Certification Action Line Answers – February 2015

Feb. 1, 2015

Answers to the February 2015 Certification Action Line quiz.

1.     d. The flux in membrane water filtration is the product or permeate that is allowed to pass through the membrane. It is defined as the amount of permeate produced per unit of time. Generally, flux is referred to as gallons per square foot of membrane surface area per day (GFD).

2.     a. In most ultrafilter applications, the membrane operation varies between 50 and 200 GFD of treated water flux at an operating pressure of about 50 psig. Reverse osmosis membranes, on the other hand, typically produce between 10 to 30 GFD at 200 to 400 psig.

3.     c. The percent recovery value represents that portion of the total liquid feed volume; (stream) this is reclaimed as permeate (or product fluid). As an example, if 100 U.S. gallons of fluid are fed to a membrane filter, and 80 gallons of that amount pass through the filter and become permeate (product fluid), then the system would be operating at 80 percent recovery.

4.     a. Retention describes the minimum particle or molecule size retained by the membrane under a given set of conditions, namely, pressure, flux, recovery and temperature.

5.     c. Colloidal silica is not removed by ion exchange. It can be removed with reverse osmosis (RO), but not as economically as with ultrafiltration.

6.     True. Trihalomethane (THM) precursors are relatively large humic and fulvic acid compounds produced from decaying vegetation. They will typically have molecular weights above 1,000 grams. The THMs, on the other hand, all have molecular weights of less than 300 grams. Ultrafiltration can effectively remove organics above molecular weight of 1,000 grams and RO is effective for organic molecules above molecular weight of 100 grams. Note that molecular weight is not the only factor in determining whether a molecule will be removed by RO.

7.     c. RO and UF membranes are cast from a solution of long chain polymers such as cellulose acetate dissolved in acetone, polysulfone, thin film composite (TFC) fluorocarbon, polyamide, polyacrylonitrile and other petrochemical-based polymeric compounds. A solvent solution of the polymer is spread onto a casting surface where the solvent begins to evaporate. When in a semi-solid state, the layer is plunged into another solvent in what is called the “quenching” step. Quenching rapidly coagulates and precipitates the remaining polymer to leave a pore spacing in a solid film formation. The size of the pores is largely determined by the drying time after the quench step.

8.     False. Asymmetric membranes have rejection occurring only on the membrane surface. The advantage of the asymmetric membrane is that pore-plugging particles cannot enter the main membrane matrix or support layer. Retained particles and macro-molecules are easily flushed away with the reject water flow. There is a tendency in microporous membranes for particles smaller than the pore size to intrude into the membrane matrix, increasing the potential for the pores to get irreversibly plugged.

9.     a. The hollow fiber type of membrane construction assembly offers a very high packing density for membrane surface area. However, because the fibers are fragile, feed velocities are kept low, thus creating laminar flow and greater susceptibility to plugging.

10.   d. The spiral-wound design is manufactured by building a basic sandwich in a sealed envelope of several layers. The first layer of construction is the out membrane, next is a spacer unit of diffusion netting and then there is a second membrane. The permeate recovery layer is the porous matting that follows the second membrane layer. The diffusion netting provides flow of the incoming fluid across the surface of the membrane. The porous matting is the permeate conduit away from the membrane. The sandwiched layers are spirally wound around a product water tube.

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