Odour Update 19 June 2026: Northacre MBT Facility; Westbury

In our previous update, we reported that Hills had an issue with their biofilter and that they had carried out maintenance work which was expected to help reduce odour levels. However, Hills have now confirmed that the new biofilter is currently not operating effectively due to a further fault with the newly installed moisture control sensor. This has resulted in the system becoming too wet and not functioning as designed.

While a back-up system enabled the issue to be identified quickly, we recognise that any increase is concerning and understand the frustration this causes within the community. We have required Hills to take action to rectify this as quickly as possible.

We have independently observed an increase in odour this week, which we attribute to these ongoing biofilter failures. The odour our officers are detecting is primarily that associated with general household waste and not hydrogen sulphide that is present at other waste sites.

Following our investigation and direct engagement with Hills, we have formally identified that the site is currently not compliant with the conditions of its environmental permit. We have made clear to the operator that this situation is unacceptable and have required immediate corrective action.

As a result, we are considering enforcement action and have instructed Hills to resolve the issues with the biofilter as a matter of urgency. Our priority is to ensure the operator meets its legal obligations and that the impact on the community is minimised.

Our officers have been proactively monitoring conditions on site and are continuing to respond promptly to odour reports. This increased level of oversight will continue until we are satisfied that the issues have been fully resolved and that control measures are working reliably.

We want to reassure residents that we are holding the operator to account and taking firm regulatory action to resolve the situation. Your reports play a vital role in supporting this work, and we encourage you to continue reporting any odour incidents.

We appreciate the community’s continued patience and will provide further updates as soon as there is meaningful progress.

Environment Agency staff monitoring odour in Westbury

Odour Update 22 June 2026: Lower Compton Landfill Site, Calne

Over the weekend, we received over 100 reports of odour. We have had staff out proactively monitoring throughout the weekend to corroborate reports.

We continue to ensure that Hills carries out improvement works to the Lower Compton landfill site to the standard we expect. Good progress has been made with the capping works, with the subsoil restoration layer now spread over approximately 60% of the permanent capping to protect the clay. Once the works are complete, we expect odour levels to improve, and we will carry out extra odour monitoring to assess the change in levels.

On modern landfills, engineering work is carried out under a Construction Quality Assurance (CQA) plan, which has to be accepted by the Environment Agency prior to work being started. Information about CQA for landfills can be found here: Landfill operators: environmental permits - Construction quality assurance (CQA) - Guidance - GOV.UK

Inspections of the construction work include:

• Assessment of field testing and lab sample results to confirm the clay used is within specification.
• Site walkovers to check for any non-conforming material (such as large rocks) and to confirm the works are being carried out as per the approved CQA plan.
• Reviewing the CQA Inspectors records and ensuring the work has the required level of supervision.

Where work has not met the required standard, or where fugitive emissions have been identified, we have required the contractor to rectify and repeat the work.

Residents may be aware that earlier this week there was a fire at a neighbouring recycling centre. This was unrelated to the permanent capping works at Lower Compton Landfill and has not affected or delayed the remedial works.

Environment Agency staff monitoring at the Lower Compton siteMonitoring

In recent weeks, we have shared information on how we are monitoring odour and air quality (see: https://engageenvironmentagency.uk.engagementhq.com/wessex-regulated-industry-sites/news_feed/odour-response-in-calne).

We have a monitoring facility at Priestley Primary School. A range of factors were considered when selecting the monitoring location. The monitoring facility has been positioned to reflect:

• The likely route that hydrogen sulphide would travel into the town; odours can move along valleys at night due to cooler, denser air
• Residential areas where high numbers of odour reports have been received on the eastern side of Calne, ensuring monitoring reflects areas of highest concern
• The prevailing wind direction, local topography, and the need for suitable hard standing and a mains power supply

The Mobile Monitoring Facility (MMF) measures ambient air quality in the area, which may include emissions from multiple local sources. As well as methane and hydrogen sulphide levels, we also record meteorological conditions, including wind direction, wind speed, temperature, and atmospheric pressure.

The data collected from the MMF has now been sent to the UKHSA to allow them to make an independent health impact assessment.

Annotated photograph of an MMF exterior

Odour Update 12 June 2026: Lower Compton Landfill Site, Calne

We recognise the ongoing impact that odour is having on residents, and we are continuing to prioritise work to address these concerns as quickly as possible.

This week we have been out on site again to ensure Hills are delivering the actions that we have required of them to bring the site back into compliance with its environmental permit. This includes the accelerated permanent capping works.

Permanent capping works progressing at the Lower Compton Hills site, June 2026Environment Agency and Wiltshire Council staff on site at Lower Compton Landfill, June 2026

We have also been out on site with the Wiltshire Council Environmental Health team who are monitoring odour for statutory nuisance and public health. By sharing data and working together we are ensuring that the odour is being investigated. While we work side by side, we have distinct roles:

• The Environment Agency assesses off-site odour to ensure compliance with environmental permit conditions
• Wiltshire Council assesses statutory nuisance impacts. You can find out more about Wiltshire Council’s role in monitoring for odour here: www.wiltshire.gov.uk/env-health-pollution

How we monitor

Our regulatory team assesses compliance in several ways, including site inspections, audits, off-site odour assessments, and reviewing operator reports and monitoring data.

We carry out monitoring in the community to substantiate odour reports and assess whether odour is occurring off-site, as required by the permit. Our presence in the community is focused on identifying and evidencing off-site odour so action can be taken where required.

The main way we assess compliance is through “sniff testing”, using trained officers to detect and assess odours. This is required by permit conditions and remains the most effective method, as the human nose can detect the full range of gases responsible for odour at very low levels. It is important to be clear that this type of monitoring is for regulatory compliance purposes and not to directly assess health impacts.

We also use a Mobile Monitoring Facility (MMF) to measure ambient air quality. This supports our assessment from sniff testing. The MMFs monitor for methane, hydrogen sulphide, and particulates.

We understand that residents have concerns about potential health effects, and we take these concerns seriously. While our role is to assess compliance with environmental permit conditions, particularly whether odour is affecting areas beyond the site, the data we collect is shared with the UK Health Security Agency (UKHSA).

UKHSA are the independent public health experts responsible for assessing any potential impacts on health. They review the monitoring data and determine whether there are any risks to the community.

It is important to recognise that odour and health impacts are assessed in different ways. Odour can be strong and disruptive at levels that are still very low in terms of measurable gas concentrations, meaning something can be clearly noticeable and unpleasant without necessarily being harmful to health.

Our approach reflects this. We use sniff testing to assess odour impact and compliance with permit conditions, and we use monitoring data to support this and to inform UKHSA’s independent health assessment

Our aim is to ensure that odour is reduced to a level where it is no longer causing unacceptable impact beyond the site boundary. The actions being taken, particularly capping and improved gas control, are expected to reduce odour, although this may take time to fully resolve. Once the capping works are complete, we will carry out additional odour monitoring to assess how conditions have improved.

We recognise that people want access to monitoring data. This will be published once UKHSA has completed its independent public health assessment, and we will continue to keep the community informed as work progresses.

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