Controlling the amount of organic matter in our streams and waterways is critical for environmental protection. A high organic content in streams and waterways leads to an increase in microorganisms, such as algae, which depletes the oxygen levels in the water to the detriment of other inhabitants of that ecosystem.

The organic matter is introduced on a daily basis. It comes from decaying natural organic sources such as plants, animals and the environment, and from synthetic sources such as insecticides, herbicides, pharmaceuticals, detergents and pesticides. Industrial plants and wastewater treatment facilities also contribute organic contaminants in various amounts.

Total Organic Carbon (TOC) is a term that encompasses all these organic compounds and is the amount of carbon present in an organic compound. Analysing TOC provides a fast and convenient way to determine the amount of organic impurities in a water source.

In particular, when it comes to processed and discharged effluent, controlling TOC is critical. Industrial, manufacturing and process plants need to be extremely careful before releasing water into the municipal system or into streams. The TOC levels must be carefully monitored and the wastewater adequately treated before being discharged into the municipal system. The consequences of pollution are financial as well as environmental as there are strict laws on tolerance limits for contaminants - and severe penalties for breaching these limits.

Three common online methods exist for determining TOC. They are chemical oxidation, high temperature oxidation and UV absorption. The selection of the method is dependant on the application and level of accuracy require.

The first step in the process of the chemical and high temperature oxidation methods is to remove the inorganic carbon from the sample. This is done by adding acid to the sample to convert the inorganic carbon into carbon dioxide (CO2) gas. The stripped CO2 is then purged from the sample by a carrier.

The next stage differentiates the chemical from the high temperature methods. After stripping out the inorganic carbon, the chemical method uses a power oxidant, sodium metabisulfite, in conjunction with a powerful UV lamp to oxidise the liquid sample and liberate the organic carbon as CO2. The CO2 concentration is then measured using an infra-red detector and is directly proportional to the concentration of TOC. This method is ideal for limit violation monitoring in water applications where high accuracy is paramount such as effluent streams feeding into rivers or oceans and water recycling plants.

In the high temperature method, after the liquid sample has the inorganic carbon stripped out it is then combusted in an oxygen-rich atmosphere in a high temperature oven at 850º C. This causes the organic carbon to be liberated as CO2. The gas is cooled and the CO2 concentration is again measured using an infra-red detector and is directly proportional to the concentration of TOC. This method is ideal for limit violation monitoring in industrial wastewater applications, where accuracy is vital and there are complex carbon chains, which need the high temperature oxidization to be broken down.

The third TOC method uses a spectrometer and measures the UV absorption in the 190 to 400 nanometre wavelength range, to determine dissolved organic carbon (DOC). The spectrometer is calibrated according to the correlating TOC content of the calibration samples DOC value. This in-situ method is therefore low maintenance and doesn’t require chemicals. As this is an inferred measurement it does not provide the same accuracy as the oxidization methods, but it does provide repeatable, continuous and instantaneous measurements required for process control. UV absorption TOC measurement is typically used in sewerage treatment plants to measure incoming load of the inlet stream and aeration process control.

TOC measurement is not only important because of its effects on the environment - human health and manufacturing processes are affected by TOC content too. The measuring processes described above are now used in a variety of other applications including monitoring drinking water and detecting contaminants in water used in manufacturing and pharmaceutical industries.

Endress+Hauser Australia focus on the supply, service and training of a complete range of water analysis measurement solutions. Encompassing more than just these three measurement methods the company are also specialists in pH, ORP, conductivity, turbidity, dissolved oxygen, chlorine, ammonium, nitrates, phosphates and BOD.