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Table 6 Aqueous emissions of cadmium from municipal sewage: households and plating works, in tons per year

EMISSIONS TO MUNICIPAL SEWERS. The two major point sources of cadmium emissions to municipal sewers are waste waters from households and small cadmium-plating operations. The contribution of these two sources during the 1970S and 1980S IS shown in table 6. The table takes into account the evolution of primary and secondary sewagetreatment plants in the basin over this time period. In 1970, only about 33 per cent of the population in the basin was connected to secondary treatment plants. By the end of the 1980S, this percentage had increased to about 84 per cent (OECD,1991). The introduction of secondary treatment has resulted in large reductions in aqueous cadmium emissions, since about 70 per cent of the cadmium inputs are trapped in digested sewage sludges generated during treatment, and about 30 per cent leave the plant as an aqueous effluent.

DeWaal Malefijt (1982), in a comprehensive study of the sources of heavy metals in sewers, estimates the cadmium load from household waste waters to be 45 mg Cd per inhabitant per year. About 20 mg is directly from human wastes, another 20 mg is from corrosion of sewer pipes, and 5 mg is present in the waste water from other sources.

As shown in table 6, aqueous plating wastes were a major source of cadmium to sewers in the 1970s. In the 1980s, however, cadmium emissions were virtually eliminated. One reason for this was that production of plate decreased by more than 80 per cent over this time period. Another reason was the implementation of strict regulations on emissions as required by the German Waste Regulation Act of 1978, and the EC Directive of 1983, which called for reduction in three steps up to 1986. To comply with these regulations, aqueous emission factors were reduced from about 0.0250 tons Cd/(ton plate produced) in 1970 to about 0.0003 tons Cd/(ton plate produced) by the mid 1980s. These laws caused a rapid restructuring in the plating industry, forcing small firms that could not afford to conform to these regulations to cease production of cadmium plate.

Solid wastes and application of agrochemicals

INDUSTRIAL AND MUNiCIPAE WASTES. Solid wastes constitute by far the largest disposal pathway of cadmium in the Rhine Basin. The following are the four major source categories of wastes containing cadmium:

- Fly ash from coal combustion.
- Flue dusts and slag from iron and steel production. Wastes from manufacture of cadmium plate.
- Disposal of consumer and commercial wastes to landfills, including fly ash from incineration of consumer wastes.

It should be noted that thermal zinc smelting plants also generate substantial amounts of solid cadmium waste. Estimates of inputs and outputs of cadmium in the two thermal smelters in the Rhine Basin indicate, however, that in the 1970s their solid wastes were discharged directly to the river. This practice, in large part, accounts for the very large aqueous emissions from the smelters given in table 5. By the early 1980s one smelter stopped producing zinc altogether and the other recycled most of its solid wastes to an electrolytic smelter.

Table 7 provides estimates of cadmium contained in solid wastes disposed in the basin by sector. For most of the categories listed in the table, the tonnage of solid wastes declined appreciably over time. One exception was waste from coal combustion. The increase in this case resulted from increasing reductions in air emissions and concomitant increases in the collection of cadmium-containing dusts and particles. Most of this waste was fly ash from combustion of lignite coal. With regard to fly ash generated from combustion of hard coal, recycling became increasingly important. In 1970, about 50 per cent of the fly ash was recycled. The percentage increased to about 65 per cent in 1980 and about 90 per cent in 1990 (Risse et al., 1991).

Table 7 Cadmium in solid wastes sent to landfills in the Rhine Basin, in tons per year and percentages

a. Sources of cadmium in fly ash and direct landfill are small consumer Ni-Cd batteries. And plastics containing cadmium pigments. Cadmium used as stabilizer in outdoor PVC window frames is not included here, since disposal of this source (in building demolition debris) is not likely to be mixed with municipal wastes.

b. Includes cadmium plate no' recycled to steel refineries. Most cadmium plate, used in automobiles. machinery, and electronic equipment, is disposed of in repositories with other hard goods and generally not mixed with municipal wastes. They are listed here because of their importance as a source of cadmium emissions.

