Antiscalant is a chemical that is used in Reverse Osmosis water purification process to prevent the RO membrane elements from scaling.

Membrane scaling occurs when concentrations of sparingly soluble salts (e.g., CaCO3, CaSO4, BaSO4, SrSO4, and silica) exceed their solubility limits. (1) 

Periodic Table of Scaling Compunds

What is Total Hardness

Total hardness is the sum of the calcium and magnesium concentrations, both expressed as calcium carbonate, in milligrams per liter (mg/L). You can determine your water’s hardness based on these concentrations of calcium carbonate: 
  • Below 75 mg/L - is generally considered soft
  • 76 to 150 mg/L - moderately hard
  • 151 to 300 mg/L - hard
  • More than 300 mg/ - very hard

What is the mechanism of Antiscalant`s scaling prevention?

Antiscalants are surface active materials that interfere with precipitation reactions in three primary ways (2):

  • Threshold inhibition: Tiny crystals (microcrystals) get bigger and turn into larger scales (macrocrystals) because certain ions like calcium and carbonate move to the surfaces of these crystals. They stick there and make the crystals grow. But if we introduce a special chemical called an antiscalant, it can block these surfaces where the crystals grow. This slows down or stops the formation of scales. Antiscalant work by stopping the tiny crystals from growing bigger right after they start forming.

  • Crystal modification: Antiscalants have a special ability to change the shape of crystals, making them softer and less likely to stick. When crystals start forming at a tiny level, certain parts of the antiscalant molecule attack the positive charges on these crystal beginnings, which stops them from growing. When we use crystal modifiers along with antiscalants, the crystals end up looking distorted, often more oval-shaped, and not tightly packed together.

  • Dispersion dispersancy - some antiscalants can stick to crystals or tiny particles floating in a liquid, giving them a strong negative charge. This negative charge keeps the crystals or particles from clumping together. It also helps to separate them from other negative charges on surfaces, like membranes.

CaCO3 crystals, untreated
(2000x, 4 microns)

CaCO3 crystals modified through treatment with a chemical Antiscalant
(2000x, 15 microns) 

What is the mechanism of a Softening system ?

A water softener is a whole-house filtration system that removes hardness-causing calcium and magnesium minerals from your water through a process called ion exchange.
When the hard water enters into the mineral tank, it flows through a bed of spherical resin beads – Na ion exchange resin. Ion-exchange resins are used to replace the magnesium and calcium ions found in hard water with sodium ions.

What is better to use before Reverse Osmosis System : Antiscalant dosing or a Softening system? 

If you are dealing with several problems in the analysis of raw water, for instance: high Total Dissolved Solids (TDS) level, high Total Hardness, high Silica content, heavy metals contamination, in this case better to install Reverse Osmosis System with Antiscalant dosing before.  

The disadvantage of using Softening system in this particular case is due to huge operating costs and low efficiency. We use Antiscalant to prevent scaling on RO membranes. Softening system is able to remove only Calcium and Magnesium from the water, but other cations, which cause scaling will stay and precipitate on the membranes surface.  

You can install Softening System and still use Antiscalant dosing before Reverse Osmosis System, but the chemical consumption will be huge and this solution from technical point of view is non-sense (3).

The table below represents a basic overview of the costs associated to operating a water softening system compared to that of applying antiscalant at various concentrations of water hardness (4).

Pretreatment: Antiscalant
Hardness (PPM) Capital Equipment Cost Antiscalant Cost ($ per year) Present Worth
10 $2,500 $1,314 $7,888
20 $2,500 $1,314 $7,888
50 $2,500 $1,314 $7,888
100 $2,500 $1,533 $8,786
250 $2,500 $1,752 $9,684
Pretreatment: Softening
Hardness (PPM) Capital Equipment Cost Salt Cost ($ per year) Present Worth
10 $25,000 $1,794 $32,356
20 $25,000 $3,588 $39,712
50 $25,000 $8,970 $86,779
100 $25,000 $22,424 $141,943
250 $25,000 $44,800 $233,688


  • Antiscalant dosages range from 3 – 4 mg/L depending on the water hardness.
  • Present worth calculations were amortized over a 5 year period at 7% interest.
  • Rock salt for softener regeneration costs $0.16 per pound.
  • Softener system with resin capacity of 40ft3, for hardness 50 ppm or above design contained two softeners.
  • Feed flow rate of 100 GPM, and an RO recovery of 75%.
  • When comparing antiscalant addition to softening, the main drawback of softening is the cost.

