Scale Inhibitors for Water Treatment: 11 Types, Mechanisms, Dosage and Selection

Scale inhibitors for water treatment are 11 chemicals in 3 classes, phosphonates, polymers and phosphorus-free green polymers, that keep calcium carbonate, calcium sulfate and other salts dissolved or dispersed at doses of a few mg/L (ppm) as active. They work far below the amount needed to bind the hardness they control, so how does 2 to 5 mg/L hold back scale in a cooling tower or an RO concentrate?

The 11 scale inhibitors are 5 phosphonates (HEDP, ATMP, PBTC, DTPMP and HPAA), 4 polymer inhibitors and dispersants (polyacrylic acid and sodium polyacrylate, polymaleic acid, AA/AMPS and phosphinocarboxylic acid) and 2 green polymers (PASP and PESA). Together they form the third of the 16 families of water treatment chemicals.

Calcite saturation, halogen exposure, calcium hardness and phosphorus limits decide the choice: HEDP and ATMP hold calcite to about 125 to 150 times saturation, PBTC to about 225 to 250 times. NSF/ANSI/CAN 60 caps potable products at product-specific maximum use levels, most commonly 20 mg/L as product for the phosphonates.

Every value on this page is checked against primary sources (NSF listings, eCFR, EPRI, CEN, PubChem, ECHA, Federal Register); see our methodology and fact-checking process.

What Is a Scale Inhibitor in Water Treatment?#

A scale inhibitor in water treatment is a chemical, such as HEDP, PBTC or a polyacrylate, that is dosed far below the stoichiometric amount needed to sequester hardness and still prevents mineral salts from depositing, because it adsorbs on forming crystals (Veolia Water Technologies Handbook, chapter 25). Chelants, by contrast, bind hardness stoichiometrically.

In reverse osmosis the same phosphonates and polymers are called antiscalants and are dosed on feed flow, while cooling and boiler programs dose them to a residual.

Which scales do scale inhibitors prevent?#

Scale inhibitors target 5 common deposits: calcium carbonate, calcium sulfate, calcium phosphate, silica and, under some conditions, magnesium sulfate, plus barium sulfate in sulfate-rich feeds.

Most calcium and magnesium salts are more soluble cold than hot, so they deposit on heat-transfer surfaces, while silica is more soluble hot and deposits on the coldest parts, such as cooling tower fill. How each salt reaches saturation is explained under scale formation in water systems.

Where are scale inhibitors used?#

Scale inhibitors are dosed in 5 kinds of industrial and municipal systems: open recirculating cooling towers, once-through cooling, reverse osmosis and thermal desalination, low-pressure boilers and closed loops. The 5 systems use the family as follows.

• Recirculating cooling: phosphonate and polymer blends.
• Once-through cooling: polyphosphates, organic phosphates and polymers below 1 to 5 mg/L (ppm) (Veolia handbook, chapter 30).
• RO and thermal desalination: antiscalants on the feed.
• Low-pressure boilers: dispersant polymers.
• Closed loops: HPAA and phosphonate blends.

Three Mechanisms of Scale Inhibitors#

Scale inhibitors work through 3 mechanisms: threshold inhibition, which delays nucleation at sub-stoichiometric doses, crystal modification, which distorts growing crystals into small, irregular, non-adherent particles, and dispersion, which keeps formed particles suspended.

1. Threshold inhibition: molecules adsorb onto growth sites of sub-microscopic crystallites (Amjad and Zuhl).
2. Crystal modification: crystals stay small, irregular and readily fractured.
3. Dispersion: the best dispersant polymers run about 4,500 to 30,000 Da, typically sulfonated (Standish 2024).

Phosphonates act mainly as threshold inhibitors, while polymers add crystal modification and dispersion, so most programs blend the two. The adsorption chemistry and laboratory evidence are covered under threshold inhibition, crystal modification and dispersion.

11 Types of Scale Inhibitors in 3 Classes#

The 11 scale inhibitors used in water treatment fall into 3 classes: 5 phosphonates (HEDP, ATMP, PBTC, DTPMP and HPAA), 4 polymer inhibitors and dispersants, and 2 green, phosphorus-free polymers (PASP and PESA).

