Waterguide - TOC
What is TOC?
The TOC value (Total Organic Carbon) refers to the total amount of carbon compounds in a sample that can be attributed to organic substances. Inorganic carbons (TIC - Total Inorganic Carbon), such as carbonates, are excluded from this measurement. Specifically, TOC is the sum of DOC (Dissolved Organic Carbon) and POC (Particulate Organic Carbon). The DOC value provides information about the dissolved, organic carbon and the POC value about the particulate, organically bound carbon in a sample. Organic compounds can have different structures and properties that influence their reactivity and behaviour in the system. As a sum parameter, TOC is not specific to a particular organic compound and therefore contains no information about the type or origin of the organic substances. However, the TOC value can be used to draw conclusions about the water quality.
Which role does the TOC value play in electrolysis?
Water electrolysis is an electrochemical redox reaction in which hydrogen molecules are decomposed by applying a DC voltage and the electricity flowing through them. The products of this process are water and oxygen gas. In addition to other factors such as the formation of hydrogen peroxide, the TOC value in the process water circuit is particularly important for the efficiency and service life of a membrane. The reason for this is that the presence of organic impurities in the water can have undesirable effects in the electrolysis cell. These impurities can impair the electrolysis process by interfering with reactions or generating undesirable products.
What are the sources of TOC?
In aquatic systems, organic substances are produced by natural metabolic products and products from decomposition and transformation processes. In addition, natural and anthropogenic leaching from soils is decisive for the input of organic substances into water systems. Accordingly, the feed water of the electrolyser is the primary source of organic compounds. Furthermore, the operating conditions of electrolysis lead to material fatigue and wear of components. This is the case, for example, in membranes, bipolar plates, electrodes, coatings, connecting elements, pipes, resins or plastics, which represent a further possible source of organic impurities that accumulate in the system over time.
How is the TOC value determined in samples?
There are three different ways to determine the TOC content in samples. In addition to the difference method, there is the addition method and the direct method. What all methods have in common is that the organic compounds are first oxidised to CO₂ and then recorded and analysed using a suitable detector. There are two types of oxidations: a) catalytic combustion oxidation and b) wet-chemical UV oxidation.
The simplest and fastest measurement is by means of an NDIR (non-dispersive infrared sensor). The gas flows constantly through the NDIR and a detector measures the concentrations and displays them as different peaks in a diagram. Another measurement option is the combustion method. In catalytic combustion oxidation, the water under investigation is burned under high heat (650 to 1200°C) in an oxygen-containing gas stream. This converts all organic substances into CO₂, which can also be analysed and quantified using an NDIR detector.
The TOC content can also be measured using UV radiation. This is known as the wet chemical method. First, the inorganic carbon (TIC) is converted to carbon dioxide in a heated digestion device using an acid. The carbon dioxide is expelled with nitrogen and the TIC is determined. To subsequently measure the organic carbon, persulphate is added and irradiated with UV light. The organic carbon is then converted to carbon dioxide in the heated reactor. The expulsion is also carried out with nitrogen and subsequent measurement of the TOC content.
Shimadzu (https://www.shimadzu.de/toc-bestimmung-reinstwasser) concludes in the method analysis: ‘Both device types with their different oxidation methods are suitable for TOC determination [...]. The advantage of the combustion method lies in the high oxidation potential, especially if particles are present in the sample. In addition, simultaneous TOC/TNb measurements can be carried out, which increases the information content of the analysis. The advantage of wet chemical oxidation is the very high injection volume, which results in a more sensitive measuring range and high accuracy in the lower ppb range.’
TOC values in water
Measuring TOC values is time-consuming and expensive. For this reason, although the TOC content is important when designing the water treatment system, it is rarely measured during subsequent operation. Here you will find some selected TOC values:
Source water: 1 – 2 mg/l
Slightly polluted rivers and streams: 2 – 5 mg/l
Heavily polluted waters: > 10 mg/l
How can the TOC value be effectively and permanently reduced?
Physical methods such as filtration and adsorption can be used to remove organic particles from the water during pre-treatment. After successful pre-treatment, reverse osmosis, in which water is forced through a semi-permeable membrane under high pressure, removes most of the dissolved organic compounds. Organic compounds can then be oxidised using a chemical oxidation process, for example using UV light, so that CO₂ and water are present at the end.
The UV radiation ‘splits’ the water, producing hydroxyl radicals. These attack the organic compounds present in the water. Depending on their size, these compounds are oxidised in one or more chain reactions through the formation of reactive compounds (atoms and radicals) until they are finally completely oxidised as CO₂ (or, due to the low conductivity, as hydrogen carbonate (CO₂ or HCO3- is pH-dependent, not LF-dependent) and water. By combining physical and chemical processes, a higher efficiency and a more comprehensive elimination of TOC can be achieved.
The TOC value (Total Organic Carbon) is particularly interesting for water treatment where the amount of organic compounds in the water needs to be reduced. This applies to various applications:
Water treatment for green hydrogen 
Water treatment for the pharmaceutical industry