Pure Water & Clean Energy
Bringing Clean Water To Every Corner On Earth
Pure Water & Clean Energy
Bringing Clean Water To Every Corner On Earth
TDS, pH, SDI, hardness, boron and more — how to read a water analysis for RO design.
Every RO project starts with a feed-water analysis. This guide explains the parameters that matter for system design, how they're measured, and the ranges typical of each water source. Pair this with the RO Basics and SWRO Design Guide when working a real project.
The mass of dissolved (filterable through 0.45 µm) inorganic and organic material in water, reported as mg/L. Determined gravimetrically by drying a filtered sample at 180 °C (APHA 2540C) or estimated from conductivity. Typical ranges:
An indirect TDS measurement via electrical conductance. The conversion factor depends on ion composition; common rules of thumb:
TDS [mg/L] ≈ 0.5 to 0.7 × Conductivity [µS/cm]
Sodium-chloride dominated waters use a higher factor (~0.65), bicarbonate-rich waters lower (~0.50). For seawater the factor approaches 0.70 (35,000 mg/L corresponds to ~50,000 µS/cm). For low-TDS permeate (< 50 mg/L), the factor approaches 0.50.
Temperature compensation to 25 °C is essential — conductivity changes ~2% per °C.
The negative log of hydrogen ion activity. Influences carbonate equilibria, scaling potential, membrane material compatibility, and biocide effectiveness. Typical:
Polyamide membranes tolerate pH 2–11 in operation and pH 1–13 during cleaning. pH adjustment is sometimes used to suppress carbonate scaling (acid feed) or improve boron rejection (alkaline feed in the second pass).
The standardized fouling index for RO feeds. A 0.45 µm filter is challenged at 30 psi feed pressure, and the time to filter 500 mL is measured at t = 0, then again after 5, 10, or 15 minutes of continuous filtration. SDI is calculated as:
SDI₁₅ = 100 × (1 − t₀ / t₁₅) / 15
Targets per membrane manufacturers:
SDI is empirical and somewhat operator-dependent. Modified Fouling Index (MFI) is an alternative.
Measures light scattering by suspended particles. Acceptable RO feed: < 0.2 NTU after pretreatment (1 NTU is achievable for well water without filtration but inadequate for open intakes). Surface waters can exceed 100 NTU during storms; coagulation + multimedia or UF is needed.
Sum of calcium and magnesium ions, expressed as mg/L CaCO₃. Classifications:
Drives CaCO₃ scaling risk on BWRO and tap RO. Calculate Langelier Saturation Index (LSI) and Stiff & Davis Stability Index (S&DSI for high-TDS) to predict scaling. Antiscalant dosing or softening pretreatment is required where indices indicate scaling.
Polyamide membranes oxidize on chlorine exposure. Tolerance is cumulative: ~1,000 ppm-hours total exposure before measurable performance loss. Practical operating limit: < 0.1 ppm continuous. Dechlorinate via:
Chloramine (NH₂Cl) is more damaging than free chlorine and not as easily removed by SMBS — design carefully when feed is municipally chloraminated.
A challenging contaminant for SWRO. Seawater contains 4–5 ppm boron as boric acid (pKa = 9.2). At natural seawater pH (~8.0), boron exists as uncharged H₃BO₃, which polyamide rejects only 50–90% — permeate boron can be 0.5–2 mg/L on a single pass. Targets:
Mitigations: high-boron-rejection membranes (FilmTec SW30HRLE, Toray TM820), second-pass RO at elevated pH (10–10.5) where boron is deprotonated and well rejected, or boron-selective ion exchange resin (Purolite S108, Lewatit MK51) polish.
Total Organic Carbon (TOC) is the most useful single index. TOC > 2 mg/L suggests need for enhanced pretreatment (coagulation, DAF, UF, or activated carbon). High molecular-weight humic substances cause membrane fouling and exacerbate biofouling by feeding biofilms.
| Ion | Seawater (mg/L) | Significance for RO |
|---|---|---|
| Sodium (Na⁺) | 10,800 | Dominant cation; balanced by Cl⁻ for charge balance |
| Chloride (Cl⁻) | 19,400 | Sets corrosion duty; demands Super Duplex 2507 for SWRO |
| Sulfate (SO₄²⁻) | 2,700 | BaSO₄ / SrSO₄ / CaSO₄ scaling risk; needs antiscalant |
| Magnesium (Mg²⁺) | 1,290 | Hardness; Mg(OH)₂ at very high pH |
| Calcium (Ca²⁺) | 410 | CaCO₃ / CaSO₄ scaling; needs LSI / S&DSI check |
| Potassium (K⁺) | 390 | Minor; rejects similarly to Na |
| Bicarbonate (HCO₃⁻) | 140 | Sets alkalinity and CaCO₃ scaling potential |
| Bromide (Br⁻) | 67 | Forms bromate if pre-oxidized with ozone |
| Strontium (Sr²⁺) | 8 | SrSO₄ scaling on BWRO at high recovery |
| Barium (Ba²⁺) | 0.02 | Trace, but very low BaSO₄ solubility |
| Silica (SiO₂) | 0.1–10 | Hard to clean; limits BWRO recovery |
| Fluoride (F⁻) | 1.3 | Brackish wells may exceed; CaF₂ scaling |
| Nitrate (NO₃⁻) | 0.5 | Drinking-water concern in agricultural wells (> 10 mg/L NO₃-N) |
HPC (heterotrophic plate count), total coliforms, and ATP (live biomass) indicate biofouling risk. Open intakes (especially at warm temperatures) need biocide strategy — intermittent chlorine + dechlorination, or DBNPA shock dosing. Cartridge filter housings and stagnant lines are biofilm hotspots.
When you receive a lab report, work through it in this order:
Feed this into projection software (DuPont WAVE, Hydranautics IMSDesign, Toray DS2, LG Q+) for membrane selection and array layout.
mg/L ÷ equivalent weight = meq/L
e.g., Ca²⁺: 40.08 / 2 = 20.04 g/eq, so 100 mg/L Ca²⁺ = 5.0 meq/L
mg/L ÷ molecular weight = mmol/L
e.g., 100 mg/L Ca²⁺ = 100 / 40.08 = 2.49 mmol/L
1 NTU ≈ 0.5–2.4 mg/L suspended solids (highly source-dependent)
1 grain/gallon hardness = 17.1 mg/L as CaCO₃
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