Heavy metal sources and contamination mechanisms in compost production

The main problem for the use of compost in agricultural applications is the high heavy metal content of this soil improver.

In this paper the potential sources of heavy metals present in the mixed domestic waste are quantified. The contamination mechanisms in compost production, leaching and direct contamination, are discussed and studied in controlled doping experiments. In these experiments a surplus dosage of specific potential heavy metal sources is added to a compost heap. The leaching process is followed by monitoring the local extra pollution of the surrounding organic waste in time. Information is obtained about the leachability of various components and its characteristic time scale.

To avoid leaching from, and direct contamination with non-compostable particles during composting, quite complete separation of these particles from the composting organic waste is required. Several methods are in principle possible. Experimental results obtained in various large scale operations in the Netherlands are reviewed. There are clear indications that the best results are obtained by source separation of the organic waste.

In the Netherlands governmental standards will be formulated for compost quality concerning heavy metal contents. These standards are likely to be increasingly restrictive in the years to come. It is argued that mechanical separation may reach these standards in the introductory phase, but eventually only source separation of organic waste has the potential to reach the final restrictive demands.