Common phosphorus-containing salts in aqueous solutions and their treatment
Phosphorus in water is mainly present as dissolved phosphate, phosphorus-bearing particles or phosphate minerals. The solubility of a phosphorus-containing compound depends on its counter-ion and solid phase; pH, temperature, ionic strength and other dissolved ions can change how much remains dissolved.
Solubility descriptions below are qualitative, not design values. “Soluble” salts dissociate into ions; low-solubility phosphates may precipitate or occur as minerals. Calcium-phosphate solubility varies with pH and mineral phase. Concentrated brines require activity-based modelling or testing.
Common phosphorus-containing salts and compounds in aqueous systems
| Compound | Formula | General solubility behaviour in water | Typical application / industry / where used | Typical aqueous streams where encountered |
|---|---|---|---|---|
| Monosodium phosphate | NaH2PO4 | Soluble; solution is generally acidic. Hydration and temperature affect tabulated values. | Food and chemical processing; pH adjustment and phosphate dosing. | Product solutions, wash water and wastewater where the salt is used. |
| Disodium phosphate | Na2HPO4 | Soluble; solution speciation depends on pH. | Food, detergent and chemical manufacture; buffering. | Formulation, cleaning and process waters. |
| Trisodium phosphate | Na3PO4 | Soluble; strongly alkaline solutions can shift phosphate speciation and precipitation behaviour. | Cleaning, surface treatment and chemical processing. | Spent cleaning baths, rinse water and phosphate-bearing wastewater. |
| Potassium phosphates | KH2PO4, K2HPO4, K3PO4 | Generally water-soluble; pH and exact salt determine speciation. | Fertilisers, food processing, buffers and specialty chemicals. | Fertiliser runoff, dosing solutions and process or wash waters. |
| Ammonium phosphates | NH4H2PO4, (NH4)2HPO4 | Generally water-soluble; ammonia/ammonium and phosphate speciation depend on pH and temperature. | Fertilisers, fire-suppression products and nutrient dosing. | Agricultural runoff, fertiliser production water and nutrient-rich wastewater. |
| Sodium polyphosphates | e.g. (NaPO3)n | Many commercial forms dissolve in water; chain length and hydrolysis affect behaviour. | Water conditioning, detergents and dispersant formulations. | Cooling, cleaning and process waters; may transform to orthophosphate. |
| Dicalcium phosphate | CaHPO4 (various hydrates) | Sparingly soluble; dissolution depends strongly on pH and mineral form. | Fertilisers, animal feed and phosphate-mineral processing. | Slurries, mineral-processing water and phosphate-treatment residuals. |
| Tricalcium phosphate / apatite | Ca3(PO4)2; apatite phases vary | Low solubility near neutral conditions; acidic conditions can increase dissolution. Exact phase matters. | Phosphate rock, fertiliser production and lime-based phosphorus removal. | Mine water, mineral-processing streams and chemical-treatment sludge. |
| Struvite (magnesium ammonium phosphate) | MgNH4PO4·6H2O | Sparingly soluble; can crystallise when magnesium, ammonium and phosphate conditions favour supersaturation. | Fertiliser recovery and nutrient management. | Digester liquor and nutrient-rich wastewater, subject to chemistry and process conditions. |
| Iron and aluminium phosphates | FePO4, AlPO4 (solid phases vary) | Often low-solubility precipitates; actual solids can include amorphous or co-precipitated phases. pH matters. | Coagulation and chemical phosphorus removal; mineral processing. | Wastewater after iron/aluminium dosing, treatment sludge and mine/process water. |
Phosphate removal and recovery options
Choose treatment based on the measured phosphorus fraction, target, water chemistry and whether the goal is removal, recovery or both. The options below require case-specific evaluation.
| Challenge or objective | Potential treatment route | Selection considerations |
|---|---|---|
| Remove dissolved orthophosphate from municipal or industrial wastewater | Iron/aluminium salt addition or lime precipitation, followed by clarification or filtration | pH, alkalinity, dose, competing reactions, solids separation, sludge volume and disposal. |
| Meet a total-phosphorus target where biological treatment is suitable | Enhanced biological phosphorus removal, often with a chemical polishing step if needed | Biological process stability, readily biodegradable carbon, nutrient balance, solids retention and effluent target. |
| Recover phosphorus from nutrient-rich sidestreams | Controlled crystallisation, including struvite where feed chemistry supports it | Magnesium, ammonium, phosphate, pH, competing solids, product quality and downstream handling. |
| Polish low residual phosphate or treat a specific water matrix | Adsorptive media, selected ion exchange or membrane separation | Capacity/selectivity, competing ions, pretreatment, fouling, concentrate and spent-media management. |
Membranes can retain phosphate under suitable conditions, but the result depends on phosphate species, membrane properties and feed chemistry. A membrane concentrate may become supersaturated with calcium, magnesium, iron or aluminium phosphate phases. Bench testing and scale assessment may therefore be important for high recoveries.
Related information: dissolved phosphate and phosphorus in water.
Discuss a phosphate-treatment challenge
Share the treatment objective and relevant water analysis with Lenntech to discuss possible precipitation, biological removal, crystallisation or polishing routes.
Discuss a water-treatment challenge
Contact Lenntech at info@lenntech.com.
Last reviewed: by Lenntech.