Silica
(endorsed 2011)
Guideline
To minimise an undesirable scale build up on surfaces, silica (SiO₂) within drinking waters should not exceed 80 mg/L.
General description
Silica present in water is usually referred to as amorphous silica (i.e. lacking any crystalline structure). When silica is dissolved within water it forms monosilicic acid:
When the concentrations of monosilicic acid increase, polymerisation of the silica occurs, forming polysilicic acids followed by formation of colloidal silica. Monosilicic acid and polysilicic acids are the forms of silica analysed when determining dissolved silica content.
The deposition of silica from solutions can occur via various mechanisms. The deposition of silica that can cause the most problems for the water industry is via silica’s ability to deposit on solid surfaces that have hydroxyl (OH) groups present. Surfaces that commonly have hydroxyl groups present are glass and metallic surfaces. For example, dissolved silica will react with the surfaces of glass and begin to form a white precipitate. The silica forms silicates on the surface, resulting in silica build-up. In cases where customer complaints occur due to scale build-up, water hardness and silica concentrations should be investigated to determine the cause.
Silica can be a problem in water treatment due to its ability to cause fouling of reverse osmosis (RO) membranes (Sheikholeslami and Tan, 1999, Ning 2002, Sahachaiyunta and Sheikholeslami 2002). This occurs when the dissolved silica of the concentrate becomes super-saturated, causing silicates to form in the presence of metals, and these deposit on the membrane surface. The silicate then dehydrates, forming hard layers on the membrane that reduce the effectiveness of the process.
Fouling of membranes can occur in two ways:
precipitation fouling – monosilicic acid polymerises at the membrane surface forming a deposit similar to silica scale on glass surfaces;
particulate fouling – accumulation of colloidal silica within the solution is then deposited on the membrane surface.
A suggested industry standard guideline is to limit the concentration of silica within the RO concentrate to ~ 120 mg/L at 25°C to limit fouling of RO membrane (Freeman and Majerle 1995).
Colloidal silica may affect ion-exchange processes in water treatment. The stability of colloidal silica as an un-ionised compound causes problems in removal using ion-exchange resins. High concentrations may also cause fouling of ion exchange units.
Typical values in Australian raw water
Dissolved silica from various source can range between 0.6 mg/L in some surface waters to 110 mg/L in ground waters.
Measurement
Silica can be determined by spectrophotometric techniques upon the addition of ammonium molybdate (EPA method 370.1, APHA Method 4500.) (Clesceri et al., 1998).
Treatment of water
The removal of silica from waters involves the use of cold lime softeners, hot process softeners, macroreticular anion resin, up-flow filters with chemical feed, cross-flow microfiltration with chemical feed, and nanofiltration.
Emerging processes in water treatment such as electrodialysis removal (EDR) and high efficiency reverse osmosis (HERO), as a by-product, have some success in silica removal. EDR is based on applying electrochemical separation processes within the water that allow for the removal of charged ions from the water. This process may offer some level of silica removal if the silica is in a charged form; however colloidal silica, which is likely to be present at large concentrations, will not be removed due to its stability as an un-ionised compound. HERO is based on normal RO processes; however pre-treatment removes divalent metals that form scale, and the operating pH has been increased to 11 to support the process. At this pH, silica is ionised, which increases its solubility and hence eliminates scaling of the membranes. It has also been reported that, at higher pH values, there is improved rejection of silica.
Health considerations
No health guideline has been set for silica as there are no data linking silica to adverse health outcomes.
Derivation of guidelines
The suggested guideline is based on the solubility of amorphous silica being between 100 and 140 mg/L at 25°C, so that, based on current date, limiting the value to below 80 mg/L should limit the formation of silica scaling.
References
Clesceri LS, Greenberg AE, Eaton AD (1998). Standard Methods for the Examination of Water and Wastewater, 20th Edition. American Public Health Association, Washington.
Freeman SDN, Majerle RJ (1995). Silica fouling revisited. Desalination, 103:113 – 115.
Ning R (2002). Discussion of silica speciation, fouling, control and maximum reduction. Desalination, 151:67-73.
Sahachaiyunta P, Sheikholeslami R (2002). Effect of several inorganic species on silica fouling in RO membranes. Desalination, 144: 373-378.
Sheikholeslami R, Tan S (1999). Effects of water quality on silica fouling of desalination plants. Desalination, 126:267-280.
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