Scale Formation in Water Systems: Calcium Carbonate, Sulfate, Silica and Phosphate

Scale formation in water systems is the precipitation of sparingly soluble salts, mainly calcium carbonate, calcium sulfate, silica and calcium phosphate, onto heat-transfer and membrane surfaces once the water holds more of them than it can dissolve. Calcium scale settles first on the hottest surfaces and silica on the coldest, so where does each scale form, and how far can a cooling tower, boiler or RO system concentrate its water before it does?

The 4 scales follow 4 triggers: calcium carbonate responds to heat and pH, calcium sulfate (also spelled calcium sulphate) to the sulfate that acid feed adds, silica to concentration and cold surfaces, and calcium phosphate to orthophosphate at high pH. Every chemical that controls them belongs to one of the 16 families of water treatment chemicals.

An RO unit at 75 % recovery concentrates its feed 4 times, a cooling tower at 15 cycles lifts silica to 150 mg/L (ppm), above the about 107 mg/L solubility of amorphous silica at 25 °C (77 °F), and a boiler at 2 % blowdown runs 50 cycles, enough to break the ASME silica limit of 40 mg/L at 600 psig (41.4 bar).

Every value on this page is checked against primary sources (Veolia Water Technologies Handbook, DuPont FilmTec manual, ASTM, eCFR, WHO); see our methodology and fact-checking process.

What Is Scale Formation in Water?#

Scale formation is the crystallization of dissolved mineral salts directly on a surface in contact with water, such as a boiler tube, a heat-exchanger wall, cooling-tower fill or an RO membrane, once their concentration passes the saturation point. Mineral scaling names the same process in membrane and cooling practice.

Scale differs from sludge, a loose precipitate. Boiler programs use the difference on purpose: calcium precipitated as hydroxyapatite at pH 11.0 to 12.0 forms a non-adherent sludge that blowdown removes (Veolia Water Technologies Handbook, chapter 12). Limescale is the everyday name for calcium carbonate scale.

What does scale do to boilers, cooling towers and membranes?#

Scale damages 3 kinds of water systems in 3 ways: it insulates boiler and heat-exchanger tubes, it carries silica into steam that deposits on turbines, and it blocks the tail end of RO membranes. The 3 effects are listed below.

• Tubes: boiler waterwalls see heat fluxes occasionally above 200,000 Btu/ft²·h (631 kW/m²) (same handbook), so an insulating deposit lifts tube-metal temperature quickly.
• Turbines: turbine deposits cost 5 % of efficiency and 20 % of capacity, and turbines may need steam purity of 10 to 30 ppb total solids.
• RO membranes: scale shows as a rising pressure drop at the tail end of the train (DuPont FilmTec manual).

How Does Scale Form? Supersaturation, Temperature and Concentration#

Scale forms when 4 conditions push a salt past its solubility: a supersaturated ion product, a pH that shifts the carbonate balance, a surface temperature that lowers solubility, and evaporation or recovery that concentrates the water.

Supersaturation: when the ion product passes the solubility product#

A salt starts to scale when the product of its ion concentrations, the ion product, exceeds its solubility product (Ksp); RO designers adjust the feed once the concentrate ion product of calcium sulfate passes 0.8 Ksp (DuPont FilmTec Reverse Osmosis Technical Manual). What decides the ion product of calcium carbonate, the most common scale?

The calcium carbonate equilibrium: bicarbonate, CO2 and pH#

Calcium carbonate scale forms by the reverse of limestone dissolution, Ca2+ + 2 HCO3- → CaCO3 + CO2 + H2O, so anything that strips carbon dioxide or raises pH, such as heating, aeration or alkali feed, pushes the balance toward scale. Boilers show the CO2 side at its extreme: bicarbonate alkalinity releases 0.79 mg/L CO2 per mg/L as CaCO3, carbonate alkalinity 0.35 mg/L (Veolia Water Technologies Handbook, chapter 19). The Langelier saturation index condenses the balance into LSI = pH - pHs.

