In Europe, as in North America, ash stream management is one of the major issues surrounding waste-to-energy (WTE). Although municipal waste combustion has a strong European presence, opinions differ as to the residue's toxicity and recyclability.
In 1992, Europe processed 24 percent of its household waste for energy recovery. The European Energy-from-Waste Coalition re-ports that levels are highest where environmental track records are strongest - such as in Denmark, Sweden and the Netherlands, which combust 48 percent, 47 percent and 34 percent, respectively, of their waste with energy recovery. The Swiss incinerate 76 percent of their municipal waste, though not always recovering energy.
Between 30 percent and 40 percent of the incinerated waste by weight remains as residue (see figure). The ash streams vary widely in their quantity, composition and characteristics and thus call for different recycling and disposal approaches in each case. For example, when a new WTE facility in Cologne, Germany, comes on line in 1998, its 129,000 metric tons of bottom ash will be used in road construction, whereas the 12,000 metric tons of salts, flyash and gypsum will be recycled by the mining industry. Only 900 metric tons a year will require disposal as a hazardous waste.
Although WTE opponents maintain that the residue cannot be managed safely or recycled without causing contamination, approximately 50 percent of all WTE slag produced in Germany today is re-cycled in road construction. Ash used for civil engineering purposes must not contain readily-leachable elements nor have a high salt content; flyash is not acceptable. After washing, which occurs in its simplest form when the ash is quenched for extraction from the incinerator, magnetic separation and screening remove metals and oversize materials. The ash then has to age for three months until volume-increasing chemical reactions are complete.
Residues that are inappropriate for direct use because of their composition, fine consistency or water solubility can serve as additives to produce concrete, bricks and other construction materials. Advantages include savings in silica-containing materials and expensive binders. Ongoing research is aimed at determining the exact behavior of ash in such applications.
In some cases, processing is required to render the residue suitable for beneficial use or inert for safe landfilling. The residue, including flyash, can be solidified into a ceramic-like material by means of screening, mixing with binders, compaction and sintering. Possible adhesives are cement or naturally occurring materials such as clay, which has the advantage of adsorbing the heavy metals contained in the ash. Residue treated by this process reportedly meets the strict Swiss leaching requirements for disposal as solid waste.
For flyash, a method that is gaining attention is vitrification at temperatures above 1,200 degrees C prior to recycling or disposal in ordinary landfills. The Swiss Federal Laboratories for Materials Testing and Research reports that vitrification "is a suitable way to destroy the organics and separate the metals and salts from the oxides." The recovered metals and salts can be reused in the metal industry, while the ash in the inert gas phase has potential applications as filler material in the construction industry, a substitute in cement production, glass wool or fibers for insulation, plastic enhancement material, container glass or industrial ceramics.
Alternatively, residues from air pollution control equipment can be vitrified without heavy metals separation. Mobilization of the heavy metals is minimized, and the resulting glass granulate reportedly can be landfilled safely.
Driving the need for ash minimization and recycling are rising collection and disposal costs, growing reliance on thermal waste treatment and decreasing political acceptance of landfilling. In Germany, for example, the cost of landfilling hazardous wastes, including some WTE ash, in-creased by 25 percent between 1991 and November 1992.
In France, WTE capacity is being greatly expanded to meet the mandate that only waste residue be landfilled as of 2002. The quantity of flyash from household waste combustion is expected to reach 1 million metric tons soon. When classified as industrial hazardous waste, ash requires disposal in the most secure (class 1) landfills.
Another impetus is the legacy of pollution. Near Frankfurt, a 19-acre unlined bottom ash landfill, impacting the ground water since the 1970s, is being sealed off by an eight-meter-deep slurry wall at a cost of about $7 million.