Ecological evaluation of long-term application of sewage sludges according to the legislative permissions

Abstract

In Germany the recycling of biogenic municipal wastes, for instance sewage sludge, as carriers of plant nutrients and organic substance in agriculture is promoted by recently published regulations. Limits for maximum application rates as well as for tolerable contents of unwanted contaminants are laid down in these regulations.The published results on effects of long-term application of sewage sludge on heavy metal accumulation in soils and their bio-transfer into the food-chain are, almost without exception, based on experiments with application rates and heavy metal contents of sewage sludge that exceed more or less these limits. Therefore, as far as the ecological value of agricultural utilization of sewage sludge within the valid frame is concerned, they lead to wrong conclusions. This is demonstrated by the results of a long-term sewage sludge treatment (1962–1992) in accordance with the actual limits on application rates as well as heavy metal contents from the very start of the experiment. After thirty years of sewage sludge application no significant influence on the heavy metal contents of the soil could be detected, although there was a tendency of slight increase in values. Furthermore, compared to the control with mineral fertilization, there was no significant effect on the heavy metal contents of wheat (grain) and ryegrass. However, both mineral fertilization and sewage sludge resulted in a significant increase in the Cd content of the test plants but on a very low and ecologically irrelevant level.