Odour Update: Northacre MBT Facility; Westbury

We have been receiving an increase in odour reports over the last couple of weeks from some residents in Westbury, and we recognise the impact this can have on daily life and activities. Ensuring that local communities are protected from environmental impacts is a key priority for the Environment Agency.

We regulate the Northacre MBT facility and, following both community feedback and our own monitoring, we acted quickly to engage with the site operator to understand what was causing the increase in odour and to ensure action was taken.

What has happened and what is being done

We are aware that some residents have recently experienced stronger or more persistent odours linked to the facility. Whilst our monitoring has suggested that these have generally remained low-level and short-lived, we appreciate that an increase in odour levels is unacceptable.

The operator’s investigation identified a fault within the site’s biofilter system, which is used to control odours. With support from an independent specialist, it was confirmed that a sensor fault caused the system to become too wet. This reduced its ability to treat odorous air effectively.

To address this, the operator has:

• Replaced the faulty sensor
• Installed a backup system to help prevent this happening again
• Regenerated the biofilter this week to restore performance

These actions are expected to improve airflow and reduce odour over the next few days.

Environment Agency staff monitoring for odour in Westbury

We continue to visit the site regularly to ensure measures are being taken to minimise off-site odour and we are maintaining increased odour monitoring to make sure that odour levels return to an acceptable level as soon as possible. If needed, we will take further action to ensure the site operates in line with its permit conditions.

We continue to work with our partners, including Wiltshire Council and we will continue enhanced odour monitoring in the area and remain focused on reducing odour levels. We are committed to keeping residents updated and ensuring residents' concerns are heard and acted on. Please keep up to date with the latest information by checking back on this website and following our social media channels Environment Agency - South and South West | Bristol | Facebook and Environment AgencySW (@EnvAgencySW) / X

What have we done so far?

We understand how unpleasant and frustrating odour can be, especially when it affects your home and day-to-day life. Addressing these concerns is a priority for us.

The Environment Agency regulates waste sites through strict permits, which set out how sites must operate. At the Lower Compton landfill site, our focus has been on making sure the operator Hills meets these requirements, particularly when it comes to reducing odour beyond the site boundary.

In response to the concerns raised by the community, we have had staff actively monitoring odour in the local area since March 2025. This work sits alongside our Ambient Air Monitoring Station, which is based locally and records odour levels around the clock, 24 hours a day.

We are also working closely with our partners:

• Wiltshire Council, which leads on statutory nuisance odour.
• UK Health Security Agency (UKHSA), which provides expert advice on any potential health impacts

Together, we are taking action to ensure that Hills improves its operations and reduces odour as quickly as possible.Environment Agency staff monitoring at the Lower Compton site

What we’ve required the site to do:

• After seeing an increase in odour reports, in March 2026 we required Hills to produce an action plan, this included bringing forward the permanent capping of the current operational area, after we established this was the main source of odour from the landfill site. This is the best long-term solution to minimise odour from a landfill.
• These works are due to be completed by the end of July. We are regularly visiting the site to check that the work is progressing properly and on time.
• We have also required Hills to produce a clear plan setting out how they will minimise odour going forward. This includes addressing changes to waste composition, management procedures on site, and sizing of future landfill cells to allow more frequent capping. We are currently reviewing this plan to make sure it is strong enough and will make a real difference.
• Once the capping works are complete, we will carry out extra odour monitoring to assess the change in odour levels.

Environment Agency staff inspecting the progress of permanent capping works at the Lower Compton Hills waste site, June 2026

What's happening with the data?

We know many residents want clear answers about odour levels and any potential impacts.

We have an air monitoring station in Calne that continuously measures levels of Hydrogen Sulphide, Methane and Particulates. Our specialist monitoring team is currently checking and validating an interim dataset collected between October 2025 and April 2026 to make sure it is accurate and reliable. This is an important step when dealing with detailed scientific information. Once complete, we will share this with the UKHSA who will make public health risk assessment.

Once this process is complete:

• UKHSA will carry out an independent public health risk assessment
• This will help explain whether the odour levels could impact health or be considered a nuisance

We appreciate that waiting for this information can be difficult. While we cannot share the data just yet, we want to reassure you that it will be made public once UKHSA have made their public risk assessment.

When the UKHSA public health risk assessment is complete, we, along with our partner organisations, will hold a community event. This will give you the opportunity to:

• Hear directly from the agencies involved
• Understand what the data shows
• Ask questions and raise concerns
• Learn what further actions are being taken

We are committed to keeping you informed and will share further updates as soon as more information becomes available. In the meantime, your reports remain vital in helping us understand when and where odour is being experienced, and we encourage you to continue reporting any issues.

