waste-related cadmium cycle in switzerland

ECOTOXICOLOGY AND ENVIRONMENTAL SAFETY 7, 14 i-150 ( 1983)

Waste-Related Cadmium Cycle in Switzerland’

LEO KELLER AND PAUL H. BRUNNER

Swiss Federal Institute for Water Resources and Water Poiiution Control, EA WAG-ETH, CH-8600 Dubendorf; Switzerland

Received August 1981

The anthropogenic contribution to the global cadmium flux exceeds natural sources by a factor of three. The most important pathway is the atmosphere; therefore, high cadmium concentrations can be found even in remote areas. On a local level, the increase in cadmium consumption can be observed in increasing concentrations in the soil, plants, and food. The question arises as to what extent the soil-plant-man-waste-soil cycle can be loaded with cadmium in order to function without negative impact on the environment. In Switzerland, 120 tonnes (t) of cadmium are consumed per year. Gf this amount, 25 t end up in municipal solid waste, 3 t in wastewater, and 19 t in precipitation and dry fallout. As a consequence of today’s waste management practice (75% incineration, 20% sanitary landfill, 5% cornposting; 75% of all sewage is purified), the annual input to tire soil is 40 t: 18 t concentrated in landfdls, 19 t dissipated via the atmosphere, and 3 t directly spread via sewage sludge, compost, and fertilizer on agricultural land. If even distribution were possible, the cadmium content of the soil would theo- retically double in 150 years. The accumulation in the soil will increase the cadmium content of plants grown on such a soil. According to a simple model, the level of 3 ppm cadmium in soils should not be surpassed. At such concentrations, plants are likely to contain > 0.4 mg Cd/ kg a concentration which can cause toxic effects in long-term experiments. The safe level in food might be even lower. In reality, cadmium is not evenly distributed over Switzerland. Ac- cording to today’s practice, it must be assumed that in only 14 years the use of compost will have enriched soils to such an extent that its cadmium content will prohibit the production of food for human consumption. For sewage sludge, this timespan is 130 years. If heavy metal limits in food are to be observed, the input of such metals to the soil has to be limited. In a steady state, the cadmium input to the soil should be equal to its output via plants, leachate, and erosion. This implies that today’s dissipative use of cadmium must be stopped.

I. GLOBAL CADMIUM CYCLE

Cadmium belongs to the atmophilic elements; therefore, the most important carrier is the atmosphere. Significant evidence for the anthropogenic cadmium loading constitutes the relationship between anthropogenic and natural emissions in the atmosphere, i.e., the so-called interjkence factor. According to Lantzy and Mackenzie ( 1979), the interference factor amounts to 19; Nriagu evaluates it at 9. Based on different assumptions, Brunner and Bacchini ( 198 1) reach a higher natural flux, and thereby an interference factor of 3 to 4 (Fig. 1).

II. REGIONAL CADMIUM CYCLE BASED ON THE EXAMPLE OF SWITZERLAND

The evaluation of global cycles renders a qualitative image. However, for an ecological evaluation of existing cadmium loadings, the quantitative, regionally differ-

’ Paper presented at the International Symposium, “Ecotoxicology of Cadmium,” May 6-8, 198 1, Neu- herberg, Federal Republic of Germany.

141 0147-6513/83/010141-10$03.00/O Copyright 0 1983 by Academic F’res, Inc. All rigbt.5 of reproduction in any form reserved.

142 KELLER AND BRUNNER

natural rnthropogrnic

0 FLUXES lo3 to/y

0 RESEW 10~ to

FIG. 1. Global cadmium cycle.

entiated balances and models are of importance. The atmospheric transport constitutes, even in the regional cycle, one of the most important cadmium fluxes.

A. Sources of Cadmium

Since Switzerland does not produce any primary heavy metals, there are no cadmium, zinc and lead emissions of such industries. No data is available regarding the scrap metal industry which constitutes an important cadmium source in Switzerland.

