A comparison of data for 2002 and 2004 shows that in the intervening two years, recycling plus composting increased by 11.8 million tons, landfilling by 6.3 million tons, and waste-to-energy by 0.5 million tons.

By Nickolas Themelis

The generation of MSW in the US has grown steadily. Every two years, the Earth Engineering Center of Columbia University conducts a survey of MSW generation and disposal in the US. The survey is based on information solicited from the waste management departments of the 50 states of the union. This survey is made in collaboration with BioCycle and is called The State of Garbage in America (SOG).

The most recent survey showed that the generation of MSW increased from 369 million tons in 2002 to 388 million tons in 2004—a rate of 2.5% per year. Landfilling accounted for 249 million tons or 64% of the MSW generated. The MSW generation per capita remained at 1.3 tons per year (3.2 kg per day), by far the highest in the world.

Overview
WTE power plants are operating in 27 states. They are fueled by nearly 29 million tons of MSW and have a generating capacity of 2,700 MW of electricity. They recover about 0.7 million tons of ferrous and nonferrous metals; also, 3 million tons of WTE are used in place of soil or stone aggregate in the maintenance of landfills.

The major WTE companies in the US are Covanta Energy (31 plants), Montenay Power (nine US plants) and Wheelabrator Technologies (17 plants). The organization corresponding to CEWEP in the US is the Integrated Waste Services Association (www.wte.org), headed by Ted Michaels. The US WTE facilities can be classified in three broad classes:

• Mass-burn plants, which generate electricity and/or steam from trash by feeding MSW as received into large furnaces dedicated solely to burning trash and producing power.
• Refuse-derived fuel (RDF) plants, which shred the MSW, recover some recyclable materials, and combust the homogenized fuel in a combustion chamber. The RDF-producing facility may be next to the furnace or at another location.
• Modular WTE plants, which are similar to mass-burn facilities but are smaller and typically prefabricated offsite and assembled where they are needed.

One can estimate the population served by WTE by assuming that in the communities having WTE facilities all the nonrecycled wastes (i.e., 71.5% of the total using the national average) are combusted.

Therefore, the total MSW in those communities amounts to 28.9x100/71.5 = 40.4 million tons. Dividing this number by the average national generation rate of 1.3 tons per capita indicates that the WTE facilities in the US serve 31 million people.

Combustion Benefits
Combusting one ton of MSW in a modern WTE power plant generates a net of 550 kWh of electricity, thus avoiding mining a quarter of a ton of coal or importing one barrel of oil. Also, WTE is the only alternative to the landfilling of nonrecyclable wastes, where the decomposing trash generates methane, a potent greenhouse gas, an estimated 40% of which escapes into the atmosphere even from modern sanitary landfills.

The non-captured methane has a greenhouse gas potential 23 times that of the same volume of CO2 (Intergovernmental Panel on Climate Change).

Taking into account the electricity generated and the methane emissions avoided has led several independent studies to conclude that WTE reduces US greenhouse gas emissions by an estimated 1.1–1.3 tons of CO2 per ton of trash combusted rather than landfilled. Therefore, in addition to the energy benefits, the combustion of MSW in WTE facilities reduces annual greenhouse gas emissions in the US by about 40 million tons of CO2.

Renewable Energy Source
In 2004, WTEs in the US generated a net of 13.5 billion kWh of electricity, greater than all other renewable sources of energy with the exception of hydroelectric and geothermal power. For comparison, wind power amounted to 5.3 billion kWh and solar energy to 0.87 billion kWh.

A regional breakdown of landfilling, recycling, and WTE

If it is assumed that the Rubber, Leather, and Textiles category of MSW, as reported by USEPA in its 1997 characterization of US MSW, is divided equally between natural (cotton, wool, leather, and rubber) and manmade (plastics, synthetic rubber, and fabrics) organics, the combustible materials in MSW consist of 82% biomass (paper, food and yard wastes, plus half of rubber, etc.) and 18% petrochemical wastes. Therefore, MSW is a renewable source of energy and, rightly so, included by the US Department of Energy in the biomass fuel category of renewable energy sources.

