BioCycle April 2008, Vol. 49, No. 4, p. 51
Study provides guidance on the most cost-effective and environmentally sustainable ways of diverting household food waste
ACCORDING to a March 2007 report from the Waste and Resources Action Program (WRAP), food waste is one of the largest single fractions in the United Kingdom’s (UK) municipal solid waste stream, but only around two percent of the available amount is collected separately for composting or anaerobic digestion. The UK, along with other members of the European Union, are required to meet a Landfill Directive that mandates reduction in the amount of biodegradable waste going to municipal landfill. By 2010, it must be reduced to 75 percent of 1995 amounts, 50 percent by 2013, and 35 percent by 2020. As the first deadline approaches, local waste authorities in the UK are evaluating their options for organics collection and processing.
“Dealing With Food Waste In The UK,” authored by Dominic Hogg, along with Josef Barth, Konrad Schleiss and Enzo Favoino, was prepared under a contract with WRAP. It provides guidance on “what is the most cost-effective and environmentally sustainable way of diverting household food waste from landfill that leads to the production of a saleable product.” The report focuses on biowaste, defined as garden and food wastes. Its evaluations include the cost of both the collection and processing of biowaste when considering management options. The full report can be downloaded at: http://www.wrap. org.uk/local_authorities/biowaste/index.html.
Financial Costs And Primary Findings
The primary findings, with regard to financial costs, are as follows:
1. Systems that include free garden waste collections tend to be more costly per household than those that target food waste only. In general, systems collecting garden waste, or mixed garden and food waste, fortnightly and free of charge are more expensive than systems collecting food waste weekly. Encouraging householders to compost garden waste at home, where possible, reduces the risk of extra material going into the waste stream. Home composting promotion is also popular with gardeners, favorable environmentally and can improve the economics of curbside garden waste collections.
2. The additional cost associated with adding food waste to an existing free garden waste collection program is significant. This is because the resulting mixture of food and garden waste must be treated by in-vessel systems, which are more expensive than windrow composting of garden waste. In general, this is a very expensive way of treating small amounts of food waste.
3. Collecting only food waste allows the processing costs to be minimized, and can increase the capture rate. There is barely any difference in cost between the systems in which: all garden waste collected at household waste recycling centers (HWRCs) is composted in windrows, and food waste digested anaerobically; and, all food waste and garden waste (collected at HWRCs) composted at aerobic in-vessel systems. If the garden waste is processed by anaerobic digestion (AD) along with the food waste collected at curbside, the costs are significantly higher. This is because windrows are a much cheaper way of treating garden waste than AD.
4. As the costs of residual waste management increase, so the benefits of separate biowaste collections increase, relative to the baseline (in which no biowaste is collected).
5. Overall, where home composting is promoted intensively by local authorities, the financial costs of the biowaste management system are likely to be lower than in situations where this does not happen, and it is a popular option for gardeners.
Food Waste Only With AD, Composting
The report concludes that if separate collection of food waste is coupled with anaerobic digestion, and followed by composting, “the environmental performance of the system is likely to be the best compared to all the other systems examined in [the] report.” Note the authors: “Collecting food waste separately from garden waste enables anaerobic digestion to be integrated into the whole waste management system in a cost-effective manner. This is because capital investments can be targeted at the material for which the investment is specifically required. Food waste exhibits less seasonal variation than garden waste and therefore anaerobic digestion of this material should enable capacity utilization to be higher.”
Experiences from plants in other countries that handle relatively high proportions of food waste were evaluated in the study. The examples, presented within the text of the report, show how various plants address some of the relevant issues related to generating marketable outputs from food waste rich feedstocks (FWRFs). Combining anaerobic and aerobic steps within the treatment process can bring benefits, explain the authors. “Integration of an additional digestion step for source separated kitchen waste can: Increase capacity; Reduce salinity of the output; Significantly reduce odors, allowing postdigestion aerobic treatment to take place in open windrows without problems; and Generate additional income through generation of energy.”
The overall findings of “Dealing With Food Waste In The UK” are summarized as follows: “Both anaerobic digestion and in-vessel composting are suitable for processing food waste with a view to generating a saleable product; Anaerobic digestion is more favorable from an environmental perspective, primarily because the process produces methane, which can be used to generate energy; Aerobic composting is a small net energy user; In overall terms, the most cost-effective, sustainable and user friendly solution will depend upon how the collection and treatment systems are configured jointly. The design of collection and treatment should be considered together.”
The authors make an important point about local waste authorities that have already embarked on an MSW management strategy – described as “path-dependence” – that could impact separate collection of food waste and treatment. “Decisions made in the past can limit the menu of choices available in the future (or at least, they make some changes more difficult and costly). Some adaptations are evidently possible in respect of treatment processes, whilst collection systems are also likely to be amenable to alteration.”
As a result, they continue, “The messages from this report are more immediately relevant for those considering, or seeking to implement, new systems for biowaste collection, particularly in more urban areas. Here, as long as adequate attention is paid to marketing strategies for the end products, and where collection and treatment systems are well integrated, consideration should be given to the possibility for collecting food waste separately for digestion. If these collection systems can be coupled to AD processes, it should prove possible to generate both energy, as electricity or heat, and quality products for use in various applications…. If the 5.5 million metric tons of UK municipal food waste were targeted for separate collection, then the total quantity of electricity generated could be of the order 477 to 761 GWh per annum if the material was digested. This is equivalent to the electricity used by between 103,000 to 164,000 households…. The net position in respect of greenhouse gases… is more than 0.22 to 0.35 million metric tons CO2 equivalent (based on an assumption that the displaced source is gas fired electricity generation). If the same amount of material had been landfilled, savings increase to 1.6 to 3.6 million metric tons CO2 equivalent, depending upon the performance of both the landfill and the digester.”
Conclude the authors: “AD plants that treat 100 percent food waste are rare at the moment, though many processes would happily receive such materials. What is being suggested here is to bring together leading-edge design of collection systems with state-of-the-art treatment systems. Some countries are using approaches to optimize collection systems, others are moving more towards digestion as an approach to biowaste treatment. Few are doing both. This report shows that the UK can learn from this experience and implement cost-effective systems for the collection and management of biowastes, which reduce greenhouse gas emissions and bring real environmental benefits.” – N.G.
April 17, 2008 | General
Food Waste Management Options In The UK
BioCycle April 2008, Vol. 49, No. 4, p. 51