Table 1 compares the 3 scale inhibitor classes and the polyphosphates by members, main strength and main limitation.

1. Phosphonate scale inhibitors (5)#

Phosphonate scale inhibitors are organophosphorus acids with C-P bonds that act as threshold inhibitors and chelants; this class has 5 inhibitors on this site: HEDP, ATMP, PBTC, DTPMP and HPAA. Amino-phosphonates such as ATMP and DTPMP are excellent sulfate inhibitors widely used in RO, while HEDP and PBTC serve mainly industrial cooling (EPRI 2014).

HEDP#

HEDP (C2H8O7P2) is the workhorse non-nitrogen phosphonate for calcium carbonate control in cooling water, sold as a 58 to 62 % acid solution and dosed at 2 to 5 mg/L (ppm) as active. It degrades with bromine over time and attacks galvanizing at high concentrations (EPRI 2014).

ATMP#

ATMP (amino trimethylene phosphonic acid) is the nitrogen-containing phosphonate most used in RO antiscalants, an excellent calcium sulfate inhibitor sold as a 50 % acid solution. Chlorine and bromine oxidize its amine linkage to orthophosphate, which limits it in halogenated cooling water.

PBTC#

PBTC (2-phosphonobutane-1,2,4-tricarboxylic acid) is the halogen-stable phosphonocarboxylic acid that holds calcite at about 225 to 250 times saturation and tolerates far more calcium than HEDP. It contains 11.5 % phosphorus by mass, so it is low in phosphorus, not phosphorus-free.

DTPMP#

DTPMP (C9H28N3O15P5) is a five-arm amino-phosphonate that inhibits carbonate, sulfate and phosphate scales, used mainly as an RO antiscalant and for barium sulfate. It is sold as a 50 % acid solution and shares the chlorine sensitivity of the amino-phosphonates.

HPAA#

HPAA (2-hydroxyphosphonoacetic acid) is a phosphonocarboxylic acid that works mainly as a carbon-steel corrosion inhibitor and only secondarily as a calcite inhibitor, dosed at 5 to 15 mg/L (ppm) as active in cooling water. Rapid reversion to orthophosphate with chlorine generally confines it to closed systems or to blends with halogen-stable inhibitors (EPRI 2014).

2. Polymer scale inhibitors and dispersants (4)#

Polymer scale inhibitors are low-molecular-weight anionic polycarboxylates and sulfonated copolymers that combine threshold activity, crystal modification and dispersion; this class has 4 inhibitors: polyacrylic acid and sodium polyacrylate, polymaleic acid, AA/AMPS and phosphinocarboxylic acid. Regulation (EU) 2023/2055 (REACH Annex XVII entry 78) excludes polymers with a water solubility above 2 g/L from the definition of synthetic polymer microparticles.

Polyacrylic acid and sodium polyacrylate#

Polyacrylic acid and its sodium salt are low-molecular-weight polycarboxylate homopolymers that inhibit calcium carbonate, calcium sulfate and barium sulfate and disperse solids without adding phosphorus. Low-molecular-weight polyacrylate is exceptionally stable with hypochlorite, but its calcium tolerance is poor.

Polymaleic acid (HPMA)#

Polymaleic acid (HPMA) is a hydrolyzed polymaleic anhydride, sold at about 50 % solids, that controls calcite at high hardness, alkalinity and temperature and resists oxidizing biocides. Enhanced polymaleic grades hold 225 to 250 times calcite saturation (LSI +2.5 to 3.0) at 8 to 12 mg/L (ppm) as active (Standish 2024).

AA/AMPS copolymer#

AA/AMPS is a sulfonated acrylic acid copolymer whose sulfonate groups give the calcium tolerance and dispersing power that plain polycarboxylates lack. It prevents calcium phosphate and calcium phosphonate precipitation where polyacrylic and polymaleic acid fail (Amjad and Zuhl).

