Phosphates in water treatment are 7 orthophosphate and polyphosphate chemicals, from phosphoric acid to sodium hexametaphosphate, that utilities and plant operators dose to control lead and copper corrosion, sequester iron and manganese, inhibit scale and condition boiler water. Their targets run from 0.3 mg/kg of phosphate in a high-pressure drum boiler to 25 mg/L as PO4 in a cooling tower, so which phosphate fits which system?
The 7 chemicals form 3 classes: 5 orthophosphates (phosphoric acid, zinc orthophosphate, MSP, TSP, DSP), 2 polyphosphates (SHMP, STPP) and blended products. Together they are one of the 16 families of water treatment chemicals.
LCRI sets tap minimums of 0.5 or 1.0 mg/L as PO4, NSF/ANSI/CAN 60 caps each product dose, and every dose here carries its basis, because 1 mg/L as PO4 is 0.326 mg/L as P.
Every value on this page is checked against primary sources (eCFR, US EPA, NSF listings, AWWA, CEN, IAPWS and the handbooks named in the text); see our methodology and fact-checking process.
What Are Phosphates in Water Treatment?#
Phosphates in water treatment are inorganic phosphate chemicals, such as phosphoric acid, zinc orthophosphate, trisodium phosphate and sodium hexametaphosphate, dosed to form protective films, hold metals in solution or precipitate hardness as a removable sludge.
Phosphonates, such as HEDP, ATMP and PBTC, are a separate organic family; both add phosphorus: 1 mg/L of HEDP adds 0.3 mg/L of total phosphorus (EPRI, 2014).
What do phosphate chemicals do in water systems?#
Phosphate chemicals do 5 jobs in water systems, listed below.
- Corrosion control of lead, copper and steel
- Sequestration of iron and manganese
- Threshold scale inhibition
- Hardness precipitation and pH buffering in boilers
- Acid or alkaline cleaning of membranes and new systems
How Do Phosphates Work in Water?#
Phosphates work in 3 ways: orthophosphate forms low-solubility metal phosphate films, polyphosphates bind divalent metals in soluble complexes and adsorb on crystal nuclei, and boiler orthophosphate precipitates calcium as a removable sludge while buffering pH.
Orthophosphate corrosion inhibition on lead, copper and steel#
Orthophosphate reacts with lead(II) and copper(II) at the pipe wall to form low-solubility phosphate solids; dose, pH, dissolved inorganic carbon and existing scale decide the effect, and aluminum and iron interfere.
Zinc, silicate, molybdate, nitrite and azole inhibitors are compared on corrosion inhibitors for water treatment.
On cooling-water steel, orthophosphate is both anodic passivator and cathodic precipitator (Veolia Water Technologies Handbook, chapter 24).
Polyphosphate sequestration and threshold scale inhibition#
Polyphosphates such as SHMP and STPP form soluble complexes with calcium, magnesium, iron and manganese and adsorb on crystal nuclei, which prevents colored water and inhibits scale at threshold doses.
Reversion to orthophosphate accelerates after 3 to 5 days of water age (Water Online, 2025). Phosphonate and polymer alternatives are compared under scale inhibitors for water systems.
Phosphate precipitation and pH buffering in boilers#
In boiler water, phosphate precipitates calcium as hydroxyapatite, Ca5(OH)(PO4)3, a non-adherent sludge removed by blowdown, while trisodium phosphate hydrolysis (Na3PO4 + H2O → Na2HPO4 + NaOH) buffers pH. Hideout is phosphate precipitating at high heat flux and returning at low load.
7 Phosphate Chemicals in 3 Classes#
The 7 phosphate chemicals fall into 3 classes: 5 orthophosphates, 2 polyphosphates and the blended products made from them. The table compares the classes.
| Class | Members | pH effect | Typical role |
|---|---|---|---|
| orthophosphates | phosphoric acid, zinc orthophosphate, MSP, TSP, DSP | acid (H3PO4); acidic (MSP about pH 4.5 at 1 %, zinc blends); mildly alkaline (DSP); strongly alkaline (TSP) | lead and copper control; boiler internal treatment |
| polyphosphates | SHMP, STPP (plus TKPP, TSPP and potassium tripolyphosphate as sections) | reverts to orthophosphate | iron and manganese sequestration; threshold scale inhibition; cleaning |
| blended phosphates | ortho-polyphosphate products | per product | corrosion control and sequestration together |
| Related family, not phosphates: phosphonate scale inhibitors | HEDP, ATMP, PBTC, DTPMP | n/a | scale inhibition |
1. Orthophosphates (5)#
Orthophosphates are phosphoric acid and its salts, each supplying the single phosphate ion: phosphoric acid, zinc orthophosphate, monosodium, trisodium and disodium phosphate.
