R E N EWABLE E N E R G Y

Waste Not,Want Not Hylton Murray-Philipson repor ts on anaerobic digestion, a still little-known process that can produce renewable energy from our domestic waste

First of all, AD is a treatment process that breaks down organic material in the absence of oxygen. It has been used in the UK to process sewage sludge for over 100 years, but its potential for processing food and farm wastes is only now being recognised.

Nature knows no waste. Falling leaves decompose, enriching the soil in which trees and other plants grow. Nutrients are recycled as the energy of the sun combines water and CO 2 to create biomass. Volatile organic compounds from the canopy seed the rain that waters the forest. The natural world is full of cycles supporting the diversity of life. Conversely, industrial societies are full of processes that lead, quite literally, to dead ends.

AD produces biogas, which can be used in three ways: to generate electricity; to be fed directly into the gas grid; or to be compressed for use as a transport fuel (as it is widely used in Sweden).

Anaerobic Digestion is a win-win-and-win-again

Approximately 7 million tonnes of food waste are sent to landfill in the UK every year (this figure is on top of commercial/industrial waste and agricultural wastes). But there is no need for this to happen at all because anaerobic digestion (AD) processes, which mimic the services of the natural world – recycling waste into energy and fertilisers to help sustain life – are now widely available.

solution whose time has now come

AD produces energy all day and all night, regardless of weather conditions, and, unlike electricity, the biogas it produces can be stored. AD overcomes the inherent problem of wind and solar being intermittent and thus requiring backup generating capacity from conventional power stations – a requirement that undermines the economic rationale of building the renewables capacity in the first place.

Energy security is becoming of increasing concern as coalfired power stations are decommissioned, and yet, despite the fact that there are over 200 AD plants already operating in the UK, media attention is focused almost solely on wind, solar and marine energy, with little or no awareness of AD.

Unlike other technologies, as well as generating renewable energy and producing organic fertilisers for agriculture, AD solves a number of additional environmental challenges and problems, including the declining availability of suitable landfill space and the unhelpful generation of methane from organic waste that has been dumped in landfill – a pollutant that is 23 times more potent than CO 2 as a greenhouse gas.

So, what exactly is AD? What contribution can it make to targets for renewables, and what is the future of the AD industry?

Apart from biogas, the only by-product of the AD process is a nutrient-rich organic fertiliser that can be applied to agricultural land to replace the more energy-intensive artificial fertilisers, currently responsible for over 1% of the UK’s total emissions of CO 2 . Depending on the feedstock and retention periods within the plant, digestate contains nitrogen and organic compounds including potash. AD therefore contributes to the sustainability of the economy as a whole, and agriculture in particular.

The UK government has now committed to the following targets in the field of energy, waste and climate change: • The EU Renewables Directive states that 15% of UK

consumer energy (not just electricity) must be derived from renewables by 2020.

12

Resurgence & Ecologist

November/December 2012