Is biostabilisation the solution for Scotland’s Municipal Waste Landfill Ban?

Biostablising organic matter in municipal waste could better reduce greenhouse gas emissions than either directly landfilling or choosing to incinerate it, according to a new report published by Zero Waste Scotland.

The Scottish Government’s Biodegradable Municipal Waste Landfill Ban requires that from 2026 local authorities will not be permitted to send untreated organic waste to landfill, due to the climate change impact of methane emissions that will otherwise result.

The report, ‘Alternative Residual Waste Treatment: Biostabilisation’, undertaken by consultants Ricardo, aims to provide analysis for government, policymakers and local authorities looking to choose the best options for managing the residual waste.

It explores the overall impact of first biostablising waste – a process of treating organic residual arisings through aerobic or anaerobic methods – and then disposing of the waste in landfill. This is a disposal option permitted by the impending landfill ban, the likely alternative being a treatment process that produces refuse-derived fuel (RDF) or solid recovered fuel (SRF) for energy-from-waste facilities.

Ricardo’s study also aims to establish the carbon footprint of treating residual waste using some form of Mechanical Biological Treatment (MBT), which is required to undertake ‘biostablisation’.

Iain Gulland, Chief Executive, Zero Waste Scotland, said: “Waste policy in Scotland is changing, with the ban on [biodegradable] waste to landfill coming in 2025. Zero Waste Scotland commissioned this research in order to help inform waste treatment choices by policymakers, local authorities and others, on how to handle their waste after this date. We also anticipate the findings being useful to the industry.

“The study shows this is a complex area, with different requirements to be balanced, and a thorough understanding required of the impacts of the different technologies available, before choosing one. It does however further underline that we should be doing everything we can to reduce waste, then reuse and recycle, in order to absolutely [minimise] the amount of waste heading for landfill in Scotland.”

ZWS conclusions on biostabilisation

The report considers different forms of MBT as treatment options to meet the requirements of Scotland’s forthcoming ban. It concludes that certain biological processes such as wet anaerobic digestion (AD), which consists of less than 20 per cent solid matter, and bio-drying, will not stabilise biological municipal waste (BMW) enough for it to meet the Scottish ban criteria. Nonetheless, these MBT approaches could assist in diverting problematic waste from landfill.

In-Vessel Composting (IVC), the process in which aerobically decomposing organic material is enclosed in a drum, silo, bin tunnel, or other containers for the purpose of producing compost, is presented as necessary to achieve the required extent of biostabilisation for landfilling biodegradable waste.

A dry anaerobic digestion system – which works for organic waste made up of 20-40 per cent solid matter – followed by IVC, can also be designed and operated to meet Scotland’s biostabilisation criteria. However, it will be necessary ‘to first remove materials that are best suited for use as refuse-derived fuel’.

Although some MBT facilities refine the IVC output for use as RDF, the report states that many more remove RDF materials through mechanical pre-treatment irrespective of what happens to the output of the biological process.

As it is ‘very common’ for MBT facilities to generate RDF during this process, Ricardo found that if RDF is combusted such that the carbon content of its ash is below the ban criteria, then such practice will help Scotland comply with the ban. But, RDF combustion has a greater CO2 emissions impact than landfilling the same material if it has first been biostabilised.

Carbon lifecycle assessment

The report also compares the carbon impacts of dry anaerobic digestion followed by IVC versus IVC on its own – both with and without the production of refuse-derived fuel (RDF) as part of the process.

IVC without RDF production was calculated to create 12kg CO2 equivalent per tonne (CO2e/t) of residual waste treated. In comparison, dry AD followed by IVC (which involves RDF production) will create 66kg CO2e/t and IVC only (with RDF production) will create 115kg CO2e/t per residual waste treated.

The report concludes that ‘the greatest influences on the carbon balance are whether RDF is produced, and subsequently combusted elsewhere for energy recovery, and whether materials are recycled’. Whereas the former negatively impacts the carbon balance, the latter benefits it.

Dry AD followed by IVC offers the benefit that biogas is produced and combusted, which can generate electricity. Yet, this requires the removal of RDF and the impact of RDF combustion.

Recommendations

To employ MBT in Scotland, ‘with landfill of most of the facility outputs’, the report says a “step-change in attitude and approach will be required” – an approach not currently practised in Scotland, with only one English facility identified as doing so.

If a process of biostabilisation using MBT were to be employed, the report says, the result would be unlikely to cause a decrease in waste landfilled and is instead more likely to increase it. It is concluded that this wouldn't keep ‘with the waste hierarchy’, wherein energy recovery is deemed preferable to landfill. However, the report’s authors note, that, due to overall emissions considerations, there is scope for challenging the orthodoxy of the waste hierarchy if MBT involves maximising the recycling of material where possible, alongside biostabilisation and landfill of organic matter.