Inverse solubility: why hot surfaces scale first#

Most calcium and magnesium salts, including calcium carbonate, calcium sulfate and calcium phosphate, have inverse solubility: they dissolve less as water heats, so they deposit first on the hottest surfaces, while silica dissolves better when hot and deposits on the coldest parts, such as cooling-tower fill. Why does an exchanger scale when the basin water looks stable? Skin temperature sets saturation at the wall: a water at pH 8.2, 400 mg/L calcium and 200 mg/L alkalinity as CaCO3 and 1,500 mg/L TDS has LSI +1.29 at a bulk 35 °C (95 °F) and +1.73 at a 60 °C (140 °F) tube wall (Carrier formula).

Concentration by evaporation, recovery and blowdown#

Every water system concentrates its scale-forming ions by a factor: cooling towers by their cycles of concentration, RO units by the concentration factor 1 / (1 - recovery), which reaches 4 at 75 % recovery, and boilers by 100 divided by the percent blowdown.

In towers, blowdown equals evaporation / (cycles - 1), and evaporation is about 1 % of recirculation per 5.6 °C (10 °F) of range; the mass balance and the blowdown arithmetic are worked through under cycles of concentration.

4 Main Types of Scale in Water Systems#

Four scales are the common scales of cooling and process water systems: calcium carbonate, calcium sulfate, silica and its silicates, and calcium phosphate, with magnesium sulfate joining them under certain conditions (Chiller and Cooling Best Practices). The 4 main scales differ in what triggers them and where they deposit.

1. Calcium carbonate scale#

Calcium carbonate scale is the most common scale in cooling and heating systems: it forms when bicarbonate alkalinity and calcium meet a high pH or a hot surface, and its tendency is the one the Langelier saturation index measures. Its solubility falls as temperature rises (Veolia Water Technologies Handbook, chapter 30), and the LSI that rates it is directional only, valid between pH 6.5 and 9.5. Tower practice is covered under scale and deposit control in cooling towers.

2. Calcium sulfate scale#

Calcium sulfate scale forms where calcium meets concentrated sulfate, most often in RO concentrate and in cooling towers that feed sulfuric acid for pH control, because acid that removes carbonate scale adds the sulfate that drives calcium sulfate. The DuPont FilmTec manual sets the 0.8 Ksp design limit and never uses sulfuric acid to clean RO membranes, because it would precipitate calcium sulfate on them.

3. Silica and silicate scale#

Silica scale forms when dissolved silica polymerizes into amorphous silica above about 107 mg/L as SiO2 at 25 °C (77 °F), or when silicate ions bind calcium, magnesium, iron or aluminum into hard metal silicates. Below neutral pH, silica sits as undissociated silicic acid that polymerizes into colloid or gel. The 107 mg/L value is Iler's solubility of about 1.8 mmol/L, nearly constant between pH 2 and 8.5; in RO concentrate, solubility rises below about pH 7.0 and above about 7.8 (same manual). Control options are covered under silica scale in RO and cooling systems.

4. Calcium phosphate scale#

Calcium phosphate scale forms where orthophosphate meets calcium at a high pH, in cooling water dosed with phosphate inhibitors and in RO concentrate from wastewater reuse, and its least soluble form, hydroxyapatite, has a pKsp of 57.74. The same manual lists pKsp 6.68 for brushite, 28.9 for tricalcium phosphate and 46.9 for octacalcium phosphate, and rates scaling with the Kubo index, SI = pHa - pHc (Kubo et al., 1979).

Boilers precipitate hydroxyapatite on purpose at pH 11.0 to 12.0, the rule behind phosphate treatment programs for boilers. Orthophosphate, polyphosphate and blended products are compared under phosphates in water treatment.

Which Less Common Scales Form? Barium, Strontium, Fluoride, Phosphonate and Magnesium Salts#

Five less common scales form in water systems: barium sulfate and strontium sulfate in RO concentrate, calcium fluoride at fluoride levels as low as 0.1 mg/L with high calcium, calcium phosphonate from overdosed inhibitors, and magnesium silicate or hydroxide in boilers. The 5 scales are listed below.

• Barium sulfate: RO feed barium below 15 µg/L for seawater, 5 µg/L for brackish water, 2 µg/L for brackish water with sulfuric acid (DuPont FilmTec manual).
• Strontium sulfate: feed adjusted at a concentrate ion product of 0.8 Ksp.
• Calcium fluoride: scales from 0.1 mg/L as F where calcium is high.
• Calcium phosphonate: calcium PBTC is more soluble than calcium HEDP and calcium ATMP.
• Magnesium silicate and hydroxide: magnesium forms silicate preferentially, hydroxide where no silica is present (Veolia Water Technologies Handbook, chapter 12).