Phosphoric acid#
Phosphoric acid (H3PO4) is the direct orthophosphate source: a 36 to 85 % liquid acid that lowers pH and alkalinity, and 1.376 mg/L of 75 % acid delivers 1 mg/L as PO4.
Zinc orthophosphate#
Zinc orthophosphate (CAS 7779-90-0) is an acidic liquid blend, typically 1:1 to 1:10 Zn:PO4 by weight, whose zinc protects cement-lined and asbestos-cement pipe.
Monosodium phosphate (MSP)#
Monosodium phosphate (MSP, NaH2PO4) is the acidic sodium orthophosphate, about pH 4.5 at 1 %, used for low-alkalinity water.
Trisodium phosphate (TSP)#
Trisodium phosphate (TSP, Na3PO4) is the alkaline boiler orthophosphate and the only phosphate IAPWS guidance allows in phosphate treatment of drum boilers.
Disodium phosphate (DSP)#
Disodium phosphate (DSP, Na2HPO4) neutralizes free caustic to TSP, the basis of coordinated phosphate control, which IAPWS TGD4-11(2015) advises against.
2. Polyphosphates (2)#
Polyphosphates are condensed phosphate chains that sequester metals and revert to orthophosphate over days; the 2 entity chemicals are SHMP and STPP. TKPP, tetrasodium pyrophosphate and potassium tripolyphosphate are covered on the polyphosphates page.
Sodium hexametaphosphate (SHMP)#
Sodium hexametaphosphate (SHMP), the glassy sodium polyphosphate (CAS 68915-31-1), sequesters iron, manganese and calcium but reverts to orthophosphate with time and temperature.
Sodium tripolyphosphate (STPP)#
Sodium tripolyphosphate (STPP, Na5P3O10) is a short-chain polyphosphate made by heating MSP with DSP, used as a sequestrant and alkaline membrane cleaner.
3. Blended phosphates (ortho-polyphosphate products)#
Blended phosphates contain at least 2 phosphate species, one polymeric, each at least 5 % of dry weight (NSF/ANSI/CAN 60-2020, 5.2.1), with 5 to 70 % orthophosphate in commercial blends (Hill and Cantor, 2011).
Which Phosphate for Which System? Selection by Application#
Phosphate selection follows the system: orthophosphate for lead and copper, polyphosphate for iron and manganese, trisodium phosphate for boilers, phosphates with polymers for cooling towers, and phosphate cleaners for membranes. The table maps each application to its phosphates.
| Application | Phosphates | Key number |
|---|---|---|
| corrosion control treatment for lead and copper | phosphoric acid, zinc orthophosphate, MSP, blends | LCRI tap minimum 0.5 or 1.0 mg/L as PO4 |
| iron and manganese removal chemistry | SHMP, STPP, blends | about 2 mg/L as PO4 per mg/L Fe(II); 5 per mg/L Mn(II) |
| phosphate treatment programs for boilers | TSP (DSP and MSP historic) | 0.3 to 1.5 mg/kg PO4 at 17 MPa (2,500 psi) |
| corrosion control in cooling water systems | ortho and poly phosphates with zinc and polymers | 2 to 10 or 10 to 25 mg/L as PO4 |
| white rust and galvanized steel passivation | SHMP | 3 to 5 mg/L as PO4 after 5 to 6 days |
| RO antiscalants | SHMP (legacy) | 20 mg/L in the concentrate, LSI below 1 |
| RO membrane cleaning chemicals | phosphoric acid, STPP | 0.5 wt% at pH 1 to 2; 2.0 % at pH 10.0 |
| pre-commission cleaning and flushing of closed systems | TSP | 1 to 2 % solution |
| chemical phosphorus removal | removal side, not dosing | WWTP phosphorus load +10 to 35 %, median 20 % |
Drinking water: lead and copper corrosion control#
Orthophosphate controls lead and copper: US EPA's OCCT guidance (EPA 816-B-16-003, 2016, updated 2019) uses 1.0 to 3.0 mg/L as PO4 (0.33 to 1.0 mg/L as P) at pH 7.2 to 7.8 as a reference target, not a limit, and 3 to 3.5 mg/L as PO4 or more with lead service lines.
