Reducing Silica in RO

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I want to treat RO reject water with silica of 80 ppm, with a double RO pass.



The recovery needs to be 80%.



By design, I have the langelier saturation index (LSI) above 4.2 due to the increase of pH as the silica is high.



How can I reduce the silica?

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13 Answers

  1. Our company has developed and manufactures a cost- effective, green-rated, non-toxic, non- hazardous, bio- degradable  scale remover. This should eliminate your scale problem without damaging surfaces, pipework, seals, membranes or other equipment.

    An alternative would be vibrating membrane solution.

    Please email me at andrew@ waterandoilsolutions . com.au  for more details.

  2. If you want to treat this water first you have to get rid of the Calcium and other scale forming ions, you could use either an NF or Ion Exchange system. Then you can rise the pH to increase the solubility and rejection of Silica by the membranes. There is a process called HERO that deals with that but I have my doubts if you will get 80% recovery with this concentration in the feed.

  3. If you want to treat this water first you have to get rid of the Calcium and other scale forming ions, you could use either an NF or Ion Exchange system. Then you can rise the pH to increase the solubility and rejection of Silica by the membranes. There is a process called HERO that deals with that but I have my doubts if you will get 80% recovery with this concentration in the feed.

  4. Desalination plays an important role in producing pure water from brackish water. Reverse osmosis (RO) is by far the most efficient way to remove colloidal and dissolved silica, which can be found in high concentrations in brackis .Reverse osmosis (RO) is by far the most efficient way to remove colloidal and dissolved silica, which can be found in high concentrations in brackish. In the monsoon season presence of colloidal silica is observed in the raw water (river source).

    Precipitation of silica with Fe(OH)3, Al(OH)3 and silica also found

    Silica, in general, is reactive  silica  and colloidal silica . Whatever form this constituent is in,  silica must be  removed  before treatment and reuse or disposal/discharge. The most familiar methods for removing silica  from a waste stream are as following..

    1.  Ion exchange and  reverse osmosis.
    2. Lime softening and
    3. RO

    The Ultrafiltration membranes are cleaned intermittently to remove any possible gel layering of any foulants. The Ultrafiltration membranes offered are also Chlorine compatible.Microfiltration (as well as sand filtration, cartridge filtration, and diatomaceous earth filtration) would not be effective in removing colloidal silica, some bacteria, smaller particulates, and large molecular weight organics.

    Reverse osmosis (RO) is a membrane-technology filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. To be “selective,” this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution, such as the solvent, to pass freely.

    Using reverse osmosis or any membrane technology to remove silica can be problematic. Silica is like glass and will cut holes in the membrane and will decrease the efficacy of a membrane because of the abrasiveness of the particles. Heavy metals foul membranes, leading to failure. Membrane systems should have a pre-treatment regimen to get rid of the constituents that foul membranes. If pre-treatment is done prior to membrane treatment, then membranes will perform optimally without expensive replacement.

    remove colloidal and dissolved silica, which can be found in high concentrations in brackish water. The presence of silica and its ability to foul membranes limits the use of silica bearing waters for desalination and when used, it has many economic penalties. This study examines the effect of silica... were carried out to examine the actual fouling mechanism in reverse osmosis units under various experimental conditions. Furthermore the effect of cleaning, with distilled water and with pulsations as well as with commercially available cleaners were examined.

     

  5. I recommend that you download either or both Winflows RO design software from GE or ROSA from DOW. The model will predict recovery based upon silica and important cations in your water. Can either add antiscalant to chelate the silica or remove it with lime soda softening, or MF/UF membranes depending reactive portion. Can also run through a SAB ion exchanger either all or blend to get to your target silica. Many options to remove the silica--usually driven by time or economics.

     

  6. May b u need t run ur RO at lower recovery since silica above 60 ppm is very high.

    Ultra filtration works when there is non ionic colloidal silica,typically a case with such high silica.

    Lime softening with Mg must give good results,at slightly higher temperatures for better absoption,given the high ph for such silica content...of course only in case of no acid feed for hardness issues

  7. Use gas absorption or remains system such as air columns

  9. Ceramic UF membranes PRIOR to RO would remove silica and increase RO membrane life.

  11. It is not clear to me what exactly you need. 

    At first reading I understand that you are designing that water treatment unit with an RO and you are taking at reject water 80 ppm SiO2, high (?) pH and LSI=4,2.

    After that you need to diminuish SiO2 concentration on Reject Water. Why? Will you have SiO2 scaling problems? What 's the SiO2 saturation value? 

    I can help you if you send me the full designing results of your RO Unit. 

    If you add acid into the feed water, you will have a lower pH at Reject Water and lower LSI, but higher SiO2 saturation. There is always risk in any action you make. 

    If you need help send me pm (personal mail). 

  13. Silica can be removed from raw water easily with standard demineralization techniques. Normally silica is present in a weak acidic form. Ion exchange will remove this as long as the anion resin is the strong base type. Silica in deionized water can easily be reduced to 20-50 ppb. Reverse osmosis will also remove silica by 90-98%. Both of these techniques are non-specific for silica, meaning they also remove all other ionic components along with the silica. Normally these methods work well for most well or surface waters where silica is below 15-20 ppm. In some cases for very large flow rates you can remove silica with lime softening techniques. In lime softening the silica is co-precipitated along with magnesium present in the water (or added if necessary). High temperature softening greatly improves silica removal. For high silica waters as seen in Mexico, Hawaii or other sandy areas where levels are 50-60 ppm or higher, the ion exchange and RO options are more troublesome because you may exceed solubility limits for one, and also you are more likely to have significant levels or non-ionic colloidal silica. This will not ion exchange and may foul an RO membrane. Typically this must be removed with ultrafiltration.

    1 Comment

    1. At that level of silica RO may not be an "only" option especially if 80% recovery is the target.  A softener followed by a desilicizer will work to remove the silica (and other minerals and ions) but it will not be an in expensive solution.  But an RO after this proper treatment should perform well.  Lime softening with magnesium present will also work but this may add ionic species that will be detrimental to the RO service.