Calcium tolerance sets the phosphonate ceiling: at 250 mg/L calcium, pH 9.0 and 45 °C (113 °F), Amjad and Zuhl measured 73 mg/L for PBTC, 13.5 mg/L for ATMP and 7.5 mg/L for HEDP, falling to 50, 7.0 and 4.8 mg/L at 750 mg/L calcium. See also struvite scale in wastewater. HEDP, ATMP, PBTC and DTPMP are compared under phosphonate scale inhibitors.

How Is Scaling Potential Calculated? 4 Worked Examples#

Scaling potential is calculated in 3 steps: express every ion on one basis, multiply by the system's concentration factor, and compare the result with the solubility or the limit that applies at the surface temperature.

Worked example 1: calcium and magnesium hardness expressed as CaCO3#

Worked example 1: water with 80 mg/L calcium and 12 mg/L magnesium carries 249.4 mg/L of total hardness as CaCO3, because calcium counts 2.5 times and magnesium 4.12 times on the CaCO3 basis. Inputs (illustrative analysis): calcium 80 mg/L as Ca, magnesium 12 mg/L as Mg, M-alkalinity 150 mg/L as CaCO3. Factors: Florida DEP (2014); carbonate hardness is the lesser of alkalinity and total hardness.

The water is very hard in the USGS classes (above 180 mg/L as CaCO3); units and classes are covered under water hardness.

Worked example 2: cooling tower water at 10 and 15 cycles#

Worked example 2: a makeup water with 10 mg/L silica and 40 mg/L alkalinity reaches 150 mg/L silica at 15 cycles, and sulfuric acid can pull its alkalinity back from 600 to 310 mg/L as CaCO3 but cannot touch the silica. Inputs: Table 31-2 of the Veolia Water Technologies Handbook (chapter 31), makeup calcium 50 and M-alkalinity 40 mg/L as CaCO3, silica 10 mg/L as SiO2, pH 7.0, pHs at 49 °C (120 °F).

Silica at 150 mg/L stays above the 107 mg/L solubility at 25 °C (77 °F), a solubility rather than a tower limit. The handbook's Table 31-1 tower circulates 50,000 gpm (11,355 m³/h), evaporates 1,000 gpm (227 m³/h) and at 15 cycles takes 1,070 gpm (243 m³/h) of makeup and 71 gpm (16.1 m³/h) of blowdown; a cooling tower water balance calculator repeats the arithmetic.

Worked example 3: RO concentrate at 75 % recovery#

Worked example 3: at 75 % recovery an RO unit concentrates its feed 4 times, so 100 mg/L calcium and 150 mg/L alkalinity as CaCO3 become 400 and 600 mg/L in the concentrate and push the concentrate LSI to +1.46. Inputs (DuPont FilmTec manual example): feed pH 7.5, 25 °C (77 °F), TDS 500 mg/L, 100 % rejection assumed for the x 4 values; concentrate pH runs about 0.5 units above feed.

Sulfuric acid adds sulfate that concentrates 4 times and tightens the brackish barium threshold from 5 to 2 µg/L; hydrochloric acid adds chloride instead.

Worked example 4: boiler water silica at 50 cycles#

Worked example 4: 1.0 mg/L of feedwater silica becomes 50 mg/L in boiler water at 2 % blowdown, which meets the ASME limit of 90 mg/L at 400 psig (27.6 bar) but breaks the 40 mg/L limit at 600 psig (41.4 bar) unless blowdown rises to 2.5 %. Inputs: feedwater silica 1.0 mg/L as SiO2 (illustrative), 2 % blowdown (C = 100 / 2 = 50), steam 2,000,000 lb/day (907,200 kg/d) as in the handbook's Table 13-4; blowdown = steam / (C - 1).

Vaporous silica carryover can occur from 400 psig (27.6 bar) and is usually not a problem below 900 psig (62.1 bar). Percent blowdown and control methods are covered under boiler blowdown.

