Samantha Arnold: Waste Management Assumptions Aren’t Forever

According to the UK’s Department of Environmental, Food and Rural Affairs (Defra), waste management accounts for three percent of the UK’s greenhouse gas emissions. However, due to uncertainties in theoretical modelling and quantification techniques, the actual amount of methane emissions remains uncertain. Golder Associates therefore conducted a study on methane gas emission levels from landfills for the Defra. The final report pinpoints the ways in which many modelling assessments are less representative than they should be, from the local and site-specific to the whole of the UK and beyond.

In an article in the Chartered Institute of Waste Management (CIWM) Journal, Samantha Arnold, senior atmospheric scientist at Golder Associates, explains why it’s therefore time to scrutinize waste emission models and change underlying assumptions based on fresh data sets.

Drawing on the findings from Golder’s research, Samantha states that while predictions from modelling techniques, such as MELMod, are applied to the waste management sector, they could be inaccurate. She highlights that one of the most surprising findings from the report was that the composition of gas emitted from UK landfill sites differs from model assumptions. Rather than the MELMod assumption of a 50:50 ratio of methane to carbon dioxide, emissions data collected by the Golder team suggests a ratio of 57:43 – with significant implications for any subsequent emissions predictions.

Samantha also suggests that there are numerous areas of improvement with regards to how data is collected. She states that it is important to use local site data, which is often collected for other purposes, to reduce the disconnect between assumptions and reality, and that it is necessary to address the blind spots with regards to flaring and fugitive emissions.

Reproduced by kind permission of CIWM and CIWM Enterprises. Visit www.ciwm-journal.co.uk.


Accuracy suffers when theoretical assumptions in environmental modelling do not get updated as new data becomes available. Samantha Arnold, Golder Associates’ Senior Atmospheric Scientist, explains why it’s time to scrutinize waste emission models and change underlying assumptions based on fresh data sets.

A number of commonly used modelling assumptions in the UK cannot be upheld when examined against contemporary empirical data – and the reliability of waste emission models could be significantly increased. This is one of the main findings of Golder Associates’ report on methane gas emission levels from UK landfills for the Department for Environment, Food and Rural Affairs (Defra).

According to Defra, waste management accounts for 3 per cent of the UK’s greenhouse gas emissions. However, due to uncertainties in theoretical modelling and quantification techniques, the actual amount of methane emissions remains uncertain. Golder’s report pinpoints the ways in which many modelling assessments are less representative than they should be, from the local and site-specific to the whole of the UK and beyond. Not an even split

In the waste management industry, computer modelling is a popular and widely used method for calculating various features. One common modelling technique is MELMod, which is used to quantify the overall amount of methane emissions from UK landfills. Another is GasSim, which is used as a landfill gas risk and resource assessment tool on a site-specific basis. Golder found that many models, however, build on theoretical assumptions about methane emissions in the UK that could be unrepresentative.

Perhaps one of the most surprising findings in Golder’s report was that the composition of gas emitted from UK landfill sites differs from model assumptions. In accordance with the standards of the Intergovernmental Panel of Climate Change (IPCC), MELMod assumes that landfill gas at the point of production is composed of 50% methane and 50% carbon dioxide. The Golder team scrutinised this assumed ratio using contemporary data collected from UK gas utilisation plants as a surrogate. The data was supplied by three UK operators that manage over 60% of the total UK landfill gas portfolio – and the results were remarkable: rather than the assumed 50:50 ratio, the data suggests a ratio of 57% methane to 43% carbon dioxide.

If this new ratio were to be adopted in waste modelling assessments for the landfill gas emitted to atmosphere, it would significantly influence the predictions. At a UK scale, for 2011, MELMod predicts around 2.2 Mt of generated methane under the existing 50:50 assumption for landfill gas. However, with the new 57:43 ratio this number rises by around 300,000 tonnes to around 2.5 Mt (an increase of 14%). This shows the great differences new data can make to existing models.