Why don`t we have Antiscalant in permeate water?

We don't have antiscalants in permeate water because antiscalants are added to the feed water before it enters the RO system. Antiscalant`s purpose is to prevent scaling of the membranes by interfering with the formation of scale or other deposits [1] [2]. Once the water passes through the the membranes, it has already been treated with antiscalant, so the permeate water that comes out is free from these chemicals.
Antiscalants are typically designed with molecules that are too large to pass through the tiny pores of the reverse osmosis (RO) membrane. RO membranes have very fine pores that allow only water molecules to pass through while blocking larger molecules and contaminants. Antiscalants are larger molecules compared to water molecules, so they are effectively blocked by the membrane during the filtration process. This ensures that antiscalants remain on the feed side of the membrane, where they can interact with scaling ions and prevent scaling or fouling from occurring on the membrane surface. As a result, the purified water that passes through the RO membrane, known as permeate water, does not contain any antiscalants.

The impact of using Antiscalant for the environment

In the past, antiscalants may negatively impact marine environments and affect fish life, coral reefs, sea-grass meadows, zooplankton, and microbial communities. Due to the multiple uses and increasing industrial applications of antiscalants, especially in desalination facilities, tons of thousands of these chemicals are discharged into the environment every year, causing detectable environmental impacts (5) (6) (7) (8). 

Currently, the formula for antiscalants has been enhanced, resulting in a biodegradable and eco-friendly antiscalant (8) (9) (10). Researchers consider antiscalants with a 60% degradation capacity within 28 days as meeting the criteria for being biodegradable (11). Although polyphosphonate-based antiscalants are considered stable, some researchers have reported that they are biodegradable by some microorganisms, including some halophilic bacteria, at different rates (12). Polyacrylate-based antiscalants are also susceptible to biodegradation in marine environments: it was found that 52% of polyacrylic acids degraded after 35 days of disposal (13).
It has taken into account, that polyphosphate-based antiscalants can be readily hydrolyzed to orthophosphate by cleaving the O–P bonds, which are considered a significant nutrient source for heterotrophic microorganisms and phytoplankton (14) (15) (16). It was the main reason to develop Phosphate-Free Antiscalant. 

In conclusion, we may say that current Antiscalants are eco-friendly, economically profitable, biodegradable and do not cause detectable environmental impacts.  

When is it better to use Softening system?

If the only problem in raw water is Total Hardness, then there is no need to install Reverse Osmosis System - for this a softening system will be enough.

List of references

(1)    Ahmad Fauzi Ismail, Kailash Chandra Khulbe, Takeshi Matsuura (2019). “RO Membrane Fouling”, Reverse Osmosis [Online], pp.189-220. Available :

(2)    M. Nasir Mangal, Sergio G. Salinas-Rodriguez, Jos Dusseldorp, Antoine J.B. Kemperman, Jan C. Schippers, Maria D. Kennedy, Walter G.J. van der Meer (April, 2021). “Effectiveness of antiscalants in preventing calcium phosphate scaling in reverse osmosis applications”, Journal of Membrane Science [Online], vol. 623. Available :

(3)    Dmitry Spitsov, Htet Zaw Aung, Alexei Pervov, Semyon Mareev, Dmitrii Butylskii, Mikhail Porozhnyy (January, 2023). “The Selection of Efficient Antiscalant for RO Facility, Control of Its Quality and Evaluation of the Economical Efficiency of Its Application”, Membranes (Basel) [Online]. Available :