Permanent capping works progressing at the Lower Compton Hills site, June 2026

Environment Agency staff monitoring at the Lower Compton site

This week we have continued to have officers out monitoring for odour in the community, including over the bank holiday weekend. We have also attended during the early hours of the morning between 2:30am-:4:00am to ensure we in the area when odour is being noticed.

During some of our recent odour rounds we have been able to detect landfill gas odour in some residential areas, however this has been generally transient and at low levels of intensity.Environment Agency staff odour monitoring during the early hours However, we appreciate that odour can enter homes and buildings causing it to remain for longer. The UKHSA advises closing your windows when outdoor odour levels are high or if you are downwind and opening them when the air clears.

While our ambient air monitoring station continues to monitor and record data 24/7, we are scaling back our proactive odour monitoring in the community over the next couple of weeks.

We are doing this because:

• We have seen a recent reduction in odour reports
• When detected, our officers have rated recent odour from Lower Compton landfill as low intensity and transient in residential areas
• Weather forecasts for the next week indicate a return to prevailing winds, meaning any emissions from Lower Compton are likely to disperse over less populated areas. We maintain the ability to attend reactively and will review the requirement for further proactive monitoring in June, as work continues to cap the main source of odour on site.

Once the engineering works are complete, we will carry out a period of daily odour monitoring to confirm that the works have minimised odour from Lower Compton landfill.

Hills capping and site update

We are closely monitoring the capping works at Hills. We routinely attend site to ensure works are progressing to time and standard. This week, contractors on site have completed the placement of the 300mm deep subsoil “regulating layer” over the area to be capped. This layer has been tested for shear strength and trial pits dug to confirm it is a minimum of 300mm deep. Work is now underway placing the first of four 250mm layers of clay to form the cap, with most of the permanent and temporary capping areas now covered.

We expect odour levels to reduce once capping works are complete and will carry out further proactive odour monitoring to verify this.

UKHSA data monitoring

As previously reported, we have provided UKHSA with an initial set of data for health analysis. We are now in the process of providing further data to UKHSA that contains more detailed data to enable a public health risk assessment to be made. We will share information on when the UKHSA public health risk assessment is available as soon as we are able to.

We have been continuing to monitor at different times of the day, from as early as 5am and as late as 10.30pm following high levels of odour reports at certain times of the day.

We do this because we know that odour levels can change at different times of the day, and it’s important that we are monitoring when odour levels are present and at their highest.

Being out in the community has also allowed us to respond very quickly to your concerns and reports of odour, often arriving at the reported odour site within 5 minutes.

Weather conditions play a key part in how odour is experienced in the community. With a return to prevailing winds this week we are noticing a drop in odour complaints, and we anticipate odour levels in Calne to reduce over the coming days. However, should odour levels change, we will increase our monitoring to ensure we are responding to the communities concerns.

Monitoring data

You have been asking us 'What are we doing with the monitoring data?' so I will answer this below:

We use our officers' observations as evidence of the impact on the community. This informs our compliance work on site, and our assessment of the actions Hills are taking to reduce odour.

We hope to be able to share more data on this page soon.

Capping

Hills Waste Solutions have now started work on the permanent capping of the current operational area.

Today officers have been on site inspecting the preparatory works, where the current temporary gas collection wellheads are being raised to accommodate the cap and maintain extraction until the permanent wells are installed. Contractors will then place 1 metre of clay in 250mm layers to form the main seal or cap, which is tied into the clay sidewalls of the landfill cell. Once the capping has been placed, the permanent vertical gas wells will be installed. When complete, this work should help to reduce levels of odour. We will continue to visit the site to ensure works are progressing as planned.

We will update this page on a weekly basis, or when there is further information to share. Please do continue to report odour as it helps us to target our monitoring response.

Image: Contractors extending temporary gas extraction lines to allow for permanent capping

Our officers continue to undertake odour monitoring in your community. We are measuring odour in several different ways, including handheld devices, ambient air monitoring systems, and by reviewing the companies' own data. This image was taken last night in Calne and shows one of our officers taking readings in the late evening when you have told us odours are often more noticeable. Please continue to report odours to us using our incident reporting number 0800 80 70 60

We have posted an update to the right of this page providing clarity on how we regulate the site and its recent history, what our inspections have found, and the action we require the site operator to undertake to address odour issues. We have included previous briefings and frequently asked questions as requested by yourselves.

