Odour Measurement and Control
Dec 06 2021
Author: Rosa Richards on behalf of SWIG
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Odour from waste-water treatment works and other operations such as waste facilities is a perennial problem particularly for those sites with houses nearby. As urban areas have expanded, pre-existing facilities on the edge of towns and cities have been surrounded by residential areas, bringing odour problems closer to people. Heatwaves increase the intensity of odour as temperature (and humidity) increases the evaporation of molecules, so odour problems may worsen with climate change increasing the frequency of extreme weather. Added to this, the regulation of pollution typically becomes stricter over time and communities demand more from regulators and industry. In the UK, a statutory nuisance abatement notice can be served by the local authority which can severely limit how a site is operated. Abatement notices stay in place with the site indefinitely. In addition, water companies are driven by the Ofwat service incentive mechanism to reduce complaints logged from customers, or face a reduction in their “Ofwat score”. As the adage goes you can only control what you can measure. Accordingly, businesses have increased their expenditure to control and track odour impacts. But what is the definition of odour pollution? And how can it be measured, modelled and controlled?
Odour pollution is defined as ‘an offence to the senses’. In terms of regulation taking England as an example, the Environment Agency (EA) regulates waste sites and installations through permits to control environmental impacts. Environmental impacts, including odour incidents, are assessed and scored under a ‘Common Incident Classification Scheme (CICS) using guidance (Box 1).
“There is an element of judgement by EA officers when assessing incidents [under the CICS] using the guidance framework, but the difference between a category 1 and category 2 odour incident is very obvious,” says Nick Sauer, Technical Advisor on waste at the EA.
Any potential breaches of permit conditions are assessed and scored under a Compliance Classification Scheme (CCS). Under the environmental permitting regulations there are two objectives that are relevant to odour. The first is that Best Available techniques (BAT) must be used to prevent, or if that is not possible, minimise pollution. (Across Europe the Industrial Emissions Directive (2010/75/EU) requires that BAT be implemented to prevent or at least minimise pollution.) The second objective is that unacceptable pollution must be prevented. If unacceptable pollution is caused and cannot be mitigated, then the EA has an obligation to revoke the permit for the site or installation. The EA can also refuse to permit sites where unacceptable pollution is anticipated.
In England, activities that cause odour are also regulated by the local authority, through statutory nuisance provisions under the Environmental Protection Act 1990. The local authority will issue a notice to control any activities, not just permitted sites, that cause a statutory nuisance. If a company or individual does not comply with the notice then the local authority can take enforcement action. For sites permitted by the EA, local authorities are required to obtain permission from the Secretary of State before they can take enforcement action to avoid dual regulation by both the EA and the local authority.
Operators of facilities at risk of odour incidents can use monitoring (or modelling) to prevent complaints, achieve and prove their compliance with their permits, and defend themselves against odour complaints, to prove what the ambient odour levels were and/or that the odour originated elsewhere for example. Odour can be monitored at site boundaries or on site where odour control systems expel air. Various technologies are available, at a 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 (H2S), 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 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 and enable the operator to respond rapidly. The same forecasting models can be used to generate an odour risk forecast for the next 72 hours, so that operational tasks can be altered or reconsidered, to help reduce the risk of the facility impacting the local community. Should offsite complaints be registered, reverse trajectory modelling (back tracking) can be deployed to model the parcel of air from the complaint back to the likely source of the odour. Therefore, reverse trajectory modelling can be used to narrow down the focus of investigations into the source of a historic odour problem.
A higher-level dashboard can be used to summarise Key Performance Indicators (KPIs) on a single interface, for example threshold breaches and odour complaints, to present the headline data to management or to site managers (Figure 3).
Besides chemical dosing, another way to control odour is to incorporate preventative measures at the design stage.The Thames Tideway Tunnel (TTT) project in London, has incorporated a ventilation and odour control system to mitigate the potential risk of odour nuisance in the vicinity of tunnel shafts. For this reason, an Air Management Plan was developed which defines how air from the proposed TTT is vented in and out of the system and how air releases are controlled and treated. Activated carbon filters were chosen for use in ventilation shafts to trap odours. Another incorporation in the design of the TTT project is the use of high ventilation columns to discharge foul air at height, to prevent the impact of odour to the surrounding area and minimise the risk of odour complaints. To allow for the proposed air management and odour control system to be sized, extensive research and development work including a series of field trials to determine the impact of septicity during tunnel operations, hydraulic modelling studies, air movement modelling and dispersion modelling studies were undertaken as part of the TTT project. Overall, the TTT air management system designed achieves over 99.7% treatment of typical year exhausts. Once operational, the TTT will use hydrogen sulphide sensors to monitor odour at all shafts to ensure system efficiency.
“We undertook field trials including a review of commercially available hydrogen sulphide monitors,” explains Dr Sofia Georgaki, Odour and Ventilation Lead for the TTT Air Management System, “we discovered that detection of hydrogen sulphide at low concentrations is difficult due to several issues identified during the TTT trials. Some of the issues include power requirement, difficulties in transmitting the data and maintenance of the equipment but also sensitivity of most sensors available to ambient temperature and humidity.” To overcome the problems identified, the team designed their own bespoke self-powered solution combining a H2S analyser (using a gold leaf sensor) with solar panels and telemetry.
To conclude, compliance with an odour monitoring plan is a vital first step, but you cannot rely solely on monitoring to reflect reality, and the human receptor is always paramount. “You can use surrogate measures to monitor odour,” explains Mr Preger, “but should always recognise that nothing is better at picking up issues than the human nose.”
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