Polyphosphates alone may increase tap lead and copper (US EPA). Orthophosphate works best below 10 mg C/L of dissolved inorganic carbon, avoiding pH 8.0 to 8.5 for lead; tap minimums come from the Lead and Copper Rule (LCRI).
For very soft water, the UK Drinking Water Inspectorate names pH 8.0 to 8.5 or orthophosphate dosing.
Drinking water: iron and manganese sequestration#
Polyphosphates prevent colored water when dosed before oxidation or chlorination, at about 2 mg/L as PO4 per mg/L of iron(II) and 5 mg/L per mg/L of manganese(II) (industry practice, Water Online, 2025).
US EPA prefers removal, which is compared under drinking water treatment chemicals.
Boilers: trisodium phosphate and precipitation programs#
Trisodium phosphate is the boiler phosphate: precipitation programs form non-adherent calcium phosphate at boiler-water pH 11.0 to 12.0 (Veolia Water Technologies Handbook, chapter 12), and in high-purity drum boilers IAPWS TGD4-11(2015) allows only TSP, between Na:PO4 3:1 and TSP plus 1 mg/kg NaOH.
IAPWS applies phosphate treatment up to 19.3 MPa (2,800 psi), notes hideout from about 10.0 to 10.5 MPa (1,450 to 1,500 psi) and sets 0.3 to 1.5 mg/kg PO4 at pH 9.0 to 9.3 at 17 MPa (2,500 psi). MSP or DSP below Na:PO4 3:1 caused acid phosphate corrosion.
Limits by pressure are covered under boiler water treatment.
Cooling towers: phosphate cooling water treatment#
Phosphate cooling water treatment holds 2 to 10 mg/L (ppm) of inorganic phosphate as PO4 at pH 8.0 to 9.2 in alkaline programs and 10 to 25 mg/L total inorganic PO4 at pH 6.8 to 7.8 in stabilized programs (the same handbook, chapter 31).
Both run at 75 to 1,200 mg/L calcium as CaCO3 with polymers against calcium phosphate.
New galvanized towers get high-polyphosphate SHMP for 5 to 6 days, then 3 to 5 mg/L as PO4 (US Army PWTB 420-49-22, 1999). Cycles and blowdown sit in cooling tower water treatment.
Membranes and closed loops: phosphate cleaners and legacy antiscalant use#
Membrane and closed-loop systems use phosphates mainly as cleaners: phosphoric acid at 0.5 wt% and pH 1 to 2 removes carbonate and iron from RO membranes (DuPont FilmTec Reverse Osmosis Technical Manual), and STPP at pH 10.0 removes sulfate scale and organic fouling.
SHMP at 20 mg/L in the concentrate allows an LSI below 1, but the same manual does not recommend it.
New closed systems are cleaned with 1 to 2 % TSP before the fill. Glycol and chilled circuits are covered under closed loop water treatment.
Phosphate Dosage Ranges and Dose Basis#
Phosphate doses are set as PO4 or as P at the point of control, not as product: 1.0 to 3.0 mg/L as PO4 at the tap, 0.3 to 1.5 mg/kg PO4 in a 17 MPa boiler and 2 to 25 mg/L as PO4 in a cooling tower each need a different product dose. The table gives each target with its basis (mg/L = ppm).
