CONTINUOUS MONITORING OF VOCS IN WATER AND WASTEWATER FOR ENVIRONMENTAL AND INDUSTRIAL APPLICATIONS
Today, more than ever, environmental health is of great concern to everyone from politicians and scientists, to the general public. All organizations must recognize the importance of protecting our ecosystems. Prosecution for polluting the environment can not only lead to expensive fi nes, but can also result in a company’s image and subsequent devaluation.
Water utility companies, industries with their own waste water treatment plants, and petrochemical industries are acknowledging the importance of controlling pollution. These organizations are investing in high-detection technologies and monitoring equipment, not just for clean water intake, but also for waste water treatment that ensures that water quality is acceptable before being returned to nature.
In cases where hydrocarbon pollutants are involved, instruments capable of measuring VOCs at an early stage can prevent and reduce the impact on the environment as well as mitigating the costs related to the cleanup process. There are several VOC detection methods already available in the market that are simple to use and that require very little maintenance and calibration. Most of VOC detection technologies are designed for ambient air monitoring, but some high-profi le companies have developed similar solutions for liquid analysis.
With a long history in gas chromatography, Chromatotec® has now launched an analytical system, the airmoVOC WMS, that incorporates liquid sample handling and has already proven its worth. This unique and effi cient Mcerts-certifi ed gas chromatography solution measures and quantifi es VOCs and BTEX (Benzene, Toluene, Ethylbenzene and Xylene) compounds dissolved in liquid matrices.
This unique instrument is comprised of several components, the most important of which is its purge and trap (P&T) system. This analytical tool, developed internally follows the US EPA 502.2 method and consists of an automatic sparger that extracts VOCs from liquid samples in 11 minutes. The loop is fi lled with 5mL of the liquids sampled using a pump and then injects them into a glass purging device. Pure nitrogen is produced by an integrated gas generator and is used to purge the water sparger with bubbles of less than 3mm of diameter at the origin of the frit (Figure 1).
In general, the P&T technique is applied to analyze low-molecular- weight (C2
-C12 ) compounds which can migrate to the vapor
phase, while other compounds remain in the liquid phase. This is especially benefi cial when high-molecular-weight species react or condense in the analytical column. P&T is the most frequently used method for the extraction and concentration of VOCs in water thanks to its high sensitivity and reduced matrix effect.
The gaseous sample extracted from the liquid matrix is then injected automatically and in continuous mode into the system with a fl ow rate of 40mL.min-1
, regulated by a critical orifi ce IET SEPTEMBER/OCTOBER 2021
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Figure 1: Purge and trap system
of 76µm. The sample passes fi rst through a dryer to remove the humidity and then hydrocarbons are pre-concentrated on a trap fi lled with a mixture of Carboxen and Carbopack. The pre-concentrated air sample is thermally desorbed at 380°C for 4 minutes and directly injected into a metallic capillary column located inside the oven of the GC. The column is temperature- regulated at a very stable temperature to obtain repeatable retention times. Within the fi rst minute of the analytical procedure, the oven temperature rises from 32°C to 34°C. Afterwards, a constant heating rate increase of 7°C.min-1 reaching 100°C, followed by a 10°C.min-1
is applied until until reaching 200°C.
The temperature is then kept at 200°C for 600s before cooling. Hydrocarbons are detected by FID at 170°C.
The typical cycle time is 60 minutes. This includes the following
steps: automatic rinse (once with deionized water and then once with the sample), sampling, liquid injection in the sparger, nitrogen purge, injection into the system, thermo-desorption, evacuation of the sparger, separation in the analytical column, detection, quantifi cation and cleaning of the system during acquisition.
This all-in-one solution is composed of an airmoVOC analyzer, hydrogen generators, nitrogen generator and zero air catalyzer. In addition, an internal calibration system can be inserted in order to validate the results and certify them in real time. The integrated multiplexing system allows the analysis of unknown gaseous samples, reference cylinders, embedded calibration (permeation tubes) and samples from the purge and trap system. The instrument is enclosed in a robust housing and can work for long periods of time without maintenance. The main goal is to improve
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