I. Commercial import. Due to unclearly differentiated customs statistics, the net import of cadmium can only be evaluated. The figures range from 40 to 300 t/year (t = tonnes), with 120 t as the most likely annual consumption. This evaluation does not take into account the substantial cadmium import via imported zinc, because there is not enough data on the cadmium content of such materials (Table 1).

2. Municipal solid wastes. Municipal solid waste treatment processes are important sources of cadmium in the environment. In Switzerland, the treatment practices for solid wastes are as follows: 75% incineration, 20% sanitary landfilling, and 5% com- posting.

Based on the assumption that, on a long-term basis, the cadmium content will remain constant, the 25 t of cadmium contained in waste materials enter in the environment by means of the following: 19 t reach incineration plants of which 13 t are deposited as slag and filter residues and approx 6 t emitted into the atmosphere;

TABLE 1

ANNUAL CADMXUM CONSUMPTION IN SWITZERLAND

Annual import/consumption rates Cadmium content in imported and processed zinc

Swiss cadmium consumption in proportion to the global consumption in comparison to other Swiss metal inputs

Cadmium consumption as a function of inhabitant Cd consumption equivalent in the ERG and France (20-40 g/person * year)

Pure cadmium and contaminants

Best evaluation (not taking into account the cadmium contamination in zinc)

1.2-125 t 0.005-0.5%

0.26-l .2% 40-190 t

120-240 t

40-300 t

120 t

CADMIUM CYCLE IN SWITZERLAND 143

5 t are directly deposited in sanitary landfills; and 1 t is spread via refuse compost on agricultural land, mostly vineyards.

3. Wastewater. Switzerland’s total wastewater contains 2-3 t of cadmium. Of this, 75% undergoes at least mechanical and biological treatment. The annual cadmium flux in sewage sludge is estimated at 1 t, the Cd residue at 50%. Hence, 1 t Cd is discharged into rivers via treated wastewater effluents. The 25% of the wastewater which is still not treated today contributes to an additional annual cadmium flux of 0.7 t. The cadmium flux in wastewater originating from precipitation and reaching the sewerage system is evaluated at 0.3 t.

4. Precipitation. Thermal processes emit the atmophilic element cadmium in the form of fine particles < 1 pm into the atmosphere where it remains for three to eight days.

In Switzerland, a network of seven different stations measures the cadmium precipitation (Zobrist and Stumm, 1979). The mean annual values vary from 0.16 to 1.1 mg/m’/year among the different stations. The total flux was estimated at 19 t/year. (This figure probably undervalues the total flux since precipitation in heavily loaded urban areas has not been taken into account.)

5. Emissions in the atmosphere. Refuse incineration plants are probably the most important atmospheric cadmium-emitting sources in Switzerland (Fig. 2). Motor vehicles, oil, and coal heating are comparatively small cadmium sources. The atmospheric industrial emissions are difficult to determine.

The total precipitation flux can be interpreted on the basis of the present data only if massive atmospheric imports are assumed. This assumption is based on the fact that the atmospheric lead import is three to four times higher than the Swiss lead emissions. In view of wind and precipitation conditions in Western Europe, this import seems possible for other metals also. Accordingly, only 30% of the quanti- tatively important cadmium load of the biosphere, i.e., via precipitation, originates in Switzerland itself.

The natural cadmium emissions (wind-blown dust, volcanic emissions, sea salt sprays) in Switzerland can be measured as surface-proportional percentages of the global natural emissions (according to Nriagu, 1980), or from the Cd/Al ratio in soils and precipitation. Values of - 1% of the total precipitation fluxes are thus obtained.

ATMOSPH. IMPORT

11 t/y

WASTE INCINERATION

6

INDUSTRIAL EMISSIONS

COMBUSTION PROCESSES (oil /coal )

40.3

MOTOR VEHICLES

0 0.01

NAT. EMISSIONS

m 0.2

FIG. 2. Sources of cadmium in the atmosphere.


