Overview
OLI software calculates Total Dissolved Solids (TDS) using two methods: Estimated and Rigorous. By default, the Estimated method is used, while the Rigorous method can be optionally enabled. This article explains both methods with examples in OLI Studio: Stream Analyzer and shows how to enable the Rigorous method in OLI Flowsheet: ESP.
Disclaimer: The user interface, calculations, and results displayed in this article are from OLI Studio Version 12. Other software versions may appear different or present slightly distinct results due to continual developments to the software and thermodynamic databanks.
Estimated Method
The Estimated TDS method is straightforward:
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Step 1: Calculate the total mass of the aqueous phase (water-rich phase).
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Step 2: Subtract the mass of pure water (H₂O) from the total mass.
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Step 3: Divide the result by the total liquid volume.
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Step 4: Report the result in mg TDS/L.
Example:
We use the Seawater (Molecular Inflows) object from the OLI Studio Object Library (see
(see: How to use the OLI Studio Object Library )
The stream composition for the object:
For this example, we will add and run an isothermal single-point calculation.
The estimated Total Dissolved Solids is 34453.0 mg/L
How Is This Value Obtained?
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Open the Report tab and scroll to Total and Phase Flows (Amounts):
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Mass of Liquid-1: 1022.7 g
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Volume of Liquid-1: ~1.0 L
Find the mass of water in Species Output (True Species):
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Change the units:
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Click the Unit Manager icon in the toolbar.
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Select Customize...
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For Liquid-1 Composition, choose Mass from the dropdown.
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Locate the Liquid-1 composition under the Output variables:
Click the OK button to return to the report. Then, scroll to Species Output (True Species) and relocate water in the Liquid-1 phase.
Perform the TDS calculation:
Mass of Liquid-1 = 1022.7 grams
Mass of H2O = 988.244 grams
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Net mass = 34.456 grams = 34456 mg
Divide this by the liquid-1 volume = 0.999999 L
TDS = 34456 mg / 0.999999 L = 34456 mg/L
This is very close to the reported value. Note: Minor differences occur due to the removal of known volatile species (e.g., CO₂).
Rigorous Method
The Rigorous method simulates the ASTM D5907-18 standard:
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A known volume of liquid is weighed.
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It is slowly dried at 185°C, driving off dissolved gases such as carbon dioxide.
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The dried sample is weighed again.
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The difference gives the solute mass, divided by the sample volume to yield TDS in mg/L.
Example:
Using the same Seawater object:
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Enable the Rigorous Calculation:
Note: The rigorous calculations increase the calculation time and are turned off by default. In addition, if the rigorous calculation fails to converge, the estimated value will be used.
- Right-click the calculation object.
- Select Calculation Options.
- Check Total Dissolved Solids (TDS) - Rigorous method.
- Click OK.
2. Run the calculation.
The reported TDS is shown in the calculation Output and in the Report.
The TDS (Rigorous) = 34230.6 mg/L, which differs from the estimated value above.
TDS in OLI Flowsheet: ESP
In OLI Flowsheet: ESP, the Estimated TDS is also available by default.
Example:
- We copied the Seawater object from OLI Studio to OLI Flowsheet: ESP.
- We added a Mixer (Adiabatic).
- We added a Callout to the Seawater stream to display the TDS estimated value (34474.1 mg/L).
To enable the Rigorous TDS:
- Click anywhere on the blank Flowsheet workspace.
- Click Optional Properties.
- Click the drop-down box next to Total Dissolved Solids (TDS) to select Rigorous.
4. Re-run the simulation.
Additional Resources
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The two example files (Version 12) are attached to this article.