Waterguide - Silica
What is silica?
Silica is a product that is formed when silicon (symbol Si, atomic number 14, atomic mass 28 g/mol) and water react. Silicon makes up 27.7 % of the Earth’s crust, and is the second most abundant element in the crust after oxygen. In normal conditions, silicon is a fixed, greyish-black material and is one of the half metals.
In what form is silica found on Earth?
Silicon is present in the Earth’s crust as quartz crystals, silicate and clay minerals. Weathering produces silicon dioxide (SiO2). When silicon dioxide reacts with water, orthosilicic acid, also known as silica, is created (Si(OH)4). You can tell from the pH value, which is 6.4, that this is a weak acid.
How does silica get into water and what concentration of silica does water have?
Given that a very large percentage of the Earth’s crust contains silicon compounds which are present in many different minerals, natural bodies of water are almost always in continuous contact with silicon compounds. Weathering can cause silicon dioxide to seep out of rocks and find its way into water. Over long periods of time, high amounts of silicon dioxide can seep out of volcanic rock, which is why groundwater in areas with strong volcanic activity (e.g. parts of Central America, Chile, Japan) has a high concentration of silica.
The concentration of silica in water can fluctuate and therefore does not have a characteristic value.
However, the typical ranges are:
- Groundwater 3 – 30 mg/l
- Groundwater in a volcanic region / thermal water 60 – 300 mg/l
- Surface water 10 mg/l
- Seawater 6.4 mg/l
Depending on the depth from which the samples are taken, the concentration may be higher.
What role does silica play in water treatment?
Little attention is paid to silica in water treatment as it is not relevant from a health perspective. It is also not essential for creating an ion balance in the design of reverse osmosis units, because it is virtually not present in a dissociated form in a neutral pH range.
However, it is precisely this missing dissociation that is problematic when treating boiler feedwater. This is because the concentration of silica already increases in the outflow of an ion exchanger before the resin bed is exhausted, which is meausured as an increase in conductivity in the outflow. This effect is also called “silica slippage". Steam power stations can only be supplied with feed water with a very low silica concentration in order to prevent the formation of a coating on the turbine. Therefore, an elaborate online measurement is sometimes used for silica in order to detect the increase of the concentration in time.
In regions with strong volcanic activity and a high concentration of silica in the groundwater, scaling caused by silica is a major problem when operating reverse osmosis units. At a neutral pH value and a temperature of 15 – 20 °C, the solubility of silica is approx. 100 mg/l. This value can quickly be exceeded by a factor of 4 – 5 in the concentrate of a reverse osmosis due to the concentration process, so that silica precipitates and a coating forms on the membrane. In this instance, you need to use an antiscalant that is specifically designed for silica-rich water in order to prevent irreversible scaling.
Increasing the temperature causes the solubility of silica to increase dramatically; for example at 60 °C the solubility is already over 200 mg/l. This is why you should always clean RO membranes at an increased temperature if the water has a high concentration of silica.
For this purpose, Herco offers manual cleaning plants (MRA) with integrated heating system.
Take a look at our product overview to find out more.