Dissolved solids in process water
Dissolved-solids terminology, residue and conductivity measurement bases, concentration units, and limits of interpretation for manufacturing water.
Tags: chemicals, process-water, measurement
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Identity and scope
TDS may be weighed residue, a constituent sum, or a conductivity estimate. The conductivity factor depends on the dissolved material and can change with concentration. The same conductivity can give different calculated mg/L for NaCl, KCl and the named 442 salt mixture. Neither conductivity nor aggregate TDS identifies chloride. USGS, Chloride, salinity, and dissolved solids
The published examples distinguish operationally defined residues from calculated quantities. Match the measurement basis before comparing process-water trends. USGS water-data definitions Fisher Scientific accumet XL reference guide, section 31
This reference covers measurement identity and interpretation in manufacturing water. It establishes no universal TDS acceptance limit, treatment setting, discharge permission, or customer qualification.
Related quantities
| Quantity | Meaning and distinction |
|---|---|
| Dissolved fraction | An operational separation. The USGS glossary uses passage through a 0.45 µm membrane for dissolved constituents. EPA Method 160.1 instead defines filterable residue through its glass-fiber filtration and drying basis. These descriptions do not establish one universal particle-size cutoff for every TDS method. USGS definitions, EPA Method 160.1, sections 2–3 |
| Total suspended solids, TSS | Nonfilterable matter, distinct from filterable TDS. ASTM D5907-18 covers both quantities and explicitly describes the results as operationally defined. ASTM D5907-18 public scope |
| Total solids or total residue | A whole-sample quantity including suspended and dissolved material. EPA Method 160.3 specifies a residue basis at 103–105 °C. Different residue temperatures prevent an unconditional claim that independently reported total solids equals TSS plus TDS. The latter limitation is synthesis. EPA Method 160.3, sections 2–3 |
| Salinity | A related description of dissolved salt content. USGS uses salinity in discussing dissolved solids, while instrument salinity modes can calculate it from conductivity and temperature. A salinity display and a weighed TDS residue are therefore not automatically interchangeable. USGS dissolved-solids overview, YSI conductivity reference |
| Individual ions and hardness | Chloride is one constituent. Hardness concerns principally calcium and magnesium. Neither is identified by the aggregate TDS number. USGS dissolved-solids overview |
Filter passage does not prove that all material exists as individual dissolved molecules or ions. Hem describes colloidal material remaining in filtrates and the effect of filter plugging on separation. “Dissolved” in a result therefore includes the method's separation convention. Hem, USGS Water-Supply Paper 2254, page 60
Three measurement bases
Gravimetric residue
EPA Method 160.1, issued in 1971, defines filterable residue by glass-fiber filtration and drying to constant mass at 180 °C. Its scope includes drinking, surface, and saline waters, and domestic and industrial wastes. These are method applicability statements, not manufacturing acceptance criteria. EPA Method 160.1, sections 1–3
The residue is affected by chemistry and retained moisture. EPA identifies hygroscopic mineral residues, bicarbonate conversion to carbonate, and trapped water as interferences. Hem also describes volatile solutes, partly volatile organic material, and water of crystallization that can remain at the drying temperature. Consequently, residue mass is not a complete inventory of the original dissolved species. EPA Method 160.1, section 5, Hem, pages 156–157
Sum of chemical analyses
A calculated dissolved-solids value totals constituents measured in a chemical analysis. The USGS definition converts bicarbonate to its carbonate equivalent for comparison with residue. An unadjusted ion sum answers a different question. An incomplete analyte list also leaves unmeasured material outside the sum. USGS definitions, Hem, page 157
Conductivity-derived estimate
Electrical conductivity describes the water's ability to carry current through dissolved ions. Specific conductance refers to conductivity at a stated reference temperature. YSI describes both the temperature dependence and the dependence of temperature compensation on sample composition. Compensation does not identify the dissolved substances. YSI conductivity reference, conductivity and temperature-coefficient sections
The Fisher Scientific accumet XL guide, revision 0, October 2012, covers TDS on its XL200, XL500, and XL600 models. Accumet XL guide, model chart on page 4
It calculates TDS by multiplying EC by a conversion factor. It states that the factor depends on dissolved material and can change with concentration. Its table gives different equivalent TDS values for KCl, NaCl, and its named 442 salt mixture at the same conductivity. Those examples do not supply a universal process-water multiplier. Accumet XL guide, section 31, including table and mixture footnote
Dissolved material need not dissociate into ions. Hanna identifies nonionic analytes as a reason conductivity-based TDS readings may be inaccurate. A low EC-derived TDS estimate therefore cannot exclude dissolved nonionic contamination. Hanna Instruments, EC/TDS measurement limitations
Units and conditional arithmetic
Milligrams per liter, mg/L, expresses mass per solution volume. Milligrams per kilogram, mg/kg, expresses mass per solution mass and equals parts per million by mass. Their numerical equality depends on density, not on the word “water.” Temperature and dissolved content affect density. Hem, pages 54–55
Derived dimensional relationship, with solution density ρ in kg/L:
concentration in mg/L = concentration in mg/kg × ρ in kg/L
For an assumed density of exactly 1.00 kg/L, 100 mg/kg corresponds to 100 mg/L. At an assumed density of 1.10 kg/L, the same mass fraction corresponds to 110 mg/L. These are arithmetic examples, not measured properties or process targets. A meter label of “ppm” alone does not establish its physical mass basis.
ASTM D5907-18's public scope separates a method reporting mg/L from a high-precision TDS method reporting µg/g. Numerically, 1 µg/g equals 1 mg/kg. The unit distinction is substantive even though both results concern dissolved solids. ASTM D5907-18, public scope section 1.4
Interpretation in process water
The following are synthesis from the measurement distinctions above:
- A feed-water result, a final-rinse result, and a used process-fluid result describe different material populations. Equal numbers do not establish equal composition or equal relevance to a part.
- A change can support a trend comparison when the sample identity, method, concentration basis, and relevant temperature treatment remain comparable. A changed EC conversion factor can change displayed TDS without changing the water.
- TDS alone does not identify chloride, hardness, scaling tendency, corrosion behavior, or organic contamination. Those conclusions require the relevant composition and conditions.
- Water concentration does not specify residue mass on a finished part. That inference also depends on the water retained on the part and on what remains after the actual process.
- A reading alone does not establish cleanliness or fitness for a manufacturing operation. Drinking-water guidance, environmental classifications, and instrument ranges do not supply a manufacturing acceptance criterion.
Uncertainty and source limits
Comparability depends on the sample's identity and state, filtration convention, drying basis or calculation model, concentration units, and temperature basis. A gravimetric value, a partial chemical sum, and an EC estimate can disagree without being measurements of the same quantity. EPA Method 160.1's reproduced section 9 provides no precision or accuracy data. It supplies no transferable uncertainty for a particular process-water matrix. EPA Method 160.1, section 9
The reviewed sources include Hem's 1985 third edition, historical EPA methods, USGS and instrument-maker public references, the 2012 accumet guide, and the public scope of ASTM D5907-18. The paid ASTM method text was not reviewed. Historical methods establish the stated definitions and limitations, not current contractual or regulatory compliance. No source here characterizes a particular manufacturing water supply, laboratory, installed meter, formulated bath, or finished-part requirement.
Related reference
Surface-finishing processes places process water in the broader manufacturing context.