Wastes from cadmium plate manufacturing decreased by nearly 80 per cent, mostly because of large reductions in production, which declined from nearly 600 tons in 1970 to about 280 tons in 1980 and about 100 tons in the late 1980s. About half of the production was used in the basin, with the balance being exported. Thus, disposal of cadmium plate to municipal landfills also decreased substantially. The declining trend not only reduced the volume of cadmium waste in plate manufacturing and municipal wastes: as was the case for aqueous cadmium emissions, it also resulted in a substantial reduction of cadmium in solid wastes from steel production.

Cadmium-coated scrap steel, although constituting only a small fraction of the total scrap, has been identified as the major source of cadmium pollution in steel production (Hutton, 1982). Assuming a tenyear lifetime of steel products coated with cadmium plate, and a recycling rate of about 40 per cent, it is estimated that about 125 tons of cadmium per year entered steel production from inputs of cadmiumcoated scrap in the 1970s. By the late 1980s, however, the inputs had decreased to around 50 tons per year. Two other factors contributing to the decline of cadmium in steel wastes were a 20 per cent reduction in steel production and a significant increase in the recycling of cadmiumcontaining flue dusts and slags generated in production processes.

Municipal wastes landfilled in 1988, including fly ash from municipal incinerators, were about half of the quantity landfilled in 1970. These reductions occurred mainly because of large reductions in the consumption of cadmium-containing products over this time period. Of particular importance was the drastic decrease in the use of cadmium in pigmented plastics, formerly the major source of cadmium in domestic wastes (Schulte-Schrepping, 1981). Use of cadmium pigments in the basin in 1988 (about 200 tons) was less than half the use in 1970 (about 440 tons). This trend was somewhat offset, however, by the increased use of small, sealed consumer Ni-Cd batteries, the cadmium content of which increased from less than 20 tons in 1970 to about 125 tons in the late 1980s.

Recycling of solid wastes ("secondary materials") in the Rhine Basin has emerged as the major alternative to land disposal and incineration. The evolution in recycling noted above for fly ash from hard-coal combustion is typical for other industrial solid wastes as well. Fly ash from coal combustion and slags from iron and steel production are used as feed stocks in the manufacture of bricks, cement, concrete, and asphalt road foundation. Flue dusts from iron and steel production and thermal zinc refining are recycled as inputs to nonferrous electrolytic metal refineries.

Recycling serves three major purposes. First, it replaces primary materials, thus conserving scarce resources. Secondly, it reduces the volume of solid wastes, thus reducing the amount of land that otherwise would have to be set aside for landfills. Thirdly, and most important from the environmental perspective, recycling the wastes into new products that effectively immobilize the cadmium may reduce the potential availability of leachable cadmium to the environment.

Some research has been conducted for testing the leaching behaviour of recycled products (Goumans et al., 1991). It appears that the recycling option may offer a viable alternative for managing cadmium-containing solid wastes, although more research is needed to determine the long-term leaching behaviour (30 or more years) of recycled products.

An important factor that needs to be taken into account when assessing the environmental impacts of disposed solid wastes is the potential for the chemical in question to leach from the waste material in which it is embodied. The concentration of the toxic material in the waste is not a good index for such an assessment, since it is not necessarily proportional to the fraction of toxic material susceptible to leaching. Very few data are available that allow for assessment and comparison of the long-term "leaching potentials" of different solid wastes under field conditions. There is, however, information from standardized laboratory leaching tests which provide estimates of leaching of heavy metals from waste materials under varying chemical conditions (Van der Sloot, 1991). The waste material is ground to a specified particle size and then subjected to an acidic solution (pH 4) at a liquid/solid ratio of 100. The amount of heavy metal leached under these conditions is defined as the "maximum availability," which refers to the total leaching that may occur over a period of 30 to 50 years.

