Silica Fouling of RO Membranes and Treatment Technologies

              pic: damaged membranes due to silica fouling

Silicon is the element which is the second most abundant in the crust of the earth by mass. Therefore, natural water sources usually contain a certain level of silica, from 1 to 40 mg/L. In some geographic areas, its concentration can reach up nearly to 100 mg/L.

As RO operation proceeds, the silica level in the concentrate stream increases and often reaches saturation, which can cause deposits of silica, or precipitation of metal silicates on the membrane surface (Scaling). Silica fouling is very difficult to remove from RO membrane, and eventually leads to performance deterioration such as permeability loss and premature system shutdown.

Occurrence

The potential of silica scaling occurs when the dissolved silica level in an RO system concentrate (i.e., reject stream) exceeds the solubility limit (≈ 120 to 150 mg/L at ambient temperature) for amorphous silica. Exceeding this saturation level in cold water (< 10°C) is not as serious a problem as silica polymerization is a very slow process at lower temperatures. However, silica in excess of 180 mg/L presents a potential problem at any temperature.

Forms of Silica in Feed water

In RO feedwaters, silica can exist in three different forms as follows:

• Monomer silica or silicic acid, Si(OH)4, is commonly referred to as ‘soluble’ (a.k.a., dissolved) or ‘reactive’ silica.
• Polymeric silica commonly referred as ‘colloidal’ or ‘unreactive’ silica that results from polymerization of silicic acid.
• Granular silica or ‘particulate’ silica.

Mechanism of Silica Fouling in RO Membranes

RO fouling relevant to silica scaling occurs mainly through deposition followed by polymerization, metal silicate precipitation, and accumulation of colloids/particulate formed in the bulk solution, respectively.

Colloidal silica in bulk phase adheres to the membrane surface forming a thin layer, where other silica colloids bind through dissolution/precipitation over time. This particle accumulation develops typically gel-like and more uniform layer.

Summary of Silica Treatment Technologies

Feed water Pretreatment	Softening/Coagulation	Adsorption/Co-precipitation by metal salts addition and pH control
	Seed precipitation /aggregation	Seed/silica agglomerate under saturation condition
	Tight ultrafiltration	Size exclusion of colloidal silica using commercial membrane
	Ion exchange	Exchange of silicate with hydroxide ion
	Adsorbent media	Sorption of silica on adsorbent such as activated alumina
	Electro-coagulation	Generation of metal coagulants by sacrifice anode. Less sludge production than chemical Treatment
RO operation under high silica conc.	Scale inhibitor	Chemicals which inhibit silica polymerization or scale particle aggregation
	Operation at alkaline pH	Increase of silica solubility in the concentrate stream (pH: 10-11)
	Operation at acidic pH	Retardation of silica polymerization at low pH condition (pH: 4-5)
	Intermediate Softening	Silica removal in the concentrate stream before feeding to next RO stage

Facing issues in your RO system performance due to silica fouling. Get in touch with us today! Nice Water Treatment Solutions 




Ref: Silica treatment technologies in reverse osmosis for industrial desalination: A review
Yong-Min Park1, Kyung-Min Yeon2, Chul-hwi Park1†
1Department of Environment Engineering, The University of Seoul, Seoul, Republic of Korea
2Engineering Center, Samsung C&T Corporation, Tower B, 26, Seoul, Republic of Korea