| Phosphate | System | Typical dose or target | Basis | Control condition |
|---|---|---|---|---|
| Phosphoric acid, MSP, zinc orthophosphate, blends | drinking water, lead and copper | 1.0 to 3.0 at the tap (EPA reference); 3 to 3.5 and higher with lead service lines | as PO4 (0.33 to 1.0 as P) | pH 7.2 to 7.8; LCRI tap minimum 0.5 or 1.0 as PO4 |
| Blended phosphates | drinking water OCCT start | at least 0.5 orthophosphate | as P (1.5 as PO4) | ortho fraction 5 to 70 % |
| SHMP, STPP | iron and manganese sequestration | 2 per mg/L Fe(II); 5 per mg/L Mn(II) | as PO4 (industry practice) | dose before oxidation |
| TSP | drum boiler, IAPWS phosphate treatment, 17 MPa (2,500 psi) | 0.3 to 1.5 mg/kg | as PO4 | pH 9.0 to 9.3; Na:PO4 3:1 to TSP + 1 mg/kg NaOH |
| TSP | HRSG LP 0.5 MPa (70 psi) / IP 2.4 MPa (350 psi) / HP 14 MPa (2,000 psi) | 0.3 to 8.0 / 0.3 to 7.0 / 0.3 to 2.0 mg/kg | as PO4 | IAPWS TGD4-11(2015) |
| TSP | precipitation program, lower pressure | residual per program | as PO4 | boiler-water pH 11.0 to 12.0 |
| Orthophosphate | alkaline cooling program | 2 to 10 inorganic PO4 | as PO4 | pH 8.0 to 9.2; Ca 75 to 1,200 as CaCO3 |
| Ortho + poly phosphate | stabilized cooling program | 10 to 25 total inorganic PO4 | as PO4 | pH 6.8 to 7.8; Ca 75 to 1,200 as CaCO3 |
| SHMP | new galvanized cooling tower | high polyphosphate 5 to 6 days, then 3 to 5 | as PO4 | white-rust passivation |
| SHMP | RO feed (legacy) | 20 in the concentrate | as product | concentrate LSI below 1 |
| Phosphoric acid | RO/NF cleaning (off-line) | 0.5 wt% | as H3PO4 | pH 1 to 2, 25 °C (77 °F) |
| STPP | RO/NF cleaning (off-line) | 2.0 % = 7.7 kg per 379 L (17.0 lb per 100 US gal) | as product | pH 10.0, 40 °C (104 °F) |
| TSP | closed-loop pre-commission cleaning | 1 to 2 % solution | as product | flush out before fill |
Potable doses stay within each product's NSF/ANSI/CAN 60 maximum use level; the STPP row follows Hydranautics TSB107.
How do phosphate doses convert between PO4, P and product?#
One mg/L as PO4 equals 0.326 mg/L as P, and the product dose that delivers 1 mg/L as PO4 ranges from 1.214 mg/L of 85 % phosphoric acid to 4.00 mg/L of trisodium phosphate dodecahydrate.
| Form | Product per 1 mg/L as PO4 | Product equal to 10 mg/L as PO4 (NSF/ANSI/CAN 60-2020 Table 5.1) |
|---|---|---|
| as P | 0.326 mg/L as P (1 mg/L as P = 3.066 mg/L as PO4) | n/a |
| Phosphoric acid, 85 % | 1.214 mg/L | 13.8 (dry basis, 100 % acid) |
| Phosphoric acid, 75 % | 1.376 mg/L | 13.8 (dry basis) |
| MSP (anhydrous) | n/a | 12.6 |
| DSP (anhydrous) | 1.49 mg/L | 14.9 |
| TSP (anhydrous) | 1.726 mg/L | 17.3 |
| TSP dodecahydrate | 4.00 mg/L | n/a |
| STPP | n/a | 12.9 |
| SHMP (glassy sodium polyphosphate) | n/a | 10.7 to 11.9 |
Factors are calculated from molecular weights (PubChem).
How is a phosphate dose turned into a feed rate?#
A phosphate feed rate equals dose times flow times 8.34 divided by purity: holding 1.0 mg/L as PO4 with 75 % phosphoric acid at 10 MGD (37,854 m³/d) needs 1.376 mg/L of product, or about 114.8 lb/day (52.1 kg/d).
Worked example inputs: flow 10 MGD (37,854 m³/d); target 1.0 mg/L as PO4; 75 % H3PO4 at 1.376 mg/L product per mg/L as PO4 (purity included).
The chemical dosage calculator converts mg/L into kg/d, lb/day and pump settings.
How Is Phosphate Treatment Controlled and Tested?#
Phosphate treatment is controlled by 3 measurements: orthophosphate as PO4 at the entry point and taps, total phosphate for polyphosphate, and pH, alkalinity or boiler-water conductivity.
Orthophosphate and total phosphate tests#
Orthophosphate is measured by the ascorbic acid method (SM 4500-P E or F, EPA 365.1, 0.01 to 1.0 mg/L as P) or by ion chromatography (EPA 300.0), and polyphosphate equals total minus orthophosphate. The 3 test rules are listed below.
- Filter within 15 minutes; analyze within 48 h at ≤ 6 °C (42.8 °F) (40 CFR 136.3).
- Report orthophosphate as PO4 and the dose rate as LCRI water quality parameters (40 CFR 141.87).
- Skip ASTM D515, withdrawn in 1997.
Passivation, reversion and feed-system limits#
A new orthophosphate program starts at 2 to 3 times the maintenance dose (Hill and Cantor, 2011). The 3 feed limits are listed below (Water Online, 2025).
- Temperature: viscous, freezing below 3.3 °C (38 °F).