Phosphinocarboxylic acid (PCA)#

Phosphinocarboxylic acid (PCA) is a telomer of acrylic acid and sodium hypophosphite whose phosphino end group adds calcium carbonate and calcium phosphate inhibition to a polycarboxylate chain. Supplier figures put its cooling dose at 5 to 20 mg/L (ppm) as product when used alone.

3. Green and phosphorus-free scale inhibitors (2)#

Green scale inhibitors are phosphorus-free polymers chosen to cut phosphorus discharge or meet biodegradability goals; this class has 2 inhibitors: polyaspartic acid (PASP) and polyepoxysuccinic acid (PESA).

Polyaspartic acid (PASP)#

Polyaspartic acid (PASP) is a phosphorus-free polyamino acid, supplied as a 40 % sodium salt solution, that gave 95 % calcium carbonate and 90 % calcium sulfate inhibition at 5 mg/L in static tests (Liu 2011). Limited thermal stability is its main weakness.

Polyepoxysuccinic acid (PESA)#

PESA (polyepoxysuccinic acid) is a polyether polycarboxylate free of phosphorus and nitrogen, supplied as a sodium salt solution of at least 40 % solids for high-alkalinity cooling water. Beyond supplier claims, its dose data are research-stage only.

Polyphosphates and other scale-control chemicals#

Sodium hexametaphosphate and other polyphosphates are inorganic threshold inhibitors of the phosphates family: they sequester iron, manganese and calcium, but they hydrolyze to orthophosphate, which can itself form calcium phosphate scale. Chelants and acids, by contrast, act stoichiometrically.

Sodium hexametaphosphate is compared under polyphosphates for sequestration and scale control.

Which Scale Inhibitor for Which System? Selection by Water and Application#

Scale inhibitor selection depends on 5 factors: the scale and its saturation level, exposure to chlorine or bromine, calcium hardness and temperature, phosphorus discharge limits and the system the inhibitor protects.

By scale type and calcite saturation#

Calcite saturation sets the first cut: HEDP and ATMP hold calcium carbonate up to about 125 to 150 times saturation (LSI below about 2.0), while PBTC holds about 225 to 250 times (LSI above 2.0 up to about 3.0) (Standish 2024). Above that range, enhanced polymaleic acid at 8 to 12 mg/L as active covers LSI +2.5 to 3.0.

Sulfate scale points to ATMP and DTPMP, and silica to high-molecular-weight polyacrylates and silica-specific antiscalants.

By chlorine and bromine exposure#

Oxidizing biocides decide between phosphonates: PBTC is highly stable to chlorine and bromine, HEDP is moderately stable and degrades with bromine over time, and ATMP, PAPEMP and HPAA revert to orthophosphate (EPRI 2014). Industrial cooling systems typically hold 0.5 to 1.0 mg/L (ppm) of oxidizing biocide residual.

How fast is the loss? With hypobromite, about 20 % of ATMP decomposed after 50 hours at 25 °C (77 °F) and about 25 % at 43 °C (109 °F) (Demadis 2006). Polyacrylates and polymaleic acid resist oxidizing biocides.

By calcium hardness and temperature#

Calcium tolerance, the phosphonate level above which calcium phosphonate precipitates, falls with hardness: at 250 mg/L Ca2+ (625 mg/L as CaCO3), pH 9.0 and 45 °C (113 °F), PBTC tolerates 73 mg/L, ATMP 13.5 mg/L and HEDP 7.5 mg/L; at 750 mg/L Ca2+ the values are 50, 7.0 and 4.8 mg/L (Amjad and Zuhl). Calcium PBTC is more soluble than calcium HEDP or calcium ATMP.

Temperature adds stress: raising water from 25 to 60 °C (77 to 140 °F) lowers phosphonate calcium tolerance 1.5 to 2 times, and precipitation risk rises with pH, temperature and calcium.

By phosphorus discharge limits#

Phosphorus limits favor low-phosphorus chemistries: each mg/L of HEDP adds 0.3 mg/L of total phosphorus to the discharge and each mg/L of PBTC adds 0.1 mg/L, while polycarboxylates, PASP and PESA add none (EPRI 2014). By mass, ATMP carries 31.1 % phosphorus, HEDP 30.1 % and PBTC 11.5 %.

