CONTENTS OF HEAVY METALS IN COMPOST FROM SOURCE SEPARATED WASTE IN DENMARK - EXAMINATION OF SAMPLING RELIABILITY

The Danish Ministry of the Environment is at present working on a new, draft Ministerial Order on the use of sludge, wastewater sludge, compost, etc. for agricultural purposes. This Order contains specifications of limit values for a number of heavy metals (mg/kg DM) : Pb = 120 (80 for private gardens), Cd = 1.2, Hg = 1.2, and Ni = 45. After 1st July, 1995 the limit values will be Pb = 120 (80), Cd = 0.8, Hg = 0.8, and Ni = 30. Alternatively the heavy metal concentrations can be stated in mg per kg P.

Results for the heavy metal content in compost from source separated waste prove that the limit values can be observed. At the same time a considerable variation in the heavy metal concentrations has been revealed - also for samples taken from the same compost windrow. This leaves an assumption that the sampling is not sufficiently reliable.

It has been calculated that up to 120 sub-samples (each of 1 kg) must be taken which are then analysed individually to ensure that the risk of error in the sampling does not exceed the risk of error involved in the analysis. The maximum deviation from the expected average is 10%.

Alternatively, up to 140 sub-samples are mixed into one absolutely homogenous aggregate sample from which a representative sample can be taken for analysis. The maximum deviation from the expexted average for this mixed sample is 20%.

The computations have been based on the variation of Pb in compost, as experience shows that this is the heavy metal with the highest variations.

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