WATER / WASTEWATER 27


Box 2: The advantages and disadvantages of different odour measurement technologies Measurement technology


Advantages Test kits, lab analysis


Redox, or Oxidation Reduction Potential (ORP) Gas detection


An innovative gaseous/ aqueous technology to monitor H2 UV analysers


S analyser using gold leaf sensor Paper tape units


H2 Electrochemical cells S


Cheap | Good at high ranges >20 ppm Relatively cheap


Disadvantages At best gas detection is an indicative measurement of dissolved H2S. Relationship between H2


Non contact therefore no fouling, Continuous sampling of water using a pump and sulphuric acid reagent to liberate H2


S gas for


Measure concentrations in ppb, Most accurate for measurement Can have flows from different directions, so can measure multiple sample points simultaneously


measurement., Accurate measurement at low concentrations of H2


Portable unit but can be adapted for use for fixed monitoring Measures concentrations in ppb Measures concentrations in ppb, H2


an average over a 15 minute sampling period


Cheapest solution | Can monitor in a stack with regular flushing with fresh air – works well in dry air streams


S specific, Readings are often S Most expensive odour or H2 S monitoring technology | Not suitable for exposure to weather so need


to be housed in a kiosk | Not ATEX approved for gas streams Expensive | Odour monitor – not specific to H2


for gas streams


Expensive: need to buy tape and pay for operator to maintain on monthly basis | Moisture or water can discolour the tape | Cannot distinguish between tape discolouration caused by factors other than H2


S (e.g. moisture, gas, water or relative humidity) | Not ATEX rated for gas streams


Does not measure down to ppb, typically measures gases down to ppm for H&S room monitoring, portable gas detectors | If differential pressures are used to create airflow, the cell can fill up with water and destroy the sensor. | Electronic noise interference because amplify electronics to read at low concentrations | Cross sensitive to other gases | Cross sensitive to temperature changes | Interference from water or high relative humidity. | Not ATEX rated for zoned gas streams


Innovative H2 S sensor


Sampling and conditioning to remove any interference from water and high relative humidity. | Monitors ambient temperature and environmental conditions and gas changes to calculate and remove any electronic noise interference | Solves the above traditional disadvantages for electrochemical cells except cross sensitivity. | Can measure pre and post odour control simultaneously


Innovative H2 S sensor for monitoring wet stacks


Pre-calibrated for ease of use Extra membrane provides protection from moisture Low maintenance, can be validated by the user Sensors can be removed for calibration and reused on rotation | Sensors only need replacement every 2-3 years | No moving parts No need to bring the sample to the sensor via sampling systems | Smart sensor technology | Relatively low cost | High performance | Measures down to 1 ppb


Table 1: The best technologies to monitor odour versus hydrogen sulphide Odour monitoring


H2 sensor


UV analysers H2


S analyser using gold leaf Electrochemical cell S monitoring


UV analysers Paper Tape


H2


S analyser using gold leaf sensor (if no mercaptan gases present which are cross reactive)


Electrochemical cell if no other odours present


range of prices with different advantages and disadvantages (Box 2). When designing an odour monitoring plan, Peter Holbrow, MD at Pollution Monitors, points out the questions that an operator needs to ask themselves:


“Besides deciding whether to monitor at the boundary and/or at the odour control unit, operators need to ask themselves other questions. What are the conditions for monitoring? Is there wet or dry air flow in the odour control stack to be monitored? Is the air flow zone rated reference the ATEX Directive? If so, to what classification?” It is worth getting expert advice on where to site sensors or monitors to ensure compliance with the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) and EU directive on ‘ATmospheres Explosible’ (ATEX) 137 and therefore avoid gas explosions, recommends Mr Holbrow, who continues, “Other fundamental questions when choosing monitoring technology will be: what are the expected gas concentrations for any gases that are present? What is the purpose of the sensor or monitor? And what is a reasonable budget?”


Another key question is whether to monitor ‘odour’ in general or hydrogen sulphide more specifically, which will narrow down which technology to use (Table 1). Problematic odour from wastewater is mostly caused by hydrogen sulphide (H2


S), which can also cause


corrosion of pipes – so by reducing this gas companies reduce corrosion of pipes as well as odour issues. Precise and accurate measurement of hydrogen sulphide can be used to control chemical dosing for wastewater treatment plants.


“Removal of hydrogen sulphide often involves expensive chemicals or expensive hydrogen peroxide,” explains Tristen Preger, Director of OEM Sales & Business Development at ATi, “and continuous measurement of hydrogen sulphide in wastewater is notoriously difficult.”


Any sensor, monitor or analyser will need recalibration and servicing. “A legal defence under an operator’s Odour Management Plan will only be valid if equipment is working and data is reliable,” points out Mr Holbrow.


“Any system is only as accurate as the person maintaining and calibrating it,” adds Mr Preger.


Odour sensors will need maintenance every 6-12 months. Most manufacturers recommend recalibration every 6 months. Operators can run a few checks themselves to keep things in good working order.


Real-time odour monitoring data can be used in a modelling platform


Cross sensitive to other gases requires suitable sampling and conditioning and/or ATEX safety devices if monitoring an ATEX air flow.


S | Cross sensitive to other gases | Not ATEX approved


Not continuous | Time lag before results | Not good at low ranges <5 ppm | Not great in wastewater Not specific to H2S | Difficult to maintain | Not accurate at low concentrations of H2 correlate Redox to H2S; not a strong relationship between ORP and H2


S | Difficult to S


S and wet well is not fixed due to fluctuations in pH because of anaerobic conditions | Turbulence affects readings | Temperature changes affect readings | Cannot control all the factors involved.


Relatively expensive | Operational costs to replace buffer, tubing, calibration of sensor


Figure 2: Predictive dispersion modelling © Envirosuite, 2021.


to gain further insights from the data by combining it with site specific information and meteorological data. A modelling software platform can take real-time monitoring data from a range of sensors, meteorological satellite data and an onsite weather station to generate an arc of influence for each device, enabling the identification of the source in real-time. Where two (or more) of the arcs overlap this helps identify the source of the odour that has been measured on affected monitors (Figure 1).


Real time alerts can be created tailored to an operator’s needs, aiding in the understanding of when an event is developing rather than receiving the data after it has already passed.


By using a sensor agnostic software platform, multiple parameters can be made available onto one dashboard, for example numerous gases (hydrogen sulphide, ammonia, sulphur dioxide), noise, water quality, network sensors and groundwater flow (Figure 2). Historical and predictive metrological information can be combined with innovative dispesion modelling to predict where and when odour problems may occur. Therefore, modelling can be used to provide an early warning of odour issues


Figure 3: High level dashboard for a modelling software platform © Envirosuite, 2021.


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