- Storage: 30-day supply; containment ≥ 110 % of the largest tank.
- Materials: HDPE, XLPE, FRP or stainless steel tanks, fed apart from lime.
Which Standards and Regulations Apply to Phosphates?#
Phosphates for drinking water are governed in 4 layers: NSF/ANSI/CAN 60 certification, AWWA and EN product standards, the Lead and Copper Rule's orthophosphate requirements and EU Article 12 rules for treatment chemicals.
NSF/ANSI/CAN 60 typical and maximum use levels for phosphates#
NSF/ANSI/CAN 60 sets phosphate typical use levels at the product dose equal to 10 mg/L as PO4 on a dry basis, from 10.7 mg/L for glassy sodium polyphosphate to 17.4 mg/L for TKPP (2020 edition, Table 5.1).
Each certified product carries its own maximum use level (MUL), explained on NSF/ANSI/CAN 60.
| Phosphate | Typical use level (2020 Table 5.1), mg/L | Product MUL (listings), mg/L | Condition |
|---|---|---|---|
| Phosphoric acid | 13.8 | commodity grades 12 to 13; formulated products up to 162 | none |
| Zinc orthophosphate | 4.0 | 6 to 40 | zinc in finished water ≤ 2.0 mg/L |
| MSP | 12.6 | most common 13; up to 62 | none |
| TSP | 17.3 | 12 to 42; most common 17 | none |
| DSP | 14.9 | 15 | none |
| SHMP (sodium polyphosphate, glassy) | 10.7 to 11.9 | 10 to 60 | none |
| STPP | 12.9 | 12 to 13 | none |
| Blended phosphates | n/a | most common 25 and 11; up to 198 | none |
MULs count all application points (NSF listings, 2026-10-20; zinc orthophosphate 2026-10-21).
AWWA and EN product standards for phosphates#
Six AWWA B-standards cover 6 of the 7 phosphate chemicals, from ANSI/AWWA B502-23 for sodium hexametaphosphate to ANSI/AWWA B507-23 for phosphoric acid, while trisodium phosphate has none on the current AWWA list.
| Phosphate | AWWA standard | EN standard (CEN/TC 164) |
|---|---|---|
| Phosphoric acid | ANSI/AWWA B507-23 | EN 974 |
| Zinc orthophosphate | ANSI/AWWA B506-25 | none identified |
| MSP | ANSI/AWWA B504-24 | EN 1198 |
| TSP | none current (B500 historic) | EN 1200 |
| DSP | ANSI/AWWA B505-24 | EN 1199 |
| SHMP | ANSI/AWWA B502-23 | EN 1212; EN 15041:2014 (membrane antiscalants) |
| STPP | ANSI/AWWA B503-23 | EN 1210 |
Editions are tracked under AWWA B-series standards.
LCRI orthophosphate requirements and EU rules#
LCRI requires at least 0.5 mg/L as PO4 of orthophosphate in tap samples for systems without prior corrosion control treatment, or 1.0 mg/L as PO4 for systems with it, and a minimum distribution pH of 7.0 (40 CFR 141.82(f), 89 FR 86418, 2024-10-30), unless the State finds otherwise.
Action levels are 0.010 mg/L lead and 1.3 mg/L copper (90th percentile, 40 CFR 141.80(c)), compliance from 2027-11-01.
The EU Drinking Water Directive 2020/2184 covers treatment chemicals in Article 12; lead is 10 µg/L until 2036-01-12, then 5 µg/L.
Complete List of Phosphates: All 7 Chemicals in One Table#
The complete list below gives all 7 phosphates with CAS number, class, function, systems, NSF/ANSI/CAN 60 MUL, AWWA and EN standard and 21 CFR 173.310 status.