EPRI defines a low-phosphorus cooling program as one that contributes less than 1.0 mg/L total P (3 mg/L as PO4), or as little as 0.5 mg/L P (1.5 mg/L as PO4), to the discharge. In an EPRI simulation, capping pH at 8.3 with sulfuric acid kept HEDP at or below 1.09 mg/L (0.33 mg/L P) up to 8 cycles. Actual limits are site-specific in NPDES and pretreatment permits.

Combined scale and corrosion inhibitor programs#

Most cooling programs are combined scale and corrosion inhibitor programs: HEDP and PBTC act as cathodic steel inhibitors at higher doses, PBTC stabilizes zinc, and alkaline phosphate programs at pH 8.0 to 9.2 carry 3 to 8 ppm of organic phosphate beside 2 to 10 ppm of inorganic phosphate (Veolia Water Technologies Handbook, chapter 31).

Molybdate works synergistically with nitrite, zinc and phosphonates in these blends. Zinc, molybdate, azoles and other metal-protecting chemistries are compared on corrosion inhibitors for water treatment.

By application: cooling towers, RO, boilers and closed loops#

Each system has a default inhibitor family: phosphonate and polymer blends in cooling towers, phosphonate or polycarboxylate antiscalants in reverse osmosis at 2 to 5 mg/L (ppm) as product, polymers in low-pressure boilers and HPAA or phosphonate blends in closed loops.

Boiler polymers carry pressure limits: the same handbook publisher's product literature gives all-polymer programs up to 600 psig (41 bar) and iron-dispersant polymers up to 900 psig (62 bar).

Table 2 lists the default inhibitors and the key number for each application.

Scale Inhibitor Dosage Ranges by Chemical#

Scale inhibitor doses run from about 2 mg/L (ppm) as active for ATMP in cooling water to 50 to 200 mg/L of HPAA in closed loops, and every range is tied to its basis: as active acid, as product or, in RO, as product on feed flow.

Table 3 gives the typical dose, basis and source for each inhibitor and system (mg/L = ppm).

Cooling values are recirculating-water residuals and RO values are doses on feed flow. Potable doses never exceed the product's NSF/ANSI/CAN 60 maximum use level.

Active versus product basis and feed rate (worked example)#

A scale inhibitor dose stated as active must be divided by the product's strength: 3 mg/L (ppm) of HEDP as active equals 5.0 mg/L of a 60 % HEDP solution.

The worked example uses lb/day = mg/L x MGD x 8.34 / purity and these inputs: dose 3 mg/L as active HEDP, flow 1 MGD (3,785 m³/d), product 60 % HEDP, and density 1.45 g/cm³ (12.1 lb/US gal) at 20 °C (68 °F). The feed is 3 x 1 x 8.34 / 0.60 = 41.7 lb/day (18.9 kg/d) of product. Dividing by the density gives about 13.0 L/d (3.4 gal/d) of liquid HEDP. The chemical dosage calculator converts any mg/L dose into kg/d, lb/day and pump settings.

How Are Scale Inhibitor Residuals Controlled and Tested?#

Scale inhibitor programs are controlled in 3 layers: a residual test or tracer confirms the dose, laboratory tests compare inhibitors before selection, and saturation indices set the limits the inhibitor must hold.

Phosphonate residuals by UV-persulfate test#

Phosphonate residuals are measured by UV-catalysed persulfate oxidation to orthophosphate (Hach Method 8007, 0.02 to 125 mg/L as PO4), and the reading is multiplied by a factor per inhibitor. The 5 conversion factors from mg/L as PO4 to mg/L of active inhibitor are listed below.

• HEDP: 1.085
• ATMP: 1.05
• PBTC: 2.84
• DTPMP: 1.207
• HPAA: 1.64

A reading of 2.0 mg/L as PO4 on a PBTC program therefore equals about 5.7 mg/L of PBTC (calculated). Polyphosphates do not interfere. Samples should be preserved at pH below 2 and at or below 6 °C (43 °F), and analyzed within 24 hours.

