Environmental Engineering Reference
In-Depth Information
6.7.3 Case study 3. Optimising methane production,
reduction in greenhouse gas (GHG) emissions and
nutrient recycling: closing the loop between urban and
agricultural wastes
The European Commission estimates that about one third of the EU's 2020
target for renewable energy in transport could be met using biogas produced
from biowaste, while around 2% of the overall EU renewable energy target
could be met if all biowastes were converted to energy, with further
economic gains from the associated recycling and waste prevention. To
achieve this requires optimisation of methane production at a far larger
scale than that of an individual plant or farm. The concept of centralised
anaerobic digesters receiving and treating urban biowastes is rapidly
becoming established, as is the practice of growing energy crops for farm-
based digestion, both driven at least in part by renewable energy subsidies.
Even with these subsidies, however, it is unlikely to be profitable to digest
the 1.25 billion tonnes of animal slurries and manures generated in the EU,
which are thought to contribute as much as 12-41% of total agricultural
CH
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emissions in most countries (Chadwick et al., 2011). To optimise GHG
capture and methane production from these requires co-digestion with
energy-rich materials, and this could be economically sustainable if based on
the polluter pays principle, reducing the level of renewable energy subsidy
needed to incentivise such an action at a regional, national or international
scale.
A successful example of this approach already exists in Denmark, where
co-digestion has been practised since the 1970s (Raven and Gregersen,
2007). These schemes have co-digested 1.51 million tonnes per year of
animal manure together with 340 000 tonnes per year of other biowastes,
mainly from food manufacturing: this was achieved using a combination of
19 centralised co-digestion plants and 56 individual farm-scale plants (Al-
Seadi, 2000).
To look at the feasibility of this approach using source-segregated
domestic food waste as a co-substrate, a regional model was created for
Hampshire in the UK in which the concept was evaluated against three
objectives - maximising the methane production from urban and rural
sources, minimising GHG emissions and closing the nutrient cycle by
replacing the nutrients exported from dairy farms in their produce with
imported urban-generated food waste. These three objectives were
optimised on the basis of a nutrient management strategy in which
no farm could import more nutrients than it exported, and neither im-
ported nor farm-generated waste could be exported to other farms after
digestion.
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