Calculator
Editable inputs
1Water analysis
Enter alkalinity or total carbonate carbon as HCO₃ equivalent, not both. For saline waters, density materially affects the conversion from mg/L to molality.
Major ions
Minor and trace ions Optional. Include Ba and Sr to screen barite and celestite.
2Concentration sweep
3Composition sensitivity optional
Assumed removals and pH or temperature states only. Removals are applied first, then ideal dilution. No reagent dose, counter-ions, precipitated solids or yield is calculated.
Screening results
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About this tool
Concentrating a wastewater pushes minerals such as gypsum, calcite, barite, celestite and silica towards saturation. Enter a full water analysis to see how each saturation index changes as solvent water is removed, and where each mineral first crosses your alert threshold.
The calculation runs on our server with PHREEQC 3.8.6 and its Pitzer database, which is suited to concentrated brines. Optional scenarios let you test assumed ion removals, dilution and pH.
Method and equations
SI = log10(IAP / K)n(H2O) = 55.506 × R mol per kg of initial solvent waterCF_nominal = 1 / (1 − R)CF_actual = initial water kg / remaining water kgR_cross = R1 + (A − SI1) / (SI2 − SI1) × (R2 − R1)CBE (%) = 100 × (Σ cations − Σ anions) / (Σ cations + Σ anions), meq/LAssumptions
- All concentrations are mg/L of the displayed species. Sulfate is entered as SO4, silica as SiO2 and boron as B. Iron is assumed to be Fe(II).
- Water removal is solvent-water mass removed from 1 kg of water, closed inventory, 1 atm and constant temperature. Nothing precipitates and no gas is exchanged.
- Enter alkalinity (mg/L as CaCO3) or total carbonate carbon as HCO3 equivalent, not both. Blank fields are left out; nothing is filled in from the example.
- A supplied density converts mg/L to molality. Without it, PHREEQC calculates the density.
- Operational minerals set the first alert. Magnesite, dolomite, chalcedony and quartz are shown for context only.
- Crossings are interpolated linearly between calculated points and are never bridged across a missing value.
Limitations
- A positive saturation index means thermodynamic supersaturation. It does not predict deposition rate, induction time, adhesion or antiscalant performance.
- An alert is not a safe operating recovery or a design limit. Use it to decide where pilot work, antiscalant review or detailed modelling is needed.
- Water removal here is not volumetric membrane recovery, rejection or flux.
- Scenario removals and pH overrides are assumptions. No reagent dose, counter-ions, sludge, product yield or energy is calculated.
- Fluoride, phosphate, aluminium, organics, inhibitors and general redox chemistry are outside this version.
- High-salinity barite results need independent validation. Beyond the first saturation point the results describe a hypothetical solution in which nothing has precipitated.
Worked example
Load the seawater example and run the screen with the default sweep (up to 95% water removal in 5% steps, alert at SI = 0).
- The PHREEQC charge error is 0.02%, so the analysis is balanced.
- Calcite and aragonite are already supersaturated in the feed (SI 0.62 and 0.44), so the first operational alert is Calcite / Aragonite, in feed.
- Barite crosses at about 69.8% water removal, celestite at about 70.0% and gypsum at about 70.6%. Anhydrite follows at about 79.6%, amorphous silica at about 88.8% and halite at about 90.5%.
- At 95% removal the actual concentration factor is 20.0×, ionic strength reaches 14.1 mol/kgw, the modelled density is 1.35 kg/L and pH falls from 8.1 to about 5.3.
The carbonate alert in the feed tells you to look at alkalinity control before anything else. The cluster of sulfate alerts around 70% is a prompt to review antiscalant limits and pilot evidence, not an operating limit.
The calculator opens empty. Use Load seawater example to fill in these values, then Run scaling screen.
Frequently asked questions
Does a positive saturation index mean scale will form?
No. A positive SI means the water is thermodynamically supersaturated with that mineral. Whether scale actually forms depends on kinetics, nucleation, surfaces, residence time and antiscalants, which this tool does not model.
Why is water removal not the same as RO recovery?
The tool removes solvent water by mass from 1 kg of water and keeps every dissolved ion in solution. Membrane recovery is a volumetric flow ratio and real membranes pass some salt, so the two are related but not identical.
Why are quartz, chalcedony, dolomite and magnesite marked as context?
They often show positive SI in natural and industrial waters but rarely form quickly at these conditions. They are shown so you can see them, but they do not set the headline alert.
What does "Not calculated" mean?
PHREEQC returned no value for that mineral, usually because a required element is missing from the analysis. A missing value is not evidence that the water is undersaturated.
Should I enter alkalinity or bicarbonate?
Use alkalinity as CaCO3 for an ordinary lab analysis. The bicarbonate field means total carbonate carbon expressed as HCO3, not the free bicarbonate ion. Enter one or the other, not both.
How accurate is it for very concentrated brines?
The Pitzer model is the right choice for brines, but interaction data are incomplete for some minerals and temperatures, especially barite at high ionic strength. Treat results as a screen and confirm critical cases with laboratory or pilot data.
Related ZLD guide chapters
More water tools
Brine Consulting screening tool. Results are not a design guarantee, vendor guarantee, regulatory determination or detailed process design, and do not replace project-specific modelling, pilot testing or professional engineering review.