LFG capture and energy recovery is a frequently applied landfill management practice. There have been many initiatives during the past few years to capture and utilize LFG in gas turbines; a number of such facilities are currently generating electricity. US regulations now require capture of an average of 40% of all landfill methane nationwide. Yet even after compliance with those regulations, it remains profitable (at a carbon price of zero or negative cost) to capture 52% of the landfill methane. At a price of US$20/tCeq (in 1996 dollars), an additional 19% of the methane could be captured, an amount that approaches the estimated maximum practical attainable level (US EPA, 1999a). Official estimates suggest that approximately half, or 35MtCeq, of landfill methane could be recovered by 2000.
Other studies have found that the methane yield from landfills is about 60-170 l/kg of dry refuse (El-Fadel et al., 1998). Some landfills produce electricity from LFG by installing cost effective gas turbines or technologically promising, but still expensive fuel cells (Siuru, 1997). Later reports dispute this claim (US EPA, 2000).
One study suggests that landfilling of branches, leaves and newspaper sequesters carbon even without LFG recovery, whereas food scraps and office paper produce a net increase in GHGs, even from landfills with methane recovery (US EPA, 1998).
Many programmatic initiatives and incentives can boost the rate of recycling. The potential gains are quite large: if everyone in the USA increased from the national average recycling rate to the per capita recycling rate achieved in Seattle, Washington, the result would be a reduction of 4% of total US GHG emissions (Ackerman, 2000). While often associated with affluent countries, recycling is also an integral part of the informal economy of developing countries; innovative approaches to recycling have been adopted in poor neighbourhoods of Curitiba, Brazil, and in other cities.
The literature on techniques for increasing the rate of recycling is too extensive for adequate citation here (see, for example, Ackerman (1997) and numerous sources cited there). One much-discussed initiative is the use of variable rates, or pay-per-bag/per-can charges for household solid waste collection. This provides a clear financial incentive to the householder to produce less waste, particularly when accompanied by free curbside recycling (Franke et al., 1999). Strict packaging and lifetime product responsibility laws for manufacturers in Germany have brought about innovations in the manufacture and marketing of a wide range of products. Other market incentives such as repayable deposits on glass containers, lead acid batteries, and other consumer products have led to major gains in recycled materials in many countries. Voluntary recycling programmes have met with a mixed range of success, with commercial and institutional recycling of office paper and cardboard, and curbside recovery of mixed household materials generally having higher recycling rates. Countries such as Austria and Switzerland successfully require separation of household waste into many disaggregated categories for high value recovery.
Increased composting of household food waste would reduce GHG emissions, but may be difficult to achieve in developed countries, where an additional separation of household waste would be required. In low-income developing countries, the high proportion of food waste in household and municipal waste makes composting attractive as a primary waste treatment technology.
Other new opportunities involve composting or anaerobic digestion of agricultural and food industry wastes. Livestock manure management accounts for 10% of US methane emissions; capture of about 70% of the methane from livestock manure appears technologically feasible. Some 20% of the feasible methane capture is profitable under existing conditions, with a carbon price of zero; 28% can be recovered at US$20/tCeq and 61% at US$50/tCeq(US EPA, 1999a).
Biogas facilities intentionally convert organic waste to methane; use of the resulting methane can substitute for fossil fuels, reducing GHG emissions. High ammonia content (e.g., in swine manure) can inhibit conversion of organic waste to methane. This problem can be avoided by mixing agricultural waste with other, less nitrogenous wastes (Hansen et al., 1998). Wastes with high fat content can, on the other hand, enhance and increase methane output. In Denmark, a number of biogas facilities have been running successfully, accepting livestock manure as well as wastes from food processing industries (Schnell, 1999). In Germany and Switzerland, pilot projects compress the methane from biogas plants and supply it to natural gas vehicles. Canadian engineers have completed a pilot project using a mixture of waste-activated sludge, food waste, industrial sludge from potato processing, and municipal waste paper. Methane production reached 50 l/kg of total solids, and heavy metal contamination was found to be far below regulatory levels (Oleszkiewicz and Poggi-Varaldo, 1998). Woody waste with high lignin content cannot be converted to methane, and yard waste is better handled by composting.
New combustion technologies with higher efficiencies of energy production and lower emissions are currently being developed:
Conventional sewage collection is very water intensive. Vacuum toilets, using less than 1 litre per flush, have long been used on ships and have now been installed in the new ICE trains in Germany. Human waste collected in this way can then be anaerobically digested. This process reduces GHG emissions and water usage is minimal. Acceptance of this technology has been slow because of cost (Schnell, 1998).
Modular anaerobic or aerobic systems are available (Hairston et al., 1997). Anaerobic digestion has the advantage of generating methane that can be used as a fuel, yet many sewage treatment plants simply flare it. The potential for energy generation is clearly very large. New York Citys 14 sewage plants, for example, generate 0.045 billion cubic metres of methane every year, most of which is flared. Cities such as Los Angeles sell methane to the local gas utility, and one New York plant and the Boston Harbor facility were equipped with fuel cells in 1997. This new technology successfully provides needed electricity and heat, but is still expensive.
Because of concerns about contamination of sewage sludge by heavy metals, policies in many countries now encourage incineration rather than soil application. However, the energy needed to dry the sludge for incineration leads to a net increase in GHGs. Alternatives to sludge incineration are anaerobic digestion, gasification, wet oxidation, and co-incineration with coal. These technologies are under development and yield improved energy efficiencies and low GHG emissions (Faaij et al., 1998).
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