Prevention of silica scale in membrane systems: removal of monomer and polymer silica

In reverse osmosis (RO) and nanofiltration silica may precipitate on the membrane surface causing the decline in mass transfer. The appearance of deposits will depend on the different forms of silica, namely monomer, polymer and colloidal present in the feed water and concentrate. Exceeding the solubility of monomer silica is commonly considered to result in the formation of silica scale, which is difficult to remove. As a consequence, deposition of silica scale is prevented avoiding supersaturations, which results in limited conversions with silica rich feedwaters, and/or more recently the addition of antiscalants. The removal of silica in a separate process may be a successful alternative to scale control if the monomer silica concentration is either removed below the solubility level or the remaining silica supersaturation will not cause the deposition in next stage of a RO system. For this purpose, silica deposition from supersaturated solutions (200-700 mg/l) was tested in seeded silica gel (size 63-200 mum) batch experiments (pH = 7, 25 degreesC). Results indicated that monomer silica deposit on seeds and form silica polymers in solution. The formation of latter strongly negatively affects the removal of silica by silica gel seeds. Addition of iron (III) in comparable experiments had a profound effect on removal of monomer and polymer silica. Removal mechanisms most likely include the formation of sparingly soluble iron silicates resulting in un-saturated amorphous silica solution and coagulation of polymer silica.