Concepts in water treatment systems: TDS, SDI, LSI, KSP
Here are the key concepts in water treatment systems, explained clearly:
- TDS (Total Dissolved Solids)
Definition: The total concentration of all inorganic and organic substances dissolved in water, measured in mg/L (or ppm).
What it includes: Minerals (calcium, magnesium, sodium, potassium), salts (chlorides, sulfates, bicarbonates), and some organic matter.
Typical ranges:
Water Type TDS (ppm)
Freshwater <500
Brackish water 500–5,000
Seawater ~35,000
RO permeate <50
Relevance: High TDS indicates hard or saline water, affecting taste, scaling potential, and membrane performance. It's a key parameter for selecting pretreatment and membrane type in RO/NF systems.
- SDI (Silt Density Index)
Definition: A measure of the fouling potential of water due to colloidal and suspended particles. It quantifies how quickly a 0.45 μm microfilter clogs under a standardized pressure (typically 30 psi).
Calculation: Based on the time it takes to filter a fixed volume (usually 500 mL) at the start and after 15 minutes of operation.
SDI classification:
SDI Value Interpretation
<3 Excellent (suitable for RO)
3–5 Acceptable with good pretreatment
5 Poor (requires extensive pretreatment)
Relevance: SDI is critical for reverse osmosis systems. High SDI means colloidal fouling will rapidly degrade membrane performance. It's the primary indicator used to design and monitor multimedia filters, UF, and cartridge filters upstream of RO membranes.
- LSI (Langelier Saturation Index)
Definition: An indicator of whether water is corrosive or scale-forming with respect to calcium carbonate (CaCO₃) at a given temperature, pH, and ionic strength.
Formula:
LSI = pH - pH_s
Where pH_s is the pH at which water is saturated with CaCO₃ (calculated from Ca²⁺, alkalinity, TDS, and temperature).
Interpretation:
LSI Value Water Tendency
< 0 Corrosive (CaCO₃ dissolves)
= 0 Balanced (neutral)
0 Scale-forming (CaCO₃ precipitates)
Relevance: In cooling towers, boilers, and RO systems, LSI helps predict scaling. RO systems often operate with LSI > 1.5–2.0 on the concentrate (reject) side, which is why antiscalant dosing is required. LSI is also used to adjust pH or add inhibitors in closed-loop systems.
- Ksp (Solubility Product Constant)
Definition: The equilibrium constant for the dissolution of a sparingly soluble salt in water. It represents the maximum product of ionic concentrations that can exist in solution before precipitation occurs.
Relevance: Ksp values are used to:
- Predict scaling potential of various salts (CaCO₃, CaSO₄, BaSO₄, SiO₂, etc.)
- Design antiscalant programs (dosing is based on how far the concentrate stream is pushed beyond Ksp)
- Determine maximum recovery rates in RO systems before scaling becomes inevitable
Common scaling salts and their Ksp (approximate, 25°C):
Salt Ksp
CaCO₃ 4.8 × 10⁻⁹
CaSO₄ 2.4 × 10⁻⁵
BaSO₄ 1.1 × 10⁻¹⁰
SrSO₄ 3.2 × 10⁻⁷
Summary Comparison
Parameter What It Measures Primary Use
TDS Total dissolved solids Water quality classification, membrane selection
SDI Colloidal/silt fouling potential RO pretreatment design & monitoring
LSI CaCO₃ scaling/corrosion tendency Scale prediction, chemical dosing
Ksp Salt solubility limit Scaling risk, recovery limits, antiscalant design