Background Methane (CH4) is a potent greenhouse gas with 21 times the global warming potential of carbon dioxide (CO2). At a global level, the methane in landfill gas contributes to climate change. The UK, and the European Union generally, has a huge legacy of closed landfill sites, many of which are derelict or under-used. They represent a significant liability because of their potential to be sources of migrating toxic contaminants and landfill gas. There are approximately 20 000 closed landfill sites in the UK. A recent trial conducted by the Environment Agency indicated accessible whole-site landfill methane volumes of around 100-150 m3/hr. Using methane from closed landfill sites to generate energy presents an alternative to non-renewable fossil fuels, reduces greenhouse gas emissions, and can contribute to the green economy. Objectives The objective of the ACUMEN project was to demonstrate how methane from expired and non-operational (closed) landfill sites can be captured and utilised. Economic and technical uncertainties have hampered the take-up of new technologies to manage methane emissions from closed landfill sites, so ACUMEN aimed to show how these can be overcome through a combination of innovative technologies and techniques. The project aimed to establish the technical and economic viability of capturing, using and mitigating methane from closed landfill sites, and to contribute to a significant reduction in the 12.9 million tonnes/year of CO2 equivalent emitted from closed landfill sites in the UK. The project also aimed to demonstrate how this approach can be easily transferred to other European countries that have to overcome similar barriers to capturing methane from old landfill sites. Results The ACUMEN project demonstrated the technical and economic viability of a range of techniques for monitoring, utilising and mitigating methane emissions from closed landfill sites. By bringing together a wide-ranging partnership involving stakeholders from the policy, regulatory, site owner and operators, and the private sector (including equipment suppliers and monitoring specialists), it created a nascent market for the utilisation of landfill gas from closed sites. As a result of its activities at five UK demonstration sites, the project found that previously operational sites are best-placed to generate energy due to existing infrastructure, namely, connections to the electricity grid and landfill gas collection infrastructure. A range of utilisation and mitigation techniques were demonstrated at the five sites, to establish best practices for utilising methane. The project showed, through cost-benefit analysis, that small-scale spark-ignition engines (150 kW) can generate around £50 000 (€65 772) per year, while micro-scale engines (18 kW combined) can generate around £9 000 (€11 841) annually. It demonstrated that low-calorific flaring is possible down to 8% methane, with low-calorific flare emissions being comparable to ‘standard’ flare emissions in the UK. The in-situ bio-oxidation of methane using a biofilter resulted in methane removal rates of 90% at the site surface. The project demonstrated that whole-surface biofilter emissions can be accurately monitored, and a monitoring programme was established. ACUMEN demonstrated a number of direct environmental benefits. On a local level, the project reduced direct methane emissions from landfill and reduced indirect carbon dioxide emissions by replacing fossil fuel-derived heat and energy to power waste sites and dry woodchips. It converted 1 339 tonnes of methane during the project lifespan, which would have been flared , to produce useful energy (594.8 MWh of electricity and 230 MWh of useable heat). In doing so, 339.98 tonnes of carbon dioxide emissions were avoided, which would have been emitted from powering landfill gas extraction systems at the sites (some of which became net electricity exporters). Also avoided was 38 tonnes of carbon dioxide emissions that would have been emitted had the heat for drying woodchips been used from standard sources. These demonstrations show the environmental benefits that could arise from the transfer and scaling up of the projects techniques. The project findings are collated in the ACUMEN technical report, which provides advice to site owners and operators responsible for managing the gas at closed landfills. Findings were disseminated through a series of webinars, workshops and seminars, and site visits organised by the project, with presentations delivered at several conferences, including the 15th International Waste Management and Land Symposium. The project generated significant interest amongst landfill site owners in the UK, and has engaged well with stakeholders in Poland. Its recent networking activities and events have enabled a good level of wider EU dissemination. The use of a range of landfill gas monitoring was demonstrated, utilisation and mitigation techniques and technologies that in some cases are relatively well established in other applications (including at operational landfills), but have not yet been used in the closed landfill context. This included only the third-ever application of Stirling engines on landfill sites (the first in the UK), the design of an innovative low calorific flare, purge and recovery tests and continuous monitoring techniques. In terms of policy, the project supports the objectives and local-level implementation of the Landfill Directive (1999/31/EC), the 2020 Climate and Energy Package and the Europe 2020 growth strategy. The beneficiaries worked with UK partner DECC to refine methodology for determining emissions from closed landfills in the UK greenhouse gas emissions inventory, reported under the UNFCCC. The project demonstrated that closed landfill sites, which are currently a financial burden on site owners, could be transformed into an income-generating asset through the sale of gas rights. With many sites owned by local authorities, this has the potential to realise economic benefits. Reduced expenditure for closed landfill site owners could result in additional funding for the restoration and rehabilitation of the site as a public amenity/open space. The project is already showing signs of stimulating a new market for private sector equipment suppliers and investment funds. Cleanergy commissioned a market survey in the UK to test the potential roll-out of Stirling engines at closed landfills, whereas Beetle Capital expressed interest in developing the sale of energy generated at closed landfills as an investment opportunity. Further information on the project can be found in the project's layman report (see "Read more" section).