Table 8 lists values of maximum availability of cadmium in the four major categories of cadmium-containing wastes. The table illustrates the wide variation in maximum availability among the various kinds of wastes. There is, for example, a striking difference between fly ash from coal combustion, with a maximum cadmium availability of 10 per cent, and fly ash from incineration of municipal waste, with a maximum availability of 90 per cent. The reason for the difference is the presence of chlorine in municipal wastes (from plastics and other consumer products), and the lack of chlorine in coal (Van der Sloot, 1991). During incineration the chlorine forms complexes with cadmium that are appreciably soluble in water.

Table 8 probably overestimates somewhat the availability of cadmium under field conditions. This is particularly true of recent decades, during which solid wastes in the Rhine Basin have been increasingly disposed of in landfills engineered to restrict their mobilization by controlling the chemical conditions within the waste site, by the use of impermeable linings, and by treatment of drainage waters.

Table 8 Estimated maximum availability of cadmium in various solid wastes (over 30- to 50-gear time period)

a. Van der Sloot et al., 1985.

b. Versluijs et al., 1990.

c. IIASA, 1992.

Table 9 provides an estimate of the availability of cadmium from solid wastes, taking into account the introduction of modern, safer landfills. For the new industrial landfills, it was assumed that the availability of industrial wastes was only 10 per cent of the maximum availability given in table 8. For municipal landfills, however, the maximum availability was assumed, even for new landfills. This assumption appears justifiable because the pH of wastes in municipal landfills is typically in the range of 4 to 5. (The maximum availability is based on leaching at pH = 4.) Moreover, municipal wastes contain high concentrations of organic acids which can form complexes with heavy metals and increase their mobility.

Two trends are evident from table 9. Firstly, there has been a better than 50 per cent reduction in the availability of cadmium. Secondly, municipal waste disposal has been the major source of available cadmium, and its share of the total available cadmium has increased over time, from about 75 per cent in 1970 to about 96 per cent in 1988.

Table 9 Estimated actual availability of cadmium contained in solid wastes generated in the Rhine Basin, in tons per years^a

a. Availability for a selected year is defined as the total amount of cadmium that may be leached to the environment over a 30- to 50-year period after the initial disposal.

The data given in table 9 assume that the new landfills function without leaks or mechanical failures. In actuality this is not always the case, as leaks from presumably safe landfills have been reported in the literature (Hjelmar et al., 1988). Moreover, in the long term (more than 30 years) even correctly functioning safe landfills may begin to leak (Foerstner, 1991), resulting in emissions of toxic materials accumulated over previous decades. In addition, complex chemical reactions can occur in landfills, resulting in the generation, in situ, of toxic chemicals such as benzene, phenols, and vinyl chloride (Fleming, 1992). Hence, constructing the "ideal landfill" that is completely safe for current and future generations is a daunting, if not impossible, task. Clearly, more research is required to understand the complex chemistry of landfills leading to the generation and mobilization of toxic chemicals, and more thought must be given to engineering design for containing the chemicals over the long term.

Table 10 Comparison of annual and cumulative annual availability of cadmium in solid wastes in the Rhine Basin, in tons per year

As noted earlier, availability corresponds to the fraction of heavy metals that may leach from the solid waste over a 30- to 50-year time period. An important consequence of this long-term leaching behaviour is that the waste may continue to be a source of toxic emissions for decades after it is deposited on the land. Thus, when assessing the total availability of a heavy metal for a given year, one must consider the cumulative availability of wastes deposited over the previous 30 to 50 years. Accounting for historical wastes is particularly important because until the 1970s most solid wastes were dumped on the land without precautions being taken to ensure their containment.

Table 10 compares the estimated annual availability, obtained by dividing the sums given in table 9 by 30, with the estimated cumulative availability, obtained by the following calculation for a given year = x:

i=x

Cumulative availability (year = x) = S (availability)/30

i=x-30

Table 10 reveals a 15-year time-lag between the year with maximum annual availability (1970) and the year of the maximum cumulative availability (1985). This result demonstrates the importance of considering the long-term cumulative effects of toxic chemicals in the environment. It suggests that measures for reducing solid wastes may not always lead to immediate improvements because of the legacy of toxic materials deposited in past decades that are still environmentally active. (For a more detailed discussion of the impacts of cumulative chemical inputs, see Stigliani, 1988.)

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