Certification Action Line — Answers — April 2014

April 14, 2014

Answers about filters.

  1. False. Cationic polymers are effective in coagulating typical submicron particles in natural waters and they also act as flocculating agents, holding together groups of coagulated solids, which further promotes the falling out of solution of the solids. Cationic coagulants would typically be injected upstream of a media or particulate filter designed for the removal of the solids. However, filters are not 100 percent effective in removing the coagulant. Polyamide membrane surfaces are typically negatively charged. Any positively charged coagulant will attach itself to such a membrane surface and become difficult to remove, thus increasing the rate of polyamide RO membrane fouling.
  2. d. The cation-exchange water softener will attract any residual coagulant that might break through the sediment filter. The water softener should be employed downstream of the sediment filter and upstream of the RO system.
  3. True. Birm is a registered trademark of Clack Corporation. It utilizes a manganese dioxide coating on a light silica pumicite core. It can be used for iron and manganese removal.
  4. e. Birm catalyzes the reaction between iron or manganese and dissolved oxygen in order to precipitate the metal (the Birm itself does not perform the oxidation). Birm does require, therefore, that a certain concentration of dissolved oxygen be present in the feedwater (some waters may require an air eduction system to increase the oxygen concentration). The water must also have a basic pH, no hydrogen sulfide and no oil as these can either strip or foul the Birm media. An advantage of Birm is that no chemicals are required for iron and manganese removal, and it tends to be less expensive than greensand.
  5. e. Activated carbon has the ability to attract and adsorb dissolved and suspended carbon-based organic molecules to the vast surface areas of its highly porous carbon granules. The attraction is associated with van der Waals forces. Activated carbon reacts with chlorine and chloramine by reducing them to chloride ions.
  6. a. Activated carbon has the capacity to react with three to five times its own weight of chlorine. Carbon also has high capacity (but not as high as that for chlorine) for higher molecular weight organics, such as most pesticides, herbicides and humic and fulvic acids. Carbon has a more limited ability to remove polar organic molecules, such as trihalomethanes.
  7. True. It is advisable to forward rinse or flush media filter beds to drain for a few bed volumes or minutes immediately following backwashing. Especially with activated carbon, backwashing will re-stratify the bed, which may result in shedding of organics and it can also create media fines. These may exit the bed in the first filtrate after backwash.
  8. c. Pleated membrane filters can be typically 98 percent efficient at the manufacturer’s pore rating. Thermally bonded spun-wound filters are about 80 to 90 percent efficient, and string-wound filters are typically 60 to 80 percent efficient.
  9. b. It is difficult to achieve an absolute sealing method between the filter and housing using a knife-edge seal. The O-ring end configuration offers a seal that is more likely to be absolute, and is especially more critical with filters rated at one micron or less. The double O-ring seal will provide a better seal than the single O-ring.
  10. c. Filters should be changed when pressure drop exceeds 15 psid, and ideally every two months to control biofilming colonizations. At a minimum, filters should be changed every six months to one year to keep them fresh from biological growths.

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