Polymer residuals, tracers and laboratory performance tests#

Polymer inhibitors are tracked with a turbidimetric polymer test or an inert tracer, and inhibitors are compared by pH-drift, turbidity, calcium mass balance or quartz crystal microbalance tests. Calcium mass balance is the least preferred (Standish 2024).

The pH-drift test shows the gap between inhibitors in minutes: at 50 °C (122 °F), untreated water failed after about 2 minutes, 5 ppm of HEDP held about 350 minutes and 5 ppm of PBTC held the full 600 minutes.

Saturation indices as control limits#

The Langelier Saturation Index (LSI = pH minus pHs) shows whether water tends to deposit calcium carbonate, but it is directional only and valid from pH 6.5 to 9.5, so inhibitor limits are written as LSI or times-saturation ceilings, not as mg/L of scale (Veolia handbook, chapter 25). A positive LSI means a deposition tendency.

In a handbook example, a tower at 10 cycles reaches LSI +2.5, and at 15 cycles without acid it reaches +3.2 and cannot operate; acid feed to pH 8.7 brings it back to +2.5, which an inhibitor can hold.

Worked calculations for LSI, Ryznar and Puckorius are on the Langelier Saturation Index page.

Which Standards and Regulations Apply to Scale Inhibitors?#

Scale inhibitors are governed in 3 layers: NSF/ANSI/CAN 60 certification where they reach drinking water, EN 15040:2014 and EN 15039 for membrane antiscalants in Europe, and 21 CFR 173.310 where boiler steam contacts food.

NSF/ANSI/CAN 60 maximum use levels for scale inhibitors#

NSF/ANSI/CAN 60 maximum use levels for scale inhibitors are product-specific and stated as product, from 0.8 mg/L for some HEDP antiscalants to 125 mg/L for some sodium polyacrylate products, with 20 mg/L the most common value for HEDP, ATMP, DTPMP, PBTC, polyacrylic acid and polymaleic acid (NSF listings, 2026-10-21). Each MUL counts the total dose across all application points.

No category exists for AA/AMPS, polyaspartic acid or PESA, and no HPAA listing was found.

Table 4 lists the MULs per inhibitor in mg/L as product (ranges retrieved 2026-10-20, most common values re-checked 2026-10-21).

HDTMPA potassium salt products carry 5 or 20 mg/L, and the one PAPEMP sodium salt listing carries 20 mg/L. How MULs are set and checked is explained on NSF/ANSI/CAN 60.

EN 15040:2014 and EN 15039 for membrane antiscalants#

EN 15040:2014 specifies phosphonic acids and salts used as membrane antiscalants for drinking water, covering ATMP, HEDP, DTPMP, EDTMP, PBTC and HDTMPA among others, and caps arsenic, cadmium, antimony and selenium at 3 mg/kg and lead, chromium and nickel at 10 mg/kg of dry product. It was published on 2014-03-18 and superseded EN 15040:2006.

The same standard caps mercury at 1 mg/kg, cyanide at 5 mg/kg, fluoride at 10 mg/kg and sulfate at 500 mg/kg of dry product.

Polycarboxylate antiscalants such as polyacrylates and polymaleic acid fall under EN 15039. All CEN/TC 164 product standards are listed under EN standards for drinking water treatment chemicals.

21 CFR 173.310 for boiler feedwater and food-contact steam#

Where boiler steam contacts food, 21 CFR 173.310 lists HEDP and sodium polyacrylate without a numeric limit and caps polymaleic acid at 1 ppm in feedwater as the acid, phosphinocarboxylic acid at 1.5 ppm and AA/AMPS at 20 ppm active (eCFR, checked 2026-10-20). The phosphinocarboxylic acid entry also caps acrylic acid monomer at 0.5 % on a dry basis.

The AA/AMPS entry applies only to grades with Mw of 9,900 or more and Mn of 5,700 or more. ATMP, PBTC, DTPMP, HPAA, PESA and polyaspartic acid are not listed in 21 CFR 173.310.

Every listed additive and limit is on 21 CFR 173.310 boiler water additives.

