition,for groundwater sources that do not needpretreatment, membrane technologies arerelatively simple to install, and the systemsrequire little more than a feed pump, a cleaningpump, the membrane modules, and someholding tanks. According to a 1997 report bythe National Research Council, most expertsforesee that membrane filtration will be usedwith greater frequency in small systems as thecomplexity of conventional treatment processesfor small systems increases.
New Regulations Favor MembraneTechnologies
Membrane processes have become more attrac-tive for potable water production in recent yearsdue to the increased stringency of drinkingwater regulations. Membrane processes haveexcellent separation capabilities and showpromise for meeting many of the existing andanticipated drinking water standards. TheSurface Water Treatment Rule (SWTR) andthe anticipated Groundwater Disinfection Rulehave led to the investigation of UF and MF forturbidity and microbial removal. The newDisinfectants/Disinfection Byproduct (D/DBP)rules have increased interest in NF andUF membranes for DBP precursor removal.Potable water treatment has traditionallyfocused on processes for liquid-solid separationrather than on processes for removing dissolvedcontaminants from water. Thus, the effect of the 1996 Safe Drinking Water Act (SDWA)amendments has been to encourage watertreatment professionals to consider the moreunconventional treatment processes, such asmembrane technologies, alone, or in conjunc-tion with liquid-solid separation, to meetcurrent regulations.
Comparing Membrane FiltrationSystems
While all types of membranes work well underproper conditions, choosing the most appropri-ate membrane for a given application stillremains crucial. (See Figure 1.) In many cases,selection is complicated by the availability of new types of membranes, applications, or bysite-specific conditions. Bench and pilot testsare powerful tools for situations where processrisks and uncertainties exist or the cost impactsfrom problems are potentially high.Membrane classification standards vary consid-erably from one filter supplier to another. What
A membrane or, more properly, a semipermeable membrane, is a thin layer of materialcapable of separating substances when a driving force is applied across the membrane.Once considered a viable technology only for desalination, membrane processes are increas-ingly employed for removal of bacteria and other microorganisms, particulate material, andnatural organic material, which can impart color, tastes, and odors to the water and reactwith disinfectants to form disinfection byproducts (DBP). As advancements are made inmembrane production and module design, capital and operating costs continue to decline.The pressure-driven membrane processes discussed in this fact sheet are microfiltration(MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO).