Permitted trace metal loads to agricultural soils

The existing EU Directive regulating the use of sewage sludge in agriculture (86/278/EEC) stipulates the highest permitted loading rate for trace metals as follows: 12 kg copper, 3 kg nickel and 30 kg zinc per ha and year as mean value over a period of 10 years. However, due to increasing concerns, several EU member states have set national limits well below the EU Directive (see Table 4.6).

A recent EU initiative (Third Draft to the Working Document on the Revision to the Sewage Sludge Directive) intends to improve the present rate of recycling of nutrients and organic matter contained in sludge by broadening the scope of the existing legislation (EU Directive 86/278/EEC) and including the management of sludge in outlets such as silviculture, green areas and reclaimed lands. It is recognised that sludge should be used when there is an agronomic interest for the crops and/or soils. To ensure that the use of sludge is carried out in such a way as to minimise the risks of negative effects on humans and the environment, the EU is discussing concentration limits for metals and organic compounds. The Working Document proposes on one hand new limit values for the immediate future (short-term limits) and, on the other, some medium and long-term objectives for metals to be reached by 2015 (medium-term limits) and 2025 (long-term limits), respectively (Table 4.6). Although the proposals for medium- and long-term limits have later been removed from the EU Working Document on sludge (I. Schoeters, ECI, pers. info, 2004), these proposals are included in Table 4.6 for reference purposes.

Table 4.6. Permitted trace metal loads (kg/ha per year) in the EU - at present and proposed for the future - as well as in some EU Member States and in some other countries. After Landner et al., 2000; Delbeke and Landner, 2000.

Region/ country

Chromium

Copper

Nickel

Zinc

EU, Directive 86/278/EEC

12

S

SG

EU, proposed short-term limits

S

S*

G.9

7.5*

(EU, proposed medium-term limits

2.4

2.4

G.6

6.G)

(EU, proposed long-term limits

1.8

1.8

G3

4.5)

Sweden (from 2000)

-

G3*

G.G25

G.6*

Denmark

-

1G

G.6

4G

U.K.

-

7.5

S

15

Germany

-

13

G3

2.5

France

-

S

G.6

9

Austria (Steiermark), arable land

-

1.25

G.25

5

Canada

-

S7G

4

S6

USA

-

75

21

14G

* The competent authority may decide to allow higher loading rate of Cu and/or Zn if shown that the soil is copper- and/or zinc-deficient and that there is a specific agronomic need for the crops.

* The competent authority may decide to allow higher loading rate of Cu and/or Zn if shown that the soil is copper- and/or zinc-deficient and that there is a specific agronomic need for the crops.

In addition to regulating the trace metal loading rate for sludge on agricultural land, most countries, as well as the EU, have also set limits for concentrations of trace metals (and of some persistent organic compounds) in sludge for use on land. Examples of such maximum trace metal concentrations in sludges in the EU (today and proposals for the future), as well as in Member States, are given in Table 4.7. It should be noted that while most concentrations are given as mg/kg dry matter (DM), the proposed short-term limits in the EU are also given in terms of mg/kg P, because one of the major incentives for a farmer of using sewage sludge on his/her land is to make use of the phosphorus in the sludge as a fertiliser. A concentration expressed on the basis of the P content of the sludge is also a much more stable base for interpretation, since sludges produced according to various stabilisation techniques will exhibit greatly varying dry contents. This means that relating the metal content to the DM content of the sludge will make it almost impossible to standardize the sludge quality.

Table 4.7. Limit values for some trace metals in sewage sludge (mg/kg DM, or when indicated, expressed as g/kg P) for use in agriculture. Values are from the EU - at present and proposed for the future - as well as from some EU Member States and from some other countries. After Landner et al., 2000; Delbeke and Landner, 2000.

Table 4.7. Limit values for some trace metals in sewage sludge (mg/kg DM, or when indicated, expressed as g/kg P) for use in agriculture. Values are from the EU - at present and proposed for the future - as well as from some EU Member States and from some other countries. After Landner et al., 2000; Delbeke and Landner, 2000.

Region/ country Chromium

Copper

Nickel

Zinc

EU, Directive 86/278/EEC

1,000-1,750

300-400

2,500-4,000

EU, proposed short-term limits

1,000

1,000

300

2,500

d:o, expressed as g/kg P

25

25

7.5

62.5

(EU, proposed medium-term limits*

800

800

200

2,000)

(EU, proposed long-term limits*

600

600

100

1,500)

Sweden** (from 2000)

100

600

50

800

Denmark

-

1,000

30

4,000

Netherlands

-

450

70

1,400

Germany, soil pH 5-6.5

-

800

200

2,000

Switzerland

-

600

50

2,000

Austria (Steiermark)

-

500

100

2,000

Canada

-

-

180

1,850

USA, high quality

-

1,500

420

2,800

Medium- and long-term limits removed from latest Working Document Mean value over a 7-year period

Medium- and long-term limits removed from latest Working Document Mean value over a 7-year period

In order to get the proposed limit values into perspective, it would be interesting to compare these values with the weighted average metal concentrations in sewage sludge as reported by the EU Member States for 1999 (all expressed in mg/kg DM) : Cr - 73 ; Cu - 330 ; Ni - 36 ; Zn -811 . As a matter of fact, it can be noted that the average metal levels in authentic sludge in 1999 were all well below the earlier proposed long-term limits for these four metals.

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