Although if one assumes that the topographic and climatic situation in Switzerland has always lead to a relative cadmium enrichment in the rainfall, it nevertheless appears that cadmium precipitation in Switzerland has increased significantly through anthropogenic sources.

B. Sinks

I. Rivers and lakes. The cadmium export via receiving waters amounts to approx 6 t. However, present data is not sufficient yet to establish a coherent balance. The remaining wastewater flux after wastewater treatment amounts to 1.7 t; the precipitation input not retained by the soil is evaluated at 0.4 t according to Bacchini (1975). The natural cadmium erosion totals approximately 2 t (J&&h, 1958). No data is available on the cadmium input via untreated street runoffs, storm overtlow, or via illegal industrial wastewater inlets which, according to Imhoff and Koppe (1980), amount to 30% of the total metal flux in the case of the Ruhr river system. The cadmium flux which reaches lakes and rivers directly via precipitation is measured at 0.7 t.

It is important to note that the present flux data does not take into account the sedimentation of cadmium in lakes and rivers. Sedimentation of erosion products certainly occurs in Alpine fringe lakes. Anthropogenic loadings set in, however, mostly down-stream (Table 2).

2. Soils. Agricultural soils (approx 25% or 11,000 km*) receive considerable cadmium input from different sources. Precipitation introduces 5.4 t. In Switzerland, as a consequence of today’s waste management practice, 2 t of cadmium are spread annually on agricultural soils via sewage sludge and refuse compost. An additional 1 t reach the fields annually via phosphate fertilizers. This relatively low value is due to the fact that approximately two-thirds of the phosphate requirements are covered by Thomas slag which has a low phosphate content (0.02 ppm). The cadmium content in superphosphate amounts to 2-6 ppm (Furrer, 198 1). The nonagricultural soils receive 12.6 t of cadmium via precipitation.

3. Cadmium cyclefir Switzerland. The Swiss cadmium cycle is illustrated in Fig. 3. It is an assumption based on the available data.

TABLE 2

CADMIUM FLUXES TO RECEIVING WATERS

Discharges into receiving waters tonnes

Wastewater Via treatment plants Untreated wastewater Storm overflow Untreated street runoffs Undetermined industrial wastewater

Soil leachate (2% is not retained by the soil) Direct precipitation input into river and lake systems Erosion (natural)

1 0.7 ? ? ?

0.4 0.7 2

River export, based on the Rhine content near Rheinfelden 6


As against the industrial and atmospheric cadmium import of 132 t, we find quan- tifiable “exports” or sinks of only 45 t to the soil and out of Switzerland. (Lake and river sedimentation was not taken into consideration.) The remaining 87 t stay in the biosphere. The half-life periods of many cadmium-containing products (PVC, accumulators, etc.) can be quite high; therefore, their emissions will not disappear rapidly even if cadmium consumption is decreased (Table 3).

III. THE WASTE-RELATED CADMIUM CYCLE IN SWITZERLAND

For the evaluation of the present ecological importance of the cadmium cycle, rapid fluxes of high concentration which end in the biosphere are of particular interest. Such important fluxes contain refuse compost and sewage sludges as welI as phosphate fertilizers (Fig. 4).

Atmospheric precipitation is responsible for 64% of the total cadmium flux to Swiss agricultural soils. An additional 12% each reaches agricultural soils via refuse compost, sewage sludge, and phosphate fertilizers. If all compost and sewage sludge were distributed evenly over the entire area of Switzerland, the cadmium content of Swiss soils would increase by 0.3 ppm in 120 years. This would double today’s cadmium content of the average Swiss soil in 120 years, if homogeneous mixing in the top 30 cm of the soil is assumed (Table 4).

In contrast with this theoretical model of homogeneous distribution, the current application rate of compost (30 t/ha) and sewage sludge (2.5 t/ha) leads to a significant metal increase per unit area (Table 5).

According to the actual application rates, the cadmium flux via refuse compost and sewage sludge exceeds the cadmium in precipitation (in t/ha) by a factor of 70 and 3, respectively.

