Air Quality Monitoring Jul 26 Questions & Answers

Since we became aware of increased odour issues, we have increased our regulatory oversight of the site. This has included carrying out more frequent inspections, undertaking odour assessments both on and off site, reviewing reports from residents, and collecting air monitoring data to help us better understand emissions from the landfill and their impact on the surrounding area.

We have also worked closely with our partners, including Wiltshire Council and the UK Health Security Agency (UKHSA), sharing information and evidence to ensure a coordinated approach. While each organisation has a different role, we have been working together to understand the issues and respond appropriately to community concerns.

Our investigations, inspections and monitoring identified concerns about aspects of the site's operation. As a result, we have required Hills to make a number of improvements and review key management documents, including its Odour Management Plan. We have also required changes to how certain waste materials are managed on site to reduce the risk of future odour emissions.

A key part of our regulatory response has been requiring Hills to undertake significant engineering works on Cell 26D. These works include the installation of permanent capping and additional gas extraction infrastructure, which has been brought forward as a result of our investigation. Permanent capping helps to seal the landfill surface and reduce the escape of landfill gas, while gas wells and associated extraction equipment improve the collection and control of gas generated within the landfill. Once collected, the gas is directed to engines and combusted to produce electricity or burnt in a flare. Together, these measures are designed to reduce emissions and provide a longer-term improvement in the environmental performance of the site.

The capping and gas infrastructure works are currently underway, and we expect them to reduce odour emissions once completed. We will continue to assess their effectiveness through site inspections, air monitoring, odour assessments and engagement with the local community.

The Environment Agency (EA) will continue to regulate the site closely and take further action where necessary. Our focus remains on ensuring that Hills complies with the requirements of its Environmental Permit and that appropriate measures are in place to minimise impacts on local residents.

We only revoke permits where we consider that the operation poses a serious risk to the environment or human health, and all other ways to reduce the odour have been exhausted.  Closing the site down would not stop the odours, work would still be required to identify and address all odour sources.

We regulate in accordance with Defra guidance on the Environmental Permitting (England and Wales) Regulations 2016 (‘EPR’) (the Core Guidance), the Regulators’ Code, and our own Enforcement and Sanctions Policy. We may consider revocation of a permit if:

• An operator in Band E or F is failing to work towards compliance in a timely manner.
• The operation poses a serious risk to the environment or human health, and the operator has exhausted all other ways to remove that risk.

At Lower Compton, the operator is undertaking engineering work on site to minimise emissions from the area we have identified as the main source of landfill gas odour, and further controls are being put in place to minimise the likelihood of significant odour recurring. If we assess that these measures are insufficient, we will require the operator to take further action or consider varying the sites permit to restrict activities or waste streams. We are not currently considering permit revocation.

The EA began air monitoring in October 2025 following a high number of odour reports from the local community.

As the regulator of waste facilities, our role is to ensure that operators comply with the conditions of their Environmental Permits. This includes requirements to prevent or minimise pollution and to manage odour appropriately. The monitoring was undertaken to support our regulatory responsibilities and help us understand whether emissions from the Lower Compton Landfill Site were being effectively controlled.

The monitoring helps us to:

• Understand what substances are present in the air.
• Identify likely sources of emissions.
• Assess whether actions taken by the operator are reducing emissions.
• Understand whether emissions from the site may be affecting local air quality.
• Inform our regulatory decisions and any action we may need to take.

The primary purpose of the EA’s monitoring was to support our regulatory role and assess compliance with the site's Environmental Permit, not for public health purposes.

Public health assessment is the responsibility of specialist health organisations, including the UK Health Security Agency (UKHSA), working alongside local authorities. Throughout this work, we have worked closely with UKHSA and Wiltshire Council, sharing monitoring data and other information to support their independent assessments.

Each organisation has a different role and area of expertise. By working together and sharing information, we can ensure that both environmental regulation and public health considerations are properly assessed using the best available evidence.

We have been monitoring for:

• Methane
• Hydrogen sulphide (H₂S)
• Particulates

Approximately 99% of landfill gas consists of methane and carbon dioxide. The remaining 1% consists of a variety of compounds, some of which are odorous, including hydrogen sulphide.

