Organic carbon analysis and the environment

Monitoring and controlling the amount of organic matter in our streams and waterways is critical for environmental protection. Total organic carbon (TOC) is the amount of carbon present in an organic compound and measuring it provides a fast and convenient way to determine water quality. This article looks at the different methods now available for analysing TOC.

A high organic content in streams and waterways leads to an increase in microorganisms, such as algae, which deplete the oxygen levels in the water to the detriment of other inhabitants of that ecosystem. This 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.

Why measure TOC?

TOC analysis is used for evaluating water quality in industry and environmental laboratories. It determines the level of organic contamination for quality control and wastewater monitoring in a range of industries including chemical, pharmaceutical, food and beverage.

Analysing TOC is essential for compliance monitoring in drinking water, groundwater, soils and wastewater. By itself, TOC isn’t harmful but it can react with the disinfectants used in drinking water processing plants to produce harmful disinfectant by-products (DBPs). Because organic matter is a precursor to DBP formation, TOC is used to determine the quantity of naturally occurring organic matter in the drinking water source so that the levels of disinfection by-products can be predicted.

Monitoring and controlling TOC is also critical when it comes to processed and discharged effluent. Industrial, manufacturing and process plants need to be extremely careful before releasing water into the municipal system or into streams. The Environmental Protection Authorities (EPAs) in each state (now known as the Department of the Environment, Climate Change and Water in NSW and Department of Environment and Resource Management in Qld), provides extensive guidelines to ensure TOC levels are correctly monitored and wastewater adequately treated before being discharged into the municipal system. The consequences of pollution are financial as well as environmental due to strict laws on tolerance limits for contaminants. There are severe financial penalties, and even the possibility of gaol, for breaching these limits. With social responsibility high on the corporate agenda, protecting reputation requires tight control of effluent strategies.

TOC as a sum parameter

The total carbon of a liquid sample can be divided into total inorganic carbon (TIC) and total organic carbon (TOC). TOC can again be divided into three groups:

• Dissolved organic carbon (DOC)
• Non-purgeable organic carbon (NPOC)
• Purgeable organic carbon (POC) or volatile organic carbon (VOC)

For the classification of organic carbon, it is important to note the difference between POC and VOC. POC is actively eliminated during online analysis of TOC (eg, during stripping). VOC is a scientific term that describes the properties of volatile organic carbons. Volatilisation of substances is a passive process strongly influenced by temperature and pressure.

 

Figure 1: Classification of carbon parameters.

Methods for measuring TOC

With today’s environmental awareness and regulatory climate to consider, continuous online measurement of TOC is the preferred option. There are three common online methods: chemical oxidation, high-temperature oxidation and UV absorption. The selection of the method is dependant on the application and level of accuracy required.

TOC analysers measure total organic carbon present in a liquid sample. However, there is always an amount of inorganic carbon (IC) in a sample that needs to be ignored. In the UV absorption method IC is not measured, whereas in the chemical and high-temperature oxidisation methods the inorganic organic carbon needs to be removed to obtain an accurate TOC measurement. So the first step in these methods is to remove the inorganic carbon from the sample.

This is done by adding acid to the sample, dropping its pH to approximately 2.0 to convert all carbonate and bicarbonate forms of inorganic carbon into carbonic acid - which is dissolved carbon dioxide (CO₂). A COCO₂-free carrier gas is bubbled through the sample, removing the carbonic acid from the sample in the form of COCO₂. This process also removes the POC through diffusion and osmosis.

Chemical oxidation

The next stage differentiates the chemical from the high-temperature methods. After stripping out the inorganic carbon, the chemical method uses the oxidant sodium metabisulfite, in conjunction with a UV lamp, to oxidise the liquid sample and liberate the organic carbon as COCO₂. The COCO₂ concentration is then measured using a non-dispersive-infrared analyser (NDIR) and is directly proportional to the concentration of TOC.

As POC is also stripped out at the inorganic carbon removal stage, the resulting measurement is a TOC direct measurement, which includes only the dissolved and suspended solid TOC.

This method is suitable 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.

High temperature oxidation

In the high-temperature method, after the liquid sample has the inorganic carbon stripped out, it is combusted in an oxygen-rich atmosphere in a high-temperature oven at 850º C. This causes all organic carbon to be liberated as COCO₂. The gas is cooled and the COCO₂ concentration is again measured using an infrared detector and is directly proportional to the concentration of TOC. This method is suitable for limit violation monitoring in industrial wastewater applications, where the sample is likely to contain complex organic compounds such as ketones which need the high-temperature oxidisation to be broken down.

In the case of wastewater with a high salt load, an optional heated salt trap can be added. The salts are caught in the salt trap so they don’t clog up the furnace. The trap can be cleaned and replaced in under five minutes while the furnace is in operation. In cases of varying pH values, pH control can be added to ensure effective acid dosing during the inorganic carbon stripping process. This minimises acid consumption and prevents salt particle formation caused by excess acid addition.

UV absorption

The third TOC method uses ultraviolet light and measures the absorption at the 254 nm wavelength to provide a correlating TOC content of the sample. 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 oxidisation and chemical methods, but it does provide the repeatable, continuous and instantaneous measurements required for process control.

The UV absorption method of TOC measurement is suited to sewerage treatment plants for controlling the aeration process and to measure the incoming load of the inlet stream.

The online future for organic carbon

The reduced biological and environmental health of some of Australia’s major water bodies, such as the Murray-Darling River system, highlights the need to protect the quality of water in our rivers, estuaries and lakes. However, 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 TOC measuring processes described above are now used in a variety of applications. While grab sampling is still being used in some industries, the online approach is the only way to provide continuous and real-time water quality information.

EPA legislation, together with increasing public awareness and concern about the environment, make online measuring and monitoring of TOC an essential part of water and wastewater management.

By Jimmy Britz