Silica removal in Industrial waste water

Question

A commercial supplier of electrocoagulation plant for removal of silica in industrial waste water. Level of Si02 is ~610 mg/L. The oil and grease levels are ~ 2,000mg/L .  It is intended to reduce this substantially with Direct Aeration Flotation (DAF) and skimming off the top system. I assume the electrocoagulation would be post the DAF system? You suggestions please

Ali Salmani · Answer

As I know, electrocoagulation is an alternative way to chemical coagulation and because of the production of hydrogen bubbles, it can act like a DAF and bring the formed sludge to the surface.
My recommendation for this source of water is:
CPI --> DAF --> Warm lime softening --> Media filtration/UF
Then the produced water can be reused.

Hossein Esmaeili · Answer

I thoroughly agree with the suggestion of Marco. In addition, using flocculants in the CPI and DAF system will remove colloidal silica in the form of flocs resulting in optimization of downstream silica removal system (to the desired level). EC can be used as the coagulant-based flocculation at DAF inlet.

Prem Baboo · Answer

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 following-
 lime softening,
ion exchange and
 reverse osmosis.

Glenn Davies · Answer

If your intention is to use a DAF and your question is to use EC as a post treatment, I would suggest EC to be a pre-treatment. EC will remove silica and add hydrogen to the water which will aid in the floatation of the particles. It will also assist in the separation of the oil which help the float. There are sodium bentonite clay blends that can assist in the agglomeration of the separated substances and provides for better dewatering characteristics.

Marco Rognoni · Answer

Dear John, I recommend to check the analysis of the raw water, because the dolubility of SiO2 at ambient temperature is about 150 mg/L and 600 mg/L may be reached only through oversaturation and solids suspended.
The elimination of SiO2 for high concentration is actually a hard task and the convenient technology should be chosen also considering the plant capacity/size. How much is the water flow to be processed ?
Probably evaporation is a good technology to be considered and a Multi effects evaporator can solve your problem. Please note that the non dissociated in ions silica flows through RO membranes thus making the RO separation rather ineffective. Moreover the use of ion exchanging (anionic) resins is not recommended because of the difficult regeneration of the resins saturated by SIO2
In any case DAF is to be installed before the SiO2 removal section. However 2000 ppm oil is much for a DAF and I suggest to install a CPI as primary treatment, and then a DAF as secondary treatment. The total oil can be thus reduced to less than 10 ppm, which is sufficient for the proper next evaporation process (not for resins nor for RO membranes)

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