Complete List of Scale Inhibitors: All 11 Chemicals in One Table#

The complete list of scale inhibitors below gives all 11 chemicals with their CAS number, class, function, systems, most common NSF/ANSI/CAN 60 maximum use level and 21 CFR 173.310 status. Table 5 follows the class order used above.

Scale inhibitors are one family among many: every other chemical is filterable by CAS, function and NSF status in the water treatment chemical database.

Who Supplies Scale Inhibitors and Scale Control Programs?#

Scale and corrosion inhibitors are the fastest-growing product segment of water treatment chemicals, at a 4.8 % CAGR in Grand View Research's 2025 estimate of a USD 35.8 billion market.

Phosphonates are made from thermal phosphorus (P4) via phosphorus trichloride hydrolyzed to phosphorous acid, and the US had no thermal phosphorus capacity in 2014 (EPRI 2014). In September 2021, Chinese energy policy cut yellow phosphorus output and phosphonate prices rose to 2 to 3 times their historical level (Standish 2024).

Phosphonate makers and distributors by region are listed under water treatment chemical suppliers.

US trade remedies shape HEDP supply. Antidumping and countervailing duty orders on HEDP from China (A-570-045 and C-570-046) took effect on 2017-05-18 (82 FR 22807, 22809) and were continued on 2022-12-01 (87 FR 73756).

On 2026-02-23 the Department of Commerce ruled that acidic solid and powder HEDP circumvents those orders (91 FR 8410).

Segment estimates by publisher and year are on water treatment chemicals market.

Plants buy scale control in 2 ways. A program provider designs the inhibitor blend, sets residual and saturation targets and reports results, while a chemical supplier delivers a named strength with an NSF/ANSI/CAN 60 listing where the water is potable.

Quotes depend on strength and basis (for example 60 % HEDP as acid), volume and packaging (drum, IBC or bulk), freight distance, contract term and the certification required.

Program providers differ in testing, reporting and service scope, and how to compare them is covered under water treatment companies.

Are Scale Inhibitors Safe to Handle and in Drinking Water?#

Scale inhibitors reach drinking water only as NSF/ANSI/CAN 60 certified products dosed at or below each product's maximum use level, while at the plant the concentrated phosphonic acids are corrosive to eyes and metals. HEDP is notified (ECHA C&L) as H318 and H302, ATMP as H319 and H290, and HPAA carries a harmonised classification (CLP Annex VI) of H302, H314, H317 and H373; for the other inhibitors, see the supplier SDS. HEDP is not stored below 0 °C (32 °F). Oxidizers and phosphonates are kept at separate dose points until diluted (HSE, HSG274).

What is the purpose of a scale inhibitor?#

The purpose of a scale inhibitor is to keep heat-transfer surfaces, membranes and pipes free of mineral deposits so a system can run at higher cycles or recovery with less blowdown or acid.

Do scale inhibitors actually work?#

Yes, within their limits: in a pH-drift test at 50 °C (122 °F) untreated water precipitated calcium carbonate within about 2 minutes, 5 ppm of HEDP delayed it to about 350 minutes, and 5 ppm of PBTC prevented it for 600 minutes (Standish 2024). Above its saturation ceiling or calcium tolerance, an inhibitor fails.

How long does a scale inhibitor last?#

In an industrial system a scale inhibitor lasts only as long as its residual: it leaves with the blowdown and is fed continuously in proportion to makeup or blowdown, and halogens degrade ATMP and HPAA faster (Veolia handbook, chapter 35). In storage, supplier data give HEDP liquid a shelf life of at least 3 years and polymaleic acid at least 1 year.

What is the best scale inhibitor?#

No single scale inhibitor is best: PBTC suits halogenated, high-calcite cooling water, ATMP and DTPMP suit sulfate-limited RO, polymers suit dispersion and phosphorus-limited discharges, and a laboratory test on the actual water confirms the choice.

Is a scale inhibitor the same as an antiscalant?#

Yes in chemistry, no in usage: antiscalant is the name used in reverse osmosis and thermal desalination, where the same phosphonates and polymers are dosed at 2 to 5 mg/L (ppm) as product on feed flow (Hydranautics TAB-111).