TDS is a useful comprehensive indicator that quickly tells you the total amount of dissolved substances in water. 

It's a good tool for assessing water taste, hardness, and the performance of certain equipment (like RO systems). However, it cannot replace professional water testing to determine water safety. For drinking water safety, attention should be paid to test reports for specific contaminants (such as heavy metals, nitrates, bacteria).

Total Dissolved Solids (TDS) refers to the total content of all inorganic salts and organic matter dissolved in water. These substances exist in water as ions, molecules, or microscopic particles. Simply put, TDS measures the total concentration of all dissolved substances in water besides pure water molecules (H₂O).

What substances primarily constitute TDS?

TDS mainly includes the following substances: inorganic salt ions (the primary component); cations: such as calcium ions, magnesium ions (these constitute water hardness), sodium ions, potassium ions, etc.; anions: such as bicarbonates, chlorides, sulfates, nitrates, etc.; and small amounts of soluble organic matter.

What are the sources of TDS?

These dissolved substances mainly come from:

Natural sources: When water flows through geological strata (such as limestone, gypsum, etc.), it dissolves minerals from rocks and soil.
Human activities: Agricultural fertilizer residues, industrial wastewater, domestic sewage, and the water pipes themselves can all introduce dissolved solids into water.

How is TDS measured?

A TDS meter is typically used for quick measurement. It works by measuring the electrical conductivity of water, because dissolved inorganic salt ions make water conductive. The more ions present, the higher the conductivity, and thus the higher the TDS value.
Unit: Parts per million (ppm) or milligrams per liter (mg/L) are typically used, and are roughly equivalent.

TDS Value Reference Guide
0-50 ppm: Very pure, e.g., RO purified water, distilled water.
50-150 ppm: Good taste, soft water.
150-300 ppm: Excellent taste, moderate hardness.
300-500 ppm: Hard water, begins to scale easily.
500-1000 ppm: High TDS water, pronounced taste, not suitable for long-term drinking.

1000 ppm: Not suitable for drinking.

TDS is an important comprehensive water quality indicator, but it does not directly represent water safety. 

Its main significance lies in:
Water Taste:
Water with very low TDS (e.g., <50 ppm) may taste "flat" or "bland".
Water with moderate TDS (approximately 150-300 ppm) usually tastes good.
Water with very high TDS (>1000 ppm) may taste salty or bitter.
Indicator of Water Hardness: Calcium and magnesium ions are the main components of water hardness and are also a significant part of TDS. Therefore, high TDS usually indicates hard water, which is prone to scaling.
Water Purifier Performance Assessment: TDS levels are commonly used to quickly check the filtration effectiveness of Reverse Osmosis (RO) water purifiers. If the TDS of the water output after the RO membrane is significantly higher than the input, it may indicate that the RO membrane is failing or needs replacement.
Key Indicator for Specific Applications:
Agricultural irrigation: Excessively high TDS can lead to soil salinization.
Aquariums: Different fish species require specific TDS ranges.
Boiler feed water, laboratory water: Require very low TDS levels.