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Remove silica from drinking water

March 3, 2010

I have a small water purification plant in Maun Botswana. The area is mostly Kalahari and very sandy.
The town council supplies us with water from boreholes where the silica content is very high.
We pump the water through several filtering stages and finally through a silica/salt softening process before sending it through our RO system.
After all this the membranes get clogged up very quickly making the process expensive.
The supplier of the membranes tells me that it is the silica clogging up the membranes.
I have but 2 questions:
a)Am I increasing the silica content even more in the water by using the above softening process (shooting myself in the foot)and if so what softening system should I use.
b)How can I cost effectively reduce the silica content in the water before the RO system
We supply 95% of the drinking water in the village, the people are poor and cannot afford expensive water
Regards
Willie

Willie Prinsloo
owner - Maun, Botswana, Africa

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March 4, 2010

Willie,

Softening the water does not increase the silica concentration.

You may want to investigate High pH reverse osmosis for your system. The solubility of silica increases significantly as the pH rises above 8. The only problem with this solution to silica fouling is calcium carbonate scaling, but since you are softening the water, this may not be a problem.

Many system and membrane suppliers are familiar with this technique to improve RO recovery and avoid silica fouling.
If the membrane does become hardness fouled, then this is much easier to clean than silica fouling, and usually only requires a short soak in very dilute hydrochloric acid at a pH of 2-3 to clean it.

Lyle Kirman
consultant - Cleveland Heights, Ohio

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March 6, 2010

Lyle

Thank you for your advice and for responding so promptly.
I will certainly follow your advice and report back on the results.
Maybe I can get to harvesting 70% of the input water in future instead of the current 40%.
We are living in a semi-desert and water is not plentiful.

Thanks again

Willie

Willie Prinsloo
- Maun, Botswana