Ranking residual waste recovery methods

A hierachy for utilising residual waste as an energy resource could prove valuable when developing future recovery strategies. Chris Coggins puts forward his case

Most people in the waste industry are aware of the waste hierarchy, with prevention as the key priority and landfill at the bottom. In between there is reuse, recycling and energy recovery. I would like to propose a similar hierarchy for residual waste as an energy resource, but also recognising the importance of source-segregated biomass waste in such a hierarchy.

A variety of criteria may be discussed and evaluated in deciding on this hierarchy in terms of local decision making. Firstly, a recognition that recycling, composting and reuse have already been maximised, leaving only residual wastes to be managed in a sustainable manner.

Secondly, waste management options – these include source segregation and targeted wastes or mixed residual waste, pressure on existing dependence on landfill. Likewise, technology options – these include scale of operations (and offsetting traffic movements and emissions), local infrastructure, links with calorific value of waste inputs and demand for electricity and/or heat.

Thirdly, emissions and the protection of human health and the environment. Science shows that public perceptions about dioxins are misplaced, but can lead to NIMBYism, and other options may also have problems concerning bioaerosols.

Recycling debate

There is also the argument that using residual waste as an energy resource diverts waste from landfill and not from recycling. The waste strategy review, published in February 2006, suggested an increased recycling/composting rate of 50% (c,20 million tonnes) for 2020 and a reduced energy from waste figure of 25% (c,10 million tonnes), compared to 33% in each case proposed in Waste Strategy 2000 for 2015.

If the municipal solid waste (MSW) recycling rate and the diversion rate from commercial and industrial waste from landfill are achieved, this will leave over 20 million tonnes of MSW and over 50 million tonnes of commercial and residual waste as residual waste. While new recycling technologies may improve recycling rates, it is better to fund energy from residual waste technologies rather than rely on landfill.

There is considerable debate about residual waste as an energy resource as part of an integrated and sustainable waste management solution, partly in the context of security and stability of clean energy supplies at affordable prices, and partly in the context of carbon emissions and climate change.

Order of importance

Under my proposed hierarchy of energy from residual waste, the following methods are ranked from best to worst:

  1. Energy efficiency – this is about design and product manufacture. For example, long-life light bulbs, designing out stand-by systems, the market transformation programme, energy labelling of fridges and about household behaviour; switching off lights and unwanted appliances.
  2. The next tier of technologies involves source segregation, reduced contamination and better quality waste inputs. In addition, the use of source-segregated biomass waste containing only ‘living’ carbon and no fossil carbon such as plastics to produce energy. These technologies include anaerobic digestion and in-vessel composting, are generally smaller scale operations resulting in less traffic movements, and more acceptable to the public.
  3. Next in the hierarchy are a range of ‘new’ technologies, referred to as advanced thermal treatments. Some of these have yet to be proven in the UK, but include the production of refuse-derived fuel from mixed residual waste. These technologies may depend on pre-treatment and process-treatment, using mechanical and biological (MBT) plants at 50,000 to 60,000 tonnes. They may involve sub-regional plants providing larger regional plants with fuel feedstock – coal-fired power stations, cement kilns as well as energy-from-waste (EfW) plants.
  4. Next are even newer technologies that might prove attractive such as pyrolysis and gasification. Defra’s waste implementation programme is funding two demonstrator gasification projects. Future additions may include plasma arc and the use of syngas from gasification leading to ‘clean’ hydrogen fuel cells.
  5. Then there are MSW EfW plants with combined heat and power, like the one at Sheffield which has a 230,000 tonne capacity and meets the Waste Framework Directive efficiency criteria of 60%.
  6. Below this are traditional large MSW EfW plants which only generate electricity at around 25% efficiency and those which produce neither heat nor power. Both would be described as disposal in the Waste Framework Directive.
  7. At the bottom of the hierarchy is landfill gas capture as methane from biodegradable waste has the most impact on climate change.

Integrate for flexibility

Until recently, most EfW plants have been proposed as part of long-term contracts (25 years or more) to be supplied with MSW, funded by 15% equity and 85% debt gate fee income over the duration of the contract, hence accentuating the debate about diverting such wastes from recycling.

Such an image can be avoided by a employing more integrated approach to managing MSW and commercial and industrial residual waste streams in the same facility. The first ‘merchant’ EfW plant is under construction at Lakeside in Slough, operated by Grundon and Viridor.

This has no long-term waste contracts, although a £600M PFI contract was signed in November 2006 by councils in Berkshire and includes a materials recovery facility to handle 50,000 tonnes and a contract to send 60,000 tonnes of residual waste to Lakeside.

In relation to carbon impact, two report were published last year which touched on how the various methodologies should be quantified in any evaluation of carbon emissions. From one of these, Friends of the Earth has outlined a hierarchy of selected technologies which partly combines the waste hierarchy (recycling) and the energy from residual waste hierarchy based on carbon emissions.

This is as follows:

  • MBT with anaerobic digestion, extracting metals and plastics for recycling and residual waste to landfill
  • MBT with aerobic digestion, with metal extraction and RDF to a cement kiln (replacing fossil fuels)
  • heat-only incineration, with metal extraction
  • MBT with anaerobic digestion, with metal extraction and stabilised residue to landfill
  • MBT with aerobic digestion, with metal extraction and stabilised residue to landfill
  • electricity-only incineration, with extraction of steel and aluminium
  • landfill, with 75% capture of methane
  • MBT with aerobic digestion, followed by fluidised bed incineration
  • landfill, with 25 to 50% capture of methane.

Professor Chris Coggins is a consultant from Wamtech (Waste Management & Technology)

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