Waterguide - TOC

What is TOC?

The TOC value is the abbreviation for total organic carbon and is a measure for the total organic carbon in a sample. Inorganic carbons (TIC) such as carbonates are excluded from this measure.

The opposite of TOC is DOC (dissolved organic carbon), which tells you how much dissolved organic carbon is in a sample.

The TOC value can help you determine the water quality.

How is the TOC value measured?

The principle of a TOC measurement is based on the following method:

Carbon compounds are oxidised though a burning process and then measured using a detector.

The fastest and easiest way of measuring the TOC is using an NDIR sensor (nondispersive infrared sensor). The gas continuously flows through the NDIR, and a detector measures the concentration levels and outputs them as different peaks in a diagram.

Another measurement option is the burning method, which involves completely burning a sample in a combustion reactor so that the carbon oxidises to carbon dioxide. The NDIR detector is also used to determine the carbon dioxide.

The TOC can also be measured using UV radiation, which is referred to as the wet chemical method. But beforehand, the inorganic carbon (TIC) is converted into carbon dioxide in a heated digestion device using an acid. The carbon dioxide is expelled using nitrogen and the TIC is measured. Next, persulphate is added and UV light is applied so that the organic carbon can be measured. The organic carbon is then converted into carbon dioxide in the heated reactor. Nitrogen is also used to expel carbon dioxide, and then the TOC content is measured.

Each of these measurement methods is time-consuming and expensive. So even though the TOC content is important for the design of a unit, it is rarely measured during operation.

TOC values in water

Source water: 1 – 2 mg/l

Slightly polluted rivers and streams: 2 – 5 mg/l

Heavily polluted waters: > 10 mg/l