Public Health
In the distant past, there were thousands of incinerators without any air-pollution controls. For example, at one time New York City had an estimated 18,000 residential incinerators and 32 municipal incinerators. The environmental impacts can still be detected in deep-lying cores of the Central Park soil.

Understandably, this has left a bad image of incineration in New York City that persists to this day. The result is that New York transports most of its MSW to distant landfills in other states. Yet the adjacent New Jersey and Long Island Sound communities depend largely on WTE, and most of the Manhattan MSW is combusted in the Essex County WTE of Covanta Energy. At this time, there are over 1,500 incinerators of all types in the US but fewer than 100 WTE plants.

In the past, when the effects of emissions on health and the environment were not well understood, all high-temperature processes, including metal smelting, cement production, coal-fired power plants, and incinerators, were the sources of enormous emissions to the atmosphere. In particular, incinerators were the major sources of toxic organic compounds, called dioxins and furans, and mercury. However, in the last 15 years and at the cost of about $1 billion, the 88 WTE facilities operating in the US have implemented air pollution control systems that have led USEPA to recognize them publicly as a source of power “with less environmental impact than almost any other source of electricity”.

 In 1995, the USEPA adopted new emissions standards for WTE facilities pursuant to the Clean Air Act. Its maximum achievable control technology (MACT) regulations dictated that WTE facilities with large units (i.e., >250 tpd) should comply with new Clean Air Act standards by Dec. 19, 2000. Small unit facilities (i.e., 35–250 tpd) represent only 5% of the US WTE capacity and by 2005 also met similar MACT rules. MACT includes dry scrubbers, fabric filter baghouses, activated-carbon injection and other measures that were implemented at the cost of over $1 billion. WTE facilities now represent less than 1% of the US emissions of dioxins and mercury, as discussed below.

Dioxin Emissions Decrease
The toxic effects of dioxins and furans were not realized, both in the US and abroad, until the late 1980s. Thanks to the implementation of MACT regulations, the “toxic equivalent” dioxin emissions of US WTE plants have decreased since 1987 by a factor of 1,000 to a total of less than 10 grams TEQ per year. In comparison, the major source of dioxin emissions reported by EPA is backyard trash burning, which emits close to 600 grams annually.

Mercury Emissions
The use of mercury in US processes and products reached a high of 3,000 tons per year in the 1970s. It decreased to less than 400 tons by 2002 because of the phasing out of most applications of this metal, as mandated by USEPA. For example, mercury-activated switches and thermostats have been substituted, and the mercury content of fluorescent lamps has been reduced substantially. Also, many communities have put in place strong recycling programs that keep older mercury-containing products out of the MSW sent to WTE facilities. This trend, plus the implementation of the MACT regulations has decreased the mercury emissions of the WTE facilities from 89 tons of mercury in 1989 to less than 1 ton today. By now the major sources of mercury in the atmosphere are the global coal-fired power plants.

The only remaining WTE emissions of concern are nitrogen oxides. However, the total WTE emissions correspond to only 0.22% of the total NOx emissions in the US. For comparison, coal-fired power plants contribute 19.5% of the US NOx emissions.

WTERT
Because of competition with low-cost landfills, the US WTE companies have not been profitable enough to support a substantial R&D function.

In recognition of this need, the Waste-to-Energy Research and Technology Council (WTERT) was cofounded in 2002 by the Earth Engineering Center of Columbia University (www.columbia.edu/cu/earth) and the Integrated Waste Services Association (IWSA; www.wte.org) that represents most of the waste-to-energy facilities in the US. WTERT brought together engineers and scientists from industry, government, and universities from all over the US and other countries.

