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Converting between mg/l and ppm (parts per million)

In upstream oil processing, the units milligram per liter (mg/l) and parts per million (ppm) are commonly used in produced water treatment plant.  It is common to have a statutory requirement for maximum oil in water concentration (either mg/l or ppm) that can be discharged into open waters.  A practice amongst many engineers is to assume these two units are equal or interchangeable such that x mg/l is equal to x ppm.  This assumption is fundamentally wrong and can only hold if the density of oil is equal to the density of water.

This brief article will show the conversion method from mg/l to ppm and vice versa.

Example 1

Convert 40mg/l oil in water to ppm

Oil density – 790 kg/m3

In Oil processing terminology, 40mg/l translates into 40mg of oil in 1 liter of oily water (oil + water)

The above calculation procedure from mg/l to ppm can be written as a simple equation of

Example 2

Convert 40 ppm oil in water to mg/l

Oil density – 850 kg/m3

In Oil processing terminology, 40ppm translates into 40 parts of oil in 1 million parts of oily water (oil + water).

This conversion can be easily carried out by using mg/l as the subject of the formulae determined in example 1 above.