The cadmium content of soils treated with refuse compost will double in a few years: in 2 years, the content wiIl have increased by 0.3 ppm, in 7 years by 1.0 ppm, and in 2 1 years, it will have exceeded the limit of 3 ppm introduced by Klocke ( 1973).

The time intervals are significantly longer for soils treated with sewage sludge. The cadmium content of such soils will double in 43 years.

FIG. 3. Cadmium cycles for Switzerland.


TABLE 3

CADMIUM BALANCE IN SWITZERLAND

t/year

Import Commercial import Atmospheric import Fertilizer

Total

120 11

1

132

Export River export Sanitary landfill Refuse compost Sewage sludge Fertilizer Rain and dry fallout

6 18 1 1 1

18

Total 45 Enrichment in the biosphere 87

An even and homogeneous distribution is merely hypothetical for cadmium fluxes from precipitation too. Measurements have shown that in the vicinity of industrial areas, the cadmium precipitation is much higher. Therefore, the atmospheric contribution to cadmium loadings in soils may locally be quite higher.

IV. ECOLOGICAL IMPORTANCE OF CADMIUM FLUXES IN SWITZERLAND

A. The Impact of Cadmium on Man

According to Friberg and Kjellstriim (1974), a daily cadmium intake of 48-60 pg/ day commonly encountered in Western Europe leads to a cadmium load above the

RlVERS *LAKES

0 FLUXES to Cd I YEAR

0 SOURCE ” * *

0 SINK ’ * -

FIG. 4. Cadmium fluxes via wastes, based on the Swiss examples.


TABLE 4

INCREASEOFCADMIUMCONTENTINTHESOILBYHOMOGENICDISTRIBUTION OVERTHEENTIREAGRICXJLTURALLAND SURFACE(~~,~OO km’)

Annual Time (years) for a Cd Total content content increase of flux Flux increase (t) (mg/m2 - year) (in ppm) 0.3 ppm 1 ppm 3 ppm

Atmospheric precipitation 5.35 0.5 0.0017 176 600 1750

Fertilizer 1 0.09 +0.0003 150 500 1500 0.0020

Refuse compost 1 0.09 +0.0003 130 400 1200 (80,000 t, 0.07 t/ha) 0.0023

Sewage sludge 1 0.09 +0.0003 115 380 1140 (189 X 103t, 0.16 t/ha) 0.0026

effect level (1% response rate). This value is reached if the basic nutrition contains more than 0.1 pg Cd/g. Accordingly, for most food products the German Federal Ministry of Health has established orientation values of ~0.1 pg Cd/g.

Two pathways are responsible for the cadmium content in plants: uptake from the soil and atmospheric precipitation (dry and wet fallout), uptake from the air. Con- sequently, the cadmium content in plants can increase by refuse incineration (via the atmosphere) as well as by application of refuse compost or sewage sludge.

B. Importance of Atmospheric Inputs

From an ecological point of view, the direct impact of the atmospheric cadmium load on the plant surface is as important as the enrichment of the soil. Evaluations

TABLE 5

CADMIUMCONTENTINCREASEINTHESOILBYCONCENTRATEDAPPLICATION OF SEWAGE SLUDGEANDCOMPOSTONALIMITEDAGRICWLTURALLANDSURFACE

Annual Time (years) for a Cd Total content content increase of: flux Flux increase (t) (mg/m* - year) (in ppm) 0.3 ppm 1 ppm 3 ppm

Atmospheric precipitation 5.35 0.5 0.0017 176 600 1750

Fertilizer 1 0.09 0.002 150 500 1500

Refuse compost (80,000 t, 30 t/ha over 27 X lo6 m2) 1 36 0.14 2 7 21

Sewage sludge (189 X IO3 t, 2.5 t/ha over 76 X lo8 m2) 1 1.4 0.007 43 143 430


TABLE 6

CADMIUMPRJXIPITATIONVALUESIN SWITZERLAND

Location mg/m* . year at/m* - &Y

Rural areas (Zobrist and Stumm, 1979)

