Gröger & Obst Analyzers for Simultaneous Measurement of TOC and VOC

Options for the Modification of TOC Standard Equipment

Dr.-Ing. Rolf Semsch

Significance of TOC and VOC

To determine the degree of organic pollution of waste water, labs prefer the kind of analytical test methods that yield useful results without being costly or demand the use of large-scale equipment. To achieve this end, labs employ methods that are sensitive to the oxidizing capability of all organic matter and record the so-called sum parameters (e.g. TC, TOC, DOC, TIC and VOC). These parameters are instrumental in water analysis and likewise in the determination of solutes and the physicochemical variables.

In recent years, continuous measuring systems (e.g. thermal catalytic oxidation systems) have been generally accepted and they have proven their efficiency compared with discontinuous systems. It is now possible to monitor the concentration gradient over an extended period of time and, by doing so, it is no longer necessary to rely on isolated results for a meaningful assessment of the quality of water.

In sewage engineering, “on-line” measuring systems for the determination of the TOC sum parameter, which reflects the level of organic pollution, have skyrocketed and are firmly established by now. For the assessment of drinking water, groundwater, surface water, leakage water from landfills and waste water containing organic matter, the total content of organic carbon (TOC) is one of the variables which is increasingly used to supplement or replace the chemical oxygen demand (COD). As a result of this situation, the supply of TOC-Analyzers on the market has increased (Fig. 1).

Fig. 1: TOC-Analysers GO-TOC 1000 (left), GO-TOC 100 P (centre) and GO-TOC P (right) (Manufacturer: Gröger & Obst Vertriebs und Service GmbH).

The TC (Total Carbon: the sum of organic and inorganic bound carbon in dissolved and undissolved compounds) represents the total load of organic matter and is mainly composed of the following sum parameters:

• TIC (Total Inorganic Carbon: the sum of inorganic carbon in dissolved and undissolved compounds)
• TOC (Total Organic Carbon: the sum of organic carbon in dissolved and undissolved compounds)
• DOC (Dissolved Organic Carbon: the sum of organic carbon in dissolved compounds)
• VOC (Volatile Organic Carbon: the sum of volatile (blow off) compounds)

VOC is the generic term for substances containing organic bound carbon that is easily volatilized. At first, all volatile organic compounds having a boiling point of 250 °C had been classified as VOCs. Now, a distinction is made between VOC and SVOC (Semi Volatile Organic Compounds = not easily volatilized organic compounds) (boiling point 240-260 °C). SVOCs are among other things phthalates, higher fatty acids and the like.

According to the WHO definition, a VOC is an organic substance having a boiling range between 60-250°C. These compounds classified as VOCs include e.g. alkanes, alkenes, aromatic compounds (benzene etc.), terpenes, halogenated hydrogen compounds, aldehydes and ketones. These easily volatilized organic compounds may leach out into the air where they are likely to become a health hazard. There are a number of other compounds that fall within the definition of VOC and have been classified as extremely toxic or rather carcinogenic (e.g. above all benzene in gasoline).

VOCs are an indirect by-product of ozone formation near the ground and of other photo-oxidants. Therefore, reduction limits must be defined for such toxic substances. The release of volatile organic compounds is also a problem in the treatment of boiler feed water. It is a potential health hazard (workplace, environment).Due to more stringent government requirements, recording of VOC levels has recently become more important. Continuous and fast determination of these critical sum parameters is of paramount importance because it allows necessary precautions and prompt reaction in case of emergencies.Thus, methods for continuous VOC determination have gained recognition for applications where specified limit values have to be met. Additional VOC readings provide better insight and understanding of the chemical and biological processes occurring in the tested sampling lines.

Fast and simple continuous determination of VOC concentrations was only possible done with conventional test methods, which were costly and time consuming and, as a consequence. Therefore, measurements were neglected even though the extent of organic pollution caused by volatile organic compounds constitutes a considerable health hazard.

The measuring principle developed by Gröger & Obst based on the modification of the proven design of continuous TOC determination with thermal catalytic oxidation can meet the requirements of continuous VOC determination.Here, the volatile organic compounds are extracted with air, purged from gaseous carbon dioxide (soda lime) and fed into the reactor. There, oxidation of the easily volatilized carbon compounds is initiated at a temperature of about 850 °C and enhanced by catalytic conversion. The carbon content of the carbon compounds is quantitatively converted into carbon dioxide. Subsequently, the carbon dioxide is purged from the vapour (condenser) and measured using an infrared detector.

Unlike the TOC analyses, no acid is used and thus, only the easily volatilized organic compounds are blown off. The inorganic carbon compounds (carbonates, hydro carbonates) remain in the aqueous phase and are removed from the system.

Because of this method, the components of the devices are much less adversely affected by VOC than by TOC measurements. No salt is added, lower concentrations are used, no acid is added, and, as a result, the service life of the individual components is extended considerably. This is reflected in extremely economical operating costs. The measuring technique is also extremely flexible and it is suitable for a wide range of applications.

The following continuous measuring methods are available:

• Consecutive measurement of TOC and VOC
• Simultaneous measurement of TOC and VOC (2nd thermal reactor und 2-channel detector, see Fig. 2)
• Two sampling flows TOC and VOC (2nd thermal reactor and 2-channel detector)

Fig. 2: Example showing two spectra obtained during simultaneous recording of TOC and VOC.

Comparative measurements conducted with conventional standardized test methods have confirmed the VOC results from thermal catalytic oxidation.

Modification of suitable test equipment

The scope of applications can be further extended e.g. to include recording of TIC and ultimately also of TC. As an extra benefit the TOC device on hand needs only a minor modification and is ready for VOC analysis. This makes the system extremely cost-effective.

When simultaneous measurements of multiple parameters are needed, it only a second reactor and a 2-channel detector are required. For consecutive measurements adjustments for the consecutive mode are just a matter of minutes.

The figure below (Fig. 3) shows a possible configuration of the equipment with a 2-channel detector and an additional reactor for simultaneous, continuous determina¬tion of VOC and TOC. Owing to the small footprint, it is possible to run 4 TOCs on-line on a surface area of 1 square metre.

Fig. 3: TOC-Analyzer GO-TOC P with continuous VOC measurement option (Manufacturer: Gröger & Obst Vertriebs und Service GmbH).

It is above all the fundamental configuration of the GO-TOC P analyzer (manufacturer Gröger & Obst) that is suitable for upgrading to do simultaneous recordings of TOC/VOC.

The configuration of the device is very flexible and it can be upgraded by adding a second oxidation reactor and a 2-channel detector. No major modifications of the basic equipment are needed. Moreover, with an alternative modification of the GO-TOC-P (manufacturer: Gröger & Obst) it is also possible to measure the VOC content of the air. Owing to the flexibility of the method, it is also possible to determine the TC contents in solids.

To sum it up!

All it takes to measure the sum parameter VOC is an appropriate test method and a suitable TOC analyzer. Simultaneous measurements of VOC and TOC are also possible; of course, the equipment has to be modified correspondingly for this purpose.

The GO-TOC P developed by Gröger & Obst is an ideal tool for these measurements and, as a result of the continuous and fast data output of crucial parameters, it is possible to act or rather react optimally in critical situations. Merits such as continuous recording of multiple parameters, enhanced quality (optimal process management and process control), high-quality safety features primarily in the area of automatic measuring and control engineering (e.g.: monitoring of valves and emergency cut-offs, etc.) are highly appreciated.

Analyzers working in continuous mode have clear advantages over gas chromatography (FID) because of their small footprint and because there are no hazardous waste problems. Moreover, there is no need for gas bottles and EXplosion proofing of rooms.