Our officers have inspected the site again today, and we are continuing to conduct daily monitoring of odour levels.

Update

We are continuing to investigate the recent and significant increase in odour complaints from residents in Calne.

While landfills and other waste activities will always generate smells, the impact on people living near them should be minimal. It is clear from the recent reports that the current controls in place at Lower Compton landfill are not performing as expected and are therefore inadequate.

We recognise the impact this is having on the community and are working closely with Wiltshire Council and the UK Health Security Agency (UKHSA) to identify specific odour sources and ensure action is taken to bring odour back to minimal levels.

This briefing is an update on the progress of our investigations with Hills Waste Solution (the operator of Lower Compton Landfill) and actions taken by them in response to recent odour reports.

Odour reports have increased significantly since the beginning of 2026, with 16 reports in late January, 101 in February, and over 1,000 in March. This trend has continued into April, with 726 reports to date.

We have determined that the initial increase in reports at the end of January and into February was likely caused by the installation of additional gas collection wells in the operational area of the landfill, which involved digging into recently deposited waste. However, we have continued to receive reports since this work was completed on 6 February.

Our recent inspections have found the main source of odour in the area is the current open landfill cell.

We are continuing to carry out offsite odour assessments, including early morning and evening visits when odours are commonly reported.

Our ambient air monitoring station has been in place in Calne since October 2025 and has now collected sufficient data on levels of Hydrogen Sulphide, Methane and Particulates in the community to produce an interim report. The data will now be validated and shared with UKHSA so they can provide updated public health advice; we will publish this report with the dataset once it is completed. Our ambient air station remains in place, and we have extended the study due to the ongoing reports of odour in the community.

Our officers have been out in the community, and have undertaken 22 odour inspections during March and 23 so far in April. These often occur in the early morning and late evening when odour is most noticeable in certain weather conditions.

We have identified landfill gas odour in residential areas and have traced this to Lower Compton landfill, specifically Cell 26D. We have also found odour from other sources, such as land spreading, grounds maintenance, and other waste activities, but have identified Cell 26D as the primary source of landfill gas odour.

On-site inspections

Following the strong odour identified in recent inspections, we have required Hills to provide a short to long-term action plan identifying how they will minimise odour and emissions from this area, and the likelihood of reoccurrence in future cells.

We are currently discussing the scope and timescale of the actions required with Hills. However, actions already underway include:

Short term actions:

• Change in material used as cover in the operational area from ‘trommel fines’ to soil. Work has been ongoing since 27 March to cover the operational area with soil, and we expect this action to be completed this week.
• Ongoing review of temporary gas collection within the cell and installation of further gas collection wells.
• Ongoing monitoring to check the effectiveness of the cover being placed

Medium term actions:

• Permanent capping of the current operational area. This has been brought forward from July and will now start in early May. The work is expected to be completed within 13 weeks. Permanent capping and installation of permanent gas collection infrastructure is the best long-term solution to minimise odour and emissions.

We are closely monitoring these works as they progress and are now undertaking weekly inspections of the site.

There are a number of factors likely to have influenced the increase in odour, and we are considering these when requiring the site’s management systems to be revised.

Previous inspection reports are available online via: environment.data.gov.uk/public-register/view/index. The standard timescale for our most recent reports to be made available on the public register is 28 days after issue. However, there may be a delay if a report is subject to a regulatory appeal.

Summary of next steps

• It is Hills Waste Solutions responsibility to improve their air quality issue.
• We will continue increased offsite odour monitoring to determine whether conditions improve.
• We will carry out weekly inspections of Lower Compton landfill as improvements are made to ensure they are effective, including further thermal imaging surveys to check for potential emissions from the operational area and surrounding cells.
• UKHSA will review the monitoring data from the ambient air unit once the report is complete. The data will then be validated and the UKHSA will provide updated public health advice.
• Regulatory measures will be considered if improvements are not delivered at pace, these can include: enforcement notices, restricting waste inputs, or other enforcement in line with the Environmental Permitting Regulations.

Our position is clear: odour impacts at this level are not acceptable, and the operator must act quickly to resolve the issues.

We will be uploading further information regarding the site and current odour issues over the next few weeks.

Please continue to report odour you believe is coming from activities we regulate. Although we cannot provide individual feedback, these reports are vital for our investigation.

24‑hour incident hotline: 0800 80 70 60 or via the online GOV.UK service: Report a smell from a waste facility, industrial site or farm in England - GOV.UK