Understanding the concepts of the water treatment environment is critical to understanding its ramifications.

In the water treatment industry, there are several parameters that can be used to express water quality or its various characteristics. For the sake of understanding and good communication, it is important to have a good understanding of the different units of measurement and chemical expression.

On this page you will find brief explanations and non-exhaustive definitions of the following concepts:

  1. Concentrations;
  2. pH;
  3. Temperature.
  4. Weight and mass;
  5. Volume;
  6. Flow rates;
  7. Conductivity and resistivity;
  8. Hardness; and
  9. Equivalencies.



The expressions PPM, mg/L and % are used to express the concentration of a solute in a known quantity of solutions.


Parts per Millions

The acronym PPM, which stands for Parts per Millions, represents 1 / 1,000,000. So, let's imagine a water that has a chlorine concentration of 2 ppm. This would mean that there would be 2 molecules of chlorine in this solution for every 1,000,000 molecules of water.

  • As an aside, in addition to PPM, the notations PPB and PPT can be used. For your information, PPB stands for "parts per billion" and PPT stands for "parts per trillion". For more information, see this article: "What Do the PPM/PPB/PPT notations mean?”

When these notations are used, mass to mass or volume to volume ratios are expressed.


The mg/L

In a similar way, like the PPM, the mg/L represents 1 / 1,000,000. However, unlike the PPM which represents a volume-volume or mass-mass ratio, the mg/L expresses a mass-volume ratio since the mass and volume of the different molecules are different.

In short, let's imagine a water with a chlorine concentration of 2 mg/L, in this case, we will find 2 mg of chlorine for 1 litre of water or 1,000,000 mg of water. In order to highlight the principle of the mass-volume ratio, here is a small comparison of certain molecules.


18,01528 g/mol


70,906 g/mol


22,98977 g/mol

As can be seen here, the molecular weight of molecules varies greatly. This is why we say that the mg/L is a mass-volume ration since 1 mg of water does not have the same mass as 1 mg of sodium or chlorine.



Although simple and widely recognized, the percentage represents 1/100th (1/100) of a solution. It is a mass to mass or volume to volume ratio that represents a fraction of an integer.


Comparison table





~ 1


10 000

~ 10 000




The pH of a solution defines its potential hydrogen. The lower the potential [0 - 7], the more acidic the solution; the higher the potential [7 - 14], the more basic the solution. The acronym pH is a unit of measurement that defines the amount of free hydrogen and hydroxyl ions in the solution. When water is highly concentrated in hydrogen ions, it is acidic [0 - 7]. On the contrary, when it is highly concentrated in hydroxyl ions, it is basic [7 - 14].

The importance of considering the pH when looking at the chemical evolution of water is that the pH can be affected by the chemicals present in or added to the water. In addition, the pH of the water will have an impact on the solubility and biological availability of its chemical constituents.



Temperature is a measure of hot or cold that is expressed on arbitrary scales. These scales can be in degrees Celsius, Fahrenheit or Kelvin. In the water treatment industry, only Celsius and Fahrenheit degrees are used.


Degrees Celsius

This scale is based on a freezing point of water of 0°C and a boiling point of 100°C. The Celsius temperature scale is the most common and widely used. It is used in Canada, Europe and almost everywhere in the world except the United States.


Degrees Fahrenheit

The Fahrenheit temperature scale is also based on two key points: freezing and boiling point of water. In the Fahrenheit scale, the freezing point of water is 32°F while the boiling point of water is 212°F.

As mentioned above, this scale is used mainly in the United States.


Weight and mass

Weight represents the gravitational pull of an object. Weight is expressed in several ways. There are two systems to express the weight and mass of an object: the metric system & the imperial system.


The Metric System

This system uses the kilogram as a unit of measurement. Originally, one gram represented the mass of one cubic centimetre of water. For practical purposes, prefixes were added to simplify the expression of the different masses.


Partial charts of the metric weight scale


0.000 001 kg


0.001 kg


1 kg


1000 kg


1 000 000 kg

  • This list is not exhaustive since the prefixes allowing the splitting of a data are very varied, but it gives you an interesting overview.

The imperial system

Originating in Britain, this system of measurement uses a large number of ramifications to define the weight of an object. Without going into too much detail, we find the ounce, the pound and the imperial ton.

1 ounce

0.0625 pounds

1 pound

1 pound = 16 ounces

1 ton

2204.62 pounds




Volume is a measure that takes into account an occupied space in all three dimensions. Units of volume are also recognized as the capacity of any container. Although there are several methods of designating volume, the most common are

  • The litre;
  • The gallon; and
  • The cubic metre.