(4)    PWT Chemicals. (2023). Water softening vs antiscalant addition in reverse osmosis systems [Online]. Available :

(5)    Elsaid K., Kamil M., Sayed E.T., Abdelkareem M.A., Wilberforce T., Olabi A.(2021). “Environmental impact of desalination technologies: A review. Sci. Total Environ”. Available : doi:10.1016/j.scitotenv.2020.141528

(6)    Petersen K.L., Frank H., Paytan A., Bar-Zeev E. (2018) “Sustainable Desalination Handbook. Elsevier; Amsterdam, The Netherlands: Impacts of seawater desalination on coastal environments”. pp. 437–463.

(7)    Petersen K.L., Paytan A., Rahav E., Levy O., Silverman J., Barzel O., Potts D., Bar-Zeev E. “Impact of brine and antiscalants on reef-building corals in the Gulf of Aqaba—Potential effects from desalination plants”. Water Res. [Online] vol. 144, pp.183–191. Available : doi:10.1016/j.watres.2018.07.009

(8)    Roberts D.A., Johnston E.L., Knott N.A. (2010). “Impacts of desalination plant discharges on the marine environment: A critical review of published studies”. Water Res.[Online], vol. 44, pp.5117–5128. Available : doi:10.1016/j.watres.2010.04.036.

(9)    Oshchepkov M.S., Rudakova G.Y., Tkachenko S.V., Larchenko V.E., Popov K.I., Tusheva M.A. (2021). “Recent State-of-the-Art of Antiscalant-Driven Scale Inhibition Theory”. Therm. Eng. [Online], vol. 68, pp.370–380. Available : doi:10.1134/S0040601521040054

(10)    Jafar Mazumder M.A. (2020). “A review of green scale inhibitors: Process, types, mechanism and properties”. Coatings [Online], vol. 10, pp.928. Available : doi:10.3390/coatings10100928

(11)    Hasson D., Shemer H., Sher A. (2011). “State of the art of friendly “green” scale control inhibitors: A review article”, Chemical Resources [Online], vol. 50, pp. 7601–7607. Available : doi:10.1021/ie200370v

(12)    Ashfaq M.Y., Al-Ghouti M.A., Qiblawey H., Rodrigues D.F., Hu Y., Zouari N.(2018). “Isolation, identification and biodiversity of antiscalant degrading seawater bacteria using MALDI-TOF-MS and multivariate analysis”. Scientific Total Environment [Online], vol. 656, pp.910–920. Available : doi:10.1016/j.scitotenv.2018.11.477

(13)    Campos E.J., Vieira F., Cavalcante G., Kjerfve B., Abouleish M., Shahriar S., Mohamed R., Gordon A.L. (2020). “Impacts of brine disposal from water desalination plants on the physical environment in the Persian/Arabian Gulf”. Environmental. Res. Commun. [Online], vol. 2. Available : doi:10.1088/2515-7620/abd0ed

(14)    Hosseini H., Saadaoui I., Moheimani N., Al Saidi M., Al Jamali F., Al Jabri H., Hamadou R.B. (2021). “Marine health of the Arabian Gulf: Drivers of pollution and assessment approaches focusing on desalination activities”. Marine Pollutants Bull.[Online], vol. 164. Available : doi:10.1016/j.marpolbul.2021.112085

(15)    Ihsanullah I., Atieh M.A., Sajid M., Nazal M.K. (2021). “Desalination and environment: A critical analysis of impacts, mitigation strategies, and greener desalination technologies”. Scientific Total Environment [Online], vol. 780. Available : doi:10.1016/j.scitotenv.2021.146585

(16)    Frank H., Fussmann K.E., Rahav E., Bar Zeev E. (2019). “Chronic effects of brine discharge form large-scale seawater reverse osmosis desalination facilities on benthic bacteria. Water Resources [Online], vol. 151, pp. 478–487. Available : doi:10.1016/j.watres.2018.12.046