TDS, Conductivity and Hardness: Making Sense of Your Water Test Report

A water analysis report lands on your desk. It lists TDS, conductivity, total hardness, calcium hardness, alkalinity, chlorides and a dozen other values. Some are in mg/l, some in microsiemens, some expressed “as CaCO3.” It is no wonder many plant owners only look at the TDS line and stop there. Understanding a few key relationships will let you get far more value from every report.

Total Dissolved Solids (TDS)

TDS is the total amount of dissolved matter in water, mostly mineral salts such as calcium, magnesium, sodium, potassium, chloride, sulphate and bicarbonate. It is expressed in mg/l, which for dilute water is essentially the same as parts per million.

TDS tells you how much is dissolved, but not what is dissolved. Two waters with the same TDS can behave completely differently. One might be high in sodium chloride and taste salty; another might be high in calcium bicarbonate and scale every kettle it touches.

Electrical Conductivity

Pure water conducts electricity poorly. Dissolved ions carry current, so the more ions present, the higher the conductivity. Conductivity is measured in microsiemens per centimetre and is the easiest and fastest water quality parameter to measure online.

The TDS and Conductivity Connection

Most handheld “TDS meters” actually measure conductivity and convert it to TDS using a factor, typically somewhere between 0.5 and 0.7 depending on the type of salts present. This is an estimate, not a precise measurement. For accurate TDS, a laboratory evaporates a filtered sample and weighs the residue. For daily plant control, conductivity is perfectly adequate, provided you know your conversion factor stays reasonably constant.

Hardness

Hardness is mainly the sum of calcium and magnesium. It is what causes scale in boilers, heaters and pipes, and the familiar white crust on taps. Reports usually express hardness “as CaCO3,” a convention that lets you compare and add different ions on a common scale.

Calcium vs Magnesium Hardness

Total hardness is often split into calcium hardness and magnesium hardness. The split matters for treatment design, especially for lime softening and for predicting specific scale types.

Temporary vs Permanent Hardness

Temporary or carbonate hardness is the portion associated with bicarbonate alkalinity and can be reduced by boiling. Permanent or non-carbonate hardness is associated with sulphates and chlorides and stays behind after boiling.

Alkalinity

Alkalinity is the water’s capacity to neutralise acid, mostly due to bicarbonate, carbonate and hydroxide. It is not the same as pH. Water can have a neutral pH and still carry high alkalinity. Alkalinity influences scaling tendency, coagulant performance and how much acid or alkali is needed for pH correction.

pH

pH measures the hydrogen ion activity, or how acidic or basic water is at that moment. It affects corrosion, scaling, disinfection efficiency and the performance of nearly every treatment process. A small pH shift can transform how a coagulant behaves or how effective chlorine is.

How These Numbers Guide Treatment Choices

• High TDS with high sodium and chloride: reverse osmosis is usually the practical route
• Moderate TDS but high hardness: softening may be enough for industrial uses like boiler feed
• High alkalinity: consider its effect on scaling and on chemical consumption
• High iron or manganese: dedicated removal is needed before membranes or resins

Checking a Report for Consistency

A useful trick is the ion balance check. The sum of positive ions, expressed in equivalents, should roughly equal the sum of negative ions. A large mismatch suggests a missing parameter or a lab error. Good engineers always perform this check before designing a plant.

Practical Advice

Get a complete analysis from an accredited laboratory before investing in any treatment system. Test again in a different season, because groundwater and surface water both change. And keep a simple conductivity meter on site for daily checks. Numbers you watch every day become numbers you understand.

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