We use methane and hydrogen sulphide as surrogates for odour because they are commonly found in landfill gas emissions and can help us understand and investigate odour impacts. If hydrogen sulphide or methane is present, it may indicate that landfill gas emissions are reaching the surrounding area. 

We use:

• Site inspections.
• Odour assessments carried out by EA officers.
• Reports from local residents.
• Information provided by the operator.

When investigating odour reports, EA officers assess:

• Whether an odour is present.
• The type and intensity of the odour.
• Whether the site is the likely source.
• Any available air quality monitoring data
• Whether the operator is taking appropriate measures to control emissions.

Odour intensity is scored on a scale from 0 to 6:

0 – No odour

3 – Distinct odour

6 – Extremely strong odour

The EA regulates the site against the conditions of its Environmental Permit.

The Environmental Permit does not have specific limits for air pollutants. Instead it includes a more powerful condition that requires the site “not to cause odour to escape the site at a level likely to cause pollution as perceived by an authorised officer. As a result officer observations are an important part of our assessment because people can often detect odours atlevels below the detection limits of monitoring equipment.

A permit breach may be identified where:

• EA officers confirm that odour from the site is present beyond the site boundary at a level likely to cause pollution; and
• The operator is not taking all appropriate measures to prevent or minimise those emissions.

If the operator is taking all appropriate measures to control odour, a permit breach may not have occurred, even if some odour remains detectable.

Reports from residents are an important part of the evidence we use. They help us identify patterns, target inspections and understand when and where odours are being experienced.

We share relevant information with the operator so they can investigate and take action where necessary. Personal information is not shared. Operators are expected to respond in line with their Odour Management Plan and inform us of any operational changes they make to reduce emissions.

Addressing community concerns requires input from a number of organisations. The EA continues to work closely with UKHSA, Wiltshire Council and the site operator, sharing information and expertise to build a clear understanding of the situation and ensure that appropriate action is taken where necessary.

While our role is to regulate the site and assess compliance with its permit, we recognise the importance of working collaboratively with our partners so that environmental, community and public health considerations are all taken into account.

We understand that residents want reassurance that ambient air quality monitoring is taking place in a location that provides meaningful information about the air they breathe.

When selecting a monitoring location, we considered a number of factors, including where odour reports were being received, how emissions from the landfill were likely to move through the local area, and the surrounding landscape and weather patterns. We also had to ensure the location was suitable for monitoring equipment, including having a secure site, power supply and an appropriate hardstanding surface.

Taking all of these factors into account, the primary school was identified as the most suitable location for the monitoring station. It also provided an opportunity to monitor air quality in an area used by the wider community.

Long‑term monitoring, rather than very close‑range monitoring, is necessary to understand how odour pollution behaves under different conditions.  We have monitored over an extended period so that we can capture a range of operating conditions and weather patterns, including different wind directions and seasonal changes. This helps us build a more complete and representative picture of air quality and odour in the area.

The monitoring data is considered alongside other important sources of information, including reports from local residents and odour assessments carried out by EA officers across the community. Together, this provides a comprehensive evidence base to support our regulatory work.

Based on the information gathered so far, we are confident that the existing monitoring, combined with officer observations and community reporting, is providing the information we need for our regulatory assessment. For that reason, we do not currently plan to install additional monitoring stations in Calne. However, we will continue to collect data from the existing monitoring station and keep our approach under review as we continue to work with local communities and partner organisations.

Yes. The air monitoring was carried out by the EA's specialist national monitoring team, who are trained and qualified to undertake this work. Their monitoring methods and processes are also independently audited to ensure they meet the required standards.

The monitoring report explains the capabilities and limitations of the equipment used, including the levels that can be detected and the degree of uncertainty in the measurements. In addition, the findings have been reviewed by experts from the UK Health Security Agency (UKHSA) as part of their assessment.

This helps ensure that the data is robust, reliable and suitable for informing decisions about potential impacts on local communities.

We understand that residents have been keen to see the results of the monitoring and to understand what the data tells us about conditions in the local area.

The monitoring programme was designed to collect data over a six-month period, from October 2025 to April 2026. Monitoring over an extended period allows us to capture a wide range of weather conditions, seasonal changes and site operating conditions, helping to build a more complete and reliable picture of emissions and odour in the community.