Which saturation index fits which scale?#

Saturation indices cover calcium carbonate only: the Langelier, Ryznar, Puckorius and Stiff-Davis indices all use pH, calcium, alkalinity, dissolved solids and temperature, so calcium sulfate, silica, calcium phosphate, barium and strontium sulfate and calcium fluoride need their own solubility checks.

For RO concentrate, ASTM D3739-19 applies the LSI from 10 to 10,000 mg/L TDS and ASTM D4582-23 the Stiff-Davis index above 10,000 mg/L. pHs, the four indices and three worked examples are explained under the Langelier saturation index.

For calcium carbonate, the Langelier saturation index calculator returns LSI, RSI and PSI in metric or US units from one water analysis.

How Is Scale Formation Measured and Monitored?#

Scale formation is measured in 2 ways: water tests give the ion concentrations that every saturation calculation needs, and deposit monitors on heat-transfer surfaces and membranes show whether scale is actually forming.

Water tests that feed every scaling calculation#

Six water tests feed scaling calculations: pH, calcium and total hardness, M-alkalinity, conductivity or TDS, sulfate and reactive silica, each reported on its own basis such as mg/L as CaCO3 or mg/L as SiO2. Editions: ASTM store, 2026-10-20; Standard Methods 24th edition (APHA Press, 2023).

The molybdate test misses polymeric and colloidal silica. Both parameters are covered under alkalinity of water and silica in water.

Deposit monitoring on heat-transfer surfaces and membranes#

Deposit monitoring tests the surface, not the water: ASTM D4778-24 measures the fouling tendency of cooling water under heat-transfer conditions, and in RO a rising pressure drop across the last elements signals scale. D4778-24 superseded D4778-15 in 2024.

Typical Scaling Limits by System: Cooling Towers, Boilers, RO and Drinking Water#

Each water system sets its scaling limits differently: cooling towers by calcium, alkalinity and silica in the circulating water, boilers by feedwater hardness and boiler-water silica per pressure, RO plants by percent saturation in the concentrate, and drinking-water utilities by hardness and dissolved solids. The table lists values, bases and sources.

Cooling towers#

Cooling towers run stabilized phosphate and alkaline phosphate programs at 75 to 1,200 mg/L calcium hardness as CaCO3 (Veolia Water Technologies Handbook, chapter 31) and hold silica near 100 mg/L as SiO2 in one published guideline.

Deposit control continues under scale and deposit control in cooling towers; programs, cycles and control limits are compared under cooling tower water treatment.

Boilers#

Boilers prevent scale by starving it: feedwater hardness stays at 0.300 mg/L as CaCO3 or less up to 450 psig (31.0 bar) and becomes not detectable above 1,000 psig (68.9 bar), while boiler-water silica falls from 150 to 1 mg/L as pressure rises. These are ASME consensus values. Every band is tabulated under boiler water limits by pressure, deposits under boiler scale, and internal treatment programs by pressure under boiler water treatment.

RO and NF membranes#

RO and NF plants express scaling limits as concentrate saturation: with antiscalant, Hydranautics TAB-111 designs conservatively to a concentrate LSI of +1.8 and typically limits gypsum to 230 % saturation. Without antiscalant, calcium sulfate and strontium sulfate stay below 0.8 Ksp. Dose ranges, chemistries and the full saturation table sit under RO antiscalants.

Drinking water distribution#

Drinking-water utilities have no enforceable US limit for scale-forming minerals: 40 CFR 143.3 sets a non-enforceable 500 mg/L secondary standard for dissolved solids and none for hardness or silica, while WHO notes that hardness of 200 mg/L as CaCO3 or more may cause scale.

What Changes Scale Formation? Chemicals and Operating Controls#

Four controls change scale formation: acid feed lowers pH and alkalinity, softening removes calcium and silica, scale inhibitors hold salts above saturation, and blowdown, recovery and temperature limit how far the water concentrates.

Acid feed and pH control#

Acid feed controls calcium carbonate scale by converting bicarbonate alkalinity to carbon dioxide, and the acid chosen decides the side effect: sulfuric acid raises sulfate and calcium sulfate risk, hydrochloric acid raises chloride and corrosion risk. Acid feed also reduces magnesium silicate scaling, while CO2 injection is generally not practical in towers because aeration raises pH again (Veolia Water Technologies Handbook, chapter 31).

