40 Water / Wastewater
Continuous Monitoring of Odourous Compounds in Waste Water Treatment Plants - Sensory approach or Physico-chemical solution?
Odour measurement is a demanding topic to manage due to the complexity of existing solution and the methodology used.
Market needs
“For years, WWTP as prestigious as SIAAP in France or stations in the UAE have placed a lot of trust in Chromatotec solutions. A measurement network has been deployed on site and data is centralised on SCADA systems.”
When an odour is perceived, the fi rst approach is to try to identify its origin. This approach requires solutions able to track very low concentrations due to air dilution effects.
Several solutions can be provided: • Sensory approach • Physicochemical approach
Sensory approach: dynamic olfactometry
Dynamic olfactometry relies on human nose evaluation. Samples are collected at the source and sent to a certifi ed laboratory working according to standard methodology (EN 13725, ASTM 679-E04, VDI 390). Samples are automatically diluted and smelt by a sensory panelist with dedicated equipment; a dynamic olfactometer.
This approach is not easy as the time between sampling and measurement must be reduced to avoid reactions and adsorption of unstable molecules in the gas mixture (i.e.: sulphur compounds).
This method offers a spot measurement which does not take into account process or meteorological variations because results are available only after a few days.
Electronic nose is an alternative and complementary approach on dynamic olfactometry as they can offer online measurement of odour emissions. Electronic nose technology may use non-specifi c sensors such as metal oxide sensors (MOS), gas chromatography or mass spectrometry. For these last two technologies, the systems provide odour and chemical fi ngerprints and can identify and quantify contaminants in comparison with MOS sensors which cannot provide such results.
In both cases, chemical profi les are compared to sensory evaluation to check possible correlation between instrumental and sensory evaluation. An odour index can be defi ned to monitor odour intensity according to specifi c methodology if needed.
These global approaches help to rank odour sources on site (dynamic olfactometry) and allow users to focus on the most emissive sources to treat odour issues with adequate deodourising process.
With e-noses, the main limit is the training of the equipment (called the learning stage) as it is needed to realise several analysis of samples with dynamic olfactometry to train the e-noses.
Authors/Contact Details:
Jean Philippe AMIET, Louis VIVOLA Chromatotec
15 rue d’Artiguelongue 33240 SAINT-ANTOINE France
Tel: +33 (0)557 940 627 / + 33(0)762 667 658
Fax : +33 (0)557 940 620 Email :
louis.vivola@
chromatotec.com IET September / October 2014
www.envirotech-online.com
On the other hand, when odour occurs it is quite diffi cult to understand which step of the process may explain the odour emission due to the lack of information on chemical concentration (i.e. in a Waste Water Treatment Plant (WWTP))
The physico-chemical approach: alternative solution to quantify odourants!
Several solutions exist for the quantifi cation of odourous molecules. Solutions are deployed and designed depending on the expected sensitivity.
Portable sensors used as indicators located in the environment provide alerts as soon as values exceed a reference value. These
indicators require annual replacement of sensors.
As they rely on electrochemical cells, electrolyte is consumed over time and also needs annual replacement. These sensors may be good indicators of process drift or chemical leakage but predictive concentration is not accurate as no calibration is provided.
Sulphur concentration is estimated with a global approach and it is not possible to discriminate H2
S from Mercaptans
concentration. If the user is interested in several molecules, several units need to be deployed (one per targeted molecule).
To quantify several molecules, it is compulsory to have multiple and sensitive analysers.
In WWTP, site managers are familiar with odour and chemical profi les as they are exclusively focused on sulphur compounds measurement. These molecules are used as process drift indicators and offer a good criteria of process performance. Hydrogen Sulphide (H2
S), Methyl Mercaptan (MM), Ethyl Mercaptan (EM), dimethyl sulphur (DMS) or Di-Methyl-di sulphur (DMDS) and sulphur dioxide (SO2
) provide a better understanding of odour
emissions. For example, high sulphur concentrations have been measured on primary clarifi ers contaminated by effl uents coming from tanneries and responsible of complaints from the neighbourhood.
So with this quantifi cation step, it is easy to link odour perception to its origin and be alerted when level of concentration exceed specifi c reference values.
In addition, as operators on site are not experts in chemical analysis, it is very important to provide turnkey solution with automatic result validation and a friendly user interface.
TRS MEDOR for on-line monitoring of Sulphurs from ppt to ppb or ppm levels
The reduced sulphur compounds, such as the Mercaptans and organic sulphides, tend to be the most odourous, based on their relatively low odour threshold concentrations.
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