WTERT believes that responsible management of wastes must be based on science and the best available technology, not on emotion or what seems to be inexpensive now but may not be sustainable even for the next 100 years. In general, the mission of WTERT is to increase the global recovery of materials and energy from used solids and, in particular, to advance both the economic and environmental performance of WTE technologies.
WTERT conducts academic research that involves both M.S. and doctoral students and investigates existing and developing technologies.

The findings are reported through presentations, publications, the North American Waste-To-Energy Conference (NAWTEC) and WTERT annual meetings, and the WTERT Web page (www.columbia.edu/cu/wtert). This page includes the database SOFOS and has become one of the best sources of information on R&D on the recovery of energy and materials from wastes and the environmental impacts of waste-processing technologies.

On the basis of several studies by graduate students in the Department of Earth and Environmental Engineering at Columbia University and other researchers, WTERT has concluded that WTE technologies are an indispensable tool in the integrated management of MSW. Both recycling and WTE conserve nonrenewable minerals and fossil fuels.

The environmental benefits of WTE derive from reducing the greenhouse gas emissions associated with landfilling putrescible materials, avoiding the conversion of greenfields to landfills, conserving fossil fuels, and recovering metals. As noted above, US WTE facilities recover 0.7 million tons of metals; in contrast, an estimated 10 million tons of metals are buried annually in US landfills.

Dioxin emissions in the United States

Research
Current WTERT research topics include the following:

• Reducing corrosion in WTE combustion chambers. Because the WTE combustion gases, before gas cleaning, contain a relatively high concentration of HCl, corrosion is more acute than in coal-fired power plants and represents a major item of maintenance. Research efforts to overcome this problem include the study of superior metal alloys and methods of application; sequestering of HCl at high temperatures; and reducing the superheater metal temperatures.
• Thermogravimetric analysis of the kinetics of drying, volatilization, and combustion of various components of MSW, including particle size and shape factor.
• Study of transport and chemical rate phenomena on WTE grates. The objective is to reduce the capital cost of future WTEs by understanding the effect of MSW size distribution and grate design on the combustion capacity of WTE units, in terms of thermal energy released (in MWh) per square meter of grate surface.
• Improving the quantity and quality of metal recovery in WTE plants. On average, only 50% of the input metal is recovered in US WTEs; also in many plants ferrous and nonferrous metals are not separated, thus reducing the value of the metal collected.
• Means to increase the beneficial use of WTE ash. For example, removal of chlorine from fly ash can increase the use of combined ash for the remediation of land used in the past for coal strip mining.
• Critical analysis of existing waste management system studies and technical economics studies on the potential for WTE implementation in Brazil, Chile, Greece, and New York City.
• Comparison of health effects, life cycle costs and benefits, and greenhouse gas effects of MSW disposal by WTE and by landfilling.

WTERT Annual Meetings
The WTERT Annual Meetings are held at Columbia University in the fall, and in 2004 and 2005 they included presentations on waste management in Brazil, China, Germany, Finland, France, India, Israel, Italy, Japan, the Netherlands, Singapore, Taiwan, and other nations. WTERT is also a partner of SWANA and ASME International in organizing the North American Waste-to-Energy Conference (NAWTEC) held each spring in Florida.
In 2004, the first WTERT awards for Outstanding Contributions to Waste Management were given at a gala dinner to Martin GmbH of Germany, a company that has continually improved the reverse-grate technology used in over 300 WTE facilities worldwide (Industry Award), and to George Tchobanoglous of the University of California-Davis for his pioneering textbooks and handbooks on waste management (Education Award).

The 2006 WTERT Industrial Award will be presented to an operating WTE facility that is judged by an international committee to be among the best in the world, based on but not limited to the following criteria: esthetic appearance of facility; energy recovery, in terms of kWe plus kWh recovered per ton of MSW and as % of thermal energy input in the MSW feed; level of emissions achieved; optimal resource recovery and beneficial use of WTE ash; and acceptance of facility by host community.     

Nickolas Themelis is chair of the Waste-to-Energy Research and Technology Council at Columbia University.

MSW - September/October 2006