In worked example 2, sulfuric acid brings the tower from LSI +3.2 to +2.5. Acids and alkalis for pH control are compared under pH adjustment chemicals.

Softening and silica removal#

Softening removes the calcium and magnesium that scale needs: sodium-cycle ion exchange swaps them for sodium, lime softening precipitates them, and hot lime with magnesium at 60 to 70 °C (140 to 158 °F) also cuts silica to about 1 mg/L (DuPont FilmTec manual). RO feed holds aluminum and ferric iron each below 0.05 mg/L even below silica saturation (same manual). See lime softening and water softening resin.

Scale inhibitors and dispersants#

Scale inhibitors let water run above saturation without depositing: threshold inhibitors adsorb on growing crystals at doses far below the stoichiometric amount, crystal modifiers distort the crystals so they do not adhere, and dispersants keep particles suspended. HEDP and ATMP hold calcite to about 125 to 150 times saturation, PBTC to about 225 to 250 times (Standish 2024).

The mechanism is explained under threshold inhibition, dispersion under polymer scale inhibitors and dispersants. Phosphonates, polymers and green inhibitors are compared under scale inhibitors.

Blowdown, recovery and temperature#

Blowdown, recovery and temperature set how far the water concentrates and how hot the scaling surface runs: fewer cycles, lower RO recovery and a cooler tube wall all keep each salt further from saturation. In worked example 4, 2.5 % instead of 2 % blowdown cuts silica from 50 to 40 mg/L for 10,466 lb/day (4,747 kg/d) more blowdown. Tower rates are covered under cooling tower blowdown.

5 Common Mistakes in Predicting Scale Formation#

Five mistakes recur in scale predictions, and the first, reading a calcium carbonate index as a verdict on every scale, hides 6 scales at once. The 5 mistakes are listed below.

1. Using the LSI for sulfate, silica or phosphate scale: check each scale against its own solubility.
2. Mixing "as Ca" and "as CaCO3": 80 mg/L as Ca equals 200 mg/L as CaCO3.
3. Calculating at bulk instead of tube-wall temperature: LSI +1.29 in the bulk becomes +1.73 at the wall.
4. Switching to sulfuric acid without checking calcium sulfate and barium: brackish RO barium limits fall from 5 to 2 µg/L.
5. Overdosing phosphonate into high-calcium, high-pH water: HEDP tolerance drops from 7.5 to 4.8 mg/L as calcium rises from 250 to 750 mg/L.

How Does Scale Formation Relate to Fouling, Corrosion and Cleaning?#

Scale formation is one of 4 fouling types and the mirror image of corrosion: the same saturation balance that deposits scale on one side dissolves protective layers on the other, and scale that forms anyway is removed by acid cleaning.

Scaling, corrosion, disinfection and coagulation are introduced together under the chemistry concepts of scaling and corrosion.

What is the difference between scaling and fouling?#

Scaling is crystallization of dissolved salts on a surface, while fouling is the wider term that also covers colloidal, organic and biological deposits, and RO operators tell them apart by where the pressure drop rises: the tail end for scale, the lead end for biofilm.

The DuPont FilmTec manual names the 4 types colloidal, biological, organic and scaling; see membrane fouling.

Is scale-forming water less corrosive?#

Scale-forming water is less aggressive to concrete and cement linings by the LSI, but a positive index does not protect steel: UFC 3-240-13FN states that the LSI "has lost its practical application for cooling water systems".

WHO (2017) notes that hardness of 100 mg/L as CaCO3 or less may make drinking water more corrosive; see corrosion in water systems.

How is scale removed once it forms?#

Scale is removed by inhibited acid cleaning: 5 % inhibited hydrochloric acid is the most common boiler solvent, and silicate scale needs a fluoride source such as ammonium bifluoride (Veolia Water Technologies Handbook, chapter 15).

Procedures sit under boiler chemical cleaning and descaling; products are compared under descaling and chemical cleaning agents.

Is calcium sulfate soluble in water?#

Calcium sulfate is sparingly soluble: it stays dissolved in most makeup waters but precipitates once concentration pushes its ion product past the solubility product, which RO designers treat as a limit at 0.8 Ksp and, with antiscalant, typically at 230 % saturation.