Silica is among the most common substances encountered on Earth, so it is no surprise that it is almost always present in source water, process water, and wastewater streams. For industrial facilities, proper management of silica levels is important to ensuring optimal function and performance life for equipment. But how do you remove silica from industrial water and wastewater?
In this article, we’ll explain what silica is, how it effects industrial facilities, and common processes for removing silica from water.
What is silica?
Also known as silicon dioxide (SiO2), silica is a crystalline compound that is an extremely common constituent in most rocks, minerals, and sand. It is also sometimes synthetically produced. Silica is used in a variety of industrial applications, including construction, as well as production of abrasives, electronics, glass, metal casting, and some types of water treatment. While most forms of silica are not particularly water soluble under normal conditions, low levels of silica (around 15ppm) tend to occur naturally in ground and surface water due to contact with surrounding rocks, and silica removal is therefore often a component of raw water treatment systems. Silica can also be introduced to water during various industrial processes, including various production activities, its application as a corrosion control agent, and during wash and rinse cycles.
Why is silica removal necessary?
Removal of silica from process or wastewater is most commonly done as a means of preventing premature wear or damage to industrial equipment. This is because silica content can accumulate on surfaces as hard mineral deposits known as scale. Scale deposits can reduce the efficiency of heating and cooling and can also interfere with the ability to operate systems within a defined pressure range. As such, silica scaling can compromise the efficiency, performance, and safety of equipment such as boilers, turbines, piping, and filtration units.
Silica can also cause membrane fouling in nanofiltration and reverse osmosis systems, or other membrane filtration units. Membrane fouling occurs when silica particles lodge in the membrane’s pores, obstructing the normal flow of liquids through the membrane. Additionally, because silica particles are abrasive, they can also cause premature tears and weakening of delicate filtration membrane materials.
How do you remove silica from water?
Silica removal is deployed for a variety of industrial applications, including treatment of raw water, cooling tower makeup water, boiler feed and blow down water. Silica can be present in the form of granules, colloids, or reactive ions, and selecting the appropriate silica removal technology depends on which physical forms of silica are present in the stream. Below, we have summarized the types of silica, and appropriate removal technologies for each.
Granular silica removal
Silica can exist in water as relatively large particles, known as silica granules or granular silicates. Granular silica removal is generally accomplished through physical chemical separation. The most common approach is lime softening, which is a process where calcium hydroxide (lime) is added to water to remove hardness. The lime works by triggering the precipitation of hardness ions (calcium and magnesium), which form flocs that can be settled out of the stream. As the flocs form, they capture silica granules, as well as other organic and inorganic particles in the solution, which effectively reduces the overall TDS and TSS content of the stream.
Colloidal silica consists of finer particles comparative to granular silica. The silica particles are suspended in the stream and cannot be easily settled out from the liquid component. Common methods of silica removal for colloidal silica include ultrafiltration (UF), flocculation, and clarification.
Ionic silica particles are much too small for effective removal by physical means; therefore, common methods of ionic silica removal include RO, electrodeionization, mixed-bed deionization, and ion exchange. Ion exchange (IX) is a process where a stream is passed through a resin substrate that facilitates removal of ionic contaminants, and replacement with similarly charged particles. There are a few types of IX, and the type used for silica removal is strong base anion (SBA) resin operated in the hydroxide cycle, as silica is a weak acid. When using IX to treat complex streams that contain silica as well as other contaminants, such as heavy metals, care must be taken to properly separate and/or dispose of any hazardous materials that may also exist in the stream. IX resins also need to be maintained through regeneration cycles and may therefore not be optimal for streams with high concentrations of silica.
Another method of ionic silica removal is reverse osmosis (RO), which uses a semipermeable membrane to separate out ions and particles from a liquid stream. The RO process works by applying enough pressure to force water through the microscopic pores in the RO membrane, leaving behind any contaminant particles, such as silica, on the retentate side of the membrane.
These methods are highly effective for targeting ionic silica; however, they can easily be fouled by other contaminants present in the stream, or by larger silica particles that may also be present. If either of these conditions apply, then additional pre-treatment technologies may be warranted to optimize overall system performance.
Can SAMCO help?
SAMCO has over 40 years’ experience in identifying appropriate silica removal technologies to help lower costs and improve system efficiency. For more information or to get in touch, contact us here to set up a consultation with an engineer or request a quote. We can walk you through the steps for developing the proper solution and realistic cost for your particular silica removal needs, or your larger raw water treatment needs.
To learn more about SAMCO’s innovative solutions, visit our page on raw water treatment technologies here.
If you want to learn more about silica removal, these other articles might be of interest to you:
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- How Much Does a Raw Water Treatment System Cost? (Pricing, Factors, Etc.)