Each of these measures represents a volume that can be used to define a capacity or flow rate.

1 litre

1 litre

1 gallon

3.785 litres

1 cubic metre

1000 litre

When any measurement is used to define a volume, it is calculated in its cubic form. To use the cubic metre as an example, we calculate a cubic metre as 1 metre x 1 metre x 1 metre.



In the water treatment community, units of capacity are used in conjunction with time to define a flow rate. A flow rate represents any volume that flows for a known unit of time.

The most common expressions for the flow rate are

  • Litres/minutes ;
  • Litres/hours.
  • Gallons per minute;
  • Gallon per hour; and
  • Gallon per day.


Comparison Table

10 L/min

14 400 litres per day

10 L/hour

240 litres per day

10 gpm

65 463.7 litres per day

10 Gph

908.5 litres per day

10 gpd

37.85 litres per day


Conductivity and resistivity

The conductivity of water represents its ability to conduct an electric current. It is important to note that in its pure state, water does not conduct electricity. To be a conductor, water must have some cations and anions that will allow the transfer of energy.

The unit of measurement used to express the conductivity of a water sample is Siemens per centimetre (S/cm) or microsiemens per centimetre (µs/cm). The expression µs/cm can be expressed in ppm to make it easier to understand.

The resistivity of a material is its characteristic to resist an electric current. As we discussed earlier, electricity is conducted by the materials dissolved in water, i.e., ions (cations/anions). This means that the higher the resistance of a water sample, the purer it is, since pure water hardly conducts electricity at all.

Interchangeability of Conductivity and Resistivity

When we talk about interchangeability, we mean that both concepts can be used to define the ability or not to conduct electricity. For example, ultra-pure water, at 25°C, has these characteristics.

  • Resistivity: ~ 18.2 MΩ × cm.
  • Conductivity: ~0.055 µs/cm

These two measurements represent the same conductivity/resistivity capacity of water.

In short, although resistivity can be used to express the ability of water to conduct an electric current, conductivity and microsiemens are generally used.

  • To learn more about conductivity and techniques for extracting it, see this article: "How to decrease the conductivity of your water."



The hardness of water is a characteristic that represents its mineral concentration. When this concentration is low, the water is described as soft. This means that the higher the concentration of minerals in the water, the harder it is. Without being exhaustive, the minerals most commonly found as a source of hardness are calcium and magnesium.


Soft water

Moderately hard water

Hard Water

0 – 60 ppm

60 – 120 ppm

120 – 180 ppm

Water hardness can be expressed in several ways. As can be seen in the table above, hardness can be expressed in parts per million of CaCO3. These results can be obtained using the titration method and an EDTA cartridge.

Grain per gallon (GPG) can also be used to define the hardness of a water sample. The grains per gallon represent the amount of calcium carbonate grain dissolved in one gallon of water (3.785litres).

  • Note that one grain represents 64.8 milligrams of CaCO3.

Finally, the milliequivalent may be used. The milliequivalents are usually expressed by the following expression: mEq/l. The milliequivalent consists of one thousandth of the equivalent of the fluid in which the minerals are dissolved. It is therefore a method of defining the concentration of hardness by taking into account the molecular weight of minerals and water.



The equivalences between the different expressions of hardness allow us to better understand the differences between each of them. The equivalence principle is simple, it is what allows us to go from one expression to another.

It is important to note that changes in the water will have an impact on the equivalencies. For example, fresh water has an equivalence of 1 mg/l to 1 ppm. This is because its specific gravity is 1.

  • Specific gravity represents the density of a substance relative to the density of water when both materials are at the same temperature.

However, when materials dissolve in water or water change temperature, its specific gravity also changes. As a result, the equivalences are no longer 1:1. When the specific gravity of the solution is not 1, the following methods should be used to define equivalencies:

  • Ppm by mass = mg/L / density
  • Mass % = mg/L / (10,000*density)

To convert from mg/L to mEq/L for an ion, the concentration in mg/L, the atomic mass of the ion and its valence must be known. Once these parameters are known, use this equation to identify the equivalence:

  • mEq/L = (mg/L)(Valence)/ (atomic mass)

Small precision, water should be electrochemically neutral and it is thanks to the data expressed in mEq/L that we can identify the different concentrations of ions present in a water sample.



Obviously, there are many other terms related to water treatment and the explanations in this article are surface level. However, with the help of the information that has been presented above, you will be able to better understand the communication jargon of water treatment experts.

In the meantime, we hope that this information will be useful to you and if you have any further questions, please do not hesitate to write to us and we will be happy to answer your questions.

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