Innovations not captured

In the Defra report, Golder also reviewed UK operators’ own data about the types of landfill gas engines that they currently deploy across the country – and found a higher rate of methane use than assumed. From a total of 490 engine units, the calculated average gross electrical efficiency was 40% across the surveyed UK engine portfolio. With an assumed 4% loss factor through parasitic loads and other inherent losses (leading to a 36% net efficiency rate), the calculated amount of methane used in landfill gas engines lies at 1 Mt for the UK in 2011.

Historically, these rates were assumed to be lower, with an estimated 34% gross electrical engine efficiency minus a 4 percentage-point loss factor, leading to a 30% net efficiency rate. Comparing the assumed rates in the theoretical modelling with the rates based on empirical data collection, it becomes obvious that current MELMod standards do not appropriately reflect innovation and efficiency gains in the UK waste management industry.

These discrepancies do not come as a surprise as waste management is considered an old industry with a traditionally strong regulatory umbrella. The landfill gas utilisation industry continues to innovate – and this innovation is often not reflected in the default regulatory assessment assumptions. Overall, what Golder’s study shows is that it is beneficial to use site data, which is often collected for other purposes, to inform the modelling assessment assumptions. Reducing the disconnect between assumptions and reality will in the end assist both parties – the industry and regulators – in obtaining defendable emissions estimates.

Blind spots: flaring and fugitive emissions

When it comes to how data is collected, Golder’s study identified numerous areas with considerable room for improvement. One big question mark in the current discussion is the issue of flaring. To know how efficient methane collection really is, it’s essential to know how much methane is combusted in flares in addition to the well-documented engine utilisation.

Unfortunately, the data available for quantities of gas flared is limited to sites that have undergone the most recent permit processes – and even here it only measures flare hours, not volumetric flow. For other (older) sites the flaring rate is entirely unknown, which means that modelling methane rates in this respect is especially difficult. To add reliability to the reported stats, it is necessary to re-visit the numbers when new data is made available.

An even larger blind spot for emission quantification is fugitive emissions. It is almost impossible to estimate the amount of emissions from flanks, leaking gas infrastructure or capped areas without models that are based on accurate monitoring. The Greenhouse Gas UK and Global Emissions project (GAUGE) uses a wide range of sensing techniques, including “unmanned aerial vehicles” (UAVs) to provide 3D methane concentration maps with thermodynamic parameters, which allows for a more accurate calculation of landfill methane emissions from an operational landfill site in the UK. Once the project’s findings are published, operators and regulators will be able to get a better idea about how valid their current default value calculations for fugitive gases really are. It will essential to stay across these findings.

The Environment Agency (EA) in the UK is also currently exploring UAVs. Last year, the EA released a feasibility study into the best techniques of measuring methane emissions from landfills through UAVs and the final report of their study is due to appear shortly. The progress that we can expect in this field is exciting – so watch this space!

Data defeats default assumptions

In the meantime, the main lesson from Golder’s investigation into landfill methane emissions and capture is that models historical assumptions have to be scrutinised in more detail. Golder’s report shows that the way forward in quantifying methane gas emitted from landfills is through using contemporary measured data to update default assumptions. This is likely to become ever more important with the fast paced technological changes in measurement techniques that the industry is embracing.


About the Author

Samantha Arnold is a senior atmospheric scientist at Golder Associates, where she has worked for the last 10 years. She is a Chartered Geographer, Chartered Scientist and a Fellow of the Royal Meteorological Society. Specialising in atmospheric science, which includes air quality and air flow, Samantha has worked on a wide range of multi­disciplinary UK and international projects, including technical due diligence, environmental impact assessments, permit compliance, and on-going research.

For more information, please contact:

Golder Associates (UK) Ltd.
Samantha Arnold, Senior Atmospheric Scientist
Tel:+44 115 937 1185
Email

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