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Numbers prove the worth of AD

With the Government heavily pushing anaerobic digestion (AD) technology for the waste industry and promising a “huge increase” in Energy from Waste from AD it seems timely to consider the role and scale that AD may play in our sector and provide some personal perspective.

Let’s start by considering the magnitude of feedstock and outputs that might be obtained.  WRAP has calculated that approx 5.8Mtpa of household food and drink waste is generated and could or is collected by local authorities. The Government AD task force estimated that between 12 and 20Mtpa of organic waste from both commercial and industrial (C&I) and municipal solid waste (MSW) markets could go to AD.  For C&I waste it can be extrapolated that more than 4Mtpa of source separated organic waste could arise from this market sector.

In total, the gross source separated organic material arising from C&I and MSW streams amounts to between 6.8 and 8.9 Mtpa depending on participation and capture rates.   Between 3.1 Mtpa and 13.2Mtpa of organic rich waste from non source segregated streams might arise but could be treated through combustion as well as AD.

From the Environment Agency 2008 returns it can be calculated that a landfill ban on organic material, applied on the European Waste Catalogue code description basis only, could give rise to around 15.7Mtpa of organic rich material.

Based on the Defra figures for waste to landfill, a ban on organic described wastes could divert up to 30% of the waste going to landfill. In the consultation document on the restrictions of waste to landfill, Defra estimates that a ban based on its options 1a and 2 could divert between 3.5Mt and 4.5Mt of organic material (6-8 per% of waste to landfill).

Currently there appears around 700k tpa of AD capacity in the UK with the ability to treat waste. Other sources such as sewage sludge and farm waste are excluded from the above number. Many of these plants are also taking a mix of wastes from MSW, C&I, agriculture and some sewage related feedstock.

There appears upwards of 2.8Mtpa of public and near public planning permissions, applications and intentions to construct waste feedstock based AD plants with the average size of these AD plants working out to be around 47ktpa.

Assuming that the average plant size is maintained (47ktpa) for the source separated feedstock, the total number of AD plants required in the UK for MSW and C&I source separated waste would be between 140 and 190. This would require investments between £1.6B and £2.5B in the sector to treat just the source separated organic waste determined above.

Should these plants be built they could produce between 2.1 and 2.6 TWh of green electricity per year (installation base of between 280 and 380MW). On a 2009 renewable generation equivalent, this could increase the renewable electricity generated by between 8 and 10% in the UK. Based on an assumption of 25% of the general waste total going to combustion with energy recovery, (comprising moving grate, fluidized bed, gasification and Pyrolysis etc) this feedstock could contribute over 15TWh of energy with potentially over 50% of the electrical output coming from renewable feedstock.

So yes, AD can play a major part in the UK waste market but it is far from a solution to all. The implications of collection costs and sustainability, feedstock quality (source separated or not), pre treatment and treatment technologies, liquid and solid digestate treatment and use, and energy outputs (electricity, heat, gas to grid, gas to vehicle etc) are complex and all inter-related. The big questions for organic waste treatment are where the sustainable limit on source separated collections is, what the future of MSW green waste services is and how to treat the remaining residual waste with organic components.

Stuart Hayward-Higham is technical director at Sita UK

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