• The process is well known and accepted in water purification systems. It is usually used where there is a high flow rate (>50 gpm or 0.18 m3/min) or high hardness (>50 ppm).
  • The incoming water is acidified before the RO unit and a degasifier is used to remove residual CO2 prior to moving on to a second pass RO or a mixed bed de-ionization (DI) unit.
  • In this pretreatment process, the incoming water is adjusted to a pH in the range of 3.8-4.2 with sulfuric acid. The acidified water is sent to a packed column degasifier for removal of free CO2 by air.
  • Removal efficiency of CO­2 is better than 98% (typical commercial degasifiers are designed to reduce outlet CO2 to less than 5 ppm). This residual CO2 should not pose a problem for downstream single and mixed bed de-ionization units or electro-deionization.
  • The residual CO2 also can be removed by addition of a base to increase the pH to 8.5 that converts it to CO3 which is removed in the second stage of the RO.
  • Commercial degasifiers are typically from 18 in. in diameter for 50 gpm to 72 in. diameter for 680 gpm (0.46 m for 0.18 m3/min to 1.83 m for 2.57 m3/min).
  • Fan power requirements will range from 1/2 HP to 10 HP for the preceding sizes. Smaller and larger units are possible to meet exact needs. Standard packed tower design methods are used.
  • The pH after removal of CO2 will be in the range of 6.5-7.0. Just prior to feeding the RO unit, the pH should be adjusted to approximately 8.0-8.5 in order to minimize the amount of free CO2 still remaining in the water and enhance removal of remaining carbonate in the second pass of the RO.
  • The acidification/degasification process has some associated problems. Air borne bacteria, if a problem or concern, can be removed by a HEPA filter in the inlet-air line. The air also may oxidize any iron present to form solids.
  • Water from the degasification column is usually collected in a holding tank. Further treatment in this tank is possible for TOC removal and microbial control.
  • A multimedia filter usually follows the degasifier for removalof initial incoming solids and any solids generated in the degasification step. When using gasification as a “softening” process, addition of an anti-scaling agent is recommended just before the RO (if used as the first stage in the final treatment step).
  • The anti-scaling agent will be removed along with minerals, high molecular weight organics, and endotoxins in the RO. Monitoring for removal of the anti-scaling agent by the RO is required.

 

Advantages:

  • Replaces softener and the handling of large amounts of salt for softener regeneration.
  • The CO2 is released to the atmosphere rather than being purged as an ion in a waste steam.
  • The added sulfate ion from acidification is easier to remove in RO than added sodium ion from softening.

 

Disadvantages:- Handling acid for acidification; instrumentation and chemical handling for two pH adjustments.