Greenhouse gas emissions are an increasingly important issue in waste policy. Concern about methane emissions from landfill has driven the European Commission's plans to reduce landfilling of biodegradable waste - a policy which in the UK means investing in incineration, composting and recycling.
The EPA study, still in draft, concludes that recycling municipal solid waste (MSW) will reduce greenhouse gas emissions. But it cautions that, if global warming is the overriding concern, then some wastes - plastics in particular - should be landfilled if they cannot be recycled. This is because burning fossil fuels in a power station produces less carbon dioxide per unit of electricity than burning plastic in WTE incinerators. In contrast, landfilling traps carbon in wastes such as plastics which do not biodegrade.
The report compares landfill with a range of alternatives - source reduction, recycling, composting and incineration - using a "streamlined" life cycle assessment (LCA) which does not consider non-greenhouse gas issues.
The report estimates that only 13.6% of the heat produced in an "average" WTE incinerator is converted into electrical energy. On top of the normal thermodynamic losses in power generation, incinerators have to drive off moisture in the waste and also lose energy in gas cleaning equipment.
The study did not consider WTE plants fitted with combined heat and power equipment - such as the Sheffield and Coventry incinerators in England - for which different results would be obtained, since more of the heat produced is captured as useful energy.
The study considered eight waste materials as well as mixed MSW and concluded that emissions are reduced significantly by both source reduction and recycling (see table ).
The reason why paper recycling scores so well is that the study took into account the amount of carbon "sequestered" in forests. It assumes that reducing paper consumption will lead to retention of larger tonnages of carbon in living trees.
The study allows for the loss of materials in recovery processes, assuming, for example, that only 66% of recovered office paper ends up as recycled product. But it does not say what sort of recycling systems - or recovery rates - were modelled. The energy consumed in transporting and sorting recovered materials can vary widely.
Where the table gives WTE a negative score, this is because incineration converts all the carbon into CO2 - at lower energy efficiency than fossil fuel combustion - whereas landfilling leaves a large proportion locked into solid waste material. The study used results from EPA-funded research showing that lignin is relatively stable and non-decomposable under anaerobic conditions in landfills, while cellulose and protein decompose only partly.
The estimates are that 40% of the dry weight of newspaper, 26% of corrugated card and 31% of food scraps - but only 4% of office paper - end up as sequestered carbon in landfill. The results were obtained from laboratory reactors designed to maximise methane generation, but it is not clear how accurately these reflect real landfill conditions over decades.
The study also assumed that 85% of the methane produced in landfills with gas recovery facilities is collected - although no research is cited to back up this relatively high estimate.
Composting fared poorly in the LCA - slightly better than landfill for garden waste and slightly worse for food scraps, despite the avoided methane emissions. The reason is that, unlike other forms of recycling, composting does not reduce energy consumption in product manufacturing.
Overall, the results are similar to those of a recent cost-benefit study of MSW options prepared for the European Commission (ENDS Report 267, pp 23-26 ), which also found considerable benefits in recycling and placed landfill just above incineration and composting in the waste hierarchy. As with that report, however, the results of the EPA study must be taken in context. A locally-based LCA - considering site-specific and regional factors and broader environmental impacts - would produce different results.
According to Friends of the Earth campaigner Mike Childs, "the EPA report demonstrates that recycling is way out on top while landfill and incineration are battling for bottom place in the waste hierarchy."