Once the monitoring period ended, the data needed to be downloaded, quality checked, analysed and interpreted by specialist staff. We also wanted to assess whether the environmental performance of the landfill site had changed over the monitoring period and ensure the findings were robust before they were shared publicly. The results were then provided to our partners at the UK Health Security Agency (UKHSA) to support their independent assessment.

Importantly, our work to require the operator to deliver improvements did not stop while the report was being prepared. The monitoring station has remained in place and continues to collect ambient air quality data.

We recognise that residents want to know whether conditions are improving. The continued monitoring is helping us assess emissions and odour while permanent capping and gas infrastructure works are being carried out on Cell 26D. It will also allow us to evaluate how effective those improvements are once completed and whether any further regulatory action may be required.

We remain committed to keeping the community informed. As more data becomes available and the ongoing work at the site progresses, we will publish further monitoring information and supporting assessments from UKHSA to help residents understand what the monitoring is showing and how it is informing our regulatory decisions.

The ambient air monitoring data produced by the EA shows that there is a source of hydrogen sulphide and methane in the direction of the Lower Compton Landfill Site.

This supports the findings of our inspections and off-site odour assessments.

The engineering work currently being completed in cell 26D is key to addressing the current odour issues on site. We will continue to monitor emissions while the capping and gas well installation works are underway and once they are completed, to assess their effectiveness and determine whether further measures are required.

We keep our monitoring arrangements under review to ensure they remain proportionate to the risk and the evidence required to support our regulatory decisions.

Based on our inspections, monitoring and other evidence gathered over recent months, we have identified concerns about aspects of the site's operation and have required Hills to take action to address them. Our priority is to secure improvements as quickly as possible to reduce the impact on the local community.

We are continuing to closely regulate the site and assess whether the operator is complying with the requirements of its Environmental Permit. Where improvements are needed, we will require the operator to make changes and, where appropriate, consider the use of formal enforcement powers.

We have already required Hills to review and update parts of its management system, including its Odour Management Plan, so that we can assess whether further measures are needed to improve odour control.

Our investigations indicate that several factors may have contributed to increased odour emissions. We are requiring Hills to address these issues to reduce the likelihood of similar problems occurring in future.

As a result of our regulatory intervention, Hills has already introduced a number of changes, including:

• Reducing the amount of trommel fines accepted at the site.
• Stopping the use of trommel fines as daily cover.
• Burying trommel fines at the operational face and ending the stockpiling of this material on site.
• Increasing the use of site-won material as intermediate cover.
• Preventing trommel fines from being placed within five metres of the base of new landfill cells.

In addition, works to install permanent capping and additional gas extraction infrastructure are nearing completion. These measures are designed to improve the collection and control of landfill gas, which we expect will help further reduce odour emissions.

We know that residents want to see lasting improvements and we will continue to explore all options with Hills, which could include permit variations. We will continue to monitor conditions, carry out inspections, review monitoring data and assess the effectiveness of the changes being made. If further action is needed, we will not hesitate to use the regulatory and enforcement powers available to us.

UKHSA Q&A Document (PDF)

As the operator, Hills have been actively engaged with the EA over this issue and are working through an improvement plan agreed with the EA.  This includes the capping of an active landfill cell with a subsoil restoration layer (now completed) and the installation of additional infrastructure such as permeant gas wells scheduled for mid-July.  Hills have also amended to type of material used for daily cover which was found to be a source of odour and have returned to the use of soils to mitigate further odours. 

No. The Hills transfer operation operates separately to the landfill and occupies a different area of their site.  Waste temporarily held at the transfer station is typically destined for landfill diversion facilities, such as the Northacre Mechanical Biological Treatment (MBT) and Lakeside Energy from Waste (EfW) plants, and can comprise household waste, commercial waste and fines from the Sands Farm Materials Recovery Facility (MRF) process which sorts recycling.

The  Northacre MBT is also an established source of odours affecting the Westbury community, and the operator (Hills) have just announced a temporary closure of that facility to undertake essential repairs. 

For the duration of the unplanned closure Hills will not be receiving household waste at the MBT.  This waste will need to be delivered to Lower Compton waste transfer site instead.  Waste from the transfer site will then either be sent to either the Lakeside Energy from Waste facility in Slough or immediately disposed of at the Lower Compton landfill.