Water Well Journal
Issue link: https://read.dmtmag.com/i/705618
Conversely, the feedwater can flow in from the exterior side of the membrane, through the membrane, and discharge into the center to flow outward in what is called outside-in flow. In dead-end filtration the direction of the fluid flow is nor- mal to the membrane surface. All membranes work under the same basic principle. The size of the sieve or port opening in a membrane is generally so small and tight that extreme values of applied pressure or head are required to force the water through the membrane. This is why most methods using mem- brane filtration for water treatment are typically operated using a pressure-driven function. Typically, membrane filtration incorporates four different types of solids removal capability, here listed in the relative order of their potential of removing larger to smaller sizes of particulate matter or dissolved ions: 1. Microfiltration 2. Ultrafiltration 3. Nanofiltration 4. Reverse osmosis Microfiltration and ultrafiltration membranes are used ex- tensively for filtration of process water and many industrial uses, as well as for pre-treatment of water, particularly ahead of reverse osmosis and nanofiltration units. Nanofiltration and reverse osmosis are more commonly used for water purifica- tion due to their superior ability to remove viruses and dis- solved ions and solids, such as salts. The relative size of particles removed for each type of membrane is shown on the chart in Figure 2 and are generally rated in micron removal capability. As you can see, the re- moval capability for bacteria and viruses with each method is indicated on the chart by the physical size of the contaminant in microns. The filtration ability of conventional particle fil- tration, including the majority of those discussed in this series, ends at about 1 micron in size, the generally accepted limita- tion to conventional filtration methods. Microfiltration overlaps some of the range associated with conventional filtration, from a high particle size of 5 microns down to a particle size of just 0.1 micron. Ultrafiltration is fur- ther capable of removing material in size between 0.5 micron down to slightly less than 0.01 micron. Nanofiltration func- tions well in the removal of particle and dissolved ion sizes be- tween 0.01 micron down to 0.001 micron. Reverse osmosis removes virtually all dissolved salts and ions between 0.001 micron down to 0.0001 micron and is the filtration process most associated with converting seawater to freshwater through removal of the saline constituents (ions) in the water. Nanofiltration is actually a form of reverse osmosis with the distinguishing factor being the size of the retained ion. Since both processes use semipermeable (non-porous) mem- branes, there is no actual pore opening or nominal pore rating, although 1-2 nanometers (1 micron=1000 nanometers) for nanofiltration and less than 1 nanometer for reverse osmosis are often accepted as the typical removal efficiency. This also make both types much more susceptible to plugging and pre- mature failure from particulate matter due to the extreme tight nature and lack of actual pore openings on the membrane. As such, the use of nanofiltration and reverse osmosis on a water supply with particulates is not to be used; the feedwater should receive an appropriate level of pre-treatment to remove these constituents before introduction into the membrane. The type of particulate rejection for each class of mem- brane is shown in Figure 3. The straight, light blue arrows in- dicate the constituents generally not removed by and therefore pass through each membrane, while the right-angled, darker blue arrows indicate the constituents rejected or typically removed by each membrane. In water treatment, the four types of membrane devices are generally classified as membrane filtration devices for the two processes involving microfiltration and ultrafiltration for larger particulate matter, and membrane desalination devices for the two processes with semipermeable membranes for removing dissolved salts and ions using nanofiltration and reverse osmosis. Due to their individual characteristics for their ability to retain very small particle sizes, the membrane classes of filtration devices can also remove various contami- nants from water. Although not usually recognized or certified for this spe- cific use in potable water, each of the membrane types can nonetheless remove many of the most common types of con- taminants from water supplies as well as lower the turbidity or cloudiness of the water. These typical contaminants again are indicated in Figure 3. All four of the processes are capable of removing or lower- ing turbidity, most bacteria and cysts, and some viruses from freshwater supplies, including pathogenic and non-pathogenic types, although many of the smaller viruses remain too small for the larger filtration methods such as microfiltration and ultrafiltration. This generally indicates the use of nanofiltration or reverse osmosis for effective virus removal. Since most protozoan cysts have a physical size between 2 microns up to 25-50 mi- crons, modified methods of conventional particle filtration, bag filtration, and microfiltration are commonly used to re- ENGINEERING continues on page 42 ENGINEERING from page 38 Figure 2. The different types of membrane filtration and the range of particle sizes removed by each type. waterwelljournal.com 40 August 2016 WWJ