The issues around composting
As illustrated in Figure 51, composting is currently being developed very rapidly. Another study conducted by
WRAP suggests that to meet the targets of the EU Landfill Directive, 5 million tonnes of municipal organic waste
will need to be composted by 2012/13. However, the results of the study have highlighted that composting may
not be an ideal solution from an environmental point of view, especially regarding climate change and energy
consumption, since it does not allow any energy recovery, unlike anaerobic digestion for example.
In the UK, home composting is very common and is being promoted by the authorities and by many other
organisations. As an example, over 75% of local authorities responsible for household waste collection and
disposal in England and Wales have promoted home composting via subsidies (Smith & Jasim, 2009). Home
composting does indeed present some significant advantages. First, home composting is a low-cost solution that
diverts organic waste from landfill and does not require specific infrastructures such as separate collection
schemes or composting centres. The compost obtained is also used as fertiliser in gardens and prevents
consumers from using chemicals instead.
However, there is some concern that if there is not enough air in the composter (if it is not mixed regularly or if
there is only food waste that does not allow the formation of air pockets), the process can become anaerobic and
produce methane, contributing significantly to climate change. Methane releases from home composting are very
hard to measure. Three studies were identified which sought to measure greenhouse gas emissions from home
composting. Of these, Wheeler and Parfitt (2002) and Colón et al. (2010) found there were negligible or no
emissions of methane from home composting, whilst Amlinger et al. (2008) identified clear emissions. There has
been no evaluation of the greenhouse potential of the 500,000 tonnes of garden and food waste that are home
composted annually in the UK. Some scientists argue that any methane formed in the composter is oxidised by
the bacteria present at the interface between aerobic and anaerobic zones and that consequently home
composting is unlikely to be a significant source of methane emissions (Smith & Jasim, 2009). However, there
seems to be little research data available on the subject; thus there is a need for more investigation in this area.
The future potential for anaerobic digestion
As anaerobic digestion is still a relatively new technology, it is not assessed as frequently as composting, landfill
and incineration in LCAs. However, the review highlighted that anaerobic digestion seems to be a very promising
option for treating food waste, in particular to tackle the climate change issue. Indeed, the biogas produced can
be burnt to generate heat and/or hand electricity or can be used as a vehicle fuel. Besides biogas, anaerobic
digestion also produces a solid and liquid residue called digestate which can be used as a soil conditioner to
fertilise land. However, end markets still need to be found for the digestate (WRAP, 2009 (c)).
In theory, anaerobic digestion is suitable for both food and garden waste but in practice, too much garden waste
in the organic mix reduces the yield of biogas, as a substance called lignin which is found in woody materials
cannot break down without oxygen (Friends of the Earth, 2007).
It therefore appears that anaerobic digestion should be promoted for food waste. The development of anaerobic
digestion for treating food waste requires the collection of source-separated food waste. One additional constraint
is that the collection frequency of food waste needs to be rather high (weekly or fortnightly) to avoid smell and
vermin problems. Kerbside collection of food waste is currently growing rapidly in the UK, which is an
encouraging sign for the development of anaerobic digestion. To encourage this channel, the construction of
more anaerobic digestion plants will be necessary because until recently anaerobic digestion has until recently
been limited to small on-farm digesters (Friends of the Earth, 2007).
Anaerobic digestion is now recognised by the UK Government for its potential in treating food waste by the UK
Government. The Waste Strategy for England 2007 sets out the important contribution that anaerobic digestion
can make to achieving the UK’s waste management goals (DEFRA, 2007 (d)). Anaerobic digestion also enters in
the framework of the Government's Renewable Energy Strategy and will contribute to the switch to a low-carbon
energy mix. DEFRA estimates that by 2020 anaerobic digestion will be an established technology in the UK and
that the country will be recognised as ‘a world leader in the cost effective, innovative and beneficial use of
anaerobic digestion and in anaerobic digestion technology and expertise’ (DEFRA, 2009 (e)).
Incineration
The study pointed out that incineration is the best option when looking at energy demand thanks to the energy
credits, despite the low heating value of wet organic waste. The analysis has also highlighted that performances
of incineration depend on the energy mix that is substituted thanks to the energy produced. Incineration was