Suburbs (Roberts et al., 1976)

Highway (Dauber et al., 1979)

Highly industrialized area (Brunner et al., 1981)

0.16-1.1 0.4-2.7 (0.3 (1.3

0.9 2.5

2.5 6.5

8-17 20-45

by different authors (Hansen and Tjell, 1980; Krause-Fabricius, 1978) show that a daily mean atmospheric flux of 0.5 ,ug Cd/m2. day already leads to a mean plant content increase of 0.02 pg Cd/g fresh wt. Higher atmospheric fluxes result in pro- portionally higher contents. The critical value of 0.1 &g fresh wt (1% response rate) is reached at a cadmium precipitation of 2.5-6 ,uglm2 * day. Such values have already been observed in Switzerland (Table 6).

C. Transfer Rates from Soil to Plant

The knowledge of transfer rates from soil to plant is important for the determi- nation of cadmium limits in soils. The establishment of limiting values raises, however, various difficulties. Transfer rates are not only dependent on soil properties (pH, cation-exchange capacity, humic substances, Ca2’) but differ greatly among plant species as well as subspecies (Fig. 5).

D. Limit for Cadmium in Soils

Today, atmospheric precipitation alone contributes already significantly to the cadmium content of soils. Therefore, soil limiting values aimed at maintaining present ecological conditions should be drawn up rather carefully. On the basis of phytotox-

LETTUCE BARLEY OAT

Tom Thumbs Aramir 6.5ppm ml.0 ppm ~kb9~rn (dwl

Webbs -5.0

Leander DO.35

Abacus 10.75

SOIL Cd-CONTENT 7.4mgILg DAVIS AND CARLTON-SMITH 1981

FIG. 5. Uptake of cadmium by plants in parts per million per dry weight as a function of differenr subspecies.


15

I

,A

ppm FG __I- ,/

Cd - Gehalt im Boden Cd-CONTENT OF THE SOIL

lPPl.1

FIG. 6. Uptake of cadmium by plants.

icological aspects, Klocke ( 1973) proposed a limit of 3 ppm for the cadmium content in soils. Transfer experiments by Hirschbeydt (1976-1978) with corn and carrots (plants with low cadmium uptake) show that for certain soils the limit of 3 ppm is too high; food grown in such a soil will have a cadmium content >O. 1 &g fresh wt (Fig. 6). Consequently, a general increase in the mean soil concentration to 3 ppm appears critical from a nutritional viewpoint.

V. CONCLUSIONS

1. In view of the cadmium accumulation in the biosphere with uncertain residence time and unknown future pathways, measures should be taken to reduce the cadmium flux to the biosphere.

2. In Switzerland, the anthropogenic emissions exceed by far the natural emissions of cadmium to the atmosphere.

3. The substantial atmospheric cadmium import indicates that the cadmium problem cannot be solved on a national level only.

4. By today’s waste Iluxes and current waste management practices soils can reach such cadmium concentrations that they become hazardous to public health. With regard to sensitive soils, plants and plant subspecies critical cadmium concentrations will be attained within a few years. The consequences will be most severe for certain parts of the population, e.g., vegetarians, home gardeners, or farming communities in close vicinity to cadmium sources.

5. The incineration of waste materials as opposed to use in agricultural rarely constitutes a real solution to the cadmium problem: in both processes, the soil is used as the final storage place. From a waste management point of view, only a reduction of the heavy metal fluxes in the wastes will lead to a long-term amelioration.

6. If, for ecological reasons, lower cadmium limits are established for soils, plants, or food products, the time to take measures will also decrease. By lowering the cadmium limits, a long recovery time will be necessary to reach the required levels due to the large cadmium reservoir in the biosphere and the low cadmium mobility in soils.


REFERENCES

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BRUNNER, P. H., AND BACCINI, P. (198 1). Neue ZUrcher Zeitung, 25.3.198 1, No. 70, p. 65. EAWAG Publication No. 840.

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waste-related cadmium cycle in switzerland

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