| # | Phosphate | CAS | Class | Function | Systems | NSF 60 MUL (as product), mg/L | AWWA / EN | 21 CFR 173.310 |
|---|---|---|---|---|---|---|---|---|
| 1 | phosphoric acid | 7664-38-2 | orthophosphate (acid) | orthophosphate source; pH adjuster; membrane cleaner | drinking water; RO/NF cleaning | 12 to 13 (commodity) | B507-23 / EN 974 | not listed |
| 2 | zinc orthophosphate | 7779-90-0 | orthophosphate (zinc blend) | corrosion inhibitor | drinking-water distribution; industrial cooling | 6 to 40 | B506-25 / none | not listed |
| 3 | monosodium phosphate (MSP) | 7558-80-7 | orthophosphate (acidic salt) | orthophosphate for low-alkalinity water | drinking water | most common 13 | B504-24 / EN 1198 | listed (sodium phosphate) |
| 4 | trisodium phosphate (TSP) | 7601-54-9 (dodecahydrate 10101-89-0) | orthophosphate (alkaline) | boiler internal treatment; alkalinity; cleaning | boilers; drinking water; closed loops | most common 17 | none / EN 1200 | listed (sodium phosphate) |
| 5 | disodium phosphate (DSP) | 7558-79-4 | orthophosphate (mildly alkaline) | boiler (historic coordinated phosphate); corrosion control | boilers; drinking water | 15 | B505-24 / EN 1199 | listed (sodium phosphate) |
| 6 | sodium hexametaphosphate (SHMP) | 68915-31-1 (also 10124-56-8, cyclic hexamer) | polyphosphate (glassy) | sequestrant; threshold inhibitor; blend component | drinking water; cooling; boiler formulations; legacy RO | 10 to 60 | B502-23 / EN 1212 | listed |
| 7 | sodium tripolyphosphate (STPP) | 7758-29-4 | polyphosphate (linear) | sequestrant; alkaline membrane cleaner | drinking water; boiler formulations; RO/NF cleaning | 12 to 13 | B503-23 / EN 1210 | listed |
Sources: NSF listings, 2026-10-20 (zinc orthophosphate 2026-10-21); eCFR 21 CFR 173.310, 2026-10-15; AWWA standards list, 2026-10-20.
Every other chemical is filterable by CAS, function and NSF status in the water treatment chemical database.
Who Supplies Phosphates and Phosphate Programs?#
Water treatment takes less than 1 % of US phosphoric acid consumption but about 40 % of sodium polyphosphate consumption (US EPA supply chain profiles, 2023).
Shelf lives are 60 months for phosphoric acid, 12 to 18 months for SHMP and STPP and about 24 months for zinc orthophosphate, a first check among water treatment chemical suppliers.
Reported 2019 US zinc orthophosphate production was 0.66 million kg (US EPA).
A program provider runs corrosion studies and LCRI reporting; a supplier delivers to an NSF/ANSI/CAN 60 listing. Providers are compared under water treatment companies.
Are Phosphates Safe in Drinking Water and at the Plant?#
Certified phosphates meet the NSF/ANSI/CAN 60 health-effects requirements at or below each product's maximum use level, with zinc capped at 2.0 mg/L in finished water, while at the plant phosphoric acid is corrosive. Its harmonised CLP Annex VI classification is Skin Corr. 1B, H314 (index 015-011-00-6); zinc phosphate carries Annex VI H400 and H410 (index 030-011-00-6).
Is there a drinking-water limit for phosphate?#
No: there is no US MCL and no WHO or EU parametric value for phosphate, and the EPA range of 1.0 to 3.0 mg/L as PO4 is a corrosion-control reference, not a limit. Zinc has a non-enforceable 5 mg/L secondary level (40 CFR 143.3).
Reporting is explained under phosphate in treated water.
Do dosed phosphates reach the wastewater plant?#
Yes: starting a phosphate inhibitor raises the wastewater plant's phosphorus load by 10 to 35 %, median 20 % (Rodgers, 2014, in EPA 816-B-16-003), which counts toward NPDES and pretreatment limits. Biosolids zinc is capped at 7,500 mg/kg (40 CFR 503.13).
Removal programs are compared under wastewater treatment chemicals.
Can phosphates cause scale?#
Yes: orthophosphate forms calcium phosphate scale where it meets calcium at high pH, so phosphoric acid is fed apart from lime and orthophosphate stays out of hard boiler feedwater lines.
Calcium phosphate sits beside carbonate and silica in scale formation in water systems.
Which phosphates are listed in 21 CFR 173.310?#
Five phosphate entries are listed in 21 CFR 173.310 without a numeric limit (eCFR, 2026-10-15).
- Potassium tripolyphosphate
- Sodium hexametaphosphate
- Sodium phosphate (mono-, di-, tri-)
- Sodium tripolyphosphate
- Tetrasodium pyrophosphate
Phosphoric acid, TKPP and zinc orthophosphate are not listed in 21 CFR 173.310; see 21 CFR 173.310 boiler water additives.
Are these phosphates used outside water treatment?#
Yes: about 85 % of US phosphoric acid goes to fertilizer (US EPA, 2023); agricultural and food uses are outside this reference.