Antiscalants are pretreatment water additives used in reverse osmosis systems to hinder the precipitation of crystallized mineral salts and prevent scale formation in membranes. Several chemicals can be used as antiscalants to enhance the operation of the reverse osmosis system.
Antiscalants are added to the feed water before it enters the reverse osmosis membrane, which causes the time delay between the calcium magnesium and bicarbonate. It thus enables water to flow through the membrane before any chemical reaction happens and results in scale formation. A research study recently cited that the Middle East Antiscalant Market has a decent growth potential during 2021-26 with around 6% CAGR. The three primary sources of water include Seawater, Brackish Water, and Portable Groundwater/Surface Water. The worldwide water usage can be summarized mainly as follows:
- Portable Groundwater/Surface Water: This type of water is used mainly in the agriculture sector/green spaces for irrigation, and some part of it is treated & consumed by households.
- Brackish and Sea Water: Seawater & brackish water are desalinated & used usually by the residential and commercial sectors. Some industries also have captive desalination plants for their water requirements.
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Different Types of Antiscalants
- Carbonate
- Phosphate
- Sulphate
- Silica
- Magnesium Hydroxide
Selecting an appropriate antiscalant is crucial to provide optimal operating conditions for either brackish or seawater in RO plants. It helps ensure the efficient working of plants and might also extend the membrane life & reduce operational expenditure.
Scaling happens when the solubility of low soluble salts is surpassed. As the water constantly flows through the membrane, it leaves behind suspended or dissolved solids and forms the concentration gradient that causes scaling at the membrane surface. This concertation at the membrane-separating surface forms a boundary layer, i.e., concentration polarization, within which salts may precipitate and suspended solids can start to deposit on the membrane surface, resulting in scaling & fouling. All natural waters, if left untreated, shall deposit calcium carbonate on the membrane surface.
The Three Stages of Scale Formation
- Scale Induction: The water passing through the membrane forms Ions in solution concentrate, particularly in the boundary layer. The ions start gathering as proto-nuclei of up to 1000 atoms, i.e., readily reversible reaction.
- Nucleation: As the proto-nuclei increases, ions start arranging themselves and a nuclei of regular shape develops.
- Crystal Growth Periods: The irreversible growth of crystals from the nuclei is the final stage, wherein crystals continue to increase as long as the salt is surpassing its solubility coefficient.
The prevention of scale formation includes several methods, including Antiscalants, pH control, and other proprietary products. Nowadays, the most extensively used scale inhibitors are based on numerous organic compounds, which work with the following three mechanisms:
Threshold Effect: These inhibitors hinder the precipitation of salts that exceed their solubility.
Crystal Distortion Effect: These inhibitors hinder the normal growth of crystals and form an irregular crystal structure with poor scaling ability.
Dispersancy: Dispersants place a surface charge on the crystal. Comparable charges make crystals repel each other and get dispersed into the water
Lastly, Crucial Properties of an Ideal Antiscalant
- Effective scale control using low concentration of Antiscalants
- Balanced adsorption/desorption for the slow & homogeneous release of chemicals into the production water
- Must have